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View Subscription Details: Letter ( 09/14/2005 ) - HTML Format

Minimum allowable condensing pressure


Space heating

One of the barriers to reducing minimum condensing pressure is the heating of high temperature coolers. The heating process is similar to hot gas defrosting of evaporator coils, only evaporator fans are running.


First step, to minimize condensing pressure, is the balancing of heating requirement and capacity of the coil. BPR (back pressure regulator) setting determines capacity of the coil.


For example. Air temperature in the cooler is 50 °F. BPR set to 93 psig (corresponding temperature 60 °F).  Temperature difference is 60 – 50 = 10 °F. Capacity of the coil is proportional to the temperature difference. By changing BPR setting to 103 psig (corresponding temperature 65 °F), temperature difference increases to 15 °F and capacity of the coil increases 50%.


BPR setting should be changed several times per year to match the heating requirements. We can have BPR setting 93 psig during the summer (low heating requirement), 98 psig for fall and spring, and 103 psig during the winter (high heating requirement).


Second step is the balancing capacity of the coil and hot gas supply. Undersupply of the hot gas will reduce capacity of the coil and part of this coil will be flooded with liquid refrigerant. If hot gas is oversupplied, only portion of this gas will condense in the coil and excessive gas will go through BPR into the suction line as parasitic refrigeration load. Pressure difference between BPR setting and condensing pressure determines the hot gas supply. Usually, this pressure difference is 10 – 15 psig.


In our example, summer BPR setting is 93 psig. To mach this lowest capacity of the coil, pressure difference should be 10 psig. Summer minimum allowable condensing pressure will be 93 + 10 = 103 psig.

BPR winter setting is 103 psig. To match this highest capacity of the coil, pressure difference should be 15 psig. Winter minimum allowable condensing pressure will be 103 + 15 = 118 psig.


To keep refrigeration plant at peak efficiency, set points should be changed from time to time, based on real operating conditions.


In our example, settings should be changed 3 times per year.

BPR (psig)

Condensing pressure (psig)

Winter

103

118

Spring, Fall

 98

111

Summer

 93

103

Traditional constant settings for this refrigeration plant are BPR-120 psig, condensing pressure -150 psig. Only variable set points can give us an opportunity to save energy.

Better result and easier adjustment can be done, if we replace BPR with liquid drainer. In this case pressure inside the coil will adjust itself and we have to change just condensing pressure.


We have 2 opportunities to reduce condensing pressure even further.

1.  Run dedicated small compressor for coil heating and the rest of the refrigeration plant run at optimum lowered condensing pressure.

2.  Run dedicated small compressor as a heat pump to increase pressure from lowered condensing pressure to 118 psig.

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