ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 27-2012 AIR-HEATING COILS.pdf

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1、27.1CHAPTER 27AIR-HEATING COILSCoil Construction and Design. 27.1Coil Selection 27.3Installation Guidelines 27.4Coil Maintenance 27.5IR-HEATING coils are used to heat air under forced con-Avection. The total coil surface may consist of a single coilsection or several coil sections assembled into a b

2、ank. The coilsdescribed in this chapter apply primarily to comfort heating and airconditioning using steam, hot water, refrigerant vapor heat reclaim(including heat pumps), and electricity. The choice between thevarious methods of heating depends greatly on the cost of the vari-ous available energy

3、sources. For instance, in areas where electricpower is cheaply available and heating requirements are limited,heat pumps are a very viable option. With available power andhigher heat requirements, electric heat is used. If electric power isconsiderably expensive, steam or hot water generated using g

4、as-fired sources is used in larger buildings and district cooling. Insmaller buildings, heat is supplied using gas furnaces, which arecovered in Chapters 33 and 34. Water and steam heating are alsowidely used where process waste heat is available.COIL CONSTRUCTION AND DESIGNExtended-surface coils co

5、nsist of a primary and a secondaryheat-transfer surface. The primary surface is the external surface ofthe tubes, generally consisting of rows of round tubes or pipes thatmay be staggered or parallel (in-line) with respect to the airflow.Flattened tubes or tubes with other nonround internal passagew

6、aysare sometimes used. The inside of the tube is usually smooth andplain, but some coil designs feature various forms of internal fins orturbulence promoters (either fabricated and then inserted, or ex-truded) to enhance fluid coil performance. The secondary surface isthe fins external surface, whic

7、h consists of thin metal plates or a spi-ral ribbon uniformly spaced or wound along the length of the primarysurface. The intimate contact with the primary surface provides goodheat transfer. Air-heating fluid and steam coils are generally availablewith different circuit arrangements and combination

8、s that offer vary-ing numbers of parallel water flow passes in the tube core.Copper and aluminum are the materials most commonly used forextended-surface coils. Tubing made of steel or various copperalloys is used in applications where corrosive forces might attackthe coils from inside or outside. T

9、he most common combination forlow-pressure applications is aluminum fins on copper tubes. Low-pressure steam coils are usually designed to operate up to 350 kPa(gage). Higher-strength tube materials such as red brass, admiraltybrass, or cupronickel assembled by brazed construction are usableup to 18

10、6C water or 1 MPa (gage) saturated steam. Higher operat-ing conditions call for electric welded stainless steel construction,designed to meet Section II and Section VIII requirements of theASME Boiler and Pressure Vessel Code.Customarily, the coil casing consists of a top and bottom channel(also kno

11、wn as baffles or side sheets), two end supports (also knownas end plates or tube sheets), and, on longer coils, intermediate sup-ports (also known as center supports or tube sheets). Designs vary,but most are mounted on ducts or built-up systems. Most often,casing material is spangled zinc-coated (g

12、alvanized) steel with aminimum coating designation of G90-U. Some corrosive air condi-tions may require stainless steel casings or corrosive-resistant coat-ing, such as a baked phenolic applied by the manufacturer to theentire coil surface. Steam coil casings should be designed to accom-modate therm

13、al expansion of the tube core during operation (a float-ing core arrangement).Common core tube diameters vary from 8 up to 25 mm outsidediameter (OD) and fin spacings from 1.4 to 6.4 mm. Fluid heatingcoils have a tube spacing from 20 to 45 mm and tube diameters from8 to 16 mm OD. Steam coils have tu

14、be spacing from 30 to 75 mmand tube diameters from 13 to 25 mm OD. The most commonarrangements are one- or two-row steam coils and two- to four-rowhot-water coils. Fins should be spaced according to the applicationrequirements, with particular attention given to any severe duty con-ditions, such as

15、inlet temperatures and contaminants in the air-stream.Tube wall thickness and the required use of alloys other than(standard) copper are determined primarily by the coils specifiedmaximum allowable working pressure (MAWP) requirements. Asecondary consideration is expected coil service life. Fin type

16、,header, and connection construction also play a large part in thisdetermination. All applicable local job site codes and nationalsafety standards should be followed in the design and application ofheating coils.Flow direction can strongly affect heat transfer surface perfor-mance. In air-heating co

