TABLE OF CONTENTS 1
2
3
Air cooled heat exchanger .................................................................................................. 1 1.1
Introduction ................................................................................................................. 1
1.2
Equipment Description ................................................................................................ 2
1.3
Major Components ...................................................................................................... 6
1.4
Equipment for cold climates ...................................................................................... 10
1.5
Advantages and Disadvantages ................................................................................. 11
1.6
General Application ................................................................................................... 13
Design of Air Cooled Heat Exchangers .............................................................................. 14 2.1
Design Guidelines ...................................................................................................... 14
2.2
Basic Design Procedure.............................................................................................. 15
2.3
Design Strategy .......................................................................................................... 17
Introduction To Simulation ............................................................................................... 28 3.1
Simulation Modeling and Analysis............................................................................. 28
3.2
Purposes of Simulation .............................................................................................. 28
3.3
Advantages to Simulation .......................................................................................... 28
3.4
Disadvantages to Simulation ..................................................................................... 29
3.5
Steps in simulation..................................................................................................... 29
3.6
Computer Software ................................................................................................... 30
3.7
Using ACOL................................................................................................................. 32
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1 AIR COOLED HEAT EXCHANGER 1.1 Introduction Air-cooled heat exchangers are second only to shell-and-tube exchangers in frequency of occurrence in chemical and petroleum processing operations. These units are used to cool and/or condense process streams with ambient air as the cooling medium rather than water. Cooling with air is often economically advantageous, e.g., in arid or semi-arid locations, in areas where the available water requires extensive treatment to reduce fouling, or when additional investment would otherwise be required to expand a plant’s existing cooling-water supply. Regulations governing water use and discharge of effluent streams to the environment also tend to favor air cooling. Although the capital cost of an air-cooled exchanger is generally higher, the operating cost is usually significantly lower compared with a water-cooled exchanger. Hence, high energy cost relative to capital cost favors air cooling. Air cooling also eliminates the fouling and corrosion problems associated with cooling water, and there is no possibility of leakage and mixing of water with the process fluid. Thus, maintenance costs are generally lower for air-cooled exchangers. Air cooled heat exchangers are used to transfer heat from a process fluid to ambient air. The process fluid is contained within heat conducting tubes. Atmospheric air, which serves as the coolant, is caused to flow perpendicularly across the tubes in order to remove heat. In a typical air cooled heat exchanger, the ambient air is either forced or induced by a fan or fans to flow vertically across a horizontal section of tubes. For condensing applications, the bundle may be sloped or vertical. Similarly, for relatively small air cooled heat exchanger, the air flow may be horizontal across vertical tube bundles. In order to improve the heat transfer characteristic of air cooled exchanger, the tubes are provided with external fins. These fins can result in a substantial increase in heat transfer surface. Parameters such as bundle length, width and number of tubes rows vary with the particular application as well as the particular finned tube design. Atmospheric air (having temperature varying from 20 to 450C, depending on season) can be used as cooling medium if the process fluid is to be cooled up to 600C. If it is to be cooled below 600C, then atmospheric air cannot be used as a cooling medium, as minimum driving force required for heat transfer in air cooled heat exchanger is 100C.
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1.2 Equipment Description 1.2.1 Overall configuration In an air-cooled heat exchanger, the hot process fluid flows through a bank of finned tubes, and ambient air is blown across the tubes by one or more axial-flow fans. For applications involving only sensible heat transfer, the tubes are oriented horizontally as shown in Figures
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For condensers, an A-frame configuration (as shown in Figure) is often used, with the condensing vapor flowing downward through the tubes, which are oriented at an angle of 60◦ with the horizontal.
In units employing horizontal tubes, the fan may be located either below (forced draft) or above (induced draft) the tube bank. In either case, the air flows upward across the tubes. The fan drive assembly in an induced-draft unit may be mounted below the tube bundle or it may be mounted above the fan. With the former arrangement, the drive assembly is easily accessible for inspection and maintenance, and it is not exposed to the heated air leaving the unit. However, the drive shaft passes through the tube bundle, requiring omission of some tubes, and the relatively long shaft is more susceptible to vibration problems.
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The forced-draft configuration provides the simplest and most convenient fan arrangement. With all blower components located below the tube bundle, they are easily accessible for maintenance and are not exposed to the heated air leaving the unit. However, these exchangers are susceptible to hot air recirculation due to the low velocity of the air leaving the unit. Induced-draft operation gives more uniform air flow over the tube bundle and the exit air velocity is several times higher than in forced-draft operation, thereby reducing the potential for hot air to be recirculated back to the intake of the unit or other nearby units. Hot air recirculation tends to reduce the capacity of the heat exchanger, thereby requiring a higher air flow rate and/or more heat-transfer surface. The induced-draft configuration also provides some protection from the elements for the tube bundle, which helps to stabilize the operation of the unit when sudden changes in ambient conditions occur. For a given mass flow rate of air, induced-draft operation in principle entails greater power consumption than forced-draft operation due to the higher volumetric flow rate of the heated air that is handled by the induced-draft fan. In practice, however, this potential disadvantage tends to be offset by the more uniform flow distribution and lower potential for hot gas recirculation obtained with induced-draft operation. As a result, induced-draft units typically do not require significantly more power than forced-draft units and in some cases may actually require less power. If the difference between fluid outlet temperature (required) and ambient air temperature is less than 300C, induced draft unit is preferred because of the less percentage recirculation of hot exhaust air. But if the same is more than 300C, forced draft unit is preferred as it consumes less power.
1.2.2 Forced vs. Induced Draft Advantages and disadvantages of forced and induced draft air-cooled heat exchangers are shown here to aid in selection. 1.2.2.1
Advantages of Induced Draft
ü Better distribution of air across the bundle. ü Less possibility of the hot effluent air recirculating around to the intake of the sections. The hot air is discharged upward at approximately 2.5 times the velocity of intake, or about 1500 ft/min. ü Better process control and stability because the plenum covers 60% of the bundle face area, reducing the effects of sun, rain, and hail.
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ü Increased capacity in the event of fan failure, since the natural draft stack effect is much greater with induced draft. 1.2.2.2
Disadvantages of Induced Draft
v Higher horsepower since the fan is located in the hot air. v Effluent air temperature should be limited to 2OO0F, to prevent potential damage to fan blades, bearings, V-belts, or other mechanical components in the hot air stream. v The fan drive components are less accessible for maintenance, which may have to be done in the hot air generated by natural convection. v For inlet process fluids above 350°F, forced draft design should be used; otherwise, fan failure could subject the fan blades and bearings to excessive temperatures. v Plenums must be removed to replace bundles 1.2.2.3
Advantages of Forced Draft
ü Slightly lower horsepower since the fan is in cold air. (Horsepower varies directly as the absolute temperature.) ü Better accessibility of fans and upper bearings for maintenance. ü Easily adaptable for warm air recirculation for cold climates. ü Better accessibility of bundles for replacement. ü Accommodates higher process inlet temperatures. 1.2.2.4
Disadvantages of Forced Draft
v Less uniform distribution of air over the bundle. v Greatly increased possibility of hot air recirculation, due to low discharge velocity from the bundles and absence of stack. v Low natural draft capability on fan failure due to small stack effect. v Complete exposure of the finned tubes to sun, rain, and hail, which results in poor process control and stability
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1.3 Major Components The major components of air cooled heat exchangers include the ü Finned tube ü Tube bundle ü Fan and drive assembly ü Air plenum chamber ü Overall structural assembly Each component is briefly described below
1.3.1 Finned Tubes Common to all air cooled heat exchangers is the tube, through which the process fluid flows. To compensate for the poor heat transfer properties of air, which flows across the outside of the tube, and to reduce the overall dimensions of the heat exchanger, external fins are added to the outside of the tube. A wide variety of finned tube types are available for use in air cooled exchangers. These vary in geometry, materials, and methods of construction, which affect both air side thermal performance and air side pressure drop. The operating environment may influence the choice of materials used and the shape of the fin. Aluminum is very often satisfactory as a fin material, although copper, steel and stainless steel fins are also used. The fin shape may be of edge-type, L-foot type or double L-foot design. The edge type is used for the grooved fin tube, and in cases, where the base tube is not subject to corrosion. Various types of high-fin tubing are available, including: • • • • •
Integrally finned (K-fin) Bimetallic (E-fin) Tension-wound fin (L-fin or I-fin) Embedded fin (G-fin) Brazed find
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1.3.2 Tube Bundle A typical tube bundle arrangement is illustrated in Figure
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Tube bundles are rectangular in shape and usually 6–12 ft wide. Since tube bundles are factory assembled and shipped to the plant site, maximum bundle width is limited by transportation requirements. The tubes are either welded to or rolled into long rectangular tube sheets that are welded to box-type headers. Both front and rear headers are equipped with screwed plugs that are aligned with the tube holes as illustrated in Figure. The plugs can be removed to provide access to the tubes for cleaning and other maintenance.
1.3.3 Fan and Drive Assemblies Fans are used, which correspond to the dimensions of the tube bundle and the performance requirements for the heat exchanger. Normally, the fan diameter is approximately equal to the bundle width, although smaller diameters may be used. For square or nearly square bundles, one fan is used. For long rectangular bundles, a number of fans operating in parallel may be used. Fans are of axial flow design, which move relatively large volumes of air at low pressure. In order to minimize air recirculation and improve fan efficiency, fan blades are set within orifice rings which provide close radial clearance between the ring and the blade tips. Fan efficiencies are typically about 65 % while drive efficiencies are 95 % or better. This power advantage for forced-draft designs generally proves to result in a more economical heat exchanger. Since the fan is close to the ground, structural costs may be less with the drive assembly, located at ground level. However, induced-draft air cooled heat exchangers offer the advantage of better air distribution across the bundle, due to relatively low air velocities approaching the tubes. Furthermore, the air exit velocities of induced-draft heat exchangers are much higher than a forced-draft design. Thus, the possibility of recirculating hot discharge air is less for the induced-draft. When cooling the process fluid to a temperature close to the inlet ambient air temperature, this may be of particular importance. The fans are situated in bays, which are self-contained sections of an air-cooled heat exchanger. A bay consists of one or more tube bundles, the fans and drive assemblies that supply air to the bundles, and the associated framework and support structures. Except in unusual circumstances, multiple tube bundles are placed side by side in the bay. Bays are usually designed for one to three fans, with two-fan bays being most common. Fan bays can be preassembled and shipped to the plant site provided they are small enough to meet transportation requirements. Otherwise, they must be assembled in the field, which adds to the cost of the heat exchanger. An air-cooled heat exchanger consists of one or more fan bays, with multiple bays operating in parallel. Some typical configurations are illustrated in Figure. 8
1.3.4 Air Plenum Chamber The velocity of the air, flowing through the fan, can be as much as 3 to 4 times the velocity across the face of the tube bundle. Also, the air, coming from the circular shape of the fan, must be distributed across the square or rectangular shape of the bundle. The air plenum chamber is intended to make this velocity and shape transition, such that the distribution of air is uniform across the bundle. Common practice is to install the fan in a chamber, such that the distance from the first row of the tube bundle to the fan is about one-half the fan diameter The plenum chamber design may be a simple box shape, formed by flat sides and bottom, or curved transition sections may be used to obtain a tapered smooth transition from the rectangular bundle to the circular fan. Either design may be used for forced-draft or induced-draft air cooled heat exchangers.
1.3.5 Structural Assembly The structural assembly of the air cooled heat exchanger is strongly dependent upon the particular plant site requirements of the user. Taken into account should be mechanical loads upon the heat exchanger structure, due to its own weight, of course, but other loadings, such as wind loads, impact loads, nozzles loading and seismic forces must be considered. The presence of equipment beneath the air cooled heat exchangers may require particular designs. Safety considerations may call for fencing or fan guards. Environmental factors could indicate the need for louvers, hail screens, or other protective devices. In addition, the physical location of the heat exchangers may require ladders, platforms, railings, safety cages and other miscellaneous items, which the user will require.
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1.4 Equipment for cold climates Air-cooled heat exchangers are designed to operate over a wide range of environmental conditions, including ambient temperatures from −600F to 1300F. Special design features are employed for operation in cold climates in order to prevent freezing of the process fluid. If the wall temperature of a tube carrying a hydrocarbon stream reaches the pour point of the hydrocarbon, the liquid will congeal around the wall, thereby reducing the flow area and increasing the tube-side pressure drop. If water is present in the process stream, ice can form around the tube wall with similar effect. Likewise, methane hydrates can form on the tube walls of natural gas coolers. The standard method for preventing freezing is to intentionally recirculate some of the warm air leaving the unit in order to raise the temperature of the intake air. This can be accomplished in a number of ways, depending on whether forced-draft or induced-draft operation is employed and the severity of the winter climate. Figure shows a typical configuration for a forced-draft unit with external recirculation, which provides the most reliable freeze protection.
The unit is completely contained in an enclosure equipped with adjustable louvers to control both exhaust and intake air rates. The manual louvers are adjusted seasonally while the automatic louvers are adjusted continuously via pneumatic mechanisms directed by a temperature controller that maintains the temperature of the air entering the tube bundle at an appropriate level. A recirculation chamber projects beyond the front and rear headers, 10
providing ducts where cold ambient air mixes with warm recirculated air. The flow rate of recirculated air is controlled by internal louvers in the ducts that open as the external intake louvers close. Either variable-pitch or variable-speed fans are used in these units. The pitch or speed is automatically adjusted by a second temperature controller that maintains the outlet temperature of the process fluid at the desired temperature. A row of steam tubes is installed below the tube bundle to warm the air stream during startups and shutdowns in cold weather. These tubes are typically the same type and size as those in the tube bundle, but with a pitch equal to twice that of the bundle. The steam tubes are commonly referred to as a steam coil.
1.5 Advantages and Disadvantages Air cooled heat exchanger has certain advantages as well as disadvantages over cooling water cooled exchanger using water.
