Air Coolers Versus Shell-and-tube Water Coolers

Engineering Practice Air Coolers Versus Shell-and-Tube Water Coolers As shown here, economical cooling is often achieved with a combination of and water cooling. Design considerations are also presented here Manas Kumar Mandal Fluor Daniel India Pvt. Ltd.

TABLE 1. PERFORMANCE COMPARISON IN AIR COOLER Air in counter-current toAir in co-current to pro­ Air in co-current process fluid in finned cess fluid in finned tubeto process fluid in bare tube(Step 3) (Step 2) tube (Step 1) No. bays/No. of bundles4/4x2 8/8 x 1 5/5x2

Particulars

or the last 40 years, air­ cooled heat exchangers 12/8 16/8 have become an indispens­ No. of tube passes/No. 12/8 able part of many chemical, of rows 12.4 12.4 12.4 petroleum-refining, petrochemical, Heat Duty, MW 64 57.4 MTD, °C 67.6 gas processing and power plants, as Hydrocarbon velocity, m/s 0.47 0.41 0.65 well as off-shore platforms. In loca­ Surface area requirement, 2.808 (bare). 35.354 3,670 (bare), 46,201 5490 2 tions such as the Middle East, where m (finned) (finned) the availability of cooling water is Hydrocarbon NRE, in/out 3.376-301 2,411-222 3,300-200 limited and expensive, air-cooled ex­ Tube skin Temp, °C 50.6 61.5 61.5 S Not estimated 1.0 changers may be a preferred choice. Exchanger cost, million 0.8 (approximate) However, even there, due to vari­ S 0.8 Not estimated ous process constraints, air cooling Installation cost, million 0.63 (approximate) alone may not always suffice, so 0.24 Operating cost, million S Not estimated 0.3 water cooling may also be required. (approximate) Similarly, where cooling water is Total cost, million S 1.67 2.1 Not estimated plentiful, shell-and-tube coolers may (approximate) not always be a straightforward so­ lution. Due to the need for elaborate water, the heat-transfer coefficient Fouling. The costs associated with cooling-water piping circuits, a cool­ will be about one third that of a fouling are usually lower for air cool­ ing tower, large cooling-water circu­ water-cooled exchanger, leading to ing compared to water cooling. lation pumps and water-conditioninghigher heat-transfer area in air cool­ Shell-and-tube coolers, the coolingsystems, the complexity and capital ers. In addition, an air cooler requirestower basin and other peripheral requirements are generally very high,elaborate structures, which further equipment require regular mainte­ leading to a preference for air cool­ increases fixed costs by anywhere nance due to extensive fouling and ers over shell-and-tube coolers. In from three to ten times that of a shell-scaling, and also biological treatment this article, with the help of a case and-tube water cooler, depending onis required, without which the perfor­ study, the author discusses situa­ materials of construction. Neverthe­ mance of the operating plant drops tions where the combination of both less, an air cooler is usually preferred substantially due to deposition or air cooler and water coolers can be to avoid, completely or partially, the fouling in the shell-and-tube coolers. used, including considerations for requirement of elaborate cooling- Air coolers may also become fouled water piping circuits, cooling tower, on the outside due to the accumula­ better overall project economics. cooling-water circulation pumps and tion of dust, insects and other debris water-conditioning systems and so on the finned surface, but less main­ Why use air coolers? Even though overall economics playon, because such additional equip­ tenance is required to handle this. a major role, an air-cooled heat ex­ ment incurs much larger fixed costs.Where shell-and-tube overhead con­ changer is used extensively in all In addition to that, the operating costdensers or trim condensers are used kinds of on-shore plants and off­ of pumping raw water, make-up for cooling or condensing column shore platforms as a first choice of water and power for cooling-tower overhead vapor, any drop in perfor­ cooling mechanism for one of the fans makes the TIC of water-cooling mance due to fouling can mean loss system much higher than an air-cool­in processing material (hydrocarbon, following reasons: Total installed cost (TIC). The TIC ing system, which only requires op­ chemical) and thermal energy. As a of an air-cooling system is less than erating costs for the fan power and result, the column pressure can be that of a cooling water system. Due some controls, such as variable fre­ affected and the hydrocarbon mate­ to lower thermal conductivity and quency drive, louver and so on (see rial is lost in slop, or flared, or the pro­ the case study, below). specific heat of air compared to duction of lower-grade material.

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CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM FEBRUARY 2019

Flexibility. An air cooler offers more flexibility for controlling the processfluid outlet temperature. There are various ways to safe energy by con­ trolling the process-fluid outlet tem­ perature in an air cooler, as follows (see also the case study below): • Switching one fan off during winter months or during the night time • Using variable-speed drive motors having 10-100% operable range • Using auto variable-pitch fans where blade angles change to draw more or less air (power) Such operational flexibility is non­ existent in shell-and-tube coolers, as rarely any control is provided for the cooling water side of water coolers. Location. No specific location is re­ quired for air coolers. However, any process plant that uses a shell-andtube cooler together with a water­ cooling system will be preferred when the location is near a source of water, such as river, lake or the sea. Power. In the event of a power failure, cooling can continue in an air cooler by natural convection. When fan mo­ tors fail to run due to mechanical or electrical problems, an air cooler can still provide cooling of 10-15% of the design heat duty by natural convec­ tion. Loss of power or other mechani­ cal issues in a shell-and-tube cooler can cause the water to be heated up more than the desired outlet temper­ ature, causing scaling and fouling. Why shell-and-tube coolers? Cooling range. Air coolers can be used mostly as primary coolers for process fluid that requires cooling before storage. If a process fluid is to be cooled or condensed from 100°C or above, to 45°C or below, an air cooler can first cool the process fluid down to 65-70°C, then further cool­ ing is provided by a shell-and-tube water-trim cooler for final cooling be­ fore it proceeds for rundown or stor­ age. A shell-and-tube cooler or con­ denser may not be a direct choice due to the probability of high tubeskin temperature, which can lead to scaling in the tubes. Where process cooling is in a lower range (70-45°C) obviously air coolers cannot be used at all, and shell-and-tube water cool­ ers are the only choice. Approach temperature. Shell-andtube water coolers can accept a lower approach temperature. For an

air cooler, an economical approach temperature between the outlet of process fluid and the ambient air temperature is generally 15-25°C, whereas for shell-and-tube water coolers, the approach temperature can be as low as 5°C. Winterization. In cold climates, air coolers require extensive winteriza­ tion arrangements to protect against congealing (due to low pour point of process fluid) or freezing for very low air-inlet design temperature. Elabo­ rate ducting with louvers, actuators, steam coil or heating fans under each fan can increase the capital cost many times. In shell-and-tube water coolers, simply switching the cooling medium to a tempered water system can prevent freezing of the process fluid. Plot areas. Air coolers require a large plot area due to the larger heat-transfer area requirement. In contrast, a shell-and-tube cooler is very compact and requires much less space. Location and performance. The performance of air coolers is affected by the proximity of other structures. The efficiency of air coolers goes down drastically when the wind di­ rection changes seasonally, affecting air inlet temperature to the bundles due to the presence of furnace stack, columns and other equipment in the path of the changing air-flow direction. Because these equip­ ment cannot be positioned very far from other structures due to space constraints in the operating plant, the air temperature may increase by few degrees. Also, if sufficient space is not allocated between air coolers and columns, furnaces and buildings in the same unit, hot-air dispersion gets hampered, leading to hot-air re­ circulating to the fresh intake air. This lowers the mean temperature differ­ ence (MTD), and the area for cooling becomes inadequate. Maintenance. Air coolers gener­ ally have higher maintenance re­ quirements than shell-and-tube condensers. An air cooler consists of many static and rotating com­ ponents that may have mainte­ nance issues, such as: 1) fan-shaft misalignment, leading to high fan/ motor vibration which stops fan; 2) high fan/motor bearing tempera­ ture, resulting in failure of coupling

