BOILER DESIGN – MARINE ENGINEERING
The main problem in boiler design is to determine the appropriate proportion of various surface heating, it is very interesting to use the maximum heat available in combustion products. an appropriate design will meet the lowest cost on its life cycle basis. each component must be integrated with other elements of the unit to provide a balanced design including the main investment costs and fuel, maintenance, and operating costs will be minimum during the life of the ship. there is absolutely no need for the reliability or safety agreed upon by these cost considerations.
For water vapor generator systems, the following must be considered:
- fuel burning equipment
- Boiler generating surface
- superheater (and reheater if used)
- Economizer and air heater
- attemperator (or control) and complement of desuperheaters
- Circulatory and steam separator system
- casing and settings
- Equipment cleaning
- Safety valves and other assistive devices
- Feed water and treatment
- foundations and support (basic and supporting tools)
- water combustion system
- system uptake gas duct and stack
this consideration requires many interrelation steps. in many cases, a number of assumptions must be made to begin the design. Like the design calculation process, assumptions must be filtered to achieve the desired accuracy in the final analysis.
The first step is the selection of the basic types of boilers, superheaters and economizers or air heaters or both used. This selection is based on a more important part and a part that is still available space or space for installation and its operating needs.
The fuel requirements needed are to determine the desired steam generator efficiency , provide vapor pressure, temperature, and current, water feed temperature and fuel heating value
K arakteristik fuel and the amount of fuel burning equipment available to do. This round turn sets excess air requirements. The combustion calculation is made later to determine each hour the number of gas flue that is now through the unit. the outlet or gas temperature stack for which the gas funnel must be cooled to achieve the desired efficiency is decisive (fig.16). and if experience shows that it is satisfactory or attainable, the design can proceed. if not, another efficiency selection must be made and the calculation repeated.
Then what will be calculated is the furnace exit gas temperature. where the value is dependent on the radiant and heat transfer / heat transfer surface installed in waterwalls, floors, roofs, and screens (radiant only) as well as the proxy of the refractory present. Next, the temperature of the gas falls and the heat absorbed by the screen and superheater is determined. the size and spacing of the tubes and the number of surfaces are assumed initially. this will then be modified to provide the desired temperature water vapor and metal temperature conservative tubes as needed. in general several screens and superheater combinations are investigated to determine the most economical solution.
Boiler banks, economizers, and air heater surfaces are sized to provide the required final take-up gas temperature. in each of the steps just described, initialed various thickness choices and types of material for tubes, water drums, and drums made.
with heating surface estabilished, draft loss losses of all components are calculated. if the draft loss exceeds the ability of the desired fan capability, the previously calculated heat transfer and draft are adjusted by changing the tube set distance, number of lines crossing or depth or for the height of the component boiler. a size or number, the number of different oil burners may need to assist in achieving a balance / need for the final draft and fan capabilities.
Pressure drops from water vapor and steam through all components from the inlet feed water economizer to the superheater outlet will be calculated later. they, in turn, estabilish the necessary boilers and the forced economizer design and safe valves that are determined. a circulation analysis is then prepared using decisive heatabsorptions from the heat transfer calculation. of this, the sizes and number of tubes and supplies are adjusted as needed.
the steps in front are followed for each design. however, with experience the designer can make very close the first estimate and substantially reduce the time needed to prepare the design.
3.2 Fuel Combustion
The basic function of a boiler kitchen will produce the maximum amount of heat from a specified quantity of specific fuel. a useful secondary function will produce water vapor in the furnace wall of the circuit tube. the theoretical aspects of combustion have been famous for years. however, the merit of burning goodness in the hearth of the navy blower requires a relatively small amount of knowledge and experience. Completing the combustion can be obtained provided there is sufficient time (a furnace volume funtion), upheaval (priovide by the geometry of the combustion assembly) and a temperature high enough to provide ignition.
combustion may be described as a chemical combination of oxygen with combustible elements in fuel. A common fuel has only three components with regard to the element which is oxygen for producing heat. the elements and their mixtures, as well as the molecular weights and their combustion constans, include blood values, are presented in [table] 1.
O ksigen combined with elements that easily lit adn mix them according to the laws of chemistry. The typical reaction for the combustible oil element, based on the assumption that the complete reaction with the right amount of oxygen needed is,
where Q is the heat that is increased by the reaction.
heat increase or heat combustion is usually called “fuel heating value” and is the sum of the heat reactions of various elements for boosting of fuel considerations. the heating value of a fuel may be calculated from theoretical considerations or it may be determined, for a real oil, by burning an example in a bomb calorimeter (see chapter 12 for additional discussion about this)
P engujian fuel by a bombing calorimater to determine heat up, two possible values (are) reported ,; thehigher (or gross gain or top) lower heating value and ornet heating value. for higher heat values, it thinks that any water vapor formed by burning hydrogen elements is all compacted and cooled to the initial temperature in the calorie meter at the end of the agreement. Steam heat, about 970 Btu / Lb of oil, is included in the reported heating value. for the lower heating value, it thinks that none of the moisture is condensing and that all combustion products remain in a gaseous state. in the United States the higher heating values are used as they are available directly from the calorie gauge determination and because the fuel purchase pratice is estabilished on a higher heating value base. lower heating values are usually used in pratice regarding Europe.
- fuel analysis.
U ntuk comparable design and purpose, the reference standard fuel oil is = 6 fuel oil (6 fuel oil (Bunker C) which has the characteristics as below
The heating value which is higher than the fuel reference standard is determined by a calorimeter bomb and corrects for specific heat at a constant pressure of 18,500 Btu / Lb. the base temperature for the heat content is estabilished like 100F. For a balanced heat design and calculation the heat value from oil correction for additional heat added (in Btu’Lb) in heating the oil to the assumed temperature (200F) (which) is important for fogging according to the following expression:
Added heat = 0.46 (parse to atom temperature -100F)
the total heating value (concerning) the reference oil is, therefore, 18,546 Btu / Lb and is used for all types of extraction which includes vaporizing fog.
- Air Combustion
oxygen is needed for combustion provided by combustion air. Other elements of air act as diluents. air is a mixture – as distinguished from chemical mixtures of oxygen, laxatives, and small amounts of carbondioxide, water vapor, argon, and other noble gases. the basic composition of dry air for combustion purposes is considered as:
gas which is rarely included as part of the elemental limp.
air is assumed to be supplied to the forced draft a temperature of 100F, a relative humadas family of 40 percent, and a barometic pressure of 29.92 in Hg. under the air such conditions have the following physical properties.
based on the fuels listed ahead and their standards, the analysis will show a stoichiometrical state or the quantity of dry air theoretical to burn a pound of fuel is 13. 75 lb. From this, the following amounts of water for various excess percentages are determined:
The final analysis of the fuel that is actually found in the variation of the standard reference fuel. illustrate 17 shows the effect of this variation on the theoretical air required for combustion. for example, a fuel consisting of 87.25C, 12.0 H2, 0.2S, 0.4o2 and 0.15N2 will require 3.0 percent more air for stoichiometric combustion (+ 3.8% for H2, – 0.4 forC, – 0.4% for S) 
UTo reduce dry gas heat losses on top of the pile, the weight / load of the gas flue must be held to a minimum consistent with providing enough air to completely burn the fuel. recognize that up front, an operator needs to observe the resultwith certain fuel oil bunkered and make excess air adjustments to achieve complete combustion. however, in the case of mose, the design of a boiler is based on an air-fuel ratio sufficient to provide 715% excess air. while many oil burners and combusition control systems can operate successfully with excess air, use 15% for design purposes to ensure sufficient surface heat transfer and force draft to blow with sufficient capacity. for / due to additional margins,
P erbandingan fuel-air or air excess is often discussed in terms of CO2, which] is ready obtained from a steam boiler operation by means of a orsat analysis. an orsat reading 14% Co2 corresponds to approximately 15% excess air. illustrate 18 shows of the relationship between Co2, O2 and excess air.
such as heat transfer and draft calculations are based on the weight of the gas flue air weight, use of the term “percent CO2” which is a volumetric measure of importance only in comparing the performance of oil burners. It is most useful where oil is used to be widely used in various analyzes of the standard fuel reference. The excess air, or air fuel ratio, can also be determined conviniently by using an oxygen analyzer, a reading of 3% oxygen corresponding to approximately 15% excess air.
The efficiency of the boiler is described as a comparison of the heat input. heat output is equivalent to heat entering less that loss.
Output heat also can be defined as the difference in enthalpy between the feedwater entered into boiler or economizer , if installed , and the steam leaving the boiler ( both superheated and desuperheated ). When a steam air heater is installed, the heat input from steam is charged to the total boiler heat input and the efficiency becomes:
Efficiency = Hi+ Ha– HLHi+Ha
Where Ha heat is added above 100 F to the combustion air by a steam air heater.
Where Ha heat is added to the beginning of the process of design , one expression is solved for the input of heat Hi , from which heavy oil fired is easily determined by dividing the value of the hot fuel design , usually 18 546 Btu / lb . All amounts are determined based on flow rates per hour .