1.5.1 Advantages Ø Generally operating cost for Air Cooled Heat Exchanger (ACHE) is less than the same of water cooled heat exchanger (WCHE). Operating cost of WCHE includes cost of make-up water for the cooling water, power required for the cooling tower fans and for the circulating pump, cooling tower maintenance, etc. while operating cost of ACHE includes cost of power consumed by fan. Day by day cost of water increases and hence the difference in operating cost between WCHE and ACHE increases. Ø Inlet temperature of water to cooling tower (or outlet temperature of cooling water from heat exchanger circuit) should not be greater than 600C otherwise it creates the scale formation and also affects the material of construction of cooling tower. While for air there is no limit for temperature. Ø Fouling or scale formation does not take place with air cooling. Ø Since water is not used as the cooling medium, the disadvantages of using water are eliminated. Ø Eliminates high cost of water including expense of treating water. Ø Thermal or chemical pollution of water resources is avoided. Ø Installation is simplified due to elimination of coolant water piping. Ø Location of the air cooled heat exchangers is independent of water supply location. 11
Ø Air cooled heat exchangers will continue to operate (but at reduced capacity) due to radiation and natural convection air circulation should a power failure occur. Ø Temperature control of the process fluid may be accomplished easily through the use of shutters, variable pitch fan blades, variable speed drives, or, in multiple fan installations, by shutting off fans as required Ø Excellent for removing high level temperatures, particularly greater than 200°F. Ø Maintenance generally claimed to be 1/3 or less than water coolers. Clean fins by compressed air and brushes, sometimes while operating. Ø Ground space often less than or equal to cooling towers; can also serve dual purpose by mounting air-cooled units above other equipment or on pipe ways or roofs of buildings. Vibration is no problem.
1.5.2 Disadvantages Ø Fixed cost of ACHE is higher than the same of WCHE. Heat transfer coefficient of air is quite less than heat transfer coefficient provided by cooling water and hence heat transfer area required by ACHE is more. Also design inlet temperature of cooling water from cooling tower is always less than design ambient temperature. It gives lesser Mean Temperature Difference (MTD) for ACHE compared to the same for WCHE and hence it increases heat transfer area required for ACHE. Ø Minimum driving force required for heat transfer, ∆tmin is 10-150C for ACHE and 3 to 50C for WCHE. Hence minimum temperature that can be achieved with ACHE is ambient temperature plus 100C, while with WCHE it is design temperature of cooling water from cooling tower plus 30C. Hence, if the fluid is to be cooled or condensed below 550C in a tropical location, ACHE cannot be used. Ø Outdoor operation in cold winter environments may require special consideration to prevent freezing of the tube side fluid or formation of ice on the outside surface. Ø The movement of large volumes of cooling air is accomplished by the rotation of large diameter fan blades rotating at high speeds. As a result, noise due to air turbulence and high fan tip speed is generated. Ø Fire and toxic vapor and liquid hazard, if leaks occur to atmosphere. Ø Not too suitable for vacuum services due to pressure drop limitations but are used in application. 12
1.6 General Application Air cooled heat exchangers are commonly used in industrial applications where a reliable source of water is not available as a cooling medium. Even if water is available, in some cases, air cooled exchangers are favored for economic or operational reasons – because dedicated cooling water circuits, pumps, water cooling systems and water conditioning systems add complexity and capital requirements, as well as operating and maintenance costs. Air cooled heat exchangers are used throughout the entire oil and gas industry from upstream production to refineries and petrochemical plants, under conditions including high pressure and temperature, as well as corrosive fluids and environments. Common applications include gas re-injection, gas lift and pipeline applications, cooling and condensing of hydrocarbon gases, and cooling of machinery oil and heavy hydrocarbons. Air cooled steam condensers are a special type of heat exchanger employed to condense steam at the exhaust end of steam turbines for both power generation and mechanical drive applications. Air-cooled units have been successfully and economically used in liquid cooling for compressor engine and jacket water and other recirculating systems, petroleum fractions, oils, etc., and also in condensing service for steam, high boiling organic vapors, petroleum still vapors, gasoline, ammonia, etc. 1) When the fluid temperatures are relatively high 2) when water is extremely scarce or expensive 3) Where a maintenance problem results from the use of cooling tower water 4) Where the industrial process or plant design is simplified by its use 5) Where maximum heat loads occur during the winter months. Air cooled exchangers are extensively used for jacket water cooling of engine driven compressors, for steam condensing, and for gas and high temperature level cooling or condensing.
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2 DESIGN OF AIR COOLED HEAT EXCHANGERS 2.1 Design Guidelines 2.1.1 Tubing Tubing selection should be based on the tube-side fluid temperature and the potential for corrosion of the external tube surface. Furthermore, it is recommended to choose one of the tubing configurations given in Table
2.1.2 Air flow distribution In order to obtain an even distribution of air flow across the tube bundle, the fan area should be at least 40% of the bundle face area as previously noted. In addition, for two-fan bays,
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the ratio of tube length to bundle width should be in the range of 3–3.5. It is also desirable to have a minimum of four tube rows.
2.1.3 Outlet air temperature For induced-draft operation, the outlet air temperature should be limited to about 2200F in order to prevent damage to fan blades and bearings. These parts may nevertheless be exposed to high temperatures in the event of fan failure. Therefore, forced-draft operation should be considered if the tube-side fluid temperature is greater than 3500F.
