ASHRAE Journal - August 2008 - (Page 50)

3.50 18 Total Compressor Volume Flow Rate (cfm/ton) Anhydrous Ammonia (R-717) Twin Screw Compressor External Oil Cooling Intermediate Pressure = 30 psig (16.5°F sat) Condensing Pressure = 150 psig (85°F sat) Single-Stage Compression and Two-Stage Liquid Expansion Anhydrous Ammonia (R-717) Twin Screw Compressor External Oil Cooling Intermediate Pressure = 30 psig (16.5°F sat) Condensing Pressure = 150 psig (85°F sat) Full-Load Compressor Efﬁciency (hp/ton) Single-Stage Compression 3.25 and Liquid Expansion 3.00 2.75 2.50 2.25 2.00 1.75 1.50 1.25 – 55 Two-Stage Compression and Liquid Expansion 16 14 12 Two-Stage Compression and Liquid Expansion 10 Single-Stage Compression and Two-Stage Liquid Expansion 8 6 Single-Stage Compression and Liquid Expansion – 45 – 35 – 25 – 15 Saturated Suction Temperature (°F) –5 4 – 55 – 45 – 35 – 25 – 15 Saturated Suction Temperature (°F) –5 Figure 2: Refrigeration system efﬁciency as a function of lowtemperature suction requirements. Figure 3: Compressor displacement per ton as a function of lowtemperature suction requirements. 3.50 (– 32°C), respectively. The condensing condition is ﬁxed at 85°F (29°C) saturated which corresponds to 150 psig (10.3 bar). The symbol indicates the theoretical optimum intermediate pressure for a two-stage compression system. Notice that in this case, the actual optimum (i.e., lowest hp/ton) occurs at intermediate pressures slightly higher than the theoretical optimum. Also note that the optimum is fairly broad, that is, the intermediate pressure does not have a large effect on overall system efﬁciency. To illustrate that point, the results shown in Figure 4 reveal that any intermediate pressure between 27 psig to 50 psig (1.9 bar to 3.4 bar) yields system efﬁciencies within 2% of the optimum. In reality, most refrigeration systems do not have only low temperature loads. Nearly all plants will have refrigeration loads that demand higher temperature refrigerant: cooler spaces, bulk product storage tanks, production air-conditioning, postpasteurization cooling, etc. The presence of these higher temperature loads will not signiﬁcantly degrade the efﬁciency of meeting the low-stage loads due to the relatively broad optimum of the intermediate pressure. This is excellent news because the high-stage load temperature requirements can now dictate the intermediate pressure without compromising the efﬁciency of meeting the low-stage loads. As a Function of Condensing Pressure 3.25 Full-Load Compressor Efﬁciency (hp/ton) 3.00 Single-Stage Compression and Liquid Expansion 2.75 Single-Stage Compression and Two-Stage Liquid Expansion 2.50 2.25 Two-Stage Compression and Liquid Expansion 2.00 1.75 1.50 1.25 10 Theoretical Optimum Intermediate Pressure Anhydrous Ammonia (R-717) Twin Screw Compressor External Oil Cooling Suction Pressure = 9 in. Hg (– 40°F sat) Condensing Pressure = 150 psig (85°F sat) 15 20 25 30 35 40 Intermediate Pressure (psig) 45 50 Figure 4: Refrigeration system efﬁciency (– 40°F [– 40°C] sat) as a function of intermediate pressure. Conclusions The last factor considered is the condensing pressure. Figure 6 shows the effect of saturated condensing temperature (pressure) on the efﬁciencies of the three systems for a ﬁxed intermediate pressure of 30 psig (2 bar) (16.5°F saturated [– 8.6°C]) and – 25°F (– 32°C) saturated suction (1.2 psig [0.08 bar]). As the condensing pressure is reduced, the compression ratio decreases and at approximately 64°F (18°C) saturated (100 psig [6.9 bar]) the efﬁciency of the two-stage system is the same as for a single-stage compression system with two stages of liquid expansion. 50 ASHRAE Journal The use of two stages of compression is common in low-temperature industrial refrigeration systems. The efﬁciency beneﬁt of a two-stage system increases as the temperature requirements are lowered and is weakly dependent on the intermediate pressure. The decision on whether to conﬁgure the system for multiple-stages of compression is one that should weigh both the advantages and disadvantages at the suction pressures required by the loads. As a general rule, two-stage compression systems should always be considered when loads demand low suction temperatures, particularly those lower than –25°F (–32°C) (1.2 psig [0.08 bar]). Regardless of the number of stages of compression, conﬁguring the liquid side for two-stages of expansion will nearly always inashrae.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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