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43

Front tube sheet side

Tube Rear tube sheet $*Je Avg. air Avg. Sk -i

Front tube sheet side Avg. air Avg Tube Rear tube sheet side I Skin temp, °C Temp °C

than one row per pass, and should preferably have at least two passes Process fad flow Ft«J enters to tube Flu*} enters to tube 60 bom tube per row, so that the fluid flowing in Process fuid entry £ i ■ • Flu*} enCs Iron lube Pa* ? ro c Row 1 ' TST h.iv. 598 Row Hni»tO 56 3*94 74'C two rows due to pass distribution, is C 141*6 ss3 128*C Row 2 [Pass io. 7*'c 619 Row 2 534 721 mixed in later passes after exchang­ 18 588 .... Row 3PsiS.SIX I TT* • ••r c St 8 627 pms Row 3Pw»* jpc Wsstr : Row 4PasMff'C. 501 594 583 629 Row 4r ing heat with air at different tempera­ # rr H Pass 5. J"Pass 6 99 C 491 57.4 633 Row 5 ROWS 92C •» c tures in different rows. This phenom­ 105C 482 Pass 4. t2t'C 538 559 658 Row 6. Row 6 ton 10 10 enon is shown in Figure 1. Pass 5 535 476 P*»11. NwJ.urc J Row 7 Row 7 67 \Sc |P*»3 73 C o Pass 12 -i7rc c 47 506 To avoid such a situation, and if V Row 8 Pass i Row 8frM'.WCcl" Air Now Process Rear chanel side the tube-skin temperature cannot Air tlow Fluid temperature in rear Fluid temperature in front header side header side fluid flow fluid temperature Front Chanel side fluid temperature be achieved in conventional flow ar­ FIGURE 1. As seen by these two examples, the tube skin temperature in an air cooler is linked to the rangement (hot fluid entering from direction of the airflow top nozzles), a co-current arrange­ and belt; 3) dust/debris/pollen built tube-side pressure drop, which can ment is tried out where process up on tubes, leading to increased be justified economically because fluid enters from bottom nozzles air pressure drop in the bundle and the increase in the operating and and moves up the bundle. Due to reduction of airflow (some reverse capital cost of the pump is small lower MTD in such an arrangement, flow also can be seen) leading to compared to surface area and cost the required surface area goes up loss in cooling capacity; 4) corro­ of air cooler saved. In general, this further. As is usually the case, this sion of finned tubes due to salty at­ design has a relatively higher surfacekind of design is tube-side resis­ mosphere or mishandling by water area requirement. Adopting a deep tance controlling, and a bare-tube washing and so on; 5) breakage bundle design also helps improve design instead of finned tube will be or stuck louver or the failure of the the air-flow distribution in the bun­ less expensive. Even after all these louver positioner, causing restriction dle. Bundles should have no more alternatives are considered, if the in air flow and reduction of cooling TABLE 2. PERFORMANCE COMPARISON WITH TEMPERED WATER capacity; and 6) mal-operation of SHELL-AND-TUBE COOLER auto-variable pitch fan blades, as Process fluid is same as as stated inHydrocarbon the cooling by tempered water in a shell-andthey can get stuck and then air flow tube cooler and cooling of same tempered water cooling case study cannot be adjusted by variablein an air cooler blade pitch angle. Type of exchanger Air cooler Shell-and-tube S"1r

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Backwash oil/ Tempered water Tempered water/Air