2.1.4 Air velocity The air velocity based on bundle face area and air at standard conditions is usually between 400 and 800 ft/min, with a value of 500–700 ft/min being typical for units with four to six tube rows. A value in this range will usually provide a reasonable balance between air-side heat transfer and pressure drop.
2.2 Basic Design Procedure The general equation for heat transfer across a surface is: Q = UA∆Tm Where Q = heat transferred per unit time U = the overall heat transfer coefficient A = heat-transfer area ∆Tm = the mean temperature difference, the temperature driving force The prime objective in the design of an exchanger is to determine the surface area required for the specified duty (rate of heat transfer) using the temperature differences available. The overall coefficient is the reciprocal of the overall resistance to heat transfer, which is the sum of several individual resistances. For heat exchange across a typical heat exchanger tube the relationship between the overall coefficient and the individual coefficients, which are the reciprocals of the individual resistances, is given by:
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Where Uo = the overall coefficient based on the outside area of the tube ho = outside fluid film coefficient hi = inside fluid film coefficient hod = outside dirt coefficient (fouling factor) hid = inside dirt coefficient kw = thermal conductivity of the tube wall material di = tube inside diameter do = tube outside diameter The steps in a typical design procedure are given below: 1. Define the duty: heat-transfer rate, fluid flow-rates, temperatures. 2. Collect together the fluid physical properties required: density, viscosity, thermal conductivity. 3. Decide on the type of exchanger to be used. 4. Select a trial value for the overall coefficient, U. 5. Calculate the mean temperature difference, ∆Tm. 6. Calculate the area required. 7. Decide the exchanger layout. 8. Calculate the individual coefficients. 9. Calculate the overall coefficient and compare with the trial value. If the calculated value differs significantly from the estimated value, substitute the calculated for the estimated value and return to step 6. 10. Calculate the exchanger pressure drop; if unsatisfactory return to steps 7 or 4 or 3, in that order of preference. 11. Optimise the design: repeat steps 4 to 10, as necessary, to determine the cheapest exchanger that will satisfy the duty. Usually this will be the one with the smallest area.
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2.3 Design Strategy The basic design procedure for air-cooled heat exchangers is similar to that for shell-andtube exchangers. An initial configuration for the unit is obtained using an approximate overall heat-transfer coefficient together with the design guidelines given above. An important preliminary step in the design process is the selection of the outlet air temperature. This parameter has a major effect on exchanger economics. Increasing the outlet air temperature reduces the amount of air required, which reduces the fan power and, hence, the operating cost. However, it also reduces the air-side heat-transfer coefficient and the mean temperature difference in the exchanger, which increases the size of the unit and, hence, the capital investment. The same situation exists with water-cooled heat exchangers, but the feasible range of outlet temperatures tends to be significantly greater for air-cooled exchangers. Thus, optimization with respect to outlet air temperature (or equivalently, air flow rate) is an important aspect of air-cooled heat-exchanger design.
Problem Statement A liquid hydrocarbon stream with a flow rate of 250,000 lb/h is to be cooled from250◦F to 150◦F in an air-cooled heat exchanger. The unit will be mounted at grade and there are no space limitations at the site. The design ambient air temperature is 95◦F and the site elevation is 250 ft above mean sea level. An outlet air temperature of 150◦F is specified for the purpose of this example. Average properties of the hydrocarbon and air are given in the table below. A fouling factor of 0.001 h·ft2·◦F/Btu is required for the hydrocarbon, which is not corrosive, and a maximum pressure drop of 20 psi is specified for this stream. Inlet pressure will be 50 psia. The maximum allowable air-side pressure drop is 0.5 in.H2O. Design an air-cooled heat exchanger for this service:
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Solution a)
Make initial specifications i.
ii.
iii.
iv.
b)
Tubing type G-fin tubing with carbon steel tubes and aluminum fins is specified based on its excellent durability. It is assumed that the environment at the plant site is not highly corrosive; otherwise, bimetallic tubing would be a better choice. Tube size and layout One inch OD, 13 BWG tubes with 10 fins per inch and a fin height of 0.625 in. are specified with reference to Table. The tube layout is triangular (30◦) with a tube pitch of 2.5 in. Draft type Since the process fluid temperature is below 350◦F, an induced-draft unit will be used. For simplicity, diffusers are not specified and it is assumed that winterization of the unit is unnecessary. Headers The pressure is low and based on the specified tube-side fouling factor, frequent cleaning is not anticipated. Therefore, plug-type headers will be used.
Energy balances
For the specified outlet air temperature of 150◦F, the required mass flow rate of air is
c)
LMTD
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d)
LMTD correction factor This factor depends on the number of tube rows and tube passes, which have not yet been established. Therefore, in order to estimate the required heat-transfer surface area, F =0.9 is assumed.
e)
Estimate UD Based on Table below, a value of 4.5Btu/h·ft2·◦F is assumed, which is in the expected range for light hydrocarbon liquid coolers.
