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Ammonia condensate in the hot gas main

 

Recently, I found that some people believe that ammonia condensate will collect in hot gas main during periods of cold weather. They think that this condensate is the reason for poor hot gas defrosting at lower condensing pressure and condensing pressure should be increased to provide adequate defrosting. Ammonia condensate will accumulate in hot gas main if the rate of ammonia condensation will be greater than the rate of ammonia condensate draining.

Assume that we have properly designed hot gas main with insulation and with liquid drainers. Liquid ammonia will be drained to intermediate pressure receiver, which has a pressure of 30 psig.

Example.

Hot gas main is located on the roof. Outside temperature is 0 degF. Condensing pressures (temperatures) are 100 psig (63.5 degF), 120 psig (72.5 degF), and 150 psig (84.4 degF).

Ammonia condensation will be proportional to the temperature difference between outside temperature and condensing temperature. Ammonia condensate draining will be proportional to the square root of the pressure difference between condensing pressure and intermediate pressure.

Increase of condensing pressure from 100 psig to 120 psig will lead to an increase of ammonia condensation by 14% and an increase of ammonia condensate draining by 13.5%.

Increase of condensing pressure from 100 psig to 150 psig will lead to an increase of the ammonia condensation by 33% and an increase of ammonia condensate draining by 31%.

This example shows that balance between ammonia condensation and ammonia condensate draining will not change at higher condensing pressure. Why is it easier to defrost evaporator at 120 psig or 150 psig, but it is not easy to do defrost at 100 psig of condensing pressure? Definitely, the reason is not ammonia condensate collection in hot gas main.

I think that misbalance of hot gas supply, ammonia condensation and ammonia condensate draining is a major reason for poor hot gas defrosting at lower condensing pressure.

March 14 - 17, 2010 IIAR conference took place in San Diego. At this conference I have presented technical paper "Optimization of Refrigeration Plant Operation. Engineering Approach". You can read this paper on the website www.ammonia21.com

 

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