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View Subscription Details: Letter ( 09/12/2007 ) - HTML Format

Water in ammonia system

 

In previous newsletter (August 2007) I told you about the penalties related to air presence in ammonia system. Outside air contains some moisture. When this air gets into the system, it will bring water into this system. Air purgers usually remove just air, but water will accumulate in ammonia system.

When water is added to ammonia the saturated temperature, at a given pressure, increases. In other words the boiling point of the ammonia rises. To maintain the desired temperature, the suction pressure must be lowered and the compressor must work harder. This means increase in power consumption. When water content in the system is low, it has very little effect on system operation. How much water is too much? There is a rule of thumb, that a 1 °F increase in suction temperature (about 2% water) corresponds to 2.5% to 3% of lost compressor capacity as well as compressor efficiency. So the goal is to keep water contain in ammonia system lower than 2%.

How can we do that? Some manufacturers suggest installing ammonia regenerators. A small amount of liquid ammonia from discharge side of the ammonia pump directed to regenerator. The ammonia will vaporize and will return to the system, while the water and impurities are left behind. These water and impurities can be warmed up and drained.

Do we really need this regenerator? I do not think so.

We already have regenerators in ammonia systems. These are oil pots. Oil pots work the same way as regenerators. The only difference is that liquid goes to regenerator from discharge side of ammonia pump, but for oil pot, the same liquid is taken from the suction side of ammonia pump. There is no difference between liquid ammonia before and after the pump.

I believe that oil pots remove the water (as well as oil) from ammonia system. Some refrigeration plants filter and reuse the drained oil. I would not recommend that, unless you are 100% sure that all impurities (water, sludge, acids and etc.) are removed from this oil.

Modern compressors have good oil separators and very little oil goes to the ammonia system. Usually, a cost to replace the drained oil is not significant. Is it worth to reuse the drained oil? I do not believe so.  

 

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