Replacing the Expansion Valve in a Chiller Cycle With a Helical Screw Expander

The potential is well known for replacing the expansion valve in the refrigeration cycle by an expander which will produce mechanical power, but until recently this potential has not been exploited. Carrier has announced a system using a two-phase turbine replacing the expansion valve to help off-set the performance penalty from using HFC-134a in centrifugal chillers. The objective of this paper is to estimate the performance of twin-screw helical screw expanders in a particular representative refrigeration cycle. Helical screw expanders have already been used with two-phase fluids in geothermal applications. This paper explores their potential for use in refrigeration cycles using R-134a. First the trade-off is explored between subcooling at the condenser exit and use of a power producing expander. Both can improve performance, but subcooling reduces the potential for power production by the expander. Then a computer simulation for helical screw expander operation is used to determine what expander performance could be expected. Depending on its design, it is found that isentropic efficiency of 60 to 70% could be achieved by the helical screw expander in this application. For the sample cycle used in the study (a 500-Ton chiller cycle) this would give a reduction in kilowatts per ton of cooling of about 3.0% (for about 60% efficiency and 8°C of subcooling) to 6.5% (for about 70% efficiency and no subcooling) as compared to the cycle using an expansion valve. For large stationary systems the savings from this performance improvement would likely more than offset the added equipment cost.