Document OEZ0MyeJ422ZEvmBemnmKybRX

692 CHAPTER 42 1965 Guide And Data Book' persture difference between superheated'vapor and air the high heat transfer coefficient obtained with the relatively high vapor velocity. In this rone, the refrigerant vapor is cooled to the condensing temperature. For efficient ' heat transfer, counterflow between air and refrigerant is used;' therefore, the desuperheating sone is preferably in the leaving' air stream. '' Condensing takes place in approximately 85 percent of the condenser area, at a substantially constant temperature. The indicated drop in condensing temperature is due to'the frio- tion loss through the cnndonaw coil. The temperature rise of the cooling air, on the basis of customary cfm per ton ratios,'' is roughly 12 F deg. Subcooling in an air-cooled condenser is usually limited because of the wm>d] temperature difference between air and refrigerant in this sone. The heat quantity transferred in the subooohng sone is relatively small (about 5 percent of the total heat transfer). In many designs, subcooling in the con- denseris held to a minimum, and an integral subcooling coil is located is the air stream entering the condenser. TYPES Air-cooled condensers may be classified as, (1) the' chassismounted condenser, which is always a component of a con densing unit, or (2) the remote condenser,-.an integral aa?, sembly remote from the compressor. Both are further sub divided into indoor and outdoor units of either the forced-air or gravity (free-convection) type. Chassis-mounted (Indoor Type) These condensers are usually mounted on a common base with the compressor, motor, and receiver. The condenser fan may-be.mounted on.the shaft of.the compressor .motor..For. small compressors up to \ hp, free-convecrion .condensers which require no fan are sometimes used. Most chassis-mounted condenser coils are. made, of steel tubes and steel fins, with the fins copper-brazed to the tubes.. The use of all-copper coils is now limited to specialiapplica^ taons because of their relatively'high cost-Steel .fins are. usually 0.012 to 0.015 in. thick, spaoed>6:to 8'per inch. Fin spacing closer than 10 per inch is seldom used in-indoor con-, densers since they are often located where the finned surfaces are subjected.to dirt clogging. The entire condenser is painted to provide protection from corrosion, and to improve appear ance. .~ Approximately 100 to 150 sq ft of coil surface, and:500-to 700 cfm of air, per compressor horsepower are.used. In most designs, air.is drawn through,the condenser^il,and,,dis-, charged, over the compressor motor for, additional cooling. The air velocity through the condenser, depends on .the relative positioning of the fan and coil. <r .; In condensing units equipped with belt-driven compressors, the compressor motor drives both the compressor and the' condenser fan. Therefore,' the power available to drive the compressor is 20 to 30 percent less than in water-cooled condensing units of equal horsepower which require no con-' denser fan. In condensing units with hermetic or direct-driven compressors, the condenser fan is powered by a . separate . motor. Condenser coils for indoor air-cooled compressors are usually sized to operate at comparatively high head pressures.. For this reason, rated capacities of factory assembled air cooled condensing units are lower than those of water-cooled condensing units. Chassis-mounted air-cooled condensers are seldom made for indoor condensing' units over 5 hp because their length and height would exceed door opening dimensions. ; The fans in indoor condensing units are usually of the pro peller type and are assembled close to the coil: Due to the high speed, of these1 fans (usually-1500 or 1750. rpm) and their proximity to the coil, their 'relatively high noise level often prevents the use of such air-cooled indoor condensing units in many locations. Chassis-mounted (Outdoor Type) Unlike the indoor condensing unit, the outdoor' chassis mounted condenser'does not occupy valuable building space/ Its dimensions are not limited by floor space or by door open ing sue.'At the same time, toe condenser has the advantage of an abundant supply of clean, cool air. Outdoor compressor units include separate condenser fans 'driven by their own motors, so that all compressor motor power is utilized to pro-, duce refrigeration. Air is drawn through toe condenser coil and, in tome designs, discharged over toe compressor to pro vide supplemental cooling of the compressor motor. The entire assembly is enclosed in.a casing which is removable for serv icing. Built-in, automatic head-pressure control is available* for units installed in cold climates. Remote Air-Cooled Condensers Remote air-cooled