Document aJDoVZraEbeELK9Okwp11qV3R

American Society of Heating and Ventilating Engineers Guide )g- Since 2546 Btu per hour = one horsepower-hour and 3415 Btu per h one. kilowatt-hour the heat extracted in performing work per p0u^i_ steam used may be obtained by dividing these figures by the Water of the units per indicated horsepower hour, or per: kilowatt hour rp <11 tively, of work done by the steam. From 1 per cent to 2 per centm deducted for radiation and other heat losses, depending upon th of unit; ................................................ s,$3 Example. A 200-kw turbine is operating on saturated steam at 150 lb gage nre MS and consuming 40 lb of steam per kilowatt hour delivered, at full load. Determi heat available in the exhaust for heating. e TMsj| Solution. The mechanical efficiency of the turbine is 0.935 and the efficiency of tpl generator is 0.934 (See Table 1), giving a combined efficiency of 0.935 X 0.934 = which would mean 40 X 0.873 = 34.92 lb per kilowatt hour of external work actualPl done by the steam. The total heat of the steam under the conditions as fed to the 111 is 1195 Btu per pound. mt The heat extracted in external work = 3415 34792 = 97,74 Btu per pound of steam. rl this add 1)4 per cent for heat lost by radiation, or 97174 + 1.47 = 99.21 Btu which 'H the total heat loss per pound of steam in passing through the turbine. This 1 1195 -- 99.21 = 1095.79 Btu in each pound of the exhaust . CHAFTER 48--Special Sources of Heat m Part II. Electric Heating HfSUlTa ntages of Electric Heating; Types of Electric Heaters; Storage 'vof Off-Peak System; Heating Costs with Electricity; Heat tffpr Equivalents. vnHE use electricity as a direct heating medium, employing soJPS^IrTcalJed space or radiant heaters, is confined largely to mild climates JiWtS ibcalities where electric rates are low. Direct electrical heaters take |^vuCscu0oa'-inilnlyccidicdoeemnntetaasIllldyy'iwwreiittchntlytinheeo"n.huecthaaetmerproewqeurirestmaeti'onnts''s, w-p-h-e--ia-c-k-hs.m-e'T-T-aHh-n---e-s--tthheartmthael l^sij^system uses off-peak electrical energy for which commercially ft^Snical rates are now being made available. Therefore;-this off-peak JfpRn-'of electrical heating is becoming practicable in many northern Twites as well as in milder climates. In some cases electricity is used as i^i^upplementary means, of heating such as in industrial plants for the 'hdabug of remote spaces. SMi;1 ADVANTAGES OF ELECTRIC HEATING following are some of the important advantages of electric heating: In a, similar manner the heat available per pound in the exhaust front* an engine driven unit for heating may be estimated by dividing the heat! equivalent per unit obwork by the number of pounds of steam fed to the! unit per unit of acttiarexternal work done, adding to this from 1 percent! to 2 per cent for radiation, and subtracting this sum from the total heat! in a pound of the steam as fed to the unit and then subtracting from thisf remainder the heat per pound in the condensate or rejected steam leaving^ the heating system. | In the ease-of an engine-driven unit,-the heat equivalent of a horsepower^ hour (2546 Btu) Should be divided by the water rate per indicated horse-! power. If the water rate is given per kilowatt hour delivered by the| generator, this may be reduced to pounds per indicated horsepower hourl by dividing by 1.34 and multiplying by the mechanical efficiency of thej unit and the efficiency of the generator.. .f Example. A 200-kw engine-driven unit operates under the same conditions as the! turbine unit in the preceding example, and delivers one kilowatt-hour on 37.8 lb of water,j The mechanical efficiency of the unit is 0.905 and the efficiency of the generator r` 0.934 (See Table 1). The water rate per indicated horsepower is therefore 37.8 ' - 07 o X 0.905 X 0:934 =-|L| x 0.845 23.84 The heat eqiiivalent.of the.external-work done per pound of steam is = 106.79 Btu.,Tb .this.adcl7l}^ per dent for radiation, pr 106.79 + 1.60 = 108.39_Btu which is the total heat lost per pound of steam in passing through the engine.. Deduct this from the total fieat per pound in the steam fed to the engine, of 1195 -- 108.39 = 1086.61. For additional.iriformatioh on this subject, see Utilization of Waste Heat, by Perry. West [A.S.H.V.E' JovatCAj, SectionXIfeaUng. Piping anA Air Conditioning), November, 1930). 288 4:"Sq. Minute and exact regulation of temperatures. Egp3sc\ *-Hvf ;,*2, Extreme simp, licity and automa,tism of opera tion ` with m, inimum maintenance. fiSYiri. Tf 'o Freedom from the dangers attending the presence of a flame. ' ..4. Freedom from handling and disposition of resulting products of combustion. V 5. Reliability of source of supply. Maximum efficiency of conversion of the energy into usefulness. 7. Cleanliness. . From the foregoing items it is apparent that electricity embraces all 7.-7 " .the requirements of an ideal heating agent, excepting low cost. ConseSifjquently, where electrical energy is sufficiently low in cost, it is being used ^( ftfmore or less for heating buildings. TYPES OF ELECTRIC HEATERS " The various commercial types7 of electric heaters may be classified as follows: 1. Direct Heaters o. Radiant Type. For quick intermittent heating. The heating element operates at a relatively high temperature, and the heat transfer is principally by radiation. b. Convection Type. For. continuous heating. The heating ele ment operates at a relatively low temperature and the heat transfer is mainly by air currents. c. Contact Type. Electric strip heaters placed in direct contact by damping to objects to be heated; They may be used to pre vent freezing of pipes and tanks'. 289