REFRIGERATION
SUPERMARKET HOT GAS DEFROST SYSTEMS R efrigeration systems in supermarkets and other large commercial-industrial applications frequently utilize several compressors connected in parallel. These parallel systems are often referred to as racks. The rack refrigeration system approach provides better capacity control when more compressors are used along with a VFD or with a compressor where all heads are unloadable. Many evaporators are connected to the rack. The evaporators are grouped into several circuits and piped into the rack’s suction and discharge headers. Each circuit can operate at a different pressure so it is important that only the evaporators which will operate at the same evaporating temperature are grouped together. The compressors are staged to maintain a pressure within the suction header that is low enough to allow flow through all the circuits. This means the suction header must be held at a pressure slightly lower than the lowest pressure circuit. To accomplish this, each circuit is equipped with an evaporator pressure regulator or EPR valve. The EPR valve is typically installed on the rack, but it could also be installed in a remote header. Some systems are set up this way so most of the solenoid valves and regulators can be separated from the rack and this often makes it easier to access and service these components. Mechanical and electronic EPR valves are upstream or inlet regulators. With mechanical EPR valves, the spring pressure is increased by turning the adjustment stem in a clockwise direction. This results in the port closing somewhat and restricting the flow from the evaporators to the suction header. Electronic valves are typically driven by a stepper motor, which in turn is fed a series of electrical pulses to drive the motor in an opening or closing direction.
Supermarket rack with EPR and DDR valves.
In addition to maintaining a minimum evaporating pressure, the EPR valve can serve as a suction stop valve. This is required during the reverse-cycle hot gas defrost cycle. The rack will also be equipped with a hot gas defrost header. Note that this defrost header is separate from the discharge header and supplies hot gas from a point that is after the oil separator.
Electronic EPR
EPR with suction stop feature Whenever the evaporator coil is cold enough to permit frost formation on its fins, the frost will reduce the capacity and efficiency of the evaporator. Frost build-up also restricts airflow through the evaporator.
Phil Boudreau
58 44
M e c h a n i c a l
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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