17、ils with only one row of tubes, the air flowsat right angles to the heating medium. Such a cross-flow arrange-ment is common in steam heating coils. The steam temperature inthe tubes remains uniform, and the mean temperature difference isthe same regardless of the direction of flow relative to the a

18、ir. Thesteam supply connection is located either in the center or at the topof the inlet header. The steam condensate outlet (return connection)is always at the lowest point in the return header. When coils have two or more tube rows in the direction of airflow,such as hot-water coils, the heating m

19、edium in the tubes may be cir-cuited in various parallel-flow and counterflow arrangements. Coun-terflow is the arrangement most preferred to obtain the highestpossible mean temperature difference, which determines the heattransfer of the coil. The greater this temperature difference, thegreater the

20、 coils heat transfer capacity. In multirow coils circuitedfor counterflow, water enters the tube row on the leaving air side ofthe coil.Steam CoilsSteam coils are generally classified, similarly to boilers, by oper-ating pressure: low (100 kPa) or high (100 kPa). However, variousorganizations use ot

21、her pressure classification schemes with differingdivisions e.g., low (100 kPa), medium (100 to 690 kPa), or high(690 kPa). Steam coils can also be categorized by operating limits ofthe tube materials:Standard steam 1030 kPa 185C copper tubeHigh-pressure 1625 kPa 204C special materialsteam e.g., cup

22、ronickel (CuNi)The preparation of this chapter is assigned to TC 8.4, Air-to-RefrigerantHeat Transfer Equipment.27.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)Although these operating conditions are allowed by code, longexposures to them will shorten coil tube life. Leaks are less likelywhe

23、n the coil tube core has thicker walls of higher-strength materi-als. Operational experience suggests preferred limits for continuous-duty steam coils (tube OD ranging from 16 to 25 mm) in commercialand institutional applications, as shown in Table 1.Steam coils also can be categorized by type as ba

24、sic steam,steam-distributing, or face-and-bypass.Basic steam coils generally have smooth tubes with fins on theair side. The steam supply connection is at one end and the tubes arepitched toward the condensate return, which is usually at the oppo-site end. For horizontal airflow, the tubes can be ei

25、ther vertical orhorizontal. Horizontal tubes should be pitched within the casingtoward the condensate return to facilitate condensate removal. Uni-form steam distribution to all tubes is accomplished by carefulselection of header size, its connection locations, and positioning ofinlet connection dis

26、tributor plates. Orifices also may be used at thecore tube entrances in the supply header.Steam-distributing coils most often incorporate perforated in-ner tubes that distribute steam evenly along the entire coil. The per-forations perform like small steam ejector jets that, when angled inthe inner

27、tube, help remove condensate from the outer tube. Analternative design for short coils is an inner tube with no distributionholes, but with an open end. On all coils, supply and return connec-tions can be at the same end or at opposite ends of the coil. For long,low-pressure coils, supply is usually

28、 at both ends and the condensatereturn on one end only.Face-and-bypass steam coils have short sections of steam coilsseparated by air bypass openings. Airflow through the coil or bypasssection is controlled by coil face-and-bypass dampers linked to-gether. As a freeze protection measure, large insta

29、llations use face-and-bypass steam coils with vertical tubes.For proper performance of all types of steam heating coils, air orother noncondensables in the steam supply must be eliminated.Equally important, condensate from the steam must easily drain frominside the coil. Air vents are located at a h

30、igh point of the piping andat the coils inlet steam header. Whether airflow is horizontal or ver-tical, the coils finned section is pitched toward the condensate returnconnection end of the coil. Installers must give particular care in theselection and installation of piping, controls, and insulatio

31、n necessaryto protect the coil from freeze-up caused by incomplete condensatedrainage.When entering air is at or below 0C, the steam supply to the coilshould not be modulated, but controlled as full on or full off. Coilslocated in series in the airstream, with each coil sized and controlledto be ful

32、l on or completely off (in a specific sequence, depending onthe entering air temperature), are not as likely to freeze. Tempera-ture control with face-and-bypass dampers is also common. Duringpart-load conditions, air is bypassed around the steam coil with fullsteam flow to the coil. In a face-and-b

33、ypass arrangement, high-velocity streams of freezing air must not impinge on the coil whenthe face dampers are partially closed. The section on OverallRequirements in this chapter and the section on Control of HVACElements (Heating Coils) in Chapter 47 of the 2011 ASHRAE Hand-bookHVAC Applications h

