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ACRC Technical Report 238PDF


Title:Applications and Control of Air Conditioning Systems Using Rapid Cycling to Modulate Capacity
Author(s):Poort, M.J.; Bullard, C.W.
Subject(s):rapid cycling
Abstract:Rapid cycling of the compressor can be used to modulate capacity as an alternative to variable speed a/c and refrigeration systems. This paper outlines design recommendations to optimize rapid cycling performance based on experimental results contrasting different heat exchangers and other components. Rapid cycling has inherent compressor lift penalties associated with larger mass flow rates relative to variable speed operation which need to be minimized. To design for optimal rapid cycling performance, it is also important to prevent dryout (superheating) in the evaporator during the off-cycle, a major penalty as cycles are lengthened. By reducing the number of starts per hour, through increasing cycle lengths, compressor performance and reliability can be improved, and efficiency increased by reducing the number of startup power spikes. To increase cycle lengths while minimizing penalties, the off cycle performance in the evaporator should be optimized. During the off cycle, two mechanisms contribute to off cycle cooling: additional boiling of refrigerant, and warming of the evaporator thermal mass. Experiments were done on a typical 2-ton residential system with a round tube and plate fin evaporator and compared to a similar 1-ton system with a microchannel evaporator to explore the tradeoffs in evaporator design. Other components including a receiver and a suction line heat exchanger were also tested. Evaporator design should be focused on preventing dryout by increasing the refrigerant side area and preventing off cycle drainage. Since the condenser does not have the dryout problem, its only design changes would be to add thermal mass if economically feasible. For optimal performance under rapid cycling conditions, it was found that it would be best to combine flash gas bypass with a suction line heat exchanger to maximize performance during the off cycle and increase cycle lengths without incurring major penalties. This would allow the careful control of refrigerant boiling during the off cycle and maximize the thermal mass contribution to cooling the air during the off cycle.
Issue Date:2005-05
Publisher:Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
Series/Report:Air Conditioning and Refrigeration Center TR-238
Genre:Technical Report
Sponsor:Air Conditioning and Refrigeration Project 161
Date Available in IDEALS:2009-06-22

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