Temp, C, In/out 204-70/60-80 80/60 Case study 12.4 12.4 As an example, we discuss the chal­ Heat duty, MW 45.3 lenges in handling a high-viscosity, MTD, °C Nil 152.106 high-fouling and low-pour-point hy­ Hydrocarbon flowrate, kg/h Tempered water flowrate, kq/h 525,668 525.668 drocarbon in an air cooler. 4A No. of bundles/shells required - / 3 in series A residue-upgrading project in Shell-and-tube area/ air cooler bare area, 1.000m2 1.500 a petroleum refinery has a back­ Reynolds number 1,330-50 wash oil cooler where hydrocarbon Exchanger cost, million S (approximate) 0.26 0.51 is cooled from 204 to 70°C. The oil Installation cost, million S (approximate) 0.32 0.61 viscosity is in the range of 1.4 to 40 Total installed cost, million S (approximate! 0.58 1.12 cP and the pour point is 38°C. In this project, the engineer has restricted TABLE 3. PERFORMANCE WITH COMBINED AIR COOLER AND TEMPERED WATER SHELL-AND-TUBE COOLER the Reynolds number to a minimum Cooling of same cooling partly by an air cooler in of 2,000. The design ambient-air Process fluid is same as usedHydrocarbon tempered water in Table 2 followed by tempered water in a shell-andtemperature is 47°C. The location of an aircooler tube cooler the refinery does not require exten­ Air cooler Aircooler Type of exchanger Shell-and-tube sive winterization for such a liquid. Fluid Type Backwash oil/Air Backwash oil/ Tempered Tempered water/ The design target for handling water Air high-viscosity, high-fouling and con­ Temp. °C, In/out 204/110 110-70/60-80 80/60 3.4 gealing (low pour point) hydrocarbon Heal duly, MW 3.4 9.01 67.6 16.2 15 is to achieve a tube skin temperature MTD. °C Nil Hydrocarbon flowrate, kg/h 152,106 152,106 of at least 15°C above pour point, 143,757 143.757 maximizing the tube velocity and the Tempered water llowrate, kg/h Nil Bundles/shells required 2/Nil/2 in series 2/heat-transfer coefficient such that z 634 375 800 the Reynolds number lies in the tur­ Bare area, m 16.985-2,870 351 at midpoint Reynolds number bulent region. 0.23 0.16 0.16 Exchanger cost, million S To get a reasonably high velocity (approximate) in the tube side for such fluid re­ Installation cost, million S 0.29 0.23 0.23 quires increasing the number of tube (approximate) passes in a deep bundle of 8 to 12 Total installed cost, million S 0.52 0.39 0.39 rows. But this also leads to a higher (approximate) 44

CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM FEBRUARY 2019

design target is a higher tube-skin temperature and a higher tube-side ul velocity, the Reynolds number in FIGURE 2. For applications where the fluid is tboth (i. the transition zone is not achieved, viscous and clean, twisted tape turbulators can be s Column so other alternatives must be con­ inserted into the tubes to increase heat transfer i sidered, such as cooling with tem­ fluid starts congealing. Z7 Pipe rack wit pered water. The following five design steps In the actual situation, the viscositywere considered for using an air of the process fluid increases as the cooler (Steps 1-3), using a tem­ temperature of the process fluid fallspered water shell-and-tube cooler while progressing in the tube bundle.(Step 4) and a combination of the FIGURE 3. Shown here is a computer model of an Also, if there is mal-distribution in thetwo (Step 5). The results of the per­ air cooler mounted on the pipe rack air side of the multi-pass air cool­ formance characteristics are sum­ ers, invariably the process fluid coolsmarized in Tables 1-3. Several key Step 2. By arranging the process more in some of tubes than in others.observations are pointed out in the fluid flow co-currently to air flow, the MTD goes down from 67°C (coun­ This will further increase the viscos­ following paragraphs: ity in the cooler tubes and therefore Step 1. The first attempt in thermal ter-current) to 57°C and therefore, reduce flow through the tubes, whichdesign, using a conventional coun­ the surface area requirement goes causes further cooling and more flowter-current flow arrangement in an airup. The existing design of four bays reduction. This process continues cooler, results in a Reynolds numberbecomes inadequate in surface until ultimately, we may notice that at the outlet of 300 (deep laminar area, so one more bay is added (5 fluid has stopped flowing in many flow) and a tube-skin temperature oftotal). As the number of bays is in­ of the tubes, and is only flowing in 50.6°C. The design also has a high creased, the tube-side velocity goes a very few tubes with higher veloc­ tube-side pressure drop and a low down further (0.47 m/s) and so does ity and turbulent flow regime. Pres­ tube-side velocity, in spite of 12 tubethe overall heat-transfer coefficient, sure drop may be substantially higher passes in an eight-tube row bundle, as tube side resistance is control­ and, unless the pump can deliver theand therefore high surface area re­ ling by 85%. Even if the tube-skin quirement (high cost). needed head, flow may stop and temperature is improved, there is not r 1

