Document zoweam37y9zVNnjoE75VgwBp3

070 CHAPTER 25 1958 Guide / thereby prevent excessive weight loss, mold and slime growth, and moisture absorption by hygroscopic materials such as dried fruits. Unit coolers, especially in the smaller sizes, are very similar in appear ance to unit heaters. Copper, aluminum, or steel prime or finned-surface tubes are used. Propeller or centrifugal fans either blow or draw room air over the tubes. The fan and coil are generally enclosed in a casing pro vided with a drip pan. The motor horsepower requirements are a function of resistance due to coil construction and arrangement, and air volume re quired. Fin spacing is based principally on operating temperatures. For operation below 35 F, fin spacing is preferably not more than four to the inch. Above 35 F it may run as high as 8 fins per in., depending on room temperature and refrigerant temperature used. Both direct expansion refrigerants and brine are used successfully as cooling mediums. Unit coolers may be arranged for either free or duct delivery. Face velocities vary, depending principally upon the intended application of the unit. In the larger sizes particularly, speed adjustment of the fan is generally provided to permit variation of the air delivery. While unit coolers are usually installed in the storage space, remote installation com bined with appropriate duct work may be required by space or other con siderations. Units are available for floor, wall, or ceiling mounting, thus providing an upward, downward, or horizontal discharge. Power, refriger ant, and drip pan connections are required, plus additional connections for defrosting, if necessary. For storage temperatures below 35 F, some positive means of defrosting is mandatory. At 35 F or higher storage temperatures, cycling of the con densing unit with low pressure control with proper settings will provide automatic defrosting each cycle. At above 35 F design refrigerant tem peratures, frost and ice formation will not normally occur. Various methods of defrosting are . used. In one method, hot gas is supplied to the interior of the tubes uniformly throughout the coil. The entire refrigerant circuit is thus contacted to obtain complete defrosting of all frosted surfaces. Electric defrosting generally involves the incor poration of heating elements within the construction of the coil, or the use of strip heaters in a dampered closed-air circuit. Warm water may be sprayed over the coil surface for defrosting. With storage temperatures above 32 F, defrosting may be accomplished by shutting down the refrigera tion system and circulating the room air over the coil. In every case, de frosting requires a cessation of refrigeration on the unit being defrosted. Where continuous operation of the system is desired, a brine spray over the coil may be used unless it might damage the product in storage, or cause too much corrosion of room fittings, ducts, etc. Ratings As various means of expressing unit cooler capacity are utilized in the industry, different manufacturers suggest different methods of selection. The engineer should be aware of the conditions and factors which affect rating, selection, and performance of a cooler. These items are discussed in following paragraphs. The refrigerating capacity of the unit may be either gross or net, the latter being less than the gross by an amount equal to the heat equivalent of the input to the unit cooler motor. In either case, the capacity should be given for a particular air volume. Air throw data are also valuable. Dry or flooded rating conditions should be stated, as well as temperature level. The temperature level determines whether the coil surface is wetted Unit Air Conditioners and Unit Air Coolers 671 or frosted, and it will also establish the refrigerant side pressure drop for any given load applied to a specific unit cooler. The refrigerant-side pres sure drop increases as the evaporating temperature decreases, and thus temperature level exercises a significant effect on the average coil-surface temperature and the consequent condensing unit selection. Coil capacity rating is usually expressed as total heat absorbed with no distinction between sensible and latent heat. The rating expresses the capacity in terms of Btu per (hour) (Fahrenheit degree temperature dif ferential between the refrigerant and the air). The term basic rating may be used. This is the Btu per hour absorbed with one Fahrenheit degree differential between room air and evaporator refrigerant temperature. When the total load has been obtained from the load calculations, a tem perature differential between the air and the refrigerant is selected. It is based on product classification previously discussed. The extent of de humidification will be a function of this temperature differential. Moisture conditions in the storage space are dependent upon the correct selection of this temperature differential. It offers a quick and sufficiently accurate practical basis for coil selection. Where close control of relative humidity is desired, heating coils or electric heaters for re-heat may have to be added. The use of air conditioning psychrometric techniques is only required for accurate humidity control. Cooling equipment is usually rated on the basis of the overall room-torefrigerant temperature differential instead of using the intermediate aver age coil-surface temperature. If the tentatively selected unit cooler does not possess the proper capacity, it may be possible to adjust the fan speed to a new air volume. At the new air volume and the new overall dif ferential, the cooler may be able to deliver the necessary cooling capacity. If not, the procedure must be repeated with another size of unit cooler, the final selection being based on the proper balance between unit cooler and the condensing unit to maintain the Btu per hour heat removal and tem perature difference at design room temperature desired. Procedures for rating and testing room coolers are given in an ASRE Standard5 which establishes four groups of conditions (numbered I to IV) under which units may be rated. Many manufacturers establish and publish their ratings in accordance with this standard in which forced circulation air coolers are classified according to air side surface conditions as (1) dry coil, (2) sprayed coil, and (3) spray-no coil; or according to type of air delivery to room as (1) free delivery fan, or (2) pressure fan. Natural convection air coolers are classified according to type as (1) external baffle, (2) built-in-baffle, and (3) without baffle. Arrangement and Operation The refrigerant is usually supplied to the evaporator through a themr static expansion valve, thus obtaining dry expansion in the evaporate In other designs, float-valve feeding is used as a means of metering the r ingerant into the evaporator. The refrigerant circuit in the coil may 1 of the continuous-flow type from inlet to outlet, or it may have means fi internal recirculation of the refrigerant. The coils having the dry-expansit th coV'^nuous fl0TM from inlet-to-outiet circuits may be of the upward feed ( ^downward feed arrangements. Upward flow is generally regarded i producing more refrigerant holding capacity and more effective use of tl urtace. However, with some refrigerants and in some systems, tl ownward-feed system has been found to produce better oil-return chara ristics. Many direct-expansion installations use liquid-vapor heat e