The efficiency of a boiler required is usually determined by specifications or heat balance . Along with the design steam pressure and temperature , it sets the number and oh settings of the heater surface mounted on the boiler and economizer . The vapor pressure design and saturation temperature are set according to the ” sink” of the boiler bank’s effective temperature , And feedwater set that from the economizer it .
In the case of an air heater installation , the sink is the inlet air temperature for it , usually 100 F. The typical curve of efficiency versus load for the steam generator is shown in Fig.19 . Note that efficiency decreases with increasing steam output . The quantity of hot exhaust gas which will increase is cooled as more fuel is consumed to increase steam output . As it happens, the effectiveness of a fixed amount of heating arises shrinkage and efficiency decreases. This is common to the surface size of boilers for commercial vessels where desired efficiency is ranked in relation to “ABS power”. Efficiency at maximum or minimum speed figures is then a function of this design point and must be at the efficiency of the characteristic curve.
A practical ceiling on boiler efficiency is imposed by the need to maintain the temperature of the uptake of gas above the dew point of the chimney gas. This minimizes the connection of sulfur deposites and corrosion from the cold end of the heat exchanger and ductwork. In economizers, corrosion results in leaks and cormorant forced outages; therefore, it is common practice to maintain at least maintaining feedwater temperatures of about 280 f, which results in a chimney gas temperature from about 315 to 320 f and limits the risk of corrosion.
In a rotating regenerative air heater, a corrosion failure is not due to non-catastrophic properties; therefore, a low temperature stack (280 f or less) is practical and boiler efficiency is obtained higher. Cycle efficiency can be further improved through the use of payment water heaters to force the altitude to provide feedwater at temperatures which are practically not high on a recurrent economizer.
- Selection of oil combustion
The choice of type and number of oil combustion used is dependent on the available draft loss, the dimensions of the furnace, and the ranking fairing boiler. High capacity, wide range burners are usually selected for installation by reducing the number of burners require and simplifying maintenance and operation. The costs of controls and safety equipment, such as maintenance, must therefore be maintained at least.
The size and arrangement of the engine room often affects the location of the burner. It is desirable to place the burner close to the control console for ease with visual monitoring and availability. In both drum boilers, burners can be installed in front of a fireplace for a wall, roof or side wall.
In front of the boiler, the gas shot latitude is parallel to the boiler bank. They compared temperatures of 90-deg directing into the screen rows, and as the depth of the hearth furnace is usually the shortest dimension the gas tends to hoard based on the back of the wall. This heavy concentration of gas is behind the confusion of the gas temperature, and making predictions from the temperature of the water vapor and the metal tube superheater temperature is more difficult. On the other hand, with the roof firing uniformly distributed gas over the depth of the boiler. Since heigth hearth furnaces are usually the longest dimensions, there is less tendency to concentrate the gases before they direct into the filter superheater.
Side firing, with an incendiary on the side wall, requires that careful attention be given to the design details. Since the gas process has no rotation before entering the screen, making the limp tends to be very long long. This can result in a flickering of the screen and superheater bank with one advers effect on the superheater temperature tube and the stearn temperature.
We usually, at least two oil burners are used so that one burner can be shot when cleaning or changing older brothers in the other.
Table 2 Oil burner clearances
|Throat Diameter, in
|Ceterline Spacing, in
|Wall Clearance, in
Ideally, a single burner per boiler would greatly simplify the installation of controls and allow optimized furnace settings. Relatively small boilers, it’s possible to obtain the oil needed to rank with a single burner. In large boilers the amount of fuel and air introduced into a furnace requires several burner installations.
Each burner size that has the least operating rate below the flame becomes unstable and there is a risk of a flammable failure. In this section there is a charateristic of the burner, but the forced draft, fuel, and control system also have an influence. The minimum rate of importance is great since a lot of simpler factories produce when all burners can be left on continuous service. When in town or during maneuvering conditions, the minimum oil flowing capability must be less than that required by the manufacturer’s request, if the valve is safe to erupt repeatedly or steaming the drain is avoided. Both of this action waste water vapor and the leadership to increase maintenance.
Burner packages can be used effectifely to follow demanding contents where burners with a limited or minimum range higher than desired flow are used. Stout status, the computer controlling the system logic is often used for the burning order; however, these equipment can greatly increase costs.
Concerns must be brought about setting up the burners to provide for even air distribution for each burner on the windbox to optimize combustion with the least amount of excess air. Inspection between the burner and the furnace wall must be sufficient to prevent interference and collisions. The volume of the furnace must be large enough to provide important time for the complete combustion to take place before the gas enters the filter superheater. Statisfactory combustion has been obtained at furnace-release rates from up to 1,500,000 Btu / Ft 2 in marine boilers.
Each burner manufacturer has its own recommended clearences and the shape of the flame can be adjusted to any extent to modify them when necessary. This is done by changing the angle of the atomizer. A wider angle is employed to shorten the flame length and produce a bushy wide flame while a shallower angle increases the flame length and shrinkage width. The burner manufacturer must always be of a certain opportunity to refuse the furnace rejected to design it very possibly the best installation can be obtained. Generally the proper burner clearences are shown in table 2. When firing the C oil bunker, it is costumary to use the minimum clearences established by experience. It might drop, perhaps by six inches, if the refined oil was shot.
The choice of oil combustion must also be interchangeable with the type of sprayer used. Alternative sprayers include: steam fogging (internal mix), mechanical steam (external mixture), wide range of open suction mechanics, rotating cups, and others. Of this type, internal mixes steaming sprayers and open-suction sprayers have the highest resistance (here and there 12 to 1) and provide the smallest and most uniform particle size over their range of operation. Perfectly decomposes into the fuel speck atoms a broad surface again probes for combustion and permissions less excess air is used, less thus draft loss, blow strength requirements, and loss of dry gas stacks
The number of burners chosen usually results in the loss of the draft burner equivalent to about 35 to 50% of the total loss of the draft boiler unit. The loss of draft burner varies with the volumetric air flow through it. At each given air flow, an in-change of air temperature will increase or decrease the draft loss ratio of absolute temperature change. Iin designing boilers with air heaters, it is a standard practice to limit the temperature of the air leaving the air heater and entering the burner no more than 600 F and preferably it does not guarantee a long life and prevents overheating of the burner section. If the premilinary design results in excessive air temperatures, the designer must reapportion the surface, perhaps adding a small economizer, to reduce the temperature of the air outlet heating air to an acceptable value.
3.3 furnace designs.
After the combustion rate and the number and type of Agis Mitra Mandiri oil burners, the furnace design is carried out. As the amount of heat absorbing surface radiation provided determines the temperature of the gas furnace to a large extent, the choice of furnace construction is possible: fireproof walls and water cooled walls. Initially all furnaces were simple brick-layered spaces. As the firing rate is pushed higher and the output of a certain size boiler increases, the life of the fireproof wall and maintenance costs become really unsatisfactory. To increase the fireproof life, cooling water in the form of spaced tubes is arranged to absorb heat by direct radiation from the cloud of fire. By this means the furnace wer lowering the temperature and it was possible to increase the rate of combustion in the given envelope furnace [2,3]. Outages for repairs are minimized and the use of lower grade fuel is possible. With the firing rate increasing, the distance tube gives way to tangent-tube construction. Tangent tubes are defined as tubes arranged so that the gap between them does not exceed ¼ in. Fire resistance and material insulation used behind the tube and, in this protected position, they have an almost unlimited life. Another form of water-cooled walls is welded walls, as illustrated by Fig. 20, which is prevalent in large size boilers. In this type of construction, tubes are assembled into panels by the spacing of the tubes and the fill shop-welding bar between them on an automatic welding machine. Tubes from assembled panels are from field-welded to stub nozzles on waterwall and drum headers when units are erected in the shipyard. From a performance standpoint, this type of construction is equivalent to a tangent-tube wall.
For a number of years the floor of the hearth furnace has been cooled with water over a horizontal rescue tube install under one refractory coverage. Water cooling, even if neglected, provided improved life for refractory floors. A refractory is definitely necessary to ensure the long life of a flooring tube since a horizontal tube cannot tolerate high heat entering into this plane. Whatever water vapor forms there tends to envelop the surface, approaching the temperature of the metal tube and lead to the initial failure.
After the selection of a type with a water-cooled surface is used and a preliminary measurement of size and contour has been estimated on the basis of the number and arrangement of the oiling burner, it is possible to estimate the temperature of the furnace to heat the temperature and heat the release rate and the rate of heat. Based on the design of the furnace and burner, the heat of the furnace releasing speed-figures must be limited to that which will result in good combustion conditions and a minimum size cage.
Gas temperature out.
For many years, an accurate estimate of the gas exiting the furnace temperature was not needed because of the conservative results of speed-type fir and its use with water vapor fulfillment. The unit which produces high-heated steam usually has several lines of boiler tube between the superheater and the furnace. As a result, one big mistake in the hearth furnace from leaving the gas temperature stage has very little effect on the performance of superheaters. In units with superheaters placed close to the hearth, however, the hearth furnace temperature must be carefully determined to ensure satisfaction superheater design. In addition, an accurate determination of the heat absorption limit in the hearths of various waterwall areas needs to provide enough water circulation with a practical number from the supply tube and the person waking up.
When estimating the temperature of a furnace gas, most designers use the underlying formula at the time of Stefan Boltzmann’s law, the status of heat absorbed by emission is proportional to the difference between the fourth removal of the absolute temperature of the scatter body and the dark limb surface (see chapter 2). However, in a fireplace furnace the precise determination of the beam beams (the average distance from the gas scattered to very interesting and about the scatter surface); partial pressure from the product of combustion; the amount, type, quantity, and heat content of a fuel; some excess air; temperature of combustion air; latent heat loss; emissivity of various surfaces and scattered collections of gases; and cheerfulness.
Tube cube, which is fired face to face, is usually limited to at least six feet even though there is a very high level of cage in service with a furnace depth of only five feet.
The selection of an oiling burner must also be interchangeable with the type of sprayer used. Alternative sprayers include: steam fogging (internal mix), mechanical steam (external mixture), wide range of open suction mechanics, rotating cups, and others. Of this type, internal mixes steaming sprayers and open-suction sprayers have the highest resistance (here and there 12 to 1) and provide the smallest and most uniform particle size over their range of operation. Perfectly decomposes into the fuel speck atoms a broad surface again probide for combustion and permissions less excess air is used, thus less draft loss, blow strength requirements, and dry gas loss stacks.
- Radiant heat absorbing surface D realm evaluate absorbs heat radiation surface, a flat projection plane tube wall and the bank used. The distance from the tube in the boiler bank adjacent to the furnace has no effect on the furnace temperature, but with the wide boiler it establishes a large percentage of the radiant heat absorbed in the row tube behind the furnace row. Furnace and roof waterwalls usually consist of bare or covered tubes (figure 20) and, with the exception of bare tangent tubes or welded walls, the effectiveness of the absorbing surface is less than the 1.0 black-body coefficient considered for the boiler tube furnace lines. Gas furnace temperatures are usually inaccurate in the initial analysis estimated since the design characteristics are of general interest, and estimates of the gas furnace temperature and heat absorption rate can be made with knowledge of the boiler and combustion conditions. Thus, with excess air assumed, the combustion product heat content and adiabatic temperature can be determined. Furthermore, the estimated furnace size gives an indication of cooled surface water and an estimate can be made of the effectiveness of the expected surface absorption and temperature of the gas furnace. In this natural approach it is usually desirable to estimate the furnace temperature and THE surface absorbs HEAT on the low side of the firing oil when. This increases the estimated furnace heat absorption and guarantees a reserve margin in the final design.
However, with coal combustion it is more important to estimate the temperature of the gas furnace on the high side to prevent possible operation with furnace temperatures above the initial ash deformation temperature. In a boiler furnace, both the gas furnace temperature and heat absorption can be changed appreciably, for a given combustion rate, by varying the amount of surface absorbing heat radiation. The gas furnace temperature and heat absorption can also be reduced, at each combustion rate, by increasing excess air (Figure 21), except when operating with lack of air. Additional air increases the weight of the product burning fuel per pound fired. This reduces the adiabatic temperature because there is less heat available per pound of combustion product, and, as demonstrated by Stefan-Boltzmann’s law, lowering the radiation temperature reduces the rate of heat absorption. In general, the radiation temperature is assumed to be equal to one third of the adiabatic temperature plus two-thirds of the gas furnace exit temperature.
- heat absorption rate
Heat furnace The radiosity of heat radians per square foot of absorbing surface increases with a greater degree of heat release. However, the percentage of heat released, which is absorbed in the boiler by radiation, decreases with increasing combustion rate, and varies from as much as 50%, or more, at lower combustion rates to around 15% at higher firing rates; see figure 22. This results from the fact that adiabatic temperatures remain practically constant, except changes due to variations in excess air temperature and combustion air, during the entire boiler operation range, while the temperature of the gas leaving the furnace and entering the tube increases the bank with the combustion rate. Although the heat absorption rate of the furnace may be conservative, the temperature of the gas furnace exit can be excessive to the temperature of ash fusion and slagging. This is especially true in coal boilers where the temperature of the gas entering the bank tube must be less than the initial ash deformation temperature. Because of the low ash fusion temperature of the oil slag, they passed from the furnace in a gas or liquid state and did not agree to control it by reducing the temperature of the furnace coming out of the gas. They must be considered in the design of the superheater.
- Tube metal temperature
Din a boiler, the rate of heat transfer in the film of boiling water on the inside of the tube may be as Btu / ft2-hrF 20,000, but when estimating the temperature of the metal tube, the transfer rate is only 2000 Btu / ft2-hrF is usually assumed in the framework assumed in order to provide a margin of resistance from possible internal deposits to heat transfer. thus, with a heat absorption rate of 120,000 Btu / ft2-hrF, the temperature drop across the entire water film is around 60 degrees F and, except for very heavy tube walls used, the temperature gradient across all metal tubes is the same. As a result, with a vapor pressure of 600 psig (489 F saturated vapor temperature) and a heat input of 120,000 Btu / ft2-hrF, the metal surface temperature of the furnace tube usually does not exceed 620 F. with a metal tube temperature of 620 F, there is a margin of around 330 degrees F between the metal temperature and the permissible oxidation temperature of carbon steel. However, the resistance to heat flowing on the internal tube scale is sufficient and it is the god of design practice to tolerate variations in feed water quality With adequate boiler circulation and proper feed water quality, heat input to the tube furnace is limited only by the temperature of the metal tube, heat creep, and pressure
- Design limitations
S hile no special furnace exit gas temperature that can be used as design criteria for all types of boilers, they must be high enough to maintain a good combustion in all ratings, including the loading port. However, at the same time they should not be too high causing high casing temperatures or excessive furnace maintenance. Because of the very light and compact requirements for naval unit installations, the evaporative rating in a naval boiler is three times greater than that which is common for the installation of most traders. As a result, the gas furnace exit in full right to an overload range of around 28000-3050 M when firing oil with about 15% excess air. Adiabatic, or theoretical, flame temperatures around 3450-3500 F with oil combustion, 15% excess air, and 100 F air combustion. With a combustion air temperature of 300 t0 350 F, the adiabatic temperature rises to around 3650-3700 F.
Even though the price of the heat release furnace varies, practically all traders’ oil fired boilers are designed for a heat release rate of 65,000 t0 125,000 Btu per cf furnace volume per hour at a normal-rating of around 15 t0 20% of the heat discharging electric tariff that matches the full naval boiler. The heat release per square meter of surface absorbs radiant heat generally ranging from 200,000 to 250,000 Btu per hour on a merchant boiler design. Naval boilers are designed to rank four to five times larger than those used for sea merchant boilers. Radiant heat absorption rates vary greatly, depending on the rate of combustion and the amount of cold (water cooled) surface in the furnace. Generally, a radiant heat absorption of 120,000 btu per square meter of cold surface per hour is considered satisfactory for the continuous operation of excess boiler traders with treated evaporated feedwater.
This results in an absorption of about 100,000 btu per square meter of cold surface per hour at full load value. There are boiler traders in continuous service with heat radiation absorption of around 150,000 btu per square meter of cold surface per hour, and most naval boilers have been designed for heat absorption circle rates of 150,000 to 200,000 btu per square foot of cold surface per hour overload rating, but operations at this level are rare.
3.4 Boiler Tube Bank
The preparation of the tube boiler bank was formed after the construction of the initial furnace size. The simplest type of tube bank is that a boiler provides saturated steam. usually two tube sizes are used at the bank. Tubes in rows adjacent to the furnace absorb far more heat than in other rows and, therefore, must be of greater diameter to increase water flow. Put the amount of heat into the tube tube furnace is the amount of radiation heat transfer and convection, in general, convection heat transfer is about 5 to 20% of the radiant heat transfer. relatively wide coverage in heat transfer convection results from variations in tube diameter, tube pitch, gas mass flow rate and temperature differences between combustion products and the tube surface. The number of tube lines installed mainly depends on the circulatory system and the desired gas temperature leaving the tube bank. The temperature of the gas leaving the boiler bank tube varies with changes in vapor pressure, combustion rate, and tube size and arrangement (arrangement of the tube may be either staggered or appropriate). However, sufficient heating boiler surfaces must be installed to obtain an outlet gas temperature which results in economical operating efficiency and does not require excessive stack and breeching insulation. Generally, the temperature of the exit gas must not exceed 750 F unless economizers or air heaters are used. Resistance to gas flow can vary appreciably by changing the pitch of the tube in a direction perpendicular to the gas flow or from changes in boiler s width, tube length, and number of tube lines. Simply rated drum boiler types usually have 1 ½ in tubes in the furnace line, but this increases to 2 in the boiler of higher rank. An inch and 1 ¼ in tube are common in major tube banks. There is no standard pitch for the tube. However, customary use of a minimum longitudinal pitch pitch (direction parallel to the drum and perpendicular to the gas flow) is consistent with good manufacturing practice and acceptable drum design, unless the draft requirements or the type of fuel fired determines the use of greater pitch. Manufacturing and fabrication practices permit the use of ½ in. Metal ligaments between 1 in or 1 ¼ in OD tubes. The circular, or back, pitch (the direction parallel to the gas flow) of the tube is usually set to maintain the efficiency of the circular or diagonal ligament equal to, or better than, the efficiency of the longitudinal ligament in the tube. Tube arrangement utilizing a minimum return pitch reduces the drum edges needed for a certain number of tube lines and allows the use of smaller diameters provided the steam drum steam drum satisfactory release speed. With these settings, the size and weight of the boiler can be reduced. When designing for high vapor pressure, it is often necessary to increase the distance of the tube in order to increase the efficiency of the ligament and reduce the thickness of the drum tube sheet. If this is not done, a large thermal voltage can be specified on the tube sheet. It is also possible to maintain a close tube distance and have not reduced the thickness of the drum sheet tube by using tubes with swaged ends for small diameters.
