ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 23-2012 AIR-COOLING AND DEHUMIDIFYING COILS.pdf

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1、23.1CHAPTER 23AIR-COOLING AND DEHUMIDIFYING COILSUses for Coils. 23.1Coil Construction and Arrangement 23.1Coil Selection. 23.5Airflow Resistance . 23.6Heat Transfer . 23.6Performance of Sensible Cooling Coils . 23.7Performance of Dehumidifying Coils. 23.9Determining Refrigeration Load 23.14Maintena

2、nce. 23.15Symbols 23.16OST equipment used today for cooling and dehumidifying anMairstream under forced convection incorporates a coil sectionthat contains one or more cooling coils assembled in a coil bankarrangement. Such coil sections are used extensively as componentsin room terminal units; larg

3、er factory-assembled, self-contained airconditioners; central station air handlers; and field built-up systems.Applications of each coil type are limited to the field within whichthe coil is rated. Other limitations are imposed by code require-ments, proper choice of materials for the fluids used, t

4、he configura-tion of the air handler, and economic analysis of the possiblealternatives for each installation.USES FOR COILSCoils are used for air cooling with or without accompanyingdehumidification. Examples of cooling applications without dehu-midification are (1) precooling coils that use well w

5、ater or other rel-atively high-temperature water to reduce load on the refrigeratingequipment and (2) chilled-water coils that remove sensible heat fromchemical moisture-absorption apparatus. The heat pipe coil is alsoused as a supplementary heat exchanger for preconditioning in air-side sensible co

6、oling (see Chapter 26). Most coil sections provide airsensible cooling and dehumidification simultaneously.The assembly usually includes a means of cleaning air to protectthe coil from dirt accumulation and to keep dust and foreign matterout of the conditioned space. Although cooling and dehumidific

7、ationare their principal functions, cooling coils can also be wetted withwater or a hygroscopic liquid to aid in air cleaning, odor absorption,or frost prevention. Coils are also evaporatively cooled with a waterspray to improve efficiency or capacity. Chapter 41 has more infor-mation on indirect ev

8、aporative cooling. For general comfort condi-tioning, cooling, and dehumidifying, the extended-surface (finned)cooling coil design is the most popular and practical.COIL CONSTRUCTION AND ARRANGEMENTIn finned coils, the external surface of the tubes is primary, andthe fin surface is secondary. The pr

9、imary surface generally consistsof rows of round tubes or pipes that may be staggered or placed inline with respect to the airflow. Flattened tubes or tubes with othernonround internal passageways are sometimes used. The inside sur-face of the tubes is usually smooth and plain, but some coil designs

10、have various forms of internal fins or turbulence promoters (eitherfabricated or extruded) to enhance performance. The individual tubepasses in a coil are usually interconnected by return bends (or hair-pin bend tubes) to form the serpentine arrangement of multipasstube circuits. Coils are usually a

11、vailable with different circuitarrangements and combinations offering varying numbers of paral-lel water flow passes within the tube core (Figure 1).Cooling coils for water, aqueous glycol, brine, or halocarbonrefrigerants usually have aluminum fins on copper tubes, althoughcopper fins on copper tub

12、es and aluminum fins on aluminum tubes(excluding water) are also used. Adhesives are sometimes used tobond header connections, return bends, and fin-tube joints, particu-larly for aluminum-to-aluminum joints. Certain special-applicationcoils feature an all-aluminum extruded tube-and-fin surface.Comm

13、on core tube outside diameters are 8, 10, 12.5, 16, 20, and25 mm, with fins spaced 1.4 to 6.4 mm apart. Tube spacing rangesfrom 15 to 75 mm on equilateral (staggered) or rectangular (in-line)centers, depending on the width of individual fins and on other per-formance considerations. Fins should be s

14、paced according to the jobto be performed, with special attention given to air friction; possi-bility of lint accumulation; and frost accumulation, especially atlower temperatures.Tube wall thickness and the required use of alloys other than cop-per are determined mainly by the coils working pressur

15、e and safetyfactor for hydrostatic burst (pressure). Maximum allowable work-ing pressure (MAWP) for a coil is derived according to ASMEsBoiler and Pressure Vessel Code, Section VIII, Division 1 and Sec-tion II (ASTM material properties and stress tables). Pressure vesselsafety standards compliance a

16、nd certifications of coil constructionmay be required by regional and local codes before field installa-tion. Fin type and header construction also play a large part in deter-mining wall thickness and material. Local job site codes andapplicable nationally recognized safety standards should be con-s

17、ulted in coil design and application.This type of air-cooling coil normally has a shiny aluminum air-side surface. For special applications, the fin surface may be copperor have a brown or blue-green dip-process coating. These coatingsprotect the fin from oxidation that occurs when common airborneco

18、rrosive contaminants are diluted on a wet (dehumidifying) sur-face. Corrosion protection is increasingly important as indoor airquality (IAQ) guidelines call for higher percentages of outside air.Baked-on or anodized coating improves the expected service lifeThe preparation of this chapter is assign

19、ed to TC 8.4, Air-to-RefrigerantHeat Transfer Equipment.Fig. 1 Typical Water Circuit Arrangements23.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)compared to plain aluminum fins under similar conditions. Un-coated fins on non-dehumidifying, dry cooling coils are generallynot affected by norma

20、l ambient airborne chemicals, except, to someextent, in a saline atmosphere. Once the coil is installed, little can bedone to improve air-side protection.Incoming airstream stratification across the coil face reduces coilperformance. Proper air distribution is defined as having a measuredairflow any

21、where on the coil face that does not vary more than 20%.Moisture carryover at the coils air leaving side or uneven air filterloading are indications of uneven airflow through the coil. Normalcorrective procedure is to install inlet air straighteners, or an airblender if several airstreams converge a

22、t the coil inlet face. Addi-tionally, condensate water should never be allowed to saturate theduct liner or stand in the drain pan (trough). The coil frame (partic-ularly its bottom sheet metal member) should not be allowed to sitin a pool of water, to prevent rusting.Water and Aqueous Glycol CoilsG

23、ood performance of water-type coils requires both eliminatingall air and water traps in the water circuit and the proper distributionof water. Unless properly vented, air may accumulate in the coil tubecircuits, reducing thermal performance and possibly causing noise orvibration in the piping system

24、. Air vent and drain connections areusually provided on coil water headers, but this does not eliminate theneed to install, operate, and maintain the coil tube core in a level posi-tion. Individual coil vents and drain plugs are often incorporated onthe headers (Figure 1). Water traps in tubing of a

25、 properly leveled coilare usually caused by (1) improper nondraining circuit design and/or(2) center-of-coil downward sag. Such a situation may cause tubefailure (e.g., freeze-up in cold climates or tube erosion because ofuntreated mineralized water).Depending on performance requirements, fluid velo

26、city insidethe tube usually ranges from approximately 0.3 to 2.4 ms for waterand 0.15 to 1.8 ms for glycol. When turbulators or grooved tubesare used, in-tube velocities should not exceed 1.2 ms. The designfluid pressure drop across the coils varies from about 15 to 150kPa. For nuclear HVAC applicat

27、ions, ASME Standard AG-1, Codeon Nuclear Air and Gas Treatment, requires a minimum tube veloc-ity of 0.6 ms. AHRI Standard 410 requires a minimum of 0.3 msor a Reynolds number of 3100 or greater. This yields more predict-able performance.In certain cases, the water may contain considerable sand ando

28、ther foreign matter (e.g., precooling coils using well water, orwhere minerals in the cooling water deposit on and foul the tube sur-face). It is best to filter out such sediment. Some coil manufacturersoffer removable water header plates or a removable plug for eachtube that allows the tube to be c

29、leaned, ensuring continuing ratedperformance while the cooling units are in service. Where build-upof scale deposits or fouling of the water-side surface is expected, ascale factor is sometimes included when calculating thermal perfor-mance of the coils. Cupronickel, red brass, bronze, and other tub

30、ealloys help protect against corrosion and erosion deteriorationcaused primarily by internal fluid flow abrasive sediment. The coretubes of properly designed and installed coils should feature circuitsthat (1) have equally developed line length, (2) are self-draining bygravity during the coils off c

31、ycle, (3) have the minimum pressuredrop to aid water distribution from the supply header without requir-ing excessive pumping pressure, and (4) have equal feed and returnby the supply and return header. Design for proper in-tube watervelocity determines the circuitry style required. Multirow coils a

32、reusually circuited to the cross-counterflow arrangement and orientedfor top-outlet/bottom-feed connection.Direct-Expansion CoilsCoils for halocarbon refrigerants present more complex cool-ing fluid distribution problems than do water or brine coils. Thecoil should cool effectively and uniformly thr

33、oughout, with evenrefrigerant distribution. Halocarbon coils are used on two types ofrefrigerated systems: flooded and direct-expansion.A flooded system is used mainly when a small temperature dif-ference between the air and refrigerant is desired. Chapter 2 of the2010 ASHRAE HandbookRefrigeration d

34、escribes flooded sys-tems in more detail.For direct-expansion systems, two of the most commonly usedrefrigerant liquid metering arrangements are the capillary tubeassembly (or restrictor orifice) and the thermostatic expansion valve(TXV) device. The capillary tube is applied in factory-assembled,sel

35、f-contained air conditioners up to approximately 35 kW capacity,but is most widely used on smaller-capacity models such as windowor room units. In this system, the bore and length of a capillary tubeare sized so that at full load, under design conditions, just enoughliquid refrigerant to be evaporat

36、ed completely is metered from thecondenser to the evaporator coil. Although this type of meteringarrangement does not operate over a wide range of conditions asefficiently as a TXV system, its performance is targeted for a spe-cific design condition.A thermostatic expansion valve system is commonly

37、used forall direct-expansion coil applications described in this chapter,particularly field-assembled coil sections and those used in centralair-handling units and larger, factory-assembled hermetic air condi-tioners. This system depends on the TXV to automatically regulatethe rate of refrigerant li

38、quid flow to the coil in direct proportion tothe evaporation rate of refrigerant liquid in the coil, thereby main-taining optimum performance over a wide range of conditions.Superheat at the coil suction outlet is continually maintained withinthe usual predetermined limits of 3 to 6 K. Because the T

39、XVresponds to the superheat at the coil outlet, superheat within the coilis produced with the least possible sacrifice of active evaporatingsurface.The length of each coils refrigerant circuits, from the TXVs dis-tributor feed tubes through the suction header, should be equal. Thelength of each circ

40、uit should be optimized to provide good heattransfer, good oil return, and a complementary pressure drop acrossthe circuit. The coil should be installed level, and coil circuitryshould be designed to self-drain by gravity toward the suctionheader connection. This is especially important on systems w

41、ithunloaders or variable-speed-drive compressor(s). When non-self-drain circuitry is used, the circuit and suction connection should bedesigned for a minimum tube velocity sufficient to avoid compres-sor lube oil trapping in the coil.To ensure reasonably uniform refrigerant distribution in multi-cir

42、cuit coils, a distributor is placed between the TXV and coilinlets to divide refrigerant equally among the coil circuits. Therefrigerant distributor must be effective in distributing both liquidand vapor because refrigerant entering the coil is usually a mixtureof the two, although mainly liquid by

43、mass. Distributors can beplaced either vertically or horizontally; however, the vertical downposition usually distributes refrigerant between coil circuits betterthan the horizontal for varying load conditions.Individual coil circuit connections from the refrigerant distribu-tor to the coil inlet ar

44、e made of small-diameter tubing; the connec-tions are all the same length and diameter so that the same flowoccurs between each refrigerant distributor tube and each coil cir-cuit. To approximate uniform refrigerant distribution, refrigerantshould flow to each refrigerant distributor circuit in prop

45、ortion tothe load on that coil. The heat load must be distributed equally toeach refrigerant circuit for optimum coil performance. If the coilload cannot be distributed uniformly, the coil should be recircuitedand connected with more than one TXV to feed the circuits (indi-vidual suction may also he

46、lp). In this way, refrigerant distribution isreduced in proportion to the number of distributors to have lesseffect on overall coil performance when design must accommodatesome unequal circuit loading. Unequal circuit loading may also becaused by uneven air velocity across the coils face, uneven ent

47、eringAir-Cooling and Dehumidifying Coils 23.3air temperature, improper coil circuiting, oversized orifice in dis-tributor, or the TXVs not being directly connected (close-coupled)to the distributor.Control of CoilsCooling capacity of water coils is controlled by varying eitherwater flow or airflow.

48、Water flow can be controlled by a three-waymixing, modulating, and/or throttling valve. For airflow control,face and bypass dampers are used. When cooling demand de-creases, the coil face damper starts to close, and the bypass damperopens. In some cases, airflow is varied by controlling fan capacity

49、with speed controls, inlet vanes, or discharge dampers.Chapter 47 of the 2011 ASHRAE HandbookHVAC Applica-tions addresses air-cooling coil control to meet system or spacerequirements and factors to consider when sizing automatic valvesfor water coils. Selection and application of refrigerant flow controldevices (e.g., thermostatic expansion valves, capillary tube types,constant-pressure expansion valves, evaporator pressure regulators,suction-pressure regulators, solenoid valves) as used with direct-expansion coils are discussed in Chapter 11 of the 2010 ASHRAEHandbookRefrige