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inside the tubes. The swirling fluid promotes greater contact with the tube wall (increasing shear stress at the walls many times more) en­ Header for tach bay (Typ.) hancing tube-side convective heat transfer efficiency. The increase in values of Nusselt number, Reynolds 1. .. number, Prandtl number, pressure drop and friction factor will depend upon the configuration of twisted FIGURE 5. When a perfectly symmetrical distribu­ tape (twisted ratio, pitch ratio, tape FIGURE 4. Normally, the inlet piping of an airtion is not possible (as in Figure 4), an equivalent width, wire diameter and so on). cooler requires a symmetrical distribution 0#

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much improvement in the Reynolds perature (110°C) from the air cooler Structural considerations number, and because it is outside is high enough to have a Reynolds Support options. For an air-cooled the user’s requirement, this method number above 2,000. The rest of theexchanger, there are two support is also not adequate. heat duty can be cooled in a closed-options: 1) to place it on pipe rack or Step 3. From the above, since loop tempered water system, as 2) to place it on a separate structure supported from the ground. Even tube-side film coefficients are very shown in Table 3. low and become controlling, there By opting for a combined system, though air-cooled exchangers re­ is generally no advantage in using the surface area is optimized sub­ quire more space than water-cooled fins on the air side to increase the stantially and the total installed cost exchangers, the majority of space overall heat-transfer rate. Bare tube is lower. An air cooler can handle a problems can be resolved if they bundles with a large number of rowshigher inlet temperature, and a high are placed on a pipe rack (Figure . Normally the tube bundle length and split passes are more practical, Reynolds number can be maintained3) as shown above. The bare surface by limiting the outlet temperature of is fixed, based on the width of the area calculated by the software is the air cooler. Then hydrocarbon pipe rack. If the pipe rack width is 9 not very high, and the power re­ is passed through the shell side m, the tube length could be 9.5 to quirement is also similar to the first of a downstream tempered water 9.7 m. The supporting legs of the air­ two designs. The real advantage of shell-and-tube cooler where higher cooler bundle are fixed on the main using bare tube bundles is that the viscosity can be tackled better by civil or structural beams, which sim­ number of bundles is reduced from considering rotated-square (45 deg) plifies the pipe rack design. At the 10 to 8, while maintaining a skin tube arrangement. As the bulk tem­ same time, it is desirable to adjust temperature of 61,5°C. perature in the shell is above 65°C, the pipe rack or the structure lon­ Step 4. Most of the challenges seen despite the low Reynolds number, gitudinal column spacing, based on in the designs of Steps 1-3 can be heat transfer coefficient and shell the width of the air-cooler bundle, so that the bundle legs sit straight on avoided if we bring tempered water side velocity are acceptable. as a coolant, since normal cooling Therefore, we see that a combina­top of the columns. Sometimes, it water is not suitable for cooling a tion of both shell-and-tube and air may not be possible to adjust spac­ process fluid with high viscosity, low cooler play a definite role in the over­ing, since each tube bundle might pour point and a limited tube-skin all cost economics, and together theyhave different widths, depending on temperature. When the total heat achieve more benefits when com­ service condition. Therefore, adjust­ duty is cooled in a shell-and-tube, bined than when used separately. ing the pipe rack columns for differ­ ent widths may not be feasible from tempered water cooler, the capital cost is very attractive, as shown in Handling lube oil in air coolers a structural design and detailing Table 2. However, because an air In off-shore platforms where space point of view. cooler is required for cooling tem­ is at a premium, air coolers are Walkways. There will be walkways pered water, the costs balance out. used for cooling lubricant oil. Lube between sets of air coolers across If a tempered water system is used oil is both viscous and clean, and the length and near the headers for elsewhere in the plant that can be therefore it is possible to use turbu­ facilitating operators to inspect the tweaked and ramped up to accom­ lence promoters (Figure 2) or tube bundle or operate the valves. The modate this cooler, the scheme can inserts (turbulators) in this type of width of these walkways is gener­ definitely be made attractive. How­ air cooler. Such inserts can increaseally 1.5-2.0 m. Air coolers must ever, in spite of more turbulence duethe tube-side heat-transfer coeffi­ have access platforms mounted all to baffling in the shell side, the Reyn­cient by 100-250% over bare tube around on the structure to provide olds number is poor, because the exchangers, with an increase in maintenance. Air coolers have mo­ bulk temperature of the process fluidpressure drop, but without much tors hanging at the bottom of the increase in velocity. Twisted tape air-cooler plenum. Hence, it is re­ is 61 °C in the coldest shell. Step 5. We can split the total cool­ turbulators are formed into a heli­ quired to have access platforms un­ ing load into two stages and use an cal fashion and they increase heat- derneath the cooler for maintenance air cooler in a series with a temperedtransfer efficiency by breaking up of the motors. A regular staircase water trim cooler. The outlet tem- the laminar flow pattern of fluids should be provided for accessing 46

CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM FEBRUARY 2019

have to be considered. The length the air cooler platforms or motor where a total of ten bays are divided maintenance platforms. Monkey lad- into five bays both sides and vapor is of all branch pipes for all tube ders are also provided in addition to distributed equally to both sides. The bundles from its header has to be more or less similar to keep the piping of the air cooler needs to be a staircase from the structure. Piping considerations. For a mini- supported, so either the structural pressure drop the same; this will ensure equal distribution of fluids mum of two fans per bay, the height columns or the pipe-rack structural to all bundles. of the underside of the fan inlet bell columns need to be extended up(for forced-draft units) or of the un- wards to properly support the pip- 2. Normally, the inlet-side header derside of the bundle (for induced- ing. Such data have to be given at a box is considered as fixed for the draft units) should be at least 2 m or very early stage in the project, since piping connection and the other header is floating. But the bundle one fan diameter (whichever is the this needs to be considered during can move in transverse direction greater) above the ground level, el- the design of the pipe rack, of tubes by a few millimeters (say, evated floor or pipe bridge. The air The air coolers for an overhead 6 mm minimum) or if it is fixed at coolers on the pipe rack should be system are normally used when a located in such a way that the bun- large quantity of vapor is required for one edge, then it can move by a higher margin (say, 12.7 mm mini­ dies are accessible with a crane — condensation or a huge quantity of at least from one side. gas or liquid needs to be cooled. The mum) in the other direction, as per American Petroleum Institute (API, Normally, the inlet piping of an major points that need to be taken 7th edition) recommendations. air cooler requires a symmetrical care of when routing the inlet and distribution for condensers. There- outlet pipes are as follows: Usually, this value can be any­ where between 5 and 60 mm. This fore, the number of bays/bundles 1. If the supply line has a very low operating pressure, which is usu­ header displacement is necessary is based on 2n = 2, 4, 8, 16, 32 ... to compensate for piping inlet/out­ (Figure 4), When such an exact sym­ ally the case for connecting a distillation column, care needs to let header expansion. The value metry is not possible due to some decided upon should be con­ constraint, such as pressure drop be taken to keep the number of or structural limitation, efforts should bends or elbows leading to the air firmed with the vendor of the air cooler, since the vendor may use be made to maximize an equivalent cooler at a minimum. But funca different displacement provision. symmetry, as shown in Figure 5, tionality and stress requirements

