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View Subscription Details: Letter ( 06/16/2009 ) - HTML Format

Hot gas supply

 

To provide efficient and safe hot gas defrosting of evaporator coils, 3 steps should be done properly:

1.     Hot gas supply

2.     Hot gas condensation

3.     Ammonia condensate draining

 

Many people are concerned about pressure drop in hot gas line and undersupply of hot gas for adequate defrosