1、14.1CHAPTER 14FORCED-CIRCULATION AIR COOLERSTypes of Forced-Circulation Air Coolers . 14.1Components. 14.2Air Movement and Distribution 14.3Unit Ratings 14.4Installation and Operation 14.6More Information 14.6ORCED-CIRCULATION unit coolers and product coolers areFdesigned to operate continuously in
2、refrigerated enclosures; acooling coil and motor-driven fan are their basic components, andprovide cooling or freezing temperatures and proper airflow to theroom. Coil defrost equipment is added for low-temperature opera-tions when coil frosting might impede performance.Any unit (e.g., blower coil,
3、unit cooler, product cooler, cold dif-fuser unit, air-conditioning air handler) is considered a forced-aircooler when operated under refrigeration conditions. Many designand construction choices are available, including (1) various coiltypes and fin spacing; (2) electric, gas, air, water, or hot-bri
4、ne de-frosting; (3) discharge air velocity and direction; (4) centrifugal orpropeller fans, either belt- or direct-driven; (5) ducted or nonducted;and/or (6) freestanding or ceiling-suspended, or penthouse (roof-mounted).Fans in these units direct air over a refrigerated coil contained inan enclosur
5、e. For nearly all applications of these units, the coil low-ers airflow temperature below its dew point, which causes conden-sate or frost to form on the coil surface. However, the normalrefrigeration load is a sensible heat load; therefore, the coil surfaceis considered dry. Rapid and frequent defr
6、osting on a timed cyclecan maintain this dry-surface condition, or the coil and airflow canbe designed to reduce frost accumulation and its effect on refriger-ation capacity.TYPES OF FORCED-CIRCULATION AIR COOLERSFigures 1 to 4 illustrate features of some types of air coolers.Sloped-front unit coole
7、rs, often called reach-in unit coolers,range from 5 to 10 in. high (Figure 1). Their distinctive slopedfronts are designed for horizontal top mounting as a single unit, orfor installation as a group of parallel connected units. Direct-drivefans are sloped to fit in the restricted return airstream, w
8、hich risespast the access doors and across the ceiling of the enclosure. Air-flows are usually less than 150 cfm per fan. Commonly, these unitsare installed in back-bar and under-the-counter fixtures, as well asin vertical, self-serve, glass door reach-in enclosures.Low-air-velocity units feature a
9、long, narrow profile (Figure 2).They have a dual-coil arrangement, and usually two or more fans.These units are used in above-freezing meat-cutting rooms and incarcass and floral walk-in enclosures, as well as 28F meat carcassholding rooms. They are designed to maintain as high a humidity aspossible
10、 in the enclosure. The units airflow velocity is low and finson the coil are amply spaced, which reduces the coils wetted surfacearea and thus the amount of dew-point contact area for the air stream.Discharge air velocities at the coil face range from 85 to 200 fpm.Medium-air-velocity unit coolers o
11、riginally had a half-roundappearance, although the more common version (often called low-profile units) features a long, narrow, dual-coil unit design (Figure 3).Both types of units are equipped with higher-volume fans. They areused in vegetable preparation rooms, walk-in rooms for wrappedfresh meat
12、, and dairy coolers. These units normally extract moremoisture from ambient air than low-velocity units do. Discharge airvelocities at the coil face range from 200 to 400 fpm.Low-silhouette units are 12 to 15 in. high. Medium- ormid-height units are 18 to 30 in. high. Those over 30 in. high areThe p
13、reparation of this chapter is assigned to TC 8.4, Air-to-RefrigerantHeat Transfer Equipment.Fig. 1 Sloped-Front Unit Cooler for Reach-In CabinetsFig. 1 Sloped-Front Unit Cooler for Reach-In CabinetsFig. 2 Low-Air-Velocity UnitFig. 2 Low-Air-Velocity Unit14.2 2010 ASHRAE HandbookRefrigerationclassifi
14、ed as high-silhouette unit coolers, which are used inwarehouse-sized coolers and freezers. Air velocity at the coil facecan be over 600 fpm. Outlet air velocities range from 1000 to2000 fpm when the unit is equipped with cone-shaped fan dischargeventuris for extended air throw.Spray coils feature a
15、saturated coil surface that can cool pro-cessed air closer to the coil surface temperature than can a regular(nonsprayed) coil. In addition, the spray continuously defrosts thelow-temperature coil. Unlike unit coolers, spray coolers are usuallyfloor-mounted and discharge air vertically. Unit section
16、s include adrain pan/sump, coil with spray section, moisture eliminators, andfan with drive. The eliminators prevent airborne spray droplets fromdischarging into the refrigerated area. Typically, belt-driven centrif-ugal fans draw air through the coil at 600 fpm or less.Water can be used as the spra
17、y medium for coil surfaces with tem-peratures above freezing. For coil surfaces with temperatures belowfreezing, a suitable chemical must be added to the water to lower thefreezing point to 12F, or below the coil surface temperature. Somesuitable recirculating solutions include the following:Sodium