The number of lines of installed tubes must be limited so that a large diameter impractically drum vapor is not needed and so that the absorption of heat in the last line tube is sufficient to maintain good circulation. The length of the tube must be such that the total absorption per tube does not result in too high a proportion of the steam in the water vapor mixture leaving the top end of the tube. Most marine boilers provide superheated steam from convection-type superheaters. In hese boilers, tube banks produce arranged in two parts. This part between the furnace and the superheater is known as the “waterscreen” and the other part, installed outside the superheater, is called the “boiler bank” or “generating bank”.
The size and arrangement of the waterscreen greatly affect the superheater design. A superheater located close to the furnace behind several rows of tubes extensively camping on the waterscreen provides relatively flat temperature steam characteristics through various ratings since radiant heat rates and transfer convection tend to complement each other. However, the superheater located further from the radiation furnace behind the waterscreen has more characteristic steam temperatures that increase sharply with an increased rating , because of the greater effect of convection and a reduction in heat radiation transfer rates.
Naval boilers usually have waterscreens consisting of three or four tube lines and sea merchant boilers generally have two or three row waterscreens in front of the superheater. Most of the heat transfer at this superheater is due to convection and inter-tube radiation, but the distance from the screen tube can usually be chosen to permit the furnace to be relatively constant steam temperature through various ratings.
The superheater must deliver the specified water vapor temperature during the life of the boiler-not only during the initial trial or prediceted test operation and achievement must be maintained continuously with minimum variations in burn rate, air pressure , the decisive burner, and the air exess. The design avoids the need for non-scheduled outeges for cleaning, etc., in order to regulate achievement. Of course, scheduling a steam boiler periodically is necessary wherever a well-organized operation is scheduled.
The design of the superheater is the most difficult and difficult part of all the components of the boiler as it affects many of the funtional boilers and mechanical shades. An adiquate fall in the pressure of water vapor through the superheater is needed for good distribution of water vapor and tubes satisfying metal temperatures and is an important factor in establising the design of the forced steam boiler. Design pressure dictates the thickness of the advanced heating pipe, which in turn is an important factor in determining superheater pressure drop and metal temperature tubes.
The superheater placement affects the tube and the size of its metal temperature [2,3]. It also influenced the design of the fireplace screen waterscreen, especially in high-temperature units. Placement and Tubearrangement have an important bearing on the possible slagging and this directly affects maintenance and outages.
There are basic considerations that are common to all types of superheaters, which are designed to have very little surface heat so that they reduce cost, size, and weight. A minimum surface can be obtained by continuously increasing the heat transfer coefficient and the differential temperature between the combution product and water vapor since since the total heat absorbed is the product of these two factors and the surface. The temperature differential level takes advantage of temperatures that can be guaranteed against potential notes, while an increase in heat transfer coefficient requires a greater resistance to install gas currents. The full advantage must be taken a high temperature difference, but entering the gas temperature must not be very high such as results in excessive metal temperature tubes or high-temperature rusts of trees with hardwood fuel (this is primarily a matter of placement). Changes in the temperature of water vapor with shot rates must be a minimum in order to prevent excessive temperatures during maneuvering and, again, this depends on placement. Water vapor Acceleration needs to provide a kind of fir good water vapor distribution, minimum minimum temperature, and accepteble water vapor pressure drops, all of which require connecting effects to the size, placement, and regulation of water vapor through.
a.Types and characteristics.
The radiant / star-shaped and convectin-type superheater are the two basic types of bases. They are, as the name implies, superheater which is heat sensitive by radiant or star-shaped heat transfer and they may be arranged hoizontally and perpendicularly .
In the starry type the temperature of the water vapor is reduced by increasing the rate of heat absorbed by the radiation not increasing according to the ratio with the flow of water vapor; see Fig. 23. In gas transfer typing, the temperature of water vapor usually increases with the maximum load rating increased because the absoption heat, due to the higher heat transfer coefficient and higher entrance gas temperature, increases at a rate compared to the output of water vapor.
Most superheaters are a combination of two basic types in which the designer builds a star-shaped component to achieve a temperature characteristic. Usually, the “hairpin” arrangement of the tube, as illustrated by Fig. 24 , where the water ram / hammer is connected to one another by U-Shape Tubes, used. Multiple-band, Ot rotates continuously, the design is frequenly used for high-pressure, high-temperature installations to reduce the number of tube joints and the thickness of the water hammer.
- Arrangement of steam passes.
water vapor through the arrangement must cause an acceptable pressure drop and the tube to satisfy the metal temperature. A change in the amount of water vapor passing greatly affects the retaliation to the water vapor stream and the temperature drop tube but, in general, has only a small influence on the rate of heat transfer rate.
The regulation of water vapor via the provider is a good distribution if the retaliation for flowing tubes is higher than for the ram. Placement of the entrance and drain connection / hammer water channel also affects the distribution of water vapor. The tubing flowing area of the pass pass should be less than the current area in the water hammer inlet in order to minimize the tendency for water vapor to bypass certain tubes.
The heat absorbing at each water vapor pass can be assumed to be proportional to the absobing heat, unless there is an uneven distribution of gas flows or the temperature difference between the combustion product and the vapor exchanges appreciably between the vapor passes through. however, the increase in the temperature of the water vapor per pass is not proportional to the heating field, because of the specific heat of the vaporwhich is heated further with less temperature.
An accurate estimate of the temperature of water vapor entering and leaving each pass is necessary to determine the retaliation for flow and to design and arrange the cavity bulkhead in the superheaders. If a large amount of heat is absorbed at each value, the temperature differential across the cavity barrier is high and can be assessed stressing provided the water hammer. These stresses may be satisfactory as long as the power is related, but they can cause leakages in the sitting-further heating pipe. When the temperature across the differential cavity barrier is more than 175 deg F, and additional water vapor cannot be used, it is cutomary to use a water sparate. although this arrangement reduces heat stresses and eliminates seated seat leaks, wide gas paths are formed at the junction of the water hammer. therefor, the tube that folds the path will have a higher metal temperature than the efficiency of the tube becouse the greater the gas flow that is past the pathway tube.
- T ube temperatures, materials, headers and attachments to.
Tabung metal temperature depends on the temperature of adjacent water vapor and gas; tube size, thickness and material; metal thermal conductivity; heat transfer vapor film rate; overall heat in. Vengeance to the heat flow through the metal is generally very small compared to that across the film rod, the metal tube raised temperature arrives mainly to drop high temperatures to the opposite side of the film water vapor. An increase in the water vapor flowing period, obtaining at the cost of a higher pressure drop, will increase the rate of displacement of the water vapor film and both the temperature gradient across the water vapor film and metal surface temperature (outside). Therefor,
Loans must be made for unequal distribution which may be both water vapor and gas flows in calculating metal temperature tubes. Unless there are exclusion conditions, it is usual to consider an uneven distribution of 20% of the side of the gas. On the water vapor side, an unbalanced aclculate , which is the depent on the tube and the water hammer adjustment, is used. The tube with the highest metal temperature is generally ancountered which is a corrugated tube which has a minimum percent of water vapor flowing and which receives about 110% of the average gas flow.
From the perspective of heat transfer, it is desirable to use an opposite current of water vapor and combution products in order to increase the temperature differential between water vapor and gas and thus reduce the required amount heat that absorbs the surface. However, with high water vapor temperatures this may result in excessive tube metal temperatures because since the largest amount of heat will be transferred to those tubes that carry the highest temperature water vapor. Therefor, Direct current is often ussed the last water vapor passing through. A small number of additional heating fields will be needed to compensate for the use of a direct current; however, a lower value metal alloy tube can be used.