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47

TABLE 4. OVERHEAD CONDENSER PERFORMANCE WITH RESPECT TO AIR AMBIENT substantial, as shown in Table 4. Such flexibility is not possible TEMPERATURE using only shell-and-tube coolers, Air cooler used alone (at lower ambient temperature! Air ambient temp, °C where operators only very rarely will 50*C 42°C 34°C 53.7 Process fluid temp, °C 65 42.6 adjust the cooling water flowrate by 58.7 Heat duty, MW 55.8 61 operating exchanger valves (manu­ Shaft power requirement. kW 390 370 350 ally or automatically). Therefore, we Air cooler and trim cooler combination (per design) with air flow optimized for lower ambient temperature see that shell-and-tube and air cool­ 558 55.8 55.8 Heat duty, MW ers both have definite roles in the 5,902,853 4,421,612(75%) 3,467,826 (59%) Air flowrate, kq/h overall cost economics. 104 180 Shaft power requirement. kW 390 Weatherizing for colder climates. Saving in shaft power per year 1,680 2,288 In some parts of the world, where (8,000 h), MW________

ambient conditions are such that in The movement of tube bundles and sometimes power is required winter, air temperature dips below in the transverse direction could for heating coils in cold climates. freezing, temperature control of the occur only when the piping con­ Normally, the design of the cooler process streams at the air-cooler nected to equipment nozzles gen­ will be such to accommodate the outlet is required to prevent freezing erates enough force to overcome highest expected ambient tempera­ of low pour-point hydrocarbons. This the friction at the bundle supports. ture (for example, 50°C), but be­ leads to a more complicated design That is why it is a common prac­ cause of seasonal changes, such a for the air cooler. When a minimum tube-wall tem­ tice to provide stainless steel, strategy would be wasteful during polytetrafluoroethylene (PTFE) or the periods when the ambient tem­ perature has to be maintained in the other type of plate at the support perature is lower (for example, 40°C)air cooler, a recirculation system is point to ease the movement. But — which can be as much as 50% employed whereby automatic lou­ this must be done in consultation for the year. Operating power costs vers at the top and sides of the air with the vendor. can therefore be much lower than cooler housing (containing the entire 3. The loads created on the bundle the installed power costs by using assembly of tube bundles, ducting, nozzle — due to thermal expan­ two-speed motors, auto-variable steam coils, plenums and fan mo­ sion, the weights of the pipe, in­ pitch fans and variable frequency tors) control the extent of recircula­ sulation and fluid, and the inside drive (VFD) fan-motor control to re­ tion. The recirculation is possible in pressure of piping — should be duce the air flow. In temperate cli­ forced-draft air coolers, since hot ex­ less than the limits given by API. mates, as much as 50% or more haust air can be recirculated through Sometimes the vendor allows a of the design power may be saved a duct-and-louver system. higher allowable load (normally over the course of a year with auto A steam coil is generally a sepa­ variable-pitch fans. two times the code value). rate tube bundle of one or two rows Locating shell-and-tube coolers. Nowadays, VFD technology has having a length and width similar to The standalone water condenser become popular and more com­ the main air cooler, that is placed and shell-and-tube trim water mon rather than auto variable-pitch below the main air cooler bundle. If cooler can be placed above the fans, which are problematic for a an electric fan heater is to be pro­ condenser drum on a structural number of reasons. The air require­ vided in place of a steam-heated platform supported from grade. ment can be adjusted from 10 to bundle, the same is placed below Even though a shell-and-tube water 100% through VFD control. Table each air cooler fan. Low-pressure cooler will require less space than 4 presents the potential savings (LP) steam being inside finned tubes, an air cooler, some area allocation that can be expected by adjusting the steam-heated bundle and lou­ is needed on the ground (or on the power requirements to different ver will involve additional pressure the platform for off-shore applica­ ambient-air temperatures, for both drop in the fan design. Closing the tions). The pressure of the cooling a standalone air cooler, and for a louver on top of a bundle allows the water reaching to the tube side of combination of air cooler and trim heating coil to warm the bundle dur­ condensers and trim coolers (when cooler. When the ambient tempera­ ing start up in freezing weather, so on a platform) is very important for ture drops to below 34°C — which that the material in the bundle will proper functioning of the condenser can occur during 5 to 6 months not freeze or solidify. A steam coil is and the trim cooler. If the pressure over the year, the trim cooler may also used to temper very cold air to drop in the cooling water circuit is not even be required at all, since the bundles in continuous operation not properly calculated while de­ the total heat duty can be handled while the fan is operating and the signing the inlet/outlet piping, the by air cooler alone. So in addition to exhaust louver is open. When two actual cooling water flowrate will be the power savings, there will be ad­ fans are operated per bay, an auto­ lower than that of the design cal­ ditional savings for not using coolingvariable-pitch fan or fan with VFD is culation, and the performance may water, which can cost around $0.4 kept at rear end of the bundle, so million/yr. If a refinery decides to that the process fluid outlet temper­ suffer as a result. Flexibility. The operating costs for keep both air coolers and trim cool­ ature can be controlled. the air cooler include the electrical ers on line, power saving through Controlling recirculation. A part power required to operate the fans reduction of air flow via VFD is very of the air leaving from the top of the

