Document OEVoq0EBOMoevXR8YmnKdQGZM

418 Chapter 22 , __________. 1945 Guide 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, Dbg F . 30 25 15 -10 . -20 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. 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 f .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 Unit Air Conditioners, Unit Air Coolers, Attic Fans 419 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 retum>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 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 pet 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 temperatures10. 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 attic, permitting air -to flow from the hall below. Outdoor air, which enters the nouse through open windows, is drawn into the attic through the grilles, and is discharged outside by the fan. An attic stairway may be used in place of the grilles. It is essential that the roof and the attic walls be free from air leaks. Boxed-in fans are units in which the fan is installed within the attic in a box or housing directly over a central ceiling grille, or in a bulkhead enclosing an attic stair. The fan may be connected by a duct system to the grilles in individual rooms. Outdoor air entering through the windows of. the rooms below is discharged into the attic space and escapes to the outside through louvers, dormer windows, or screened openings under the eaves. Another version of the attic fan is the window fan for use when attic application is not feasible or no attic is available! Supplied with a perforated or expanded metal enclosure and mounted in either the upper or lower window section, this fan is easy to install or move to another location. The locations of the fan, the outlet openings, qnd grilles should be selected after consideration of the room and attic arrangements in order to give uniform air distribution in the individual rooms served. If the outlet for the air is not bn the side away from the direction of the pre vailing wind as in the case of the boxed-in fan, openings should be provided on all sides. Kitchens should be separately ventilated because of the fire hazard, and to prevent the spread of cooking odors. 1#Comfort Cooling with Attic Ventilating Fans, by G. B. Helmrich and G. H. Tuttle (A.S.H.V.E. Trans actions. Vol. 40. 1934, p. 155). A.S.H.V.E. Research Report No. 979--Study of Summer Cooling in the Residence for the Summer of 1933, by A. P. Kratz and S. Konzo (A.S.H.V.E. Transactions, vl 40,1934, p. 167). A.S.H.V.E. Research Report No. 1198--The Effect of Attic Fan Operation on the Jj^olmg of a Structure, by W. A. Hinton and A. F. Poor (A.S.H.V.E. Transactions, VoJ-48, 1942. p. 145). r ne Installation and Use of Attic Fans, by W. H. Badgett (Agricultural and Mechanical' College of Texas, 5tfrfm,No. 52, 1940). .