Document 06r0w88RKbdNzVq8xJqNQbaNR

238 CHAPTER 16 1960 Guide Table 1 .... Standard Rating Basis for Self-Contained Air-Conditioning Units Type* of Uaft* ttecn CatnQ Condition Description Vahje All All a Barometric pressure 29.92 In. Hg Water-cooled, air-cooled, and evaporatively cooled condens- b [Jnit ambient and air entering room--air inlet (1) Dry-bulb (2) Wet-bulb 80 F 67 F Cooling Water-cooled condensers c Ventilation air d Water temperature entering unit See Note 75 F e Water temperature leaving unit 95 F Air-cooled and evaporatively cooled condens ers f Air entering outdoor air inlet (1) Dry-bulb (2) Wet-bulb 95 F 75 F Beating vided with heating feme-. tion g Unit ambient and to tal air entering unit 70 F h Beating medium, pressure or temper ature (1) Dry saturated 16.7 psia steam (2) Water in 180 F (3) Water out 160 F . Unit ambient 70 F Humidi vided with hu j fying midifying func tion Total air entering unit (X) Dry-bulb (2) Wet-bulb 70 F 53 F Air cir All k Filters culation New and clean NoU: ikall be beaed os both end win'Aihtod room air Marin* at 80 P dry-bath end 87 P wet-bulb temperature. (The NeUaa given materia) not pertinent to conditioners are selected for the usual range of external re sistances nwimniily used with the specific size of conditioner. The larger conditioners are capable of serving larger areas and therefore require fan and motor capacity for the greater external duct resistances that may be imposed on such condi tioners. When locating unitary equipment, floor and beam loadings should be carefully checked. Suspended horizontal units can add SO 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 sdow load, excess beam deflection may occur and cause cracking of plaster or concrete fireproofing. A small fire, normally of little consequence, may cause a rupture of a heavily loaded structure and permit the equipment to drop with extensive property damage. Self-contained units should be carefully installed rincft 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 base ment is an inexpensive 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. Twisting buildings seldom are wired ade quately for the electrical load imposed by the starting of an air-conditioning compressor on any branch 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 sepa rate electrical circuit of adequate capacity from the main panel is more than justified; it is a necessary expense in the majority of installations. . A water supply of adequate capacity and pressure is nec essary 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 psi pressure, in cluding 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 pressures on the city water system in the event of condenser failure. A check valve should also be in stalled in the water supply as a further precaution against contamination. When installing anall remote or self-contained units with outdoor 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 dis tribution. It can also cause annoying whistling at door open ings 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 in stalled in the outdoor air intakes and that they have ade quate locking devices. One further consideration when installing self-contained units in conditioned areas is to ensure that space is adequate for any maintenance or repairs to be required in future years. Sound Isolation Both suspended and vertical floor-mounted units can trans mit vibration through the supports. Wherever such trans mission of sound might be objectionable, the supports should be isolated through rubber-in-shear or other sound deadeners. (For design of suitable sound deadeners see section Con trolling Vibration from Machine Mountings in Chapter 25.) UNIT AIR COOLERS Unit air coolers are intended principally for product cool ing, but are often used for cooling spaces to low temperatures. They differ from normal air-conditioning units only in fea tures required to produce lower temperatures. In using such units, dry-bulb temperatures should be considered first, after which room moisture conditions and air distribution should Unitary Air-Conditioning Equipment 239 be considered, depending on the product being cooled or stored. Products to be cooled or stored may be divided into four general classifications based on their sensitiveness to moisture loss." Selection of the proper temperature differential be tween storage-room air temperature and evaporating re frigerant temperature will produce adequate moisture con trol for all practical purposes. The differentials required for gravity circulation and for forced air circulation are differ ent, the latter requiring closer differentials. The four classes and differentials which have been successfully used are: Class 1. Products requiring high room-moisture content, which are highly susceptible to moisture loss and are being stored for extended periods, for example, eggs. Gravity Circulation Forced Air Circulation 18 F or less. 6 to 9 F. Class t. Products similar to Clam 1 but which are being stored only for a limited time as in retail stores where the turn-over is usually 3 days or less, for example, cut meats. Gravity Circulation Forced Air Circulation 18 to 22 F. 9 to 12 F. Class S. Products requiring moderate room moisture content and which are moderately susceptible to moisture loss, for example, carcass meats and some fruits. Gravity Circulation Forced Air Circulation 21 to 27 F. 12 to 20 F. Class 4 Products from which there is a very low moisture loss or none at all and where room moisture content is not important, for example: canned goods, furs, woolens, bottled goods, dried fruits, foods packaged in good'moisture