Document 5LOpyM62N5VraqYEjEeJe2p6e

862 CHAPTER 79 1962 Guide And Data Table A .... Approximate Combined Operating and Maintenance Cost for Large Air-Conditioning installations. Using High-Quality Equipment* DaOan per(Ton) (VooH Repairs for refrigeration machinery.......... Painting (Water boxes and dehumidifiera). Filters, dean and re-oil 4 times per year.. Controls, outside service.............................. Total............................................................... L07 0.40 0.11 0.45 1.50 0.28 1.38 5.17 operations is well justified in medium and larger plants by economies achieved in maintenance and energy costs. ENERGY COSTS Energy costs include the costs for power, steam, gas, coal, oil, etc., consumed to operate the system. Methods of calculating the energy costs for heating are discussed in Chapter 12. Constant Loads In any air-conditioning system, a large part of the auxiliary equipment will be in constant operation regardless of frac tional loading of the refrigerating plant, and these auxiliaries will usually be driven by electric motors. Their annual power costs can.be radwtlftfAd from the following equation: where annual power cost = -746(bhp)ffg 9. n) bhp " brake horsepower. B " annual operating hours. B power rate, dollars per kwhr. 9 " motor 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 structure is such that it is largely the demand charge which determines the proper value of A to use in Equation 1. Electrically-Operated Refrigeration In an air-conditioning system the refrigerating equipts* * is usually the largest power consuming item to be considered ' Also, tiie prediction of operating cost is more difficult be- cause the power required for summer cooling is affected bv many factors of a variable nature. ^' The major difficulty is that the refrigeration equipn^ often operates at its full capacity during no more ^ 20 percent of its running time. During the balance of tfe running time, it will be operating at varying fractions of ha full-load capacity with an energy consumption different for each point of fractional loading. A precise method for determining annual energy cost in volves examination of weather records for a period of yean to determine the houriy occurrence of outdoor temperature during the cooling season.-A procedure of this type is recom mended for heat pump systems and is described in Chapter 4. A similar procedure could be used to calculate bp^tuI energy consumption for other electrically-operated air-con ditioning systems. It should be noted that the use of weather records for a specific year may lead to large inaccuracies, since there may be wide variations from year to year. For air-conditioning systems other than heat pumps, an approximation of annual energy costs, much more easily arrived at and sufficiently precise for most purposes, will re sult from the equivalent full-load hours concept. Table 5 gives the equivalent full-load operating hours of refrigerating equipment used for summer cooling for the period of May 15th to October 15th. This table was cal culated from the following equation: B. - m(5 + cf) (2) B, -- 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 open for business. b *= fraction of maximum load from internal heat under average operating conditions. e ** fraction of maximum load that is due to external sources at maximum design conditions. / " ratio of the number of hours for a particular city, when the outdoor wet-bulb exceeds 65 F, during the period June 1 to October 1 to the total number of hours Table 5.... Equivalent Full Load Operating Hours of Refrigeration Equipment Used for Summer Cooling __________________ May 15 to Oct 15/ AppCcotioo Hr Op for Atlanta Business Chicago Detroit lot Angolas Nor Orleans New York noodoiphta Oklaho Si. ma City 1^1, Wadr mgtoo Barber Shops.......................... 1280 1010 650 720 720 680 1080 830 860 1020 890 940 Department Stores................. 940 840 560 610 610 580 890 700 720 840 750 780 Drug Stores............................. 2100 1630 950 1060 1060 980 1790 1280 1330 1650 1420 1530 Funeral Parlors...................... 600 440 300 330 330 310 470 370 380 440 400 410 Offices........................................ 1100 1030 660 720 720 680 1060 840 880 1040 910 960 Restaurant (Short Hour) Restaurant (Long Hour).... Speciality Shops (5 & 10) Theatere--Continuous........... Theaters--Neighborhood.... 1290 2100 1090 1500 900 970 535 1510 820 800 530 1010 700 640 420 620 930 590 750 450 620 570 1060 760 800 980 830 880 930 850 1690 1170 1210 1530 1300 1400 590 560 860 670 690 810 720 750 750 720 1080 850 870 1020 910 950 450 430 650 500 520 650 550 580 JfWcr* Air Comditimint. Bmpine atU V*ntOati*4, by W. H. Carrier, R. E. Choree, W. A. Grant. end W. R. Roberta (Pitman Publiaium Corp., IBM. p. *19). and Operating Costs 863 ' ta 'rV* dazing tt1*1 same period. Total hours are assumed as 8 kf 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.. ghouid be pointed out that certain southern cities may ^-have seasons longer than the 5-month period indicated m " Table 5- If it is desired to consider a longer season of opera- ratio of full-load operating hours to hours open for `bosines is smaller; in other words, the refrigeration load iador. 