34、ave more details.Water/Aqueous Glycol Heating CoilsNormal-temperature hot-water heating coils can be categorizedas booster coils or standard heating coils. Booster (duct-mounted orreheat) coils are commonly found in variable-air-volume systems.They are one or two rows deep, have minimal water flow,

35、and pro-vide a small air temperature rise. Casings can be either flanged orslip-and-drive construction. Standard heating coils are used in run-around systems, makeup air units, and heating and ventilating sys-tems. All use standard construction materials of copper tube andaluminum fins.High-temperat

36、ure water coils may operate with up to 200Cwater, with pressures comparable or somewhat higher than the satu-rated vapor temperature of the water supply. The temperature dropacross the coil may be as high as 85 K. To safely accommodate thesefluid temperatures and thermal stresses, the coil requires

37、industrial-grade construction that conforms to applicable boiler and safetycodes. These requirements should be listed in detail by the specify-ing engineer, along with the inspection and certification require-ments and a compliance check before coil installation and operation.Proper water coil perfo

38、rmance depends on eliminating air and ongood water distribution in the coil and its interconnecting piping.Unless properly vented, air may accumulate in the coil circuits,which reduces heat transfer and possibly causes noise and vibrationin the pipes. For this reason, water coils should be construct

39、ed withself-venting, drainable circuits. The self-venting design is main-tained by field-connecting the water supply connection to the bot-tom and the water return connection to the top of the coil. Ideally,water is supplied at the bottom, flows upward through the coil, andforces any air out the ret

40、urn connection. Complete fluid draining atthe supply connection indicates that coils are self-draining and with-out air or water traps. Such a design ensures that the coil is alwaysfilled with water, and it should completely drain when it is requiredto be empty. Most manufacturers provide vent and d

41、rain fittings onthe supply and return headers of each water coil.When water does get trapped in the coil core, it is usually causedby a sag in the coil core or by a nondraining circuit design. Duringfreezing periods, even a small amount of water in the coil core canrupture a tube. Also, such a stati

42、c accumulation of either water orglycol can corrode the tube over an extended period. Large multirow,multicircuited coils may not drain rapidly, even with self-drainingcircuitry; if they are not installed level, complete self draining willnot take place. This problem can be prevented by including in

43、terme-diate drain headers and installing the coil so that it is pitched towardthe connections.To produce desired ratings without excessive water pressuredrop, manufacturers use various circuit arrangements. A single-feedserpentine circuit is commonly used on booster coils with low waterflows. With t

44、his arrangement, a single feed carrying the entire waterflow makes a number of passes across the airstream. The more com-mon circuit arrangement is called a full row feed or standard cir-cuit. With this design, all the core tubes of a row are fed with anequal amount of water from the supply header.

45、Others, such as quar-ter, half, and double-row feed circuit arrangements, may be avail-able, depending on the total number of tubes and rows of the coil.Uniform flow in each water circuit is obtained by designing eachcircuits length as equal to the other as possible.Generally, higher velocity provid

46、es greater capacity and moreeven discharge air temperature across the coil face, but with dimin-ishing returns. To prevent erosion, 1.8 m/s should not be exceededfor copper coils. At higher velocities, only modest gains in capacitycan be achieved at increasingly higher pumping power penalties.Above

47、2.4 m/s, any gain is negligible.Table 1 Preferred Operating Limits for Continuous-Duty Steam Coil Materials in Commercial and Institutional ApplicationsPressure, kPa Material Tube Wall Thickness, mm35 Copper 0.5135 to 100 0.64100 to 200 0.89200 to 345 1.25345 to 515 Red brass 0.64515 to 690 0.89690

48、to 1035 90/10 CuNi 0.891035 to 1380 1.25Note: Red brass and CuNi may be interchanged, depending on coil manufacturersspecifications.Air-Heating Coils 27.3Velocities with fluid flow Reynolds numbers (Re) between 2000and 10 000 fall into a transition range where heat transfer capacitypredictions are l

49、ess likely to be accurately computed. Below Re =2000, flow is laminar, where heat transfer prediction is again reliable,but coil capacity is greatly diminished and tube fouling can becomea problem. For further insight on the transition flow effect on capac-ity, refer to Figure 16 of Air-Conditioning, Heating, and Refrigera-tion Institute (AHRI) Standard 410. Methods of controlling watercoils to produce a uniform exit air temperature are discussed inChapter 47 of the 2011 ASHRAE HandbookHVAC Applications.In some cases, the hot water circulated may contain a consider-ab