REFRIGERATION
CONTROLLING DISCHARGE AND LIQUID PRESSURE AT LOW HEAT SINK TEMPERATURES – PART II
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n Part I of this two-part series in the November/December 2021 issue, we discussed the condensate or liquid drop leg regulation method. In this article, we will cover the discharge gas regulation method.
Discharge Gas Regulation As the name implies, the discharge regulation method involves the use of a regulator in the discharge line between the compressor and the condenser. Like the condensate regulator, the discharge regulator used in the discharge gas regulation method is an inlet regulator. One can think of this pressure regulator as a hold-back regulator since it works to maintain a certain upstream pressure. One potential advantage of this method over the condensate drain regulation method is that the pressure regulator setting is reached very quickly on compressor start-up. With the condensate drain method, the process of building pressure tends to take longer, particularly when the air-cooled condenser is subject to low ambient temperatures. The discharge gas regulation method can be very useful for starting up cold systems. Also, during operation, the discharge gas regulation method is often preferred in systems using heat reclaim, hot gas defrost/ice harvest, and so on. Since this regulator is installed in the discharge line, and must regulate hot gas instead of liquid, it will need to have a larger pipe connection and port size than if a condensate regulator was used. On system start-up the regulator modulates to maintain at least the minimum set point pressure and the discharge pressure upstream from the valve increases rapidly. In the meantime, the pressure will begin to climb. The rate of
Phil Boudreau
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Discharge Regulation Method pressure increase within the condenser will typically lag the discharge pressure as the vapour entering the condenser experiences desuperheating, condensing and then subcooling. As the system warms up, liquid refrigerant will accumulate in the condenser, effectively reducing its available heat transfer surface. Since the amount of refrigerant leaving the condenser is somewhat limited, the receiver pressure may be too low to provide the differential pressure required by the expansion valves. It is also important to keep in mind that as the liquid is flooding the condenser, the expansion valves are also trying to feed liquid from the liquid line to the evaporator. This also contributes to the lowering of the liquid level and pressure in the receiver. In the condensate regulation method, we discussed how a bypass regulator installed between the compressor discharge line and the receiver inlet, works to maintain a desired receiver pressure. The discharge gas regulation employs this bypass or receiver regulator in the same manner. In fact, for a given system design and load, both methods would use the same regulator size. This regulator is an outlet regulator, so it is always sensing its downstream or receiver pressure. The receiver pressure
Phil provides sales and application support for Bitzer Canada Inc. and provides training and technical support for Bitzer’s clientele. He can be contacted at pboudreau@bitzer.ca.
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