Document x1vdnNXOyRpXgjZdLq1bzeBB6

American Society of Heating and Ventilating Engineers Guide, 1937 The unit of refrigeration is the ton, based on the latent heat of fusion of one ton of ice in 24 hours. Thus one ton = 200 Btu per minute = 12,000 Btu per hour. Coefficient of Performance The coefficient of performance of a refrigeration system is the ratio of the refrigerating effect to the work of compression, both expressed in the same units. The ideal or Carnot coefficient of performance depends upon the tem peratures Ti and in much the same way as the ideal efficiency of a steam engine depends upon its working temperature, with an inverse relationship. Ideal C. of P. = Tl it -- 1 l (5) Evidently the smaller the compression range, the less power will be required to produce a given refrigerating effect. Table 7. Theoretical Comparison of Various Refrigerants3 Refrigerant Wore per Lb Ft Lb Equiva lent Head Ft Gas Temp. Leaving Coup. F ReFRIG. Effect per Lb vLb per Min per Ton Ammonia'.,-................... ,..... 53,900 53,900 209 489.8 0.408 Carbon Dioxide6. .......... 9,940 9,940 146 64.0 3.125 Dichlorodifluoro- methane (Fi,)................. 6,280 6,280 134 55.74 3.638 Methyl Chloride_________ 16,500 16,500 183 156.9 1.275 Monofluorotrichloro- methane (Fn) 7,050 7,050 101 74.0 2.605 . Water0..... ............................ 114,300 114,300 360 1010.8 0.181 COBFP. OF Pebfor. 7.06. 5.00 . 6.90 7.38 7.53 6.85' 8.33' . Cycle Eff. per CENT 84.8 46.9 82.9 88.8 `r 90.4 82.3 100.0 eEvapi Refrig. Temp. = 40 F. Cond. Refrig. Temp. = 100 F. Suction Vapor. Temp.'*='65 F. Liquid Temp. = 98 F. bBased on 1057 lb per sq in. (87 F) Condenser Pressure, and 85 F liquid Temperature. cBased on 40 F Temperature. The theoretical coefficient of performance of actual refrigerants is always less than the ideal due to the tendency of most refrigerants to superheat when compressed, and due to the heat of the liquid which must be removed. The cycle efficiency is the theoretical C. of P. divided by the ideal for the same temperatures. The cycle efficiency usually changes as the compression temperatures change. .. Comparative results of modified theoretical cycles of the refrigerants, are given in Table 7. Practical Cycle Fig. 3 illustrates the pressure-volume and temperature-entropy dia grams for an actual cycle. These diagrams are based upon the com pressor receiving vapor superheated and upon sub-cooling of the liquid going to the evaporator. The theoretical cycle is aibiCCiejaj. However, the vapor during compression actually follows line aj>2 due to superheating as a result of the inefficient work of compression. The theoretical work of 42 Chapter 2--Refrigeration compression is a-})\cda\. Added to this is the area bjbigjhbz on the tem- oerature-entropy diagram which represents the inefficient work of com pression (assuming no compressor heat losses). The sum of these areas represents the total work of the compressor per pound of refrigerant, and the ratio of theoretical cycle work to the actual work represents the over all efficiency. It should be noted that area is considered as part of the inefficient work and is commonly termed the superheat loss. The refrigerating effect per pound is the same for the practical as for the theoretical cycle, working with the same sub-cooling of liquid and super heating of vapor, that is, area eiOig^ei. Sources of loss which are usually recognized as reflected by the overall Fig. 3. Practical Dichlorodifluoromethane (F,) Cycles efficiency referring particularly to reciprocating and rotary systems, are as follows: 1. The superheat loss. 2. A pressure loss to and from the cylinder of the compressor. (The line pressure drop between the compressor and the evaporator and condenser, respectively, is usually taken into account separately in the design of the refrigeration system.) 3. Leakage loss through valves and past pistons is quite small -in most compressors. 4. With an oil soluble refrigerant, there may be an absorption loss due to absorption and re-evaporation of refrigerant in the oil of the cylinder. 5. Mechanical losses are always present and are usually a large part of the total. Reciprocating and rotary compressors always take in less vapor than that which corresponds to the displacement. The overall volumetric efficiency is the ratio of the suction vapor volume to the piston displace ment. Part of this is the re-expansion volumetric efficiency which is the volume, at suction pressure, of the usefully re-expanded vapor which was in the clearance volume. This is expressed by ihe following equation: Volumetric Efficiency = 1 -- Pc d 43 -. t [(*)-] (6)