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View Subscription Details: Letter ( 12/28/2012 ) - HTML Format

Hot gas defrosting and flow

 

During winter operation, many refrigeration plants can save a lot of energy by lowering condensing pressure below 100 psig. However, hot gas defrosting is a major barrier to operate plants at such low head pressure. I believe that properly adjusted hot gas defrosting can be done at condensing pressure below 100 psig.

Hot gas defrosting is a triple process: hot gas supply, hot gas condensation and ammonia condensate draining. These three parts should be adjusted properly to provide adequate defrosting at low condensing pressure. It is well known in our industry, that defrost BPR (back pressure regulator) should be set to 70 - 80 psig. This setting provides hot gas condensation at the required temperature level. However, ammonia should be continuously drained from the evaporator to provide flow of the hot gas. Majority of the refrigeration plants have bottom feed overfed evaporators. The bottom of this evaporator will not defrost properly, unless it is free of liquid ammonia.  How can we achieve proper ammonia condensate draining?

Required condensate draining can be provided by proper pressure difference between evaporator defrost pressure and BPR setting. Typically, this pressure difference should be 25 - 30 psig for the freezers and 10 - 15 psig for the coolers. Why do we need this pressure difference?  During defrosting, hot gas condenses inside the coil and flows down to the bottom of this coil. To be drained out of the coil, ammonia condensate should be pushed through small orifices located at the bottom of each circuit. At the mentioned pressure differences, adequate defrosting and ammonia condensate draining will be provided and the minimum blow by gas will be produced. Blow by gas is the gas that did not condense inside the coil and went to the suction as parasitic refrigeration load.

How can we get the right pressure difference? Two variables should be adjusted: hot gas supply and BPR setting. Many refrigeration plants oversupply hot gas for the defrosting. This oversupply will require the refrigeration plant to operate at higher condensing pressure to provide adequate draining of ammonia condensate and an energy efficiency of this plant will suffer. To keep balance supply of hot gas, pressure in the hot gas main should be constant all year round. To control this pressure, an outlet pressure regulator should be installed.

 

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