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Vapor injection has been available on large commercial screw and multistage centrifugal compressors, but not on small hermetic compressors. The recent introduction of vapor-injected scroll compressors allows the economized refrigeration cycle to be applied with individual small compressors, and achieve the benefits of increased capacity and efficiency. This article describes the economized vapor-injection (EVI) scroll compressor concept, its key components, and the associated performance and application aspects for commercial refrigeration systems.
The economized scroll compressor cycle is similar to a two-stage cycle with interstage cooling, but is accomplished with a single scroll compressor as shown in Figure 1a. The high-stage of compression is accomplished by extracting a portion of the condenser liquid and expanding it through a thermostatic expansion valve (TXVi) into a counterflow brazed-plate heat exchanger acting as a subcooler. The superheated vapor is then injected into an intermediate vapor injection port in the scroll compressor as shown in Figure 1b. The additional subcooling increases the evaporator capacity by reducing its inlet enthalpy, thereby increasing refrigeration effect and decreasing mass and volume flow.
This capacity gain can be improved by decreasing the injection pressure, thereby increasing the second-stage injection mass flow. The lowest injection pressure achievable is governed by the pressure drop in the injection passage and the scroll porting location.
It is well known that the efficiency of a two-stage cycle is higher than the conventional single-stage delivering the same capacity because the added capacity from subcooling is achieved with less power. This is because incremental vapor created in the subcooling process is compressed only from the higher interstage pressure rather than From the lower suction pressure. The interstage cooling also decreases the first-stage suction pocket and increases the mass flow. By integrating vapor injection into one scroll compressor, the isentropic efficiency is comparable for both stages, providing high efficiency without requiring dedicated low-stage compressor optimization. The compressor designer has a choice of injection location. A lower pressure location gives more capacity gain while a higher pressure location provides more efficiency gain.
Since the added capacity achieved by enhanced subcooling provides higher enthalpy gain across the evaporator, the compressor displacement required can be reduced by the percentage enthalpy gain for the same evaporator capacity. This reduction in displacement can provide a means for...