ASHRAE Journal - August 2008 - (Page 48)

sion ratio increases (suction pressure decreasing), the advantages of two-stages of compression and liquid expansion increases. Two key points are identified in Figure 2. The point designated by j indicates the point (– 18.5°F [– 25°C]) where the efficiency of the two-stage system (Figure 1c) is 10% better (lower hp/ton) than a single-stage system (Figure 1a) while k indicates the point (– 32°F [– 35.6°C]) where the two-stage system is 10% better than the single-stage system with two-stages of liquid expansion (Figure 1b). It is also noteworthy that the efficiency advantage of the two-stage compression increases as the suction temperature decreases. At a saturated suction temperature of – 12°F (– 24°C) (8 psig [0.6 bar]) the two-stage and single-stage system with twostages of liquid expansion have equal operating efficiency. The advantage of a two-stage compression system is improved system efficiency, especially for lower temperature process requirements. The efficiency advantage does come with a price: more compressors. Figure 3 shows the total compressor size (i.e., volume flow rate, cfm) per ton of low temperature refrigeration load. Across the range of saturated suction temperatures (– 45°F to – 5°F [– 43°C to – 21°C]), the two-stage system has an incremental compressor size of 1.5 cfm/ton to 2 cfm/ton (0.20 L/[s · kW] to (0.27 L/[s · kW]) compared to a single-stage system and 2.5 cfm/ton (0.34 L/[s · kW]) compared to a single-stage system with two-stages of liquid expansion. More compressors mean more maintenance, more machinery room floor space, etc. The two-stage system will result in lower compression ratio operation on each individual compressor which reduces wear and tear compared to a single stage system. These trade-offs are relatively complex and plant-dependent; however, they will affect the total life-cycle cost of the system. As a Function of Intermediate Pressure A Evaporative Condenser(s) High Pressure Receiver Low Temperature Recirculator Low Temperature Evaporator(s) Compressor(s) B Evaporative Condenser(s) Low Temperature Evaporator(s) High Pressure Receiver Medium Temperature Recirculator Low Temperature Recirculator Medium Temperature Evaporator(s) In the context of two-stage compression systems, the concept of an “optimum” interstage pressure arises. An optimum interstage pressure is one that will minimize the total power consumption for the system. A frequently applied approximation for this optimum intermediate pressure is given by:1 Popt,int = Low Temperature Compressor(s) Medium Temperature Compressor(s) C Psuction , sat ·Pdischarge,sat Evaporative Condenser(s) (1) High Pressure Receiver Medium Temperature Evaporator(s) Medium Temperature Recirculator/ Intercooler where Popt,int represents an estimate of the optimum intermediate pressure, Psuction,sat is the absolute saturation pressure corresponding to the suction conditions, and Pdischarge,sat is the absolute saturation pressure corresponding to the discharge or condensing conditions. This relationship equally divides the compression ratio between the low-stage and high-stage of a two-stage compression system. The actual optimum will depend on the operating efficiencies of individual compressors attached to each suction pressure level. In addition, there are other factors that may restrict the ability to vary the intermediate pressure such as temperature requirements for loads served by the intermediate pressure level of the system. Free from any constraints on intermediate pressure, Figures 4 and 5 show the effect of intermediate pressure on the efficiencies of the three systems for a fixed saturated suction temperature of – 40°F (–40°C) and –25°F 48 ASHRAE Journal Low Temperature Evaporator(s) Low Temperature Recirculator Booster Compressor(s) High Stage Compressor(s) Figure 1: Schematics of refrigeration systems evaluated. 1a (top): Single-stage; 1b (center): Single-stage with two-stage liquid expansion; 1c (bottom): Two-stage. ashrae.org August 2008 http://ashrae.org

Table of Contents Feed for the Digital Edition of ASHRAE Journal - August 2008

ASHRAE Journal - August 2008 Contents Commentary Industry News Letters Meetings and Shows Maintain to Sustain—Delivering ASHRAE’s Sustainability Promise Ultraviolet Germicidal Irradiation: Current Best Practices Improving Humidity Control With Energy Recovery Ventilation Single- or Two-Stage Compression Data Center Cooling: Using Wet-Bulb Economizers Building Sciences InfoCenter Practical Pointers Products Emerging Technologies Washington Report People Special Products Classified Advertising Advertising Index

ASHRAE Journal - August 2008

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