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f)
Calculate heat-transfer area
g)
Number of tube rows, tube length, and number of tubes The bundle face area required for a given (standard) face velocity is
Assuming a (standard) face velocity of 600 ft/min from the design guidelines gives
Thus, the ratio of heat-transfer surface area to bundle face area is
From Table
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The closest ratio is 107.2 for four tube rows. Using this value, the required face area is:
Based on the design guidelines, a tube length, L, of three times the bundle width, W, is assumed, giving
Thus
The number of tubes is found using the value of ATot/L =5.58 from Table
Taking the closest integer divisible by four gives 224 tubes with 56 tubes per row. The corresponding bundle width is the tube pitch times the number of tubes per row. Allowing 2 in. for side clearances gives:
The actual bundle face area and (standard) face velocity are
h)
Number of tube passes The tube-side fluid velocity is: 21
Two, three, or four passes will give a velocity in the range of 3–8 ft/s. Since the tubeside pressure drop allowance is fairly generous, four passes are chosen for the first trial in order to maximize the heat-transfer coefficient and minimize fouling. Checking the Reynolds number
The flow is fully turbulent and, hence, the configuration is satisfactory. This completes the preliminary design calculations.
i)
LMTD correction factor The correct value of the LMTD correction factor can now be determined using Figure12.A5 of “Process Heat Transfer; Principles and Applications” by “Robert W. Serth”, Page Number 674.
j)
Calculate required overall coefficient
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k)
Calculate hi For turbulent flow (ReǦ104), the Seider–Tate equation is used in the form
l)
Calculate ho The maximum air velocity in the tube bundle is calculated using Equation
The face velocity is first converted from standard conditions to conditions at the average air temperature. The fin thickness is taken as 0.013 in.
Equation below is used to calculate the air-side heat-transfer coefficient with ATot/Ao =21.4
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m) Wall temperatures and viscosity correction factors No viscosity correction is required for the air-side heat transfer coefficient, so only Tp is needed for the tube-side correction. In the present case, no viscosity data were given for the tube-side fluid. Therefore, φi is assumed to be 1.0 and the wall temperature is not calculated.
n)
Calculate the clean overall coefficient The clean overall coefficient is given by Equation below with the fouling factors omitted. The contact resistance is neglected and ATot/Ai =26.3 is obtained from Table.
Since UC >Ureq, continue.
o)
Fouling allowance The tube-side fouling factor was specified in the problem statement as 0.001Btu/hft2◦F. except in unusual circumstances, air-side fouling is minimal and, therefore, RDo is taken as zero. Thus, the total fouling allowance is:
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p)
Calculate the design overall coefficient
Since UD >Ureq =4.10 Btu/hft2.◦F, the heat exchanger is thermally workable.
q)
Tube-side pressure drop Following equations are used to calculate the tube side pressure drop
For laminar flow, the friction factor is given by:
For turbulent flow in commercial heat-exchanger tubes, the following equation can be used for Re≥3000:
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Assuming 5-in. schedule 40 nozzles are used, the flow area per nozzle from Table B.1 of “Process Heat Transfer; Principles and Applications” by “Robert W. Serth”, Page Number 718. is 0.1390 ft2. Hence,
r) Air-side pressure drop
Thus
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Design Summary Number of fan bays: 1 Number of tube bundles per bay: 1 Number of fans per bay: 2 Bundle width and length: 11.8 ft×37 ft (including headers) Number of tube rows: 4 Number of tube passes: 4 Number of tubes: 224 Tubing type: G-fin Tube size: 1 in. OD, 13 BWG, 36 ft long Tube layout: Equilateral triangular with 2.5-in. pitch Fins: 10 fpi, 0.625 in. high, 0.013 in. thick Heat-transfer surface area: 45,000 ft2 Draft type: Induced draft Fan diameter: 10.5 ft Motor size: 20 hp Tube-side nozzles: 5-in. schedule 40 Headers: Plug-type box headers Materials: Carbon steel tubes, aluminum fins, carbon steel headers, tube sheets, pass partitions and nozzles
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3 INTRODUCTION TO SIMULATION 3.1 Simulation Modeling and Analysis Simulation modeling and analysis is becoming increasingly popular as a technique for improving or investigating process performance. It is a cost-effective method for evaluating the performance of resource allocation and alternative operating policies. In addition, it may also be used to evaluate the performance of capital equipment before investment. These benefits have resulted in simulation modeling and analysis projects in virtually every service and manufacturing sector. Simulation modeling and analysis is the process of creating and experimenting with a computerized mathematical model of a physical system.