vapor barrier packages. Gravity Circulation Forced Air Circulation 25 F and up. 20 F and up. When using Class 4 application, with finned evaporatora and automatic defrosting at each cycle of the condensing unit, it is important to avoid using too wide differentials with very low refrigerant temperatures. In many applications, prime surfaced evaporators prove unsuitable for tbie type of defrost ing and will require some positive method of defrosting. Unit coolers can perform satisfactorily in installations re quiring accurate control of relative humidity, air motion, and dry-bulb temperature, and thereby prevent excessive weight loss, mold and slime growth, and moisture absorption by hygroscopic materials such as dried fruits. Unit coolers, especially in the smaller sizes, are very simi lar in appearance to unit heaters. Copper, aluminum, or steel prime or finned-surface tubes are used. Propeller or centrifu gal fans either blow or draw room air over the tubes. The fan and coil are generally enclosed in a taring provided with a drip pan. The motor horsepower requirements are a func tion of resistance due to coil construction and arrangement, and air volume required. Fin spacing is based principally on operating temperatures. For operation below 35 F, fin spac ing is'preferably not more than four to the inch. Above 35 F it may run as high as 8 fins per in., depending on room tem perature and refrigerant temperature used. Both direct ex pansion refrigerants and brine are used successfully as cooling mediums. Unit coolers may be arranged for either free or duct de livery. Face velocities vary, depending principally upon the intended application of the unit. In the larger sizes particu larly, speed adjustment of the fan is generally provided to permit variation of the air delivery. While unit coolers are usually installed in the storage space, remote installation com bined with appropriate ductwork may be required by space or other considerations. Units are available for floor, wall, or ceiling mounting, thus providing an upward, downward, or horizontal discharge. Power, refrigerant, and drip pan con nections are required, plus additional connections for de frosting, if necessary. For storage temperatures below 35 F, some positive means of defrosting is mandatory. At 35 F or higher storage tem peratures, cycling of the condensing unit with low-pressure control at proper settings will provide automatic defrosting at each cycle. At above 35 F design refrigerant temperatures, frost and ice formation will not normally occur. Various methods of defrosting are used. In one method, hot gas is supplied to the interior of the tubes uniformly through out the coil. The entire refrigerant circuit is thus contacted to obtain complete defrosting of all frosted surfaces. Electric defrosting generally involves the incorporation of heating ele ments within tiie construction of the coil, or the use of strip heaters in a dampered closed-air circuit. Warm water may be sprayed over the coil surface for defrosting. With storage temperatures above 32 F, defrosting may be accomplished by shutting down the refrigeration system and circulating the room air over the coil. In every case, defrosting requires a cessation of refrigeration an the unit being defrosted. Where continuous operation of the system is desired., & brine spray over the coil may be used unless it might damage the product in storage, or cause too much corrosion of room fittings, ducts, and equipment. Ratings Since various means of expressing unit cooler capacity are utilized in the industry, different manufacturers suggest dif ferent methods of selection. The engineer should be aware of the conditions and factors which affect rating, selection, and performance of a cooler. These items are discussed in follow ing paragraphs. The refrigerating capacity of the unit may be either gross or net, the latter being less than the gross by an amount equal to the heat equivalent of the input to the unit cooler motor. In either case, the capacity should be given for a particular air volume. Air throw data are also valuable. Dry or flooded rating conditions should be stated, as well as temperature level. The temperature level determines whether the coil surface is wetted or frosted, and it will also establish the refrigerant-side pressure drop for any given load applied to a specific unit coder. The refrigerant-side pressure drop increases as the evaporating temperature de creases, and thus temperature level exercises a significant ef fect on the average coil-surface temperature and the conse quent condensing-unit selection. Coil capacity rating is usually expressed as total heat ab sorbed with no distinction between sensible and latent heat. The rating expresses the capacity in terms of Btu per (hour) (Fahrenheit degree temperature differential between the re- S frigerant and the air). The term basic rating may be used. This is the Btu per hour absorbed with one Fahrenheit de gree differential between room air and evaporator refrigerant temperature. When the total load has been obtained from the load calculations, a temperature differential between the air and the refrigerant is selected. It is based on product classi fication previously discussed. The extent of dehuxnidific&tion will be a function of this temperature differential. Moisture conditions in the storage space are dependent upon the cor rect selection of this temperature differential. It offers a quick and sufficiently accurate practical baas for coil selection. Where close control of relative humidity is desired, heating coils or electric heaters for reheat may have to be added. The