Document 2aogj6g1vKpK7JKjeOwLXN2b

638 CHAPTER 36 1946 Guide Ratings ., In order to rate and test equipment of this kind which normally operates below the frost line, a proposed code. Standard Methods of Rating and Testing Forced-Circulation Air' Coolers for Commercial arid Industrial Refrigeration 3, has been issued. This proposed standard covers only the modifications of the Standard Method of Rating and Testing Air Con ditioning Equipmentl, as it is related to the different applications of unit air coolers. In this standard, the gross cooling effects are taken since the motor power input equivalent is to be computed as part of the load. From this code Table 2 is abstracted to show standard .rating con ditions for forced-circulation air coolers. Other modifications take into' . Table 2. Standard Rating Conditions for Air Coolers . Group No. i ii hi IV V Entering Dry-Bulb Temperature. . F Deg 45 35 35 0 0 Entering Relative Humidity, Per Cent 85 85 85 85 85 Evaporating Temperature. F Deg 30 25 15 -10 -20 consideration the effect of frost formation on the coils and the change in length of test runs required to meet such conditions. Defrosting Unit air coolers are often required to operate in rooms where air and refrigerant temperatures are below the freezing point. This results in the freezing of the condensation on the coils and this accumulation of frost. builds up to such an extent that there is a loss of capacity. This deposit is removed from the coils by the process known as defrosting which may be accomplished by several methods: 1. Where the room temperature is above the freezing point, the flow of refrigeration to the coils is halted and the fan continued in operation until the coils are defrosted. 2. The hot gas defrosting method is accomplished by a valving arrangement whereby hot compressed gases from the compressor are pumped directly into the evaporator. This operation is continued until defrosting is complete, when the system is returned to normal operation. __ 3. Where brine is used as a refrigerant, hot brine may be circulated .through the coils. Where the frosting is particularly heavy it is sometimes more advisable to apply the source of heat externally. This principle of defrosting is accomplished by: 4. Defrosting by warm air is accomplished by dampering arrangements which permit . the cooler fan to draw warm air from a source outside of the refrigerated space', pass it over the coils and discharge it outside of the refrigerated space. 5. Constant wetting of the coils with a brine or eutectic solution prevents the for mation of frost. This is accomplished on a vertical unit air cooler as shown.in Fig. 10. - 6. Electric heating elements, placed in such a manner that the fan forces.the heated . air over the coils. < - 7. Water sprays, placed so that the coil is thoroughly wetted during the defrosting, process, are a simple means of defrosting. City water at ordinary pressures is used. The fan is-not running during this operation. ................... ' Those systems using method 5 have the problem; of removing con- i Unit Air Conditioners, Unit Air Coolers, Attic Fans 639 densation from the eutectic solution. One -way is to waste the entire charge when dilution has rendered it ineffective. . Another method employs a device which boils off the water and returns the eutectic solution to the system for further use. ECONOMICS In the planning and designing of a unit system or in comparing units with central plant application, a systematic approach to the problem should be made from the economic viewpoint. First Costs. The question of first cost is but one factor in the economic approach of a skillful designer or an intelligent buyer. 1. Equipment. The use of a large percentage of factory fabricated equipment to maintain installation costs at a minimum, as represented by self-contained units, should be contrasted with the use of systems with a large percentage of installation labor and material, such as a central station system. The remote unit offers a compromise between these two.. . . . . 2. Installation costs. The influence of existing codes and ordinances, and installation conditions, involving new construction or treating an existing structure deserve careful consideration. Methods which are suitable for new construction often may not be * applied on buildings erected in the past. Sprinkler system rearrangements, fire doors and dampers, restrictions on multiple direct expansion units, rewiring or increasing service, cutting or reinforcing of ceilings and roofs are some items which should receive attention in this respect. ;; Operation and Maintenance. Tenants or occupants usually operate the self-contained equipment, which also lends itself readily to contract service for maintenance. Central plants require operating engineers in ' attendance who also frequently service the equipment. The remote units require a combination of these two methods of operation with maintenance by building personnel. Water and power costs per season can be tabulated and compared for the various systeriis and an estimate of the costs of replacement material such as filters, belts, oil, refrigerant and wearing parts should be included on ari annual basis. Such questions as obsolescence, depreciation arid return on investment are subjects for special study arid investigation. Due consideratiori should be given to the value of resale of equipriierit arid its portability in the event'of removal to new locations. ; ATTIC FANS ' Attic fans are used during the warm months of the year to draw large volumes of outside air through a house and offer a means of using the comparative coolness of outside evening and night air to lower the inside temperature: Because the low static pressures involved, are usually less than ]/% in. of water, disc or propeller fans are generally used instead of the blower types. The fans should have quiet operating characteristics, and they should be capable of giving,about 20 to 30 air changes per hour in northern areas. In the South the usual specification requires one air change per ' minute which provides appreciable air movement in addition to lowering the inside air temperatures 6. Types Open attic fans are units in which the fan is installed in a gable or .'dormer of the attic and one or more grilles are provided in the floor of the /