3.2 Purposes of Simulation The simulation modeling and analysis of different types of systems are conducted for the purposes of • Gaining insight into the operation of a system • Developing operating or resource policies to improve system performance • Testing new concepts and/or systems before implementation • Gaining information without disturbing the actual system
3.3 Advantages to Simulation • Experimentation in compressed time • Reduced analytic requirements • Easily demonstrated models
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3.4 Disadvantages to Simulation Although simulation has many advantages, there are also some disadvantages of which the simulation practitioner should be aware. These disadvantages are not really directly associated with the modeling and analysis of a system but rather with the expectations associated with simulation projects. These disadvantages include the following: • Simulation cannot give accurate results when the input data are inaccurate. • Simulation cannot provide easy answers to complex problems. • Simulation cannot solve problems by itself.
3.5 Steps in simulation •Problem formulation • Project planning • System definition • Input data collection • Model translation • Verification, validation •Experimental design •Analysis • Presenting conclusions and results
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3.6 Computer Software 3.6.1 Introduction to HTFS/Aspen The HTFS software was originally developed by the Heat Transfer and Fluid Flow Service Division of the Atomic Energy Research Establishment, UK. The organization was later privatized and eventually became a part of Hyprotech, Ltd., developer of the HYSYS chemical process simulator. Hyprotech was subsequently assimilated by Aspen Technology, Inc. (www.aspentech.com), which now markets the software as part of the Aspen Engineering Suite. The software interfaces with HYSYS and the Aspen Plus process simulator so that the extensive thermodynamics and physical properties (including petroleum properties) packages available in the simulators can be utilized. AspenTech is a leading provider of award-winning process optimization software and services. AspenTech’s integrated aspenONE™ solutions enable manufacturers to reduce costs, increase capacity, and optimize operational performance end-to-end throughout the engineering, plant operations, and supply chain management processes. Heat transfer equipment accounts for up to 30% of process capital costs and 90% of operating costs. Designing or rating heat exchangers effectively is a complicated task. Multiple engineering groups with different requirements are typically involved in each aspect of the design process, using standalone tools with a minimum of integration. Manual transfer of information results in costly errors, delays, and inconsistencies. An integrated solution that meets the needs of all functional groups is required to provide a truly optimized design that drives capital and operational efficiency. The HTFS/Aspen program called ACOL is used for design and rating of air-cooled heat exchangers. ACOL can operate in checking, simulation, and design modes. However, there are nine simulation options available. The Standard Simulation option is the one most often used. In this mode the outlet temperatures and pressures of the two streams are computed from the given inlet temperatures, pressures, flow rates, and exchanger configuration. The other simulation options allow different sets of specified and computed parameters. Other parameters that can be calculated by the program are: • Air mass flow rate • Tube-side mass flow rate • Tube-side inlet temperature • Tube-side fouling factor 30
There is also an option for natural convection that can be used to simulate an air-cooled exchanger with the fans turned off. ACOL generates a cost estimate and approximate setting plan for the heat exchanger. The setting plan is useful for determining the overall dimensions of the unit and visualizing the layout.
3.6.2 Calculation Modes ACOL can perform a wide range of crossflow heat exchanger calculations. There are three basic calculation modes ACOL Calculation Modes
Design (only for air-cooled exchangers)
Checking
The program offers a range of exchanger widths and tube lengths, which satisfy the required thermal duty for a given X-side pressure drop and tube and fin geometry. The designer can select a design subject to constraints of maximum tubeside pressure drop and maximum and minimum tubeside velocities. Calculates a heat transfer area ratio (actual/required) to indicate whether a given exchanger will meet a required duty, calculated from the tubeside flow rate and inlet and outlet conditions. An area ratio equal to or greater than unity indicates that the exchanger will meet the required duty ACOL offers nine simulation options:
Simulation (for all types of cross-flow heat exchanger)
• SIMU - Standard Simulation. Calculates the tubeside outlet temperature from the tubeside inlet temperature and flowrate, and the X-side inlet temperature and flowrate. Similar to the Tubeside Outlet Temperature option described below, but uses a combined forward and backward iteration technique. This is the default option and has shown to run quicker and to be more accurate than the Tubeside Outlet Temperature option. • SIM0 - Tubeside Outlet Temperature. Calculated from the tubeside inlet temperature and flow rate, and the X-side inlet temperature and flow rate. Similar to the Standard Simulation, described above, 31
but uses a purely forward iteration technique. • SIM1 - Tubeside Inlet Temperature. Calculated from the tubeside outlet temperature and flowrate, and the X-side inlet temperature and flowrate. • SIM2 - Natural Convection. Calculates the X-side flowrate during natural convection. Can be used when all fans in an air-cooled heat exchanger are switched off. • SIM3 - Tubeside Flowrate. Calculated from the tubeside stream inlet and outlet temperatures and the X-side inlet temperature and flowrate. • SIM4 - X-side Flowrate. Calculated from the specified tubeside conditions and the X-side inlet temperature. • SIM5 - Tubeside Fouling Resistance. Calculates a hypothetical tubeside fouling resistance to give a process duty that matches the duty calculated from the input tubeside conditions. This gives an indication of the surplus heat transfer surface area available. • SIM6 - X-side face velocity. Calculates the X-side mass flowrate from a given X-side face velocity (at actual conditions) and the bundle geometry and then carries out a Standard Simulation. • SIM7 - Bundle pressure drop. Calculates the X-side mass flowrate from a given bundle pressure drop (at actual conditions), inlet X-side stream density and the bundle geometry.