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. ^ should also be pointed out that there is an increasing tendency in large office buildings to operate the refrigeration p)nta through hours much longer than those of normal qyppanry, often 24 hr a day. Where tins practice is to be ^observed, another set of variables is introduced and .Table 5 b oot applicable. However, where Table 5 is applicable, the s^son electrical power cost for refrigerating equipment is ' then given by the following equation: season power costs = 0--.-7-4--6--(-b--h--p)--,-r---f-^-2- . 9 (3) where (bbp)i -- brake horsepower per too (see Fig. 1) tor average load duriag period. (Due allowance should be made for poorer compressor efficiency at light load.) T " maximum refrigeration design load, tons. B, " equivalent full load refrigeration operating time, hours (from Table 5). R -- power cost, including demand and energy charges, dollars per kwhr. 9 " motor efficiency at average load (decimal). Heat-Operated Refrigeration If the refrigeration is heat operated, whether by steam turbine, gas turbine, absorption, or combination thereof, the same general method can be followed, taking into ac count energy consumption per ton of refrigeration and cost. WATER COSTS The quantity of water used in an air-conditioning installa tion for purposes other than heat rejection is a negligible cost item. In a refrigerating plant utilizing water-cooled condensers, beat is generally rejected either to purchased water which is subsequently wasted, to well or surface water, or by means of cooling towers or spray ponds. Often, water conservation equipment must be used regardless of economics because of: (1) a lack of adequate water supply, (2) local regulations in tended to conserve existing supply, or (3) taxes or service charges attendant upon wasting to sewer. Certain munic ipalities will remit the sewer tax or service charge if the con densing water is discharged into a storm sewer or used for process purposes. Whatever the of producing refrigeration and what ever the ultimate heat sink, if water-cooled condensers are involved, the quantity of water through the condenser is a function of its inlet and outlet temperatures and their telationship to each other. Fig. 1 shows the typical variation *(th condensing temperature of the brake horsepower per ton required by an electric driven compressor. Since con densing temperature never equal leaving water tempera- tore but can only approach it, it follows that if, in the interest of economy, the leaving temperature of the condensing water is raised by reducing its quantity per ton, there is an offsetting increase in the cost of producing refrigerating effect. Where condensing water rejects its heat to the atmosphere anH is recirculated, the inlet temperature is a function of the heat rejection equipment and of the wet-bulb temperature. In the case of either purchased water or of surface or well water, inlet temperature rn vary widely and the quan tity selected must be based on its probable maximum value. Approximate maximum water main temperatures are given in Chapter 32 of the 1961 Guide And Data Book but they should always be verified locally. In the case of purchased water which is wasted after use, control is usually provided to vary the quantity supplied so as to hold the leaving water temperature (or condensing tem perature) constant. The cost of water, once its maximum quantity per ton has been established, can also be estimated on a hang of equivalent fuel load operating hours of the re frigerating equipment. In this connection the following for mula is useful: B - 0.060 aTH.C (4) where B -- cost of water for refrigeration during period, dollars. a " average gallons per (minute) (ton). T " tons of refrigeration at maximum design load. B% -- equivalent full load refrigeration operating hours (Table 5). C " water cost, dollars per 1000 gal. In the case of a well or lake, the cost of condensing water is usually simply the cost of pumping. Depending on the plant design, energy costs for pumping may vary with the quantity of refrigeration developed or they may be oonst&nt for all degrees of loading. Vdu*i given ora rapraranfofrra of dicMorodiAooroaatbaa* (Sefrigm) 12) recprecrting aocbtne* of about 25 toot capacity to air-coadfc~tioaiog oppficgffom. Itaquireaeaf* of maflor aodwae* era tmtafly higher, and for larger aoduoo* may bo (over. Valuo* shown ora for hqtad re frigerant at condenser temperafora Uo cubcooGng). Sebcoofing of (ho . 69aid may docraeio (bora vafuas approximately 0.3 porcoef to 0.5 per cent for oath Fahrenheit degree tho liquid temperature U lotmod. Fig. 1 .... Typical Brake Horsepower Requirements for. Refrigeration*