Document GKOek2dwB25K8X6mNe53NvZan

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 Table 2. Standard Rating Conditions for Air Coolers Group No. i ii iii IV V Entering Dry-Bulb Temperature, Deg F 45 35 35 0 0 Entering Relative Humidity, Per Cent . 85 85 85 85 85 Evaporating Temperature, Deg F 30 25 15 -10 -20 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 and Industrial Refrigeration8, has been issued. This proposed standard covers only the modifications of the Standard Method of Rating and Testing Air Con ditioning Equipment9, as it is related 1;o the different application 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 consideration the effect of frost formation on the coils and the time of the test runs are adjusted to meet these 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 : i 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. Loc. Cit. Note 3. Loc. Cit. Note 1. 454 CHAPTER 23. UNIT AIR CONDITIONERS, UNIT AIR COOLERS, ATTIC FANS 6. Electric heating elements, placed in such a manner that the fan forces the heated air over the coils. 7. A simple means of defrosting is by means of water sprays placed so the coil is, thoroughly wetted during the defrosting process. 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 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 systems and an estimate of the costs of replacement material such as filters, belts, oil, refrigerant and wearing parts should be included on an annual basis. Such questions as obsolescence, depreciation and return on investment are subjects for special study and investigation. Due consideration should be given to the value of. resale of equipment and 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 ]/g in. 455