Typically, the tube is bound with affection with a water hammer curled when the water vapor temperatuers is below 850 F, and is welded above this temperature. If, because the temperature or service intends, it is necessary to weld the tube to the headers, special consideration must be given to the material and the welding method. Similar material presents a slight problem; however, different materials are used tubes and water hammer. various difficulties of meeting the weld tube to ramming / hammer the water is reduced with making the tube “safe-ended”. A safe-end is a short tube section of a material compatible with its water hammer, which facilitates the welding area. different increasingly difficult Materials and integration between safe and ended cylinders are made under controlled conditions in the store.
Most superhaeters have a water-cooled support consisting of a test tube with a number of metal alloy brackets. Frequently, the tip temperature of the brackets of these metal alloys is more than 1700 F and the metal alloys must be able to withstand higher temperatures in order to provide an operating edge for the distribution of uneven distribution of gas flows and temperature.
- Locations of headers.
in steam boilers with horizontal superheaters, water hammers are usually placed; located behind, and renewal of the tube is the center of attention of the steam boiler. Where vertival superheaters are used, the tube screen can be set to allow the heating pipe to be renewed through the center of the fireplace or the boiler buttocks. In other settings, further heating pipes.
Can be renewed think superheater cavity. The use of the firing tunnel for the extension of the superheater tube reduces the space needed for boiler installation. f. high temperature slagging and corrosion.
Slagging and high temperature corrosion of the tubes and supports varies considerably with the type of fuel oil used and the amount of fuel oil contamination, especially by sodium chloride and vanadium salt, which is constant which results in heavy slagging and corrosion.
The investigative laboratory shows that for tube materials given the corrosion rate increases with the temperature of the gas increases or the temperature of the metal. Furthermore, the level of corrsion increases greatly with the increase in vanadium in fuel oil as shown by Fig. 25.
In oil-fired units, boiler tubes and superheaters are usually pitched at the center of a minimum practical to reduce boiler size and weight. Consquently, as shooting and epavorative improve the ranking and quality of the fuel oil deteriorates, the accumulation of slag in the superheater becomes the main design and maintenance problem. To overcome this, the superheater design has been developed with an “in-line” istead of setting “staggered” tubes, increased tube pitch, and with superheaters located within the lowest partical gas temperature zone so that the gas provider provides the most favorable metal-temperature relationship. For commonly used materials, this means that the temperature of the metal is limited to 1050 F. This cavity or “walk in” superheater typer (Fig. 4 and 5) combining these features and greatly increases accessibility for cleaning. A further infrastructure for the cleaning and maintenance of superheater-running supplies in the Beetwan boiler tank cavity. This cavity facilitates the removal of corrosive slag and soot deposits which accumulate on the drum water. Although the boiler features result in large and heavy, economical units are more operational since maintenance and outages are reduced.
Experience has shown that the use of diligent sootblowing tools (especially mass-action retractable units) can usually keep a superheater surface satisfactorily clean for one year, or more, operations and manuals that cleaning and washing absorb the external surface heat needed only during scheduled overhauls.
The design of the heater involves the same procedure and consideration that the patient has to design the superheater. However, the distribution of steam and the temperature problem of metal tubes is more critical because heaters must be designed for very low vapor pressure losses if high cycle efficiencies are to be obtained.
Steam or combustion gases can be used as heating reheaters. When steam is used, the temperature of heated steam is usually limited to 550-600 F, because it is customary to use condensation rather than superheated steam as a heating medium because the rate is much higher than heat transfer.
The use of gas reheaters is necessary if high steam temperatures and reheat cycle efficiency are required. The heater can be fired sparetely or installed in the right boiler. Separately fired reheaters are not common because they require individual firing tunnels and low clearance, as well as additional pipes, controls, breechings, firing equitments, fans, etc. 3.6 air heaters and economizers.
P emanas economizersor air or both are required to obtain a high efficiency boiler. The saturated vapor temperature at 850 psig is 528 F and the temperature of the combustion product leaving the boiler pipe tank will be, for a conversative boiler design, around 150 deg F above this value, or around 675 F. When burning oil, and operating with CO2 14 .0 percent in combustion products (about 15 percent excess air), the temperature of the uptake gas will result in an operating efficiency of only around 80 percent as can be seen from Figure 16. If the absorption gas can be cooled to a temperature equal to the temperature of the saturation of the vapor to which the use of an infinite amount absorbs surface heat, the increase in efficiency will only be 83.75%. Therefore, air heaters or economizers must be installed to increase the full load efficiecies to the 88-90% range normally desired. Furthermore, the use of a high evaporative rating at any given steam pressure increases the need for additional heat-reclaiming equitment.
When air heaters or economizers are installed, the proportions of the boiler, air heater, and surface of the economizer must be balanced. Usuall, the temperature difference between products from combustion and heat = absorbing liquid in the economizer and air heater is greater than in the last part of the boiler tube bank. This is advatageous in reducing the surface absorbing heat needed for heat recovery. In air heaters, the nominal advantage resulting from an improved temperature difference is offside by high resistance to heat flowing in the air film (19). Therefore, the proportion of surface components must be studied carefully to obtain the most economical overall arrangement. The minimum feed water temperature for traders economizers at sea varies between 270 and 280 F. The standard feed water temperature for naval installations is at most 246 F. This satisfies lower temperatures because premium fuels with sulfur fuel content are used. The keeper of the gas temperature leaving the economizer cannot be less than the temperature of the incoming water, so follow that high feed water temperatures limit the efficacy obtained. Consquently, with high feed water temperatures, the economizer is not often used unless it is installed conjuntion with an air heater.
In an air heater, the minimum temperature of the gas uptake depends on the temperature of the incoming air. Therefore, the attraction of Thye air heater installations is because of the possibility of operating with high boiler efficiency if using feed water temperatures in the range of 300-450 F.
When the steam tubines bleed to defeat the regenerative feed, the efficiency plan increases by about 1% for every 100 degree F rise in feed temperature because the condenser heat loss decreases. Does this guarantee an increase in the expenditure efficiency needed for heating additional feed and other equipment must be carefully for each application.
The use of air heaters requires an increase in air pressure for the boiler unit because of the additional resistance to the flow of air through the air heater, the air pressure must also be increased when using economizers because the resistance is relatively high for gas flow in the economizer, but, for boilers that operate in a size that is Same with comparable firing rates, an air heater installation will usually require a high air pressure that will total the unit equipped with an ecconomizer. P. Air heat is not pressure vessel, so the tubes can be made of mechanical pipes (cheaper then pressure tubes) which are lightly expanded into sheet tubes. However, economizers are part of the pressure system and must be designed to withstand the main raw pump pressure being depleted, to operate without leakagle, and withstand themal shock.
- Air heater.
Heated air can increase combustion, reduce boiling sooting, and reduce the possibility of ignition losses especially at the extreme low end of the firing range.
Practically all marine oil heaters are older than the tubular type, but the regenerative rotating air preheater is shown in the figure. 14. Its gas-tight casing forms part of the boiler forcing air-draft and gas drain absorption. The heater is separately installed above the boiler and the expansion joint is suitable for use in two joining channels.
An important component of the heater is the rotor where the heat transfer plate elements are packed. Air for combustion is axially packed through one side of the rotor while the exhaust gas is passed through the other side in the opposite direction. As the rotor turns. Heat continues to be transferred from the gas to the heating surface; heat also continues to be given up to the air as the plate is heated across the side of the air. Opposing gas and air counterflow guarantees efficient heat transfer.
The heat-transfer element is made of corrugated and flat sheets, which are alternately packaged in the main part of the heater and in a cold-end basket. This cold-end basket is designed to be easily removed for cleaning or replacement while guaranteeing conditions. For daily cleaning, cleaning devices consisting of soot mass-action installed.water blowers and bypass gas dampers are an integral part of the preheater and are useful in maintaining metal surface heat transfer temperatures above the dew point of the gas. This minimizes corrosion at low levels of operation and also helps minimize soot buildup. This damper operation can be made fully automatic.
P emanas Most tubular-type aerial horizontally (Fig. 13). The vertical type is not often used because it is needed to install cosiderably more surfaces for the adsorption of heat needed than is needed for the horizontal type. It is customary to pass air through the cylinder and the gas in the cylinder. In the vertical type the gas usually passes through the tube and air passes through the tube.
Horizontal air heaters usually utilize tubular tube settings in-line. This facilitates cleaning of external heating surfaces, a feature considered to be far more beneficial than slightly higher heat transfer obtained by staggered-tube arrangement.
T Abung air heaters range in size from 1.5-in. For 2.5-in. outer diameter, with most installations using 1.5-in. Tube. If resistance to internal flow must be reduced by a large cylinder is preferred because, for a gas flow given mass, resistance is inversely proportional to the inner diameter.However, compactness is a key requirement of marine boilers and, therefore, the use of small cylinders may be better to permit installation from the maximum surface in the available space. Furthermore, the heat transfer coefficient of gas and air flowing throughout and trought the tubes, respectively, varies inversely as about the power of 0.34 and 0.22 of the tube diameter and, thus, the heat transfer at each mass flow rate increases with a reduction with a tube diameter (2.19).
