Document 2J4XvzGMkmv8X46BaRxO1rLzN

974 CHAPTER 44 1954 Guide m = total hours during period May 15 to October 15 that the establishment is open for business. I b = fraction of maximum load from internal heat under average operating con ditions. . 'fruit 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 wetbulb exceeds 65 F, during the period June1 to_ October 1 to the total number . of hours during that sime 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. , It should be pointed out that certain'southhrh citiek 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 sm^ler; in: other words,'the re/rt^erofton load factor 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 (bhpO TJh H season power costs = .--------------,-------- 4 tof where bhpt = brake horsepower per ton (see .Fig. 1) for average load during period. (Due allowance should be made for poorer compressor efficiency at light load.) T = maximum refrigeration design load, tons: ` ' Hc = equivalent full load refrigeration operating time, hours (from Table 5)'., R = power cost, including demand and energy Charges, dollars per kwhr. , y = 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 aTHtC (4) Owning and Operating Costs i'-* *>>.:- 975 where . '- ;* ' - `v.,-.......... B -- cost of water for refrigeration during period, dollars. a = average gallons per (minute) (ton)., 7* = tons of-refrigeration at maximum design, load. Bo = 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 entering, and leaving .temperature8 are, considered constant, .the, average gallons per,minute per ton. obviously, are equal to the design gallons per minute per tom However, when the source is river or lake water, its max imum seasonal temperature )yill generally be reached at the same time that - ;o. .. .. ; > v;',--:- . , - - r - d. riou Fig. 1. Typical Brake Horsepower Requirements for Refrigeration* _ __J_________________ ..wu.v.vuiuuvivuKisiiuic; icvipiwamig UiacomeS OI aDOUt 23 tons capacity in air conditioning applications. Requirements of smaller machines are usually higher, and for larger machines may be lower. Values shown are for liquid refrigerant at condenser temperature (no subcooling). Subcooling of the liquid may decrease these values approximately 0.3 percent to 0.5 percent for each Fahrenheit degree the liquid temperature is lowered. the refrigeration load factor is highest. The average gallons per minute per bm should be calculated from known or estimated water temperatures, be cause they vary through the season. Maximum water main temperatures are given in Chapter 35, but should always be verified locally. In lieu of this tedious work, the average gallons per (minute) (ton) may be taken as 80 percent of design gallons per (minute) (ton) with reasonable accuracy, for the condition of variable temperature of entering water obtained from rivers and lakes. Cooling towers and evaporative condensers virtually eliminate condens]ng water charges since the windage and evaporation losses are seldom over two or three percent of the water circulated. The savings in water consumed often times will not in itself justify the