condensers are now successfully used for, refrigeration systems of all.types, with capacities from.one-. hlf to 500 tons and larger. Remote air-cooled condensers are generally of the forced-air type (unit condenser). A gravity (free-convection) condenser is offered by one manufacturer. Remote gravity condenser* consist of a single,row condensing coil of large face area, usually mounted on,toe roof at a 7 deg angle with toe horizontal. Since air movement through the cou takes place due to gravity or wind,' the condenser capacity is highly * sensitive to wind velocity (Fig. 15). Selection of these condensers is made on toe basis of minimum wind velocity, or by correlating wind velocity with simultaneous dry-bulb frequencies. These condensers are recommended where: (1) ample roof or floor sur face is available for installation, (2) fan noise must be completely avoided, and (3) power oost is relatively high so that the saving in fan motor current becomes significant. Gravity condensers are usually constructed of copper tubes with aluminum fins spaced 4 per inch. The face area, based on a rated temperature difference of 20 F deg between condensing and air temperature, varies from 17 to 22 sq ft per air condition- ton, with single row coils. -- arced-Air unil condenser* consist of three main components: (1) a finned condensing coil, (2) a direct-driven pr belt-driven fan with its motor, and (3) an aluminum or steel casing. In smaller condensers, toe casing can be formed by extending the frame of the coil. The fan is usually of the free delivery mulUblade propel ler type.' .For. indoor applications where ductwork h required, centrifugal fan* are used. fiq. 14 .... Temperature and Enlhalpy Changes in 'an Air-cooled Condenser ,v Air may either be drawn through or blown through the coil by the bn and motor. In the draw-through unit, the fan and and motor are located on the air discharge side of the coil. Os a blow-through unit, the motor is located in the entering Condensers. 693 `.T.refrigerant flow and the difference in enthalpy of the entering refrigerant vapor and the enthalpy of toe leaving refrigerant > liquid, expressed in Btuh. A unit oondenser may also be rated in terms of Net-Re frigeration Effect (NRE), which is the total heat rejection -less the heat added to the refrigerant in the compressor by the ; heat of compression. The Net Refrigeration Effect is a practical ' expression of the capacity of a refrigeration system of which i.the unitcondenser isa component part. Some manufacturers have incorporated in the design of toe - air-cooled condenser a liquid subcooling coil. The addition 1 of a liquid subcooling coil results in an increase in compressor- .. condenser capacity of; approximately 0.5 percent for each ' degree of suboooling. Subcooling coils are generally sized to air stream. In meet designs, the motor is located inside the ; - 'give from 10 to' 15 degrees of subcooling or 5 to 7$percent casingforprotection from toe weather. 0 ' increase in system capacity. Since subcooling increases the refrigeration effect per pound of refrigerant, a smaller com -Unit condensers are suitable for outdoor and roof installation; ; wfatfe an ample supply of cooling sir is available at a low tem perature.-- - ' pressor may be used for a given refrigeration load and will ; operateat a higherBtu'per KWinput. Subcooling alsopermits Condensing coils for remote unit condensers are installed either ; .'.greater liquid lift in vertical risers .without; flashing of the in a vertical plane for horizontal air flow or in a horizontal plane : = liquid and permits use of smaller pipe sizes. for vertical upward air flow. Certain.designs for vertical upward air flow, orient the cofl face at a 45 deg angle from the vertical, to reduce'the overall bright of the unit condenser. The, slanted j` . The Total proportional Heat Rejection of an air-cooled condenser is to the- difference between entering dry-bulb cofl is also favored by designers of beat pump systems because' it 1 .temperature and saturated condensing temperature. For rWlifj>tAa drainage whm the condenser serves as a heat source ; - instance,'a condenser with a 30 F deg temperature difference rinpng the heating cycle. Coils are constructed of coppertubes I . will have a condensing temperature of 125 F, with an entering and aluminum fbm, although all-copper and all-aluminum are *lan available. Most coils are of the vertical serpentine cofls type '-> dry-bulb temperature of 95 F, or 120 F with an entering dry- with parallel circuits and horizontal inlet and outlet headers. bulb temperature of 90 F. The Gross Heat Rejection at 30 F Tube sizes range from } to l in. diameter, with fin spacing from deg temperature difference is 50 percent greater than the 6 to 13 per in. The condensing coil surface for 20 F deg tempera ture difference between entering air and refrigerant,' is from 170 . to 200 sq ft per ton, while a cofl designed for-30 F deg tempera- same condenser selected for 20 F deg temperature difference. It is accepted practice to publish ratings in terms of Gross ;ture difference requires 100 to' 125 sq ft per ton.