18、chloride solution is limited to a room temperature of10F or higher. Its minimum freezing point is 6F.Calcium chloride solution can be used for enclosure tempera-tures down to about 10F, but its use may be prohibited in enclo-sures containing food products.Aqueous glycol solutions are commonly used i
19、n water and/orsprayed-coil coolers operating below freezing. Food-grade pro-pylene glycol solutions are commonly used because of their loworal toxicity, but they generally become too viscous to pump attemperatures below 13F. Ethylene glycol solutions may bepumped at temperatures as low as 40F. Becau
20、se of its toxicity,sprayed ethylene glycol in other than sealed tunnels or freezers(no human access allowed during process) is usually prohibitedby most jurisdictions. When a glycol mix is sprayed in food stor-age rooms, any spray carryover must be maintained within thelimits prescribed by all appli
21、cable regulations.All brines are hygroscopic; that is, they absorb condensate andbecome progressively weaker. This dilution can be corrected by con-tinually adding salt to the solution to maintain a sufficient below-freezing temperature. Salt is extremely corrosive, and must becontained in the spray
22、ed-coil unit with suitable corrosive-resistantmaterials or coatings, which must be periodically inspected andmaintained. All untreated brines are corrosive: neutralizing thespray solution relative to its contact material is required.Sprayed-coil units are usually installed in refrigerated enclosures
23、requiring high humidity (e.g., chill coolers). Paradoxically, the samesprayed-coil units can be used in special applications requiring lowrelative humidity. For these applications, both a high brine concen-trate (near its eutectic point) and a large difference between the pro-cess air and the refrig
24、erant temperature are maintained. Process airis reheated downstream from the sprayed coil to correct the dry-bulbtemperature.COMPONENTSDraw-Through and Blow-Through AirflowUnit fans may draw air through the cooling coil and discharge itthrough the fan outlet into the enclosure, or they may blow airt
25、hrough the cooling coil and discharge it from the coil face into theenclosure. Blow-through units have a slightly higher thermal effi-ciency because heat from the fan is removed from the forced air-stream by the coil, but their air distribution pattern is less effectivethan the draw-through design.
26、Draw-through fan energy adds to theheat load of the refrigerated enclosure, but heat gain from fractionalhorsepower or small three-phase integral fan motors is not signifi-cant. Selection of draw-through or blow-through depends more on amanufacturers design features for the unit size required, air t
27、hrowrequired for the particular enclosure, and accessibility of the coil forperiodic surface cleaning.The blow-through design has a lower discharge air velocity be-cause the entire coil face area is usually the discharge opening(grilles and diffusers not considered). Throw of 33 ft or less is com-mo
28、n for the average standard air velocity from a blow-through unit.Greater throw, in excess of 100 ft, is normal for draw-through cen-trifugal units. The propeller fan in the high-silhouette draw-throughunit cooler is popular for intermediate ranges of air throw.Fan AssembliesDirect-drive propeller fa
29、ns (motor plus blade) are popularbecause they are simple, economical, and can be installed in multi-ple assemblies in a unit cooler housing. Additionally, they requireless motor power for a given airflow capacity.The centrifugal fan assembly usually includes belts, bearings,sheaves, and coupler driv
30、es, each with inherent maintenance prob-lems. This design is necessary, however, for applications with highair distribution static pressure losses (e.g., enclosures with ductworkruns, tunnel conveyors, and densely stacked products). Centrifugal-fan-equipped units are also used in produce-ripening ro
31、oms, whereFig. 3 Low-Profile CoolerFig. 3 Low-Profile CoolerFig. 4 Liquid Overfeed Type Unit CoolerFig. 4 Liquid Overfeed Unit CoolerForced-Circulation Air Coolers 14.3a large air blast and 0.5 to 0.75 in. discharge air static is needed forproper air circulation around all the product in the enclosu
32、re, toensure uniform batch ripening.CasingCasing materials are selected for compatibility with the enclo-sure environment. Aluminum (coated or uncoated) or steel (galva-nized or suitably coated) are typical casing materials. Stainless steelis also used in food storage or preparation enclosures where
33、 sanita-tion must be maintained. On larger cooler units, internal framing isfabricated of sufficiently substantial material, such as galvanizedsteel, and casings are usually made with similar material. Someplastic casings are used in small unit coolers, whereas some large,ceiling-suspended units may
34、 have all-aluminum construction toreduce weight.Coil ConstructionCoil construction varies from uncoated (all) aluminum tube andfin to hot-dipped galvanized (all) steel tube and fin, depending on thetype of refrigerant used and the environmental exposure of the coil.The most popular unit coolers have
35、 coils with copper tubes and alu-minum fins. Ammonia refrigerant evaporators never use coppertubes because ammonia corrodes copper. Also, sprayed coils are notconstructed with aluminum fins unless they are completely protectedwith a baked-on phenolic dip coating or similar protection appliedafter fa
36、brication. Coils constructed with stainless steel tubes and finsare preferred in corrosive environments, and all-stainless construc-tion, or with aluminum fins, is preferred in environments where highstandards of sanitation are maintained.Fin spacings vary from 6 to 8 fins per inch for coils with su
37、rfacesabove 32F when latent loads are insignificant. Otherwise, 3 to 6fins per inch is the accepted spacing for coil surfaces below 32F,with a spacing of 4 fins per inch when latent loads exceed 15% of thetotal load. One and two fin(s) per inch are used when defrosting isset for once a day, such as
38、in low-temperature supermarket displaycases. Staged fins in a row of coils, such as a 1-2-4 fins per inchspacing combination, greatly reduce fin blockage by frost accumu-lation (Ogawa et al. 1993).Even distribution of the refrigerant flow to each circuit of the coilis vital for maximizing cooler coi
39、l performance. Distributor assem-blies are used for direct-expansion halocarbon refrigerants and occa-sionally for large, medium-temperature ammonia units. Applicationrequires that they be precisely sized. Distributor design and construc-tion material may vary by refrigerant type and application. Ap
40、plica-tion information from the distributor manufacturer should be closelyfollowed, particularly regarding orifice sizing and assembly mount-ing orientation on the coil.For liquid pumped recirculating systems, orifice disks are usu-ally used in lieu of a distributor assembly. These disks are sized a
41、ndinstalled by the coil manufacturer. They fit in the inlet (supply)header, at the connection spuds of each coil circuit. The specifyingengineer may require a down-feed distributor assembly, less any ori-fice, if significant flash gas is anticipated.Headers and their piping connections are part of t
42、he coil assem-bly. Usually, header lengths equal the coil height dimension; there-fore each header is sized to the coil capacity for the application,based on refrigerant flow velocities and not on the temperatureequivalent of the saturated suction temperature drop. Velocities ofapproximately 1500 fp
43、m are used to compute the size of the returngas header and its connection size. In the field, connection size isoften mistaken to be the recommended return line size, but the sizeof lines installed in the field should be based on the suction drop cal-culation method (see Chapters 1 to 4).Frost Contr
44、olCoils must be defrosted when frost accumulates on their sur-faces. The frost (or ice) is usually greatest at the coils air entryside; therefore, the required defrost cycle is determined by the inletsurface condition. In contrast, a reduced secondary-surface-to-primary-surface ratio produces greate
45、r frost accumulations at thecoil outlet face. A long-held theory is that accumulation of rela-tively more frost at the coil entry air surface somewhat improvesthe heat transfer capacity of the coil. However, overall accumu-lated coil frost usually has two negative effects: it (1) impedes heattransfe
46、r because of its insulating effect, and (2) reduces airflow be-cause it restricts the free airflow area within the coil. Both effects,to different degrees, result from combinations of airflow, fin spac-ing, frost density, and ambient air conditions.Depending on the defrost method, as much as 80% of
47、the de-frost head load of the unit could be transferred into the enclosure.This heat load is not normally included as part of the enclosureheat gain calculation. The units refrigeration capacity rating is av-eraged over a 24 h period, by a factor that estimates the typicalhours per day of refrigerat
48、ion running time, including the defrostcycles.As previously mentioned, time between defrost cycles can beincreased by using more coil tube rows and wider fin spacing. Iceaccumulation, which interferes with airflow, should be avoided toreduce both the frequency and duration of the defrost cycles. For
49、example, in low-temperature applications having high latent loads,unit coolers should not be located above freezer entry or exit doors.Operational ControlsIn the simplest form, electromechanical controls cycle the refrig-eration system components to maintain the desired enclosure tem-perature and defrost cycle. Pressure-responsive modulating controlvalves, such as evaporator-pressure regulators and head-pressurecontrols, are also used. A temperature control could be a thermostatmounted in the enclosure, used to cycle the compressor on and off,or a liquid-line solenoid valv