3.7 Using ACOL Use ACOL to rate the air-cooled heat exchanger designed in Example and compare the results with the hand calculations
Solution ACOL is run in checking mode for this problem and dry air is selected as the X-side option on the Startup form for consistency with the hand calculations.
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Data obtained from Example are entered on the appropriate input forms as indicated below. Items not listed are either left at their default settings or left blank to be computed by the program.
(a) Bundle geometry (i) Nozzles/Headers The number (1) and inside diameter (5.047 in.) of the inlet and outlet nozzles are entered and plug is selected from the list of header types. (ii) Bundle Setup Number of Passes: 4
Number of Rows: 4
Number of Tubes: 224
Rows per Pass: 1
(iii) Tube Details Tube ID: 0.81 in.
Total Length: 432 in.
Tube OD: 1 in.
Transverse Pitch: 2.5 in.
Longitudinal Pitch: 2.165 in. Note: Either longitudinal pitch or tube layout angle can be given here. (iv) Materials Tube: Carbon Steel
First Fin: Aluminum
Header: Carbon Steel
(b) Extended Surfaces First type of fin or stud: G-fin (default) Fin or stud crown frequency: 10/in. Mean fin or stud thickness: 0.013 in. Fin or stud tip diameter: 2.25 in.
(c) ACHE Geometry (i)
Unit Configuration Number of Bays per Unit: 1
Number of Bundles per Bay: 1 33
Number of Fans per Bay: 2
Fan Configuration: Induced Draught
(ii) Fan Details Fan Support Loss Coefficient: 0.2 Fan Drive Eefficiency: 95% Approximate Fan Static Efficiency: 60% Exchanger Fan Diameter: 126 in. Fan Inlet Type: Conical
(d) Process Data (i)
Process Streams
(ii)
Tube-side Fouling Fouling Resistance: 0.001 h · ft2·◦F/Btu
(iii)
Air Stream Conditions Inlet Dry Bulb Design Temperature: 95◦F
Altitude: 250 ft
(e) Options (i)
Main Output Options Units of Output: British/US
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(f) Physical properties The bulk properties of the hydrocarbon stream are entered by selecting for the Stream Data Source. A single level (50 psia) is entered and values for the density (49.94 lb/ft3), liquid specific heat (0.55 Btu/lb ·◦F), liquid viscosity (0.5 cp), and liquid thermal conductivity (0.082 Btu/h · ft ·◦F) are entered on the spreadsheet at a temperature of 200◦F. No entries are required for the air stream. Hence, this completes the data entry for this problem. Running ACOL with the above input data generates the results summary shown below.
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36
37
38
39
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The manner in which the heat-transfer coefficients are presented in this file requires some explanation. All coefficients are referenced to the external bare (unfinned) tube surface. Thus, the value of 400Btu/h.ft2.◦F listed for the tube-side coefficient is actually hiDi/Do. The value of hi is therefore:
The value of 400Btu/h.ft2.◦F listed for the dirty overall coefficient represents UD(ATot/Ao), where UD is the overall coefficient based on the total external surface area. Hence, this value must be divided by the area ratio to obtain UD. The area ratio calculated by ACOL is given as 21.5 in the second line of the results summary. Therefore:
The listed value of 162Btu/h.ft2.◦F for the air-side film coefficient is referenced to the bare tube surface, i.e.,
The weighted efficiency of the finned surface computed by ACOL is given in the full output file as 0.818. Therefore:
3.8 Results Data from the results summary and the full output file were used to prepare the results comparison shown in the following table. Item
Hand
ACOL
Rei
69594
69622
Reo
8115
8442
hi (Btu/h.ft2.oF)
420
494
2 o
9.12
9.29
2 o
UD (Btu/h.ft . F)
4.45
4.56
∆Pi (psi)
15.3
15.6
∆Po (in. H2O)
0.36
0.42
ho (Btu/h.ft . F)
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REFERENCES R.W. Serth: Air cooled heat exchanger, Process Heat Transfer Principles and Applications, First edition, 2007, pp.630-668 Nicholas P. Cheremisinoff: Handbook of chemical processing equipment, 2000, pp. 12-19 Thakore/bhatt: Introduction to Process Engineering and Design, February 2008, pp. 159-160, 249-250 Ernest E. Ludwig: Applied process design for chemical and petrochemical plants, Volume 3, 3/e, pp. 252-253 Carl Branan: Rules of thumb for chemical engineers: a manual of quick, accurate solutions 3/e, section # 1, pp. 42 - 43 The Hague: Third World Petroleum Congress, 3/e, 1951, Proceedings Section VIII, PP. 201 Christopher A. Chung: HANDBOOK of simulation and modeling, 2004.
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