Document ZnMNjyVJ5DwNj6pdkgo7EDJyV

928 CHAPTER 43 1952 Guide i ' -large air conditioning installations,, using high quality equipment, some approximate costs per ton are given in Table 4. SERVICE COSTS Service costs include the costs for power, water, steam, coal, oil, etc., consumed to operate the system. ' From the selected equipment and type'Of installation, it is possible to segregate the relatively-constant power loads and the total brake horse power. Annual power cost can then be figured from the following formula: .. 1 x 0.746(bhp)//K 1 annual'power cost = --:------------ -- (1) where bhp = brake horsepower. H -- annual operating hours. R = power rate, dollars per/kwhr. ri = motpr efficiency (decimal). In using Equation 1 it must be pointed out that the electric rate, R must reflect the proper combination of energy and demand rates. These vary widely between the utility companies, and sometimes the rate struc ture is such that it is largely the demand charge which determines the proper value of R to use in Equation 1. Operating Refrigerating Equipment In an air conditioning system the refrigerating equipment is usually the largest power consuming item to be considered. Also, the prediction of operating cost is more difficult because the power required for summer cooling is affected by many factors of a variable nature. Table 5 gives the equivalent full load operating hours of refrigerating equipment used for summer cooling for the period of May 15th to Octo ber 15th. This table was calculated from the following equation: //. = m(b + cf) . i - (2) where . ;- j. H. = equivalent full load operating, hours of refrigeration equipment used for summer cooling during period May 15 to October 15. m = total hours during period May: 15 to October 15 that the establishment' is i. open for business. b = fraction of maximum load from internal heat under average operating con ditions. . c-= fraction:of.maximum load which is due to external sources at maximum : design conditions. : / = ratio of the number of hours for a particular city, when the outside wet-bulb i I : exceeds 65 F, during the period June 1 to October ! to the total, number of hours during that same period. Total hours are assumed, as 8 hr per day . period for barber shops, department stores, funeral parlors, offices, short hour restaurants, and specialty shops, and 12 hr per day .period for drug stores, long hour restaurants, and theaters. . Owning,and Operating Costs 929 It should be pointed out that certain southern cities.may have seasons longer than the 5-month period indicated in Table 5. If it is desired to consider a longer season of operation, the ratio of full load operating hours to hours open for business is smaller; in other words, the refrigeration loadfactor is lower. This is true because the extra increment of days added will be a" relatively light load, since the table already includes the more severe part of the season. The season electrical power cost for refrigerating equipment is then given by the following equation: 0,746 (bhp,) TH. R season power costs = V (3) where brake horsepower per ton (see Fig. 1) for average load during period. (Due allowance should be made for poorer compressor efficiency at light load.) tons of refrigeration at maximum design load, equivalent full load refrigeration operating hours (Table 5). power cost, dollars per kwhr, including demand and energy charges, motor efficiency at. average load (decimal).' In considering refrigeration power consumption, it should be noted that the use of weather records for a specific-year may lead to large inaccuracies in estimating operating costs, since there may be wide variations from year to year, and therefore, average yearly weather records should, be used rather than those for any individual year. If the refrigeration compressor is steam turbine driven, the same general method can be followed, taking into account average water rate per brake horsepower-hour and the cost of steam. Condenser Water Condenser water cost estimates can also be based on equivalent full load operating hours of the refrigeration equipment. The varying temperature of the water at its source, as well as the temperature of the discarded water, must, however, be taken into account. In general, when water is pur chased, control is provided to hold the leaving water temperature (or condensing temperature) constant; and in such case the entering water temperature becomes the major variable, and the gallons per minute per ton can readily be calculated for any water temperature rise. The following equation for cost of condenser water is useful: B = 0.060 aTH.C 1 .(4) where B = cost of water for refrigeration during period, dollars, o = average gallons per minute (ton). T = tons of refrigeration at maximum design load. H. = equivalent full load refrigeration operating hours (Table 5). C = water cost, dollars per 1000 gal. The average gallons per minute per ton must take into account the variable water temperature. When well water is used as a source, and