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CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM FEBRUARY 2019

system is recirculated and mixes regulate the opening and closing 6. "Heat Exchanger and Cootng Tower Manual," Chevron Corp., 1989. with fresh air entering from the sidesof the outlet louver. The duct sizing 6. Thomas, J.W., Air Wins Even with Water Plentiful," 'Heat so that the combined temperature is calculation and differential pres­ Exchanger Design Handbook.'Gulf Publish**) Co.. 1968. precisely the design ambient tem­ sure drop across the dampers is 7. Basic Considerations for Equipment and Piping Layout perature. The lower the ambient carefully done to avoid failure of of Ax-cooled Exchanger Piping, from www.whatisprping temperature, the greater will be the the system. The vendor’s scope of com/piprig-layoul-air-cooled-heat-exchanger. work will include the design of the 8. Exchanger Optimizer software (Trial version). Heal Trans­ extent of recirculation. For startup, when ambient tem­ louvers. In general, air-cooler tube- fer Research Institute (HTRI), Navasota, Tex., vwvw.htri. net/exchanger-optimizer. perature is lower than design am­ skin temperature is kept 15-20°C bient temperature, ambient air can above the tube-side fluid pourbe passed through a live-steam point temperature through this Author coil, located below the bundle, elaborate system. Manas K Manual is a principal design engineer for Fluor Daniel Although the cost of such an air such that air approaching the bun­ India Pvt. Ltd. (6th Floor, Infinity dle is heated up to the design tem­ cooler is quite high, it is still pre­ Tower B. Cyber City, DLF City perature. The recirculation of exit ferred in colder climates to prevent Phase II, Gurgaon 122 002. Hary­ ana, India; Phone: +91-124■ air is gradually increased, which icing or frosting. 4570700; Fax; +91-124Edited by Gerald Ondrey reduces the steam requirement. 4263269; Email: manas. Eventually, the steam supply can [email protected]). He has more than 34 years of experience in the be stopped altogether. References of process heat transfer, cost optimization studies, The air cooler is fitted with a duct 1. Giammaruti. R.. Performance Improvement held to Existing process design and operations, process revamp, project Air-Cooled, Heal Exchangers, Gas Machinery Research leading from the top outlet to the control and energy management. Prior to Fluor, he Council. Dallas Tex.. 2006. bottom inlet. Louvers are placed in worked at Engineers India Ltd. Delhi and Hindustan Pe­ Perkins B.C., Which Cooling Medium-Watertroleum or Air. "Heat Mumbai Refinery. Mandal has presented many the bypass duct, at the air inlet and 2.Exchanger Design Handbook," Gulf Publishing Co.. in 1968. papers various seminars on heat transfer, energy at the air outlet. The temperature of 3. Basics of Air Cooled Heat Exchangers Amarcool management Manu- and process improvement. Mandal holds air just below the tube bundles will Wiring Inc Tulsa, Okla. a B.Tech. degree in chemical engineering from the In­ regulate the opening and closing of 4. Williams Jr., GL.and Damron, R.D., Which dian CoolsInstitute of Technology, Delhi and Masters degree in financial inlet and bypass louvers, whereas Cheaper: Water or Air," "Heat Exchanger Design Hand­management from Jamnalal Bajaj Institute ot Management Studies. Mumbai. India. the process outlet temperature will book," Gull Publishing Co., 1968.

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AFPM

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