Chapter 10 System Balancing Copy Right By: Thomas T.S. Wan 温到祥著 Sept. 3, 2008 All rights reserved The major components of a refrigeration system are the compressor, condenser and the evaporator. These three components are the structure of the triangle of the refrigeration system. Each component has its performance curve under the design operating conditions and the balance point of these three curves shall be the exact refrigeration capacity of the system at the design conditions. Changing any one of the three components might change the shape of the triangle and therefore, changes the character and the capacity of the system. Figure 10-1 is the typical performance curves for a compressor. At the constant condensing temperature, say 110 F, the compressor capacities at various saturated suction temperatures are: Table A-1 Constant Condensing Temperature, 110 F Saturated Suction Compressor Capacity, TR 30 F 185 TR 20 F 147 TR 10 F 116 TR 0 F 90 TR -10 F 67 TR -20 F 49 TR From the Table A-1, at constant condensing temperature, the compressor capacity is higher if the suction temperature is higher and the capacity is smaller if the suction temperature is lower. From Figure 10-1, if the suction temperature is constant, the compressor capacities at various condensing temperatures are as the following: - 121 -
Table A-2 Condensing Temperature Constant Saturated Suction Temp. 0 F 80 F 112 TR 178 TR 90 F 105 TR 168 TR 100 F 98 TR 158 TR Constant Saturated Suction Temp. 20 F 110 F 90 TR 148 TR 120 F 82 TR 138 TR 130 F 74 TR 127 TR From Table A-2, if the compressor suction temperature is constant, the compressor capacity is higher if the condensing temperature is lower and the compressor capacity is smaller if the condensing temperature is higher. In general speaking, the compressor capacities are higher at various condensing temperatures if the suction temperature of the compressor is higher as shown. - 122 -
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The Figure 10-2 is the characteristic and performance lines representing the major components of compressor, condenser and evaporator. The diagram [A] of the Figure 10-2 is the compressor general curves. The horizontal axis represents the capacity and the vertical axis is the condensing temperature. The performance line at the left is the compressor performance line of a lower constant suction temperature and the right hand side line is the compressor performance of a higher constant suction temperature. The diagram [B] of Figure 10-2 is the condenser characteristic line at a fixed GPM/TR and fixed cooling water temperatures range, a larger condenser with higher Sq.Ft./TR heat transfer surface will have higher capacity and have lower condensing temperatures; likewise, a smaller condenser will have higher condensing temperature and smaller capacities. The diagram [C] of Figure 10-2 is the general curves for an evaporator. At the same capacity, the smaller size evaporator will result in lower evaporative temperature while higher ET for the larger evaporator; larger size evaporator produces more TR than the smaller evaporator is having smaller tonnage at the same ET. By combining all the general characteristics of the major components as described as the above, by plotting the actual performance data of each compressor, condenser and the evaporator, a typical system balancing chart is shown in Figure 10-3; the horizontal axis is the TR of the system; the upper vertical line is the condensing temperature and the lower vertical axis is the evaporative temperature of the system. The Figure 10-3 shows the system balance chart for the compressor of model R-blv2, R-22 at the compressor speed of 1450 RPM, the condenser is a model HC-xlb6, 2-Pass arrangement, 85 to 95 cooling water range at 3 GPM/TR, 0.001 scale factor and the brine cooler is model C-R2B2 at leaving brine temperature of -5, 40% by wt. of Ethanol brine, 0.001 scale factor, 2-pass arrangement. The compressor model R-blv2 performance lines are shown; the left hand side line (1) is the compressor at constant suction temperature of -20 and the right hand side line (2) is the compressor at the constant suction temperature of -10. The condenser performance line is the line (3); the line (3) intersects with the lines (1) and (2), the line between points of (A) and (B) is the combined performance curve of the condenser HC-xlb6 and the compressor R-blv2., representing the compressor and the condenser at the various capacities at the corresponding condensing temperature at the design operating condition for the compressor and the condenser. For example, the compressor and condenser will produce 29 TR at condensing temperature of 101.6. Locate point (C) by projecting point (A) which is the compressor suction at -20, also locate point (D) by projecting point (B) which is the compressor suction at -10. Connecting point (C) and (D), this line represents the combined compressor and the condenser performance at various points for the compressor and the condenser with - 124 -
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suction temperature added. Now, plotting the brine cooler performance line (4) against the TR and ET on the chart, the intersection point of line (4) and line (C)-(D) is the point (E) which shall the balance point of the compressor, condenser and the brine cooler at the design operating conditions. The balance point of the system using the specified three components is shown as: System Capacity: 31.1 TR Saturated Suction: -13.8 Condensing Temperature: 102.3 Refrigerant: R-22 Compressor Model: R-blv2 Compressor Speed: 1450 RPM Condenser Model: HC-xlb6 Condenser Pass: 2-Pass Cooling Water Temperature: 85 to 95 Condenser Scale: 0.001 Cooler Model: C-R2B2 Leaving Brine Temperature: -5 Brine: 40% by wt. of Ethanol brine Cooler Scale Factor: 0.001 Cooler Pass: 2-Pass - 127 -