* These figures Heat Rejection. The-ratings are based o'h temperature dif vary widely, depending on toe air quantity, air velocity, fin ference between condensing temperature and ' entering' air sptoing,'and the coil heat-transfer characteristics.' The cofl of a unit condenser may have several-separate con densing circuits for use with several compressors. The capacity dry-bulb temperature. Most compressor ratings include Total Heat Rejection'as.well as Net Refrigeration Effect and KW and location of individual circuits is adapted to the combination Input. This eliminates the need for converting Net Re of compressors used. Each circuit has its own islet and outlet connection and uses a separate receiver. Air and power requirements. For most refrigeration applications in the United States, remote unit condensers-are selected for a temperature difference of 20 to 25 F deg between air and con densing refrigerant. For instance, with air at 90 F entering the condenser, the design condensing temperature is 110 to 115 F. 'For economical design, a temperature'difference.of.6 toTOF^deg should be maintained between the condensing refrigerant and the leaving air temperature At 14,400 Btuh per evaporator ton, 70 to frigeration Effect to Gross Heat Rejection by applying fac tors based on suction and condensing temperature. \ A standard method11 of testing-for rating remote air-cooled condensers has been prepared by ASHRAE. Many > manu facturers publish ratings in terms of Net-Refrigeration-Effect at 40 F evaporating-temperature and 90 F air temperature mitering the condenser and -120 F condensing temperature (30 F deg temperature difference), in order to obviate Gross 100 cfm of air per 1000 Btuh of condenser heat.rejection will be required. ,Free delivery propeller, fans require approximately 0.1 motor'horsepower per air-conditioning horsepower. Centrifugal fans require 0.15 to 0.25 hp per air-conditioning horsepower, 'dependingon delivery resistance. - For air conditioning applications, economic and dimensional considerations have lea to use of remote condensers which operate at 30 F deg and higher difference between air and condensing temperature Such condensers deliver 50 to 75 cfm of air per 1000 Btuh beat rejection and require from 0.1 to 0.2 hp per ton. The elevated condensing pressures produced by such condensers at occasional high ambient temperatures-'can- be tolerated by most present-day compressors at toe expense of greater power consumption at a relatively small capacity loss. Thus, while initial cost saving* are made possible by reduced size condensers, operating cost* may be greater, depending on the geographic location,-due to the greater power demand of the compressor, particularly if frequent operation at elevated bead pressures`is required.** Heat Rejection from the selection procedure. No attempt is made to correct such ratings for open versus hermetic com pressors. Condenser capacities published in such terms as bone rating can be converted to other than baste rating conditions by using toe factors in Table 3. These factors can be applied with sufficient precision for Refrigerants 12, 22, 500, and 717, when used with belt-driven or direct-driven compressors, for roost applications. Condenser selections for hermetic compres sors are best made on the basis of gross heat rejection ratings, if published by the compressor manufacturer, and the gross heat rejection effect of the air-cooled condenser. In the absence of published compressor heat rejection ratings for hermetic compressors, an approximate condenser selection can be made by usiog basic ratings and the factors in Table 3. This is because the additional heat of the motor APPLICATION AND RATING OF REMOTE UNIT CONDENSERS in a hermetic compressor is dissipated in the desuperheating zone (Fig. 14), where high temperature differentials are available. The result is only a small possible error in toe ulti It is current practice to rate remote unit condensers in terms of Gross Heat Rejection (GHR), which is toe total heat removed in desuperheating, condensing, and subcooling of the refrigerant! Hus value is toe. product of the weight rate of mate selection. .. More precise-selection of unit-condensers is possible by cross-plotting Gross Heat Ejection of the .oondenser against Gross Heat Rejection of. the compresor as'shownIn Fig.J 16.