ASHRAE Journal - September 2009 - 84

What’s Easiest Is Not Always Best By John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE Mechanical Subcooling M echanical subcooling, despite its additional complexity and equipment costs, actually can enhance refrigeration performance in certain situations. When compared to a simple refrigeration cycle system, mechanical subcooling often proves easiest is not always best. In most refrigeration and air-conditioning systems, a simple vapor compression cycle performs the cooling. Refrigerant travels in a closed loop from an evaporator, where heat is removed from the cooling load as the refrigerant boils, to an electric motoror engine-driven compressor; where refrigerant vapor is compressed, to a condenser; where compressed vapor is condensed, to a throttling device; where condensed liquid is expanded at constant enthalpy, and then back to the evaporator. All of the commonly used expansion devices (e.g., thermostatic, constant pressure, or electronic expansion valves, capillary tubes, orifice tubes, and orifices) are constant enthalpy expansion devices, differing by how they control refrigerant flow. If the refrigerant is expanded by a work recovery expander, some shaft power can be recovered and applied to the compressor power input or other useful load; and the work removed from the refrigerant can reduce its energy content, increasing the refrigeration effect, and thereby improving the coefficient of performance of the cooling cycle. 84 ASHRAE Journal Alternative Cooling Cycles With conventional cooling cycles using typical refrigerants, the incremental performance improvement is typically too small to justify the added cost and complexity of a work recovery expansion device. However, two noteworthy exceptions exist: • Transcritical carbon dioxide-based cooling cycles have a large enough constant enthalpy expansion loss such that a work recovery expander will provide a significant (25% to 30%) increase in the COP; and • Some chillers are large enough such that the small increase in COP justifies the addition of a specially designed expansion turbine. These exceptions aside, the majority of cooling systems use a constant enthalpy, throttled expansion. Another way of looking at the expansion loss is in terms of the amount of vapor that is flashed during the expansion. Before the condensed liquid refrigerant can do any useful cooling via evaporation, the liquid refrigerant itself must be cooled down to the evaporating temperature. This occurs during the expansion process by evaporating (flashing, as it ashrae.org happens nearly instantaneously) enough of the refrigerant to remove enough heat to cool it down from the temperature leaving the condenser to the evaporating temperature. The fraction of flashed vapor varies with refrigerant properties and with the temperature difference between the refrigerant leaving the condenser and the evaporating temperature. Table 1 summarizes the calculated fraction of flashed vapor for R-22, R-410A, and R-134a at a range of air-conditioning conditions (hot climate, AHRI standard conditions, and the DOE B test condition), with no subcooling and with 15°F (–9°C) subcooling leaving the condenser. Particularly in hot climates, a significant fraction of the refrigerant is flashed just to get from the condensing to the evaporating temperature. A simpler alternative to using work recovery expansion to improve the efficiency of the expansion process is using two stages of constant enthalpy expansion, commonly called a liquid economizer cycle, as shown in Figure 1a. The vapor flashed after the first expansion is at a pressure and saturation temperature between those of the condenser and evaporator, and can be compressed independently from this intermediate pressure back to the condenser pressure with less work input (about half depending on the specific pressure levels) than having to compress it all the way from the evaporator pressure up to the condenser pressure. After the second expansion, the cooling capacity of the refrigerant is slightly higher than it would have been after a single expansion. The intermediate pressure vapor can be compressed separately in a small compressor, or in systems having two-stage compression, the intermediate vapor can be compressed in the second compressor stage. In most small systems (e.g., residential and light/medium commercial), the compressor is single-stage, and a drawback of introducing a second small compressor with a higher inlet pressure than the main compressor is that there is no simple way September 2009
ASHRAE Journal - September 2009

Table of Contents for the Digital Edition of ASHRAE Journal - September 2009