In the initial airheater design, both tube and pitch tube sizes were assumed. It is satisfying, in many cases, to consider 1.5-di. For 2-in. Tube with 0.5-di. Tube ligament. This estimate is then made from the length of the tube, the number of tubes per row, the number of tube lines, and the number that passes through gas and air. This makes it easy to determine the level of heat-transfer and surface heating. The initial assumptions are then adjusted, if necessary, so that the surface and heat-transfer settings provide the required heat absorption. Gas and air flow patterns must also be analyzed since the distribution can reduce heat absorption, increase fan power, reduce heat absorption, increase fan power, reduce or increase the temperature of metal tubes, or limit the capacity of boiler units.
The design of the air heater is usually predicted at an inlet of around 100F and at an output of around 300-450 F under normal operating conditions. Design gas output at a temperature of 290-320 F is used for the tubular version of the water heater and the result is the efficiency of the boiler is 88.5-88%. The renewal of the air heater can be designed to reduce gas intake which causes corrosion. Air temperature from 240-260F for renewal of air heater with boiler efficiency is 90-89.5%
Both the weight of the production at specific heat is better than the combustion air. When injecting oil is about 15%, the reduction of the combustion product temperature of the heater is about 13%, less than the air temperature.
The arine economizer can be classified into 2 groups, “bare tubes” and extended surfaces. Both types are usually designed counterflow from water and combustion products. The result of the differential magnitude of temperature, and better absorption. . depicting counterflow will increase the efficiency of the boiler because the discharge approaches the inlet water.
The size of the economizer ranges from 1.5-2 inches. Many types of extended surface economizer. The most striking thing is having spiral welded steel fins. Some features include a joint on the tube used by the header. Marine economizer uses a counterflow design where the upward flow of gas and downward gas. Pressure drop ranges from 25% of normal operating conditions. The minimum condition of the required pressure drop is not in a parallel flow condition, the flow rate to the top of gas and water.
Although the economizer is operated in feedwater with temperatures below 180F, it can be set to 220F. This temperature will minimize corrosion. With residual oil used in feedwater at a temperature of 270-280 F. From that, we will get an economic counterflow designed to reduce the temperature of the outgoing gas in the range of 320F. The economizer of counter flow (down flow water, upflow gas) differential temperature at least 50 deg F must at least 50F must be maintained. in the bypass economizer, the total of efficiency losses ranges from 5-7%. The gas temperature entering the economizer is usually less than 950F. The most important thing, when operating with an economizer, will increase the superheater outlet temperature. The result will improve the flow of gas at turns, increases the mass ratio of steam and transfers a lot of steam.
If the feedwater is from an economizer that contains oxygen, the oxygen will release when heat is generated for the tube and the corrosion header.
3.7 D esuperheater and A ttemperator
Desuperheater and attemperator will produce a heat excangers which will reduce and control the steam temperature of the superheater. The two distinguishing things are the drum (or internal), where will be installed steam or water drum and external type, where it is located in the external pipe system of the boiler.
The internal desuperheater consists of a single pipe or there are several small diameters of rotating or welded tubes in the manifold and installed below the water level.
The auxiliary superheater function will reduce the temperature of the superheated steam output of the boiler used for auxiliary machinery, general heating. Usually designed not more than 50-75F where the residual of the superheat and designed maximum flow with a pressure drop of 75-100psi under the superheater outlet.
The type of desuperheater flow rate is relatively 5000-20000 lb / hr. To improve flow, a desuperheater type bundle is used. The maximum flow that occurs around 150,000 lb / hr in large tankers where cargo heating and machining pumps will be higher. No control needed in desuperheated machinery cannot be cooled. An external desuperheater is normally sprayed to increase the quantity of desuperheated.
Control of the desuperheater is used to control the steam condition in the end. Superheated temperature which is a function of rating and used in marine boilers. To more effectively use the material in superheated and main pipes in steam so that it can control the temperature,
Desuperheated steam is returned to the last path of the superheater where it mixes with the main flow to provide a design temperature [2,3,8]
A manually operated valve or an autiomatically controlled valve is used to regulate the temperature at allrates above the “control point” (the point on the characteristic uncontrolled steam temperature curve that crosses the controlled cooled temperature line).
3.8 circulation and steam baffles.
The confusing characteristics of the natural circulation of the boiler and steam drum are determined after the arrangement of absorbing surface heat has been established. Generally, due to the influence of steam drum baffles on the circulation system, simultaneous analysis is carried out. Circulation calculation procedures are in the empirical section and in the theoretical section. The purpose of this analysis is to establish a system of downcomers, increase, and produce tubes, which will ensure that each tube receives an ade-quated supply of water with respect to the maximum heat absorbed.
boiler water tube banks and wall-furnaces. The characteristics of the circulation of furnace waterwalls and tube boiler banks are determined by the same procedure and, because the water-vapor ratio decreases with an increased rating, characteristics must be set for maximum contempiated rating.
In analyzing boiler circulation, it can be assumed that each circulating system is, in essence, a U-tube [8,20]. The riser portion of the U-tube is the part of the tube bank where the upward flow of steam and water as heat is applied. The downcomer section at the top of the column consists of a heated tube or parts of a banksin tube that absorb heat lower than in the riser section. Due to differences in fluid density, the heated tube can act as a downcomers for the riser section and there is a definite transition zone between the heated dowencomers and the riser tubes, locations that vary with changes in boiler combustion rates.
In the U-tube analogy, there is intianlly a vertical pressure field at the bottm where the pressure exerted by hot and cold water feet is the same. As heat is applied and so much water circulates, resistance to flow is encountered. Thus, in the same hypothetical pressure field in the lower water drum, or header, the pressure corresponding to the flow of water through downcomers is the same as the product of the head of water in the riser and its density minus resistance to flow. Pressure must balance the product of the head of water on the steps and its density plus resistance to flow. By equating both the number and the solution to the friction loss in the downcomers, it is evident that the friction loss for the downcomers is the same as the product of the water head and the difference in the density of the downcomer at the top of the column and rung,
In the circulation analysis most of the steam produced in the riser tube is calculated and the water-vapor flow, as well as the head are clean, is then determined for the ratio of water-various vapors. In analyzing circulation characteristics, it is costumary to plot graphically both the downcomer friction loss at the top of the column and the net circulation head available for the assumed water-vapor flow mixture. As shown by Fig. 31, the flow at which the head is available minus the resistance to flow through the steam baffle equals resistance to the downcomer flow at the top of the column is needed to balance the circulatory system. From the current at the balance point the stean percentage by volume at the top of the riser tube can be calculated.
The percentage of steam by volume at the top of the riser tube must be like to prevent overheating of the tube. If the gthge quantity is excessive, the circulation system must be redesigned to provide additional downcomers, or the size and contour of the muast downcomers are changed to reduce resistance to flow. It may also be necessary to change the location, size, and contour of the bopiler tube to distribute heat absorption and reduce flow resistance.
In a satisfactory circulation system, an adequate amount of water must be given for every pound of steam produced. Therefore, if the percentage of steam by volume at the exit of the riser tube is used as a design criterion, it is necessary to vary the allowable percentage as a change in pressure since the percentage of steam by volume will increase because the pressure decreases due to the specific volume of steam increasing. Naval boilers are usually designed for a water-steam ratio (weight or water / weight of steam passing through a power plant tube) ranging between 5.0, and 10.0 and merchant units usually fall in the range of 15.0-20.0 at overload rates operation. Lower water-steam ratio is used on naval boilers in order to reduce boiler size and weight by minimizing the requirements of the upper column downcomer.
- Heated Downcomers.
If the conservative evaporative rating and the temperature of the gas leaving the boiler do the ONT exceeds around 750 F, the first few lines of the tube will serve as an additional enhancer by serving as heated downcomers. As the combustion tariff increases, the zone of high temperature gas moves further back to the tube banks and additional tyubes because the temporary rungs act according to smaller numbers as downcomers. If the combustion rate is further increased, the number of downcomers becomes insufficient, circulation prevents, and the victim tube can occur, when the design design shows such circumstances, the external or internal heated downcomers must be installed.
- External and internal heated downcomers.
With a conservative evaporative rating, external downcomers are needed for only parts of the boiler where the tubes cannot act as downcomers (a single tube line forms the furnace boundary, a shallow tube bank is installed between two furnaces, or a tube bank protects the superheater from two furnaces) .
If downcomers are needed for the main tube bank, they are usually located outside the tube bank even though the arrangement requires more drum boilers. The use of a heated internal downcomers minimizes drum lengths and removes tubes in the main boiler bank, but not heated internal downcomers usuallyenter steam drums at high water levels and they may lose water during roll weight or intentional reduction at water level. Furthermore, the use of internal heated tube downcomers complicates bank settings, increases resistance to gas flow, and reduces the surface of the boiler absorbing heat. Heat transfer for internal downcomers can be minimized by using plates, stud-tubes, or finned-tube baffle protection.
- confusing steam drum.
U ap drum confusing is used in the simplest marine boilers in construction and arrangement. The type commonly used in header type boilers is “vertical insulation,” which is located between the dry pipe and the circulator drain tube. The only checking design needed when using a vertical baffle is the determination of the velocity of the rear vapor and around the insulating end. This speed, based on the maximum boiler steam output, must be less then the critical speed at which the steam takes water, while the velocity of steam can be reduced by increasing the size of the drum steam or by tilting the baffle.
Single and multiple hollow-plate bulkheads, as in Fig.32 (a), are used in most drum-type boilers operating at conservative steam ratings; This baffle depends on the natural separation of steam and water. For higher boiler ratings a positive means of steam separation is required and a compartment-type bulkhead, FIig.32 (b), is often used.
Centrifugal steam separators are used mainly in highly rated merchants and Navy drum-type boilers; they are highly desirable for subjectedto fast boilers, fluctuations in water level, or high solid concentration in boiler water. Steam centrifugal separators can be adjusted both hirozontally and vertically in the steam drum as in Fig. 32 ©.
Resistance flowing through centrifugal separators is greater than that through baffle-type plates orcompartments. This will tend to increase the need for downcomers at the top of the column, or obstruct circulation, but the bootom discharge from centrifugal separators is practically water vapor free, and thus the avaiable head for circulation increases because the density of water supplied to the downcomers is greater than that of “foamy” water- Mixed steam is removed from perforated plates and compartment-type bulkheads.
The flow of water vapor through drum baffles is in series with all the flow circuits in the circulatory system. So, if flow through one of the circuits is increased, for example, by the installation of additional downcomers, the flow through the steam baffles also increases. This causes an additional resistance in the Overal circulation system with the result that the flow in downcomers will not increase in direct proportion with the addition made.
3.9 construstion and physical requirements.
The structural design of the drums, headers and tubes must be in accordance with the governing body rules governing shipbuilding (USCG, ABS, USN, Lloyd’s, etc.)
- Drum. The construction of steam and water drums is basically the same.
A cylindrical drum with the tip closed by the head of one of the semi-elliptical or hemispherical shapes. Drum shells are usually made of plates called wrapping sheets and tubes. Tube sheet thickness is greater than the wrapping sheet to provide the required strength in the way of the tube hole. For the most part, the drum is a welding construction although in a smaller size one section of hollow forgings can be used. Drum heads are usually forged.
For commercial works 70,000 tensile steels are widely used in construction drums; As a weight reduction, 80,000 tensile steels are used.
Steam drums range in diameter from 36-72 in. With the most influential 48-in to 54-in. drum and naval boilers use 46-in to 60-in drums. As the power level increases, drums 60 to 72 in diameter are used more frequenly to provide the space needed for the steam baffles and to provide the ability to accommodate the shrinking and swelling that occurs when maneuvering.
- Headers and tubes.
Headers for waterwalls or economizers are usually made from stock pipes. Hollow forgings can also be used primarily for superheaters. They even become round or forget quadrilateral or other to facilitate tube installation. Tubes are installed by extending or by welding.
Standard and economizer boiler tubes made from electrical resistance whether welded or seamless stock. expensiveand less welded electric resistance tubes have proven to be as reliable as seamless vessels in boilers and economizers. Superheater tubes are made of seamless stock alloy steel or tubes, as required by the temperature of the metal involved.
Air heater gas cylinders are usually made from welded mechanical pipes because the defferential pressure between air and gas is minimal, and does not guarantee the cost of pressure pipes.
- Design Case.
` Envelopes containing combustion air and exhaust gases are known as casings. The main function is for the content and air ducts and combustion of the exhaust gas through the pressure section. An important secondary function is to reduce heat losses to the engine room, thereby increasing both boiler efficiency and engine room occupancy. Good design practice limits the average temperature of the outer surface of the casing to 130 F or less. Local areas, for example where the superheater inlet or outlet nozzle penetrates the casing, may be hotter because “through steel” which does not allow insulation to be effectively applied.
Most boilers are double-casing construction. An inner and outer casing is used to form the air space surrounding the boiler. This space is pressurized by combustion air, which is at a higher pressure tha exhaust gas (by no less than the loss of a draft oil burner). Any tendency of leakage in the doorway, access door, etc. will result in leakage of combustion air into the engine room or conducting side fire. On the other hand, with a single-cased kettle, the combustion gas discharges into the engine room during a leak.
envelope from the combustion gas is channeled using the casing. This function is to reduce pressure. Another function is to reduce heat losses in the engine room. Good design around under 130F. Most boilers use double construction design. The inside and outside are used to circulate air. This space is also used for pressure with combustion air, where the gas pressures are below the fuel. Tendency of seals, doors etc. In the picture 33 double casing in boiler construction. Thickness of the insulation and refractory material.
The structure of the strength of the casing supports several pressure parts. This casing will support rolling and pitching on the ship. It is usually practical to support the header of the superheater in the case structure. In a large boiler, there is a double wall where this is one of the conditions.
3:10 O il burner
There are two basic principles of oil burner, atomizer and air register assembly. Atomizer is used to service furnaces in the form of particles. The air register is used for throat, air doors, vanes, impeller or air diffuser. Atomizer and air register use valve, fitting and safety coupling. Safety couplings are designed to prevent cracks in the steam or oil from the atomizer where it is used for cleaning. Fuel from a marine boiler. Where for the old boiler use a mechanical type that uses a pressure of 100-300 psi on land, if at sea requires 600psi. Oil is sprayed to the plate after that. In some cases the mechanical atomizer basically continues where it increases turbulence.
This type of rotation is another example of a mechanical part where the air that is sprayed by a jet will increase the effectiveness for propulsion from the boiler.
3.11 B mounting oiler
Boiler mounting covers
- Stop valve
- Feed check valve
- Feedwater regulator.
- Safety valve
- Sentinel valve
- High and low water level alarm
- Pressure gauge
- Vent and drain valve
- Blowdown valve
- Water level indicator
- Water sampling connection
- Soot blower
- Burner flame scanner and ignitor
In general Buffer boiler ” accessories,” the pitch is important for the control and operation of boilers and safety in operation . The term ” mounting” is a synonym of the term “fitting .” Reliable boiler operation can be obtained only if the Buffer is chosen with due regard to quality and functional compatibility . When installing a stand , the thing to consider is the function and location for easy operation and maintenance . A discussion of some of the most important supports will be discussed further next .
- safety valves.
Each boiler must be equipped with a safety valve that is of sufficient type and capacity to meet the design codes and applicable regulations. The purpose of the valve is to prevent the increase in boiler pressure above the specified safety limit. Typically , two tubes attached valve seat and the valve outlet superheater used . Where the reducing capability needed exceeds that provided by the valve , additional cylinder valves are installed .
There are two basic types of safety valve systems . One , the oldest and the most common , uses a Joaded spring valve designed to open or ” pop” at a regulated pressure and remain open until the desired pressure drop or ” blowdown” has been reached. The lowest valve set
always safety superheater valves , which are usually set to open at pressure
around 4 % above the superheater outlet design pressure . Valve superheater must be opened before the tube valve can guarantee the flow of steam through the superheater to prevent from the overheating . The first valve on the steam tube is usually set at a pressure of 2 % above the amount of the regulating superheater outlet valve plus a decrease superheater pressure at the maximum level . If one or more valve tubes must provide the ability to reduce , they are set at a pressure of 5 to 10 psi apart which can guarantee sharp operation at opening and closing . The valve spring superheater arrangement can be pulled out frequently in situations of rapid maneuvering due to control ( or operation ) response time . Often opera tion of the valve causes the problem of leakage and maintenance as the valve particularly ” safety” and valves are not designed or constructed to be a ” pressure control ” valve , to increase the spread between the outlet superheater pressure and pressure set valve seat superheater ( thus providing further margin for transient pressurized visits ), a pilot operated valve safety system must be used . A pilot operated valve consists of a small safety guard
valve in the steam tube ( called the pilot valve tube ) and the load valve at the superheater outlet .
The pilot tube is set at & pressurized equal to the number of superheater steam outlets
THC is pressurized . the superheater maximum pressure drop on overload , with a margin of 4 % to 5 % on a steam outlet . In operation , the pilot appears when the Company adjusts the pressurized ls to reach and almost immediately triggers the opening of the superheater unioading valve . If the pressure continues to rise , pop safety valve tubes in their sets ‘ order
- Water level indicator .
It is important that the water which is enough to be in the boiler setiapsaat for the operation of the safe ‘ range is working normally shown in visualdengan how to directly read gauges installed on steam tubes and with indicators of the remote is not directly installed for example in the control of the main console ( or, in the case of control bridges
from the main engine , on the bridge ). Typical direct reading and remote – reading gage shown in Figs . 36 . Too low a water level can result in loss of circulation head and tube failure if downcomers are revealed . Too high a level can cause carry-over with thermal shock after superheater or , if it is severe enough , it can be the main engine . Maintain
close water level control is mandatory and water level indicators must be kept clean and in good condition at all times . Each boiler must have at least two independent tools indicating the water level .
The usual practice is to use two separate direct reading gage glasses and a remote gauge as many warrant specific operating conditions .
- Smoke Indicator .
The ability to see gas discharge piles is a great help in boiler operation . Sudden changes from clean to dark gas stack may be evidence of maloperations such as dirty oil burners or forced drafts or control difficulties . For permission to view the stack without leaving the operating station , a smoke indicator is installed . The direct reading indicator is basically a periscope set to give the operator a direct view of the light source that shines through the boiler absorption and combustion gas . The other type employs ceilphotoelectricity and provides readings on a meter scale calibrated in the smoke density unit , but can also be installed to sound an alarm when certain smoke densities are reached .
- Instrumentation and control.
The need for operating instruments and manuals or automatic controls varies with the size and type of equipment, the method of combustion, increasing the capability of the operating personnel, and the desired level of automation. For safe operation and efficient performance, the information needed is relative to the water level in the boiler tube, burner performance, steam pressure and feedwater; superheated temperature (and heated) steam; gas and air pressure entering and leaving the main component; feedwater and boiler water chemistry conditions and accumulation particie; operating feed pump, fan, combustion of fuel, equipment and materials fuel preparabion, combustion air links were actually passing through the furnace to the theoretically required for fuel burning, temperature, water fuel gas, and air entering and leaving the main component parts of the unit, feedwater, steam, fuel, and air currents. Over the years , marine boilers have been equipped with control equipment allowing stable operation at sea with the participation of small operators except when maneuvering . However, complete automation of the boiler might be desirable so , with the exception of getting started , they can be operated throughout the full range from standby to full load without manual adjustment . To achieve fully automatic operation , adequate control components are essential. The operating characteristics of the basic items and the additional equipment that produces uapharus are truly known since these characteristics affect the level of control , the scope of the control needed , and the response obtained . As a test , for example , where the burner has a range of operations or turndown capability equal to or greater than those required by the boiler , the need for a burner sequence ( or taking them out of service ) is eliminated . This, in turn , Eliminating many of the decisions and functions that others will need a system management burner automatically , and the system is simple can be selected .
The degree of control that can be achieved , in order to ride sophistication , it manually , manually controlled locally , user controlled remote , automated ( non – recycled again ), and automatic (recycling ). Various types of this control can best be described by associating their functions to pengoperasianburner . With Manual type control , Fig . 37 , the burner is manually cleaned and signaled . This might be automatically modulated but stopped manually . Although there is no function of the operator performed automatically , width kisaranpembakar can be used to control the combustion automatically to facilitate the operation of the pier – kedermaga without manual participation . However, without monitoring the boiler and burner , the operator must remain near the boiler to provide the necessary supervision . In a supervised local manual system, Fig. 38, the burner is cleaned and signal manually, but surely the procedures and conditions are monitored by safety interlocks. All manual functions are performed and checked by the operator at the burner station during normal operation, and if the demand for steam is within the capability of the burner, unattended boiler operation is achieved. Monitoring and security of interlocks are provided to change operation if safety conditions develop, and to the boiler burner or, if necessary. After out-trip, the operator must take corrective measures needed to remove interlocks and recycle burners or boilers. Manually controlled remote systems, Fig. 39, allows the burner to be cleaned and ignited by a push button or selector switch, automatically modulated, and guaranteed by a remote driven manual switch or switch selector. It also provides procedure supervision by the security interlocks. The burner is mechanical and all functions of the operation are carried out by mechanical devices. The burner is mechanical and all the functions of the operation are carried out by mechanical devices starts from a remote control station , which shows whether each function has been done correctly . The control system does not free the burner manipulation operator . He must devote attention penuhkepada step – by step procedure to start and secure the burner , which takes time process . System control can ijustified only in installations where the ability of the turndown of the burner does not fit with the requirements turndown dariboiler and dimanaburner to be manipulated
to cover the operating range . Its application will not meet the requirements
USCG for automatic boilers .
3.12 Sample Design Problem (sample design problem)
Steps that will be followed in the development of heating surfaces of a generator of steam that has been designed to meet the requirements of the determination of the cycle of performance best be illustrated with an example . Basic boiler performance is usually provided by ship specifications , however , for example the requirements of the 30,000 shp plant (see example heat balance in chapter 2) to be used.
It is assumed that a single boiler is used to provide the steam needed to drive main propulsion and service on other vessels. Divided into 2 parts, the integral drum and boiler furnace equipped with a steam air heater and economizer. In the furnace must tersediar water cooled and two burners large (wide range burners) that will digunakan.super heater will be installed vertically and the access cavity (cavities) will be provided.
From the preliminary heat balance to an ABS rating of 30,000 shp, the following operating requirements must be met:
Steam pressure, drum, approx ………………………………….. 960 psig
Steam pressure, superheater outlet …………………………… 875 psig
Steam temperature, superheater outlet ……………………… 955 F
Superheated 185,520 lb / hr
Desuperheated 16,870 lb / hr
Total 202,390 lb / day
Feed water temperature ……………………………………….. … 284 F
Efficiency (based on 1.5% radiation and 15% countless losses for excess air) ………………………. ……………………… 88.5%
Fuel total heating value (standard C bunker + added heat in water) ………… 19,264 Btu / lb
Fuel required ………………………………………… …………….. 14,349 lb / hr
Water temperature, leaving steam air heater ……………………… 278 F
Air flow (16.07 lb / lb oil at 15% excess air) ……………….. 230,600
Flue gas flow = 244,937, say ……………………………………. … 245,000 lb / hr
From the example above, one level of operation will be calculated even though for an actual boiler design it is not usual to calculate three or more rates to make a characteristic curve of its performance. The rated power will be calculated since the design meeting was formed of the specified efficiency and steam temperature. Data transfer heats have been provided in the curves and procedures in Chapter 2
Fig 4.1 Boiler layout for sample problems
- Boiler Lay out
2 oil burners will be used to supply a total oil flow of 14,349 lb / hr, rated power is 8000 lb / hr for each overload. The necessary clearances for the burner capacity have been provided from the selected boiler manufacturing. From the information above, approximately the furnace and boiler layout will be presented as in fig 41. with a note that the furnace volume and heating surface are ignored.
- Furnace calculation
Furnace volume, cold surface, and radiant heat absorbing surface (RHAS) will be explained by the method in section 2 of capter 2 or T & R Bulletin 3-14 :
Furnace volume = 2655 ft 3
Projected surface = 1200 ft 3
RHAS = 1175 ft 3
With a high fuel rating of 18,500 Btu / lb, the furnace rating of rated power is:
Release rate = 14,349 x 18,5002655 = 99,985 Btu / ft 3
Oil Rate / RHAS = 14,3491175 = 12.2 lb / ft 3
From the satisfactory, Furnace exit gas temperature and heat absorption can be calculated according to the furnace surface area as below:
PROJECTED ARE SURFACE EFECTIVENESS FACTOR RHAS
Rear waterwall 190 100 190
Front waterwall 175 0.856 150
Screen and floor 435 1.00 435
Side waterwall and roof 400 1.00 400
1200 ft 3 1175ft 3
from figure 2 chapter 2 adiabatic flame temperature, T A can be obtained at 3990 F or 4450 R, with 15% excess air
Qta = 17,500 + 46 +278–80 (0.2445–16.07)16.07 + 1
= 1073 Btu / lb
to determine the shape emissivity factor F E F A available data is
V F = 2655 ft 3
S T = 1200 ft 3
therefore Firing Density equal to
WFP.FVF = 14,3491 x 2655 = 5.40
From subsection 1.2 (d) chapter 2, the tube surface temperature of the furnace is
T S = t S + ( QcS.c ) x e / k = 592 F
Furthermore, from the estimated correct furnace exit temperature, T E to be 2200 F, tube film temperature can be obtained with the following approach:
T F = Te+Te‘2+ Ts2 = 2298 + 22002+ 5922 = 1412 F
- Heating Surface
After determining the furnace exit temperature, the performance of the boiler on the screen, the super heater and the generating bank can be evaluated in order. From the boiler layot approach Fig41 the following data can be determined
The final steam temperature, which is then determined, the heat absorbed by steam at the superheater by direct radiation from the furnace is as follows:
Q SHR = AF L Q C / S C
A = screen projected area = 12.9 x 14 = 180 ft 3
F L = screen leakage factor for tube pitch / diameter ratio
Q C / S C = 88,100 Btu / day
Q SHR = (180) (0.09) (88,100) = 1,427,000 Btu / day
The heat absorbed at the super heater by convection from the flue gas can be calculated by:
Q SHC = WG C P (T 1 – T 2 )
= (245,000) (0.316) (2084-1339) = 57,680,000 Btu / hr
Extended surface economizer is used for final heat recovery, economizer will be designed to reduce the temperature of the exhaust gas to a temperature of 316
F is required to obtain the desired 88.5% boiler efficiency.
The preliminary design of the boiler is now complete. Additional boiler rating such as part load or overload will be calculated later, from here draft losses, circulation characteristics, metal tube temperature, control and auxiliary desuperheater sizes, safety valve settings etc.