ASHRAE Journal - August 2008 - (Page 47)

pressure of 196 psia (1351 kPa) and a compression ratio limit of 8:1, the resulting lower limit on suction pressure would be 24.5 psia (169 kPa), which corresponds to a saturated suction temperature of approximately – 8.5°F (– 22.5°C). With many food processing facilities operating low-temperature freezing systems below – 40°F (– 40°C), it is clear that reciprocating compressors only could be used in a two-stage or compound arrangement to overcome the compression ratio limit. Although modern day screw compressors are capable of serving a – 40°F (–40°C) load in a single stage of compression, the overall system operating efﬁciency may be signiﬁcantly lower than a two-stage arrangement. The second factor limiting single-stage compression systems is the behavior exhibited by industrial refrigerants such as ammonia during the compression process. As ammonia is compressed, its pressure increases and because of its low heat capacity, it experiences a dramatic increase in temperature. With reciprocating compression technologies, high compression ratio operation requires an external source of cooling for the compressor (water or refrigerant-cooled heads). A high discharge temperature for a reciprocating compressor would be 250°F (121°C). High discharge temperatures tend to increase the rate of compressor lubricating oil breakdown as well as increasing the likelihood of compressor material fatigue. The combination of compression ratio limits and refrigerant discharge superheat conspire to limit our ability to provide useful refrigeration in a single-stage compression arrangement with reciprocating and rotary vane compressors. As a result, early refrigeration pioneers overcame these constraints by conceiving, implementing, and reﬁning multistage (compound) compression systems. In this article, we explore single versus two-stage compression arrangements from an efﬁciency perspective. Operating efﬁciency changes with varying suction and discharge pressures are determined. System Conﬁgurations sors by direct-contact with liquid refrigerant maintained at the intercooling (high-stage suction) pressure. We consider three system conﬁgurations in our efﬁciency comparison. The simplest conﬁguration is a single-stage compression system with no loads present at the intermediate pressure. The next level of complexity is a single-stage compression system with the presence of loads at the intermediate pressure. The presence of intermediate loads allows evaluation of the efﬁciency beneﬁts associated with two-stages of liquid expansion. And ﬁnally, a full-blown two-stage compression system with two-stages of liquid expansion and intercooling is considered. The three systems are shown in Figure 1. Another option that is not evaluated in this article is economized single-stage compression system arrangements. In some applications and operating situations, single-stage compression systems equipped with economized screw compressors can achieve efﬁciencies approaching two-stage compression arrangements. In the present analysis, we focus on screw compressors equipped with external oil cooling because they are clearly the most commonly speciﬁed technology for industrial refrigeration applications today. All of the compressor performance information included in the analysis is based on manufacturers’ selection programs across the ranges of operating pressures considered. The performance of the compressor is optimized (i.e., variable volume ratio or a properly chosen ﬁxed volume ratio machine) for the given application conditions. In each case, the compressor’s performance is based on the entire package, which includes pressure losses associated with suction and discharge trim. The refrigerant state entering the compressor is assumed to be saturated vapor and the liquid state leaving the evaporative condensers saturated. Efﬁciency Comparison Before proceeding with an evaluation of the energy-efﬁciency characteristics of two-stage compression systems, we need to establish some terminology that will be used during our discussion. Multiple stages of compression are often combined with multiple stages of liquid expansion and intercooling. Stages of compression represent the number of compression steps required to raise the refrigerant pressure from suction to condensing. The term liquid expansion used here refers to the number of times liquid refrigerant expanded (reduced in pressure) from the condensing pressure until it reaches the lowest pressure level in the system. At each stage of liquid expansion (or throttling), the resulting ﬂash gas is recompressed to a higher pressure level within the system. Two stages of liquid expansion can be implemented on two-stage compression systems and single-stage compression systems conﬁgured with two or more suction pressure levels. The term intercooling only applies to compound systems and represents the process of desuperheating the discharge gas from the low stage (or booster) compresAugust 2008 The operating efﬁciency expressed as hp/ton for each of the three system arrangements shown in Figure 1 will be necessarily affected by a number of factors. Some of these can be considered constraints such as the temperature requirements of the refrigeration loads at low suction and intermediate suction (if present). Other variables are uncontrolled such as the ambient conditions which will inﬂuence condensing pressure. We will investigate the comparison of single- versus two-stage compression arrangements over a range of suction pressures, intermediate pressures, and condensing pressures to accommodate this variability. As a Function of Suction Pressure The most obvious factor that affects the efﬁciency is the lowtemperature suction pressure. Figure 2 shows the effect of suction pressure on the efﬁciencies of the three systems for a ﬁxed saturated intermediate pressure (SIP) of 30 psig (16.5°F saturated) (2 bar or – 8.6°C saturated) and a ﬁxed saturated condensing temperature (SCT) of 85°F (150 psig) (29°C or 10.3 bar). The condensing condition is chosen to approximate a yearly average condensing pressure for a refrigeration system and the 30 psig (2 bar) intermediate pressure as reﬂective of a typical high-stage suction pressure setpoint. The ﬁgure shows that as the compresASHRAE Journal 47

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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