Document 66mj96axepBDyYK43JOVnj0m

568 CHAPTER 25 1951 Guide essential. Wherever there is danger of freezing of coils or clogging with dirt, sufficient clearance should be available for replacing them without removing parts of the building. The outstanding source of difficulties in unitary systems is usually dirty filters. The characteristics of the light weight fans used are such that air volume drops off rapidly with increase in static resistance. Since changing of filters is an unpleasant duty likely to be neglected unless it can be done easily, the operator should be at the same level as the filters, rather than under them, when they are being removed. Fan speeds should be selected accurately for the system resistance. Variable pitch motor pulleys are often provided in a unit for minor field adjustment of air volume. Any such field adjustment should be made when filters are dirty, to simulate average operating conditions. Because varying sizes of coils are used within the same casing, it is very important that coil safing be carefully installed to prevent by-passing of unconditioned air. If coils are not equipped with individual casings, additional safing may be required on top to prevent short circuiting air down through the upper edges of the fins. In this same category is the need for careful installation of the various sections of sectional)zed units, using a sealing compound if necessary to prevent air leakage into the fan section of the unit. - Since the drain connection is usually made on the exit side of the coil, it is important that the drain line be properly sealed. This seal should be at least twice as deep as the suction' on the fan in inches of water, to prevent gurgling sounds, and to insure a positive seal against infiltration of odors and moisture laden air. Drain pans should not be used to sup port the coils, unless they are designed to hold this weight without sagging. As the movement of air draws the condensate or excess humidification water toward the fan, drain connections are usually placed on the exit side of the coil. The advantages of quick drainage are lost if improperly supported coils distort drain pans and cause water to accumulate in the center or back of the pan. When the fans of vertical units are stopped condensate, that has been held up in the coils by fan suction, drops into the drain pan and splashes against the casing. If water damage is to be avoided, flashings should be provided to prevent this water from running out of the unit at the seams. When locating unitary equipment, floor and beam loadings should be carefully checked. Suspended horizontal units can add 50 to 100 lb per square foot to the loading on the floor above. ' Should this floor be already heavily loaded, or be a roof structure designed for a 40 lb per square foot snow load, excess beam deflection may occur and cause cracking of plaster or concrete fire-proofing. A small fire, normally of little consequence, may cause a rupture of a heavily loaded structure and permit the equip ment to drop with extensive property damage. Self-contained units should be carefully installed since their weights, run as high as 200 lb per square foot. When they are installed in street floor.shops, the extra precaution of placing a column beneath them in the basement is an inex pensive method of reducing vibration, as well as providing insurance against overloaded floor beams. - The services required for operation of unitary equipment should conform to the many restrictive, but necessary, local municipal codes. Existing buildings seldom are wired adequately for the electrical load imposed by the starting of an air conditioning compressor on any branch Unit Air Conditioners and Unit Air Coolers 569 circuit. Even the smallest room cooler can draw enough current to reduce the voltage of a lighting circuit to the point where it is visibly apparent. This voltage drop may even affect the life of the unit due to the relatively slow starting. The cost of a separate electrical- circuit of adequate capacity from the main panel is more than justified; it is a neces sary expense in the majority of installations. A water supply of adequate capacity and pressure is necessary to prevent overloading of electrical equipment by high head pressures. The average city water supply pressure is adequate for installations up to the third floor. Since most water cooled units require about 20 lb pressure,including control valve losses, it is important that any units served by gravity from roof tanks be checked carefully if located less than 40 ft below the tank. Drain connections from condensers should flow to an open and properly, trapped sink as required by most city codes. This prevents back pres sures on the city water system in the event of condenser failure. A check valve should also be installed in the water supply as a further precaution against contamination. When installing small remote or self-contained units with outside air connections in buildings more than 6 stories high, the effect of wintertime stack action in elevator and stairwells requires special attention. This stack action is the cause of negative pressures on the lower floors, tending to draw cold air through the units, and positive pressures on the upper floors preventing adequate ventilation and disrupting air distribution. It can also cause annoying whistling at door openings and lead to serious complaints in hotels and offices. Wherever the removal of such units is impracticable, it is important that carefully fitted, felt-edged dampers be installed in the outside air intakes with adequate locking devices. One further consideration when installing self-contained units in con ditioned areas is that any maintenance or repairs to be required in future years must be carried on in occupied space. If this is kept in mind in .locating units, much inconvenience can be avoided. UNIT AIR COOLERS This type of unit is primarily intended to perform the main function of cooling air, with humidity control a secondary function within the limita tions of the design. The main application of this equipment is in process' and product refrigeration, such as cold storage warehousing, fruit and vegetable packing, in breweries, and in wholesale and retail food markets. Application of the unit method of air cooling with mechanical circu lation is comparatively recent, being an improvement over the pipe or finned coil, , which depended on gravity for circulation. Bunkers were sometimes constructed around the coils to direct the air flow, and some times fains were used for forcing air over the coils. The unit air cooler is usually placed within the refrigerated area, but the. larger, blower type models may be remotely located. Design and Performance. Greater application and use of commercial refrigeration have resulted from the development of the unit air cooler. Flexibility of design has permitted almost any condition to be met. Fin ned type coils are usually employed, with continuous fan operation. By' varying such physical features as tube size, fin spacing, refrigerant circuit ing, the depth of coil rows, and air volume over the coils, the designer is able to produce a wide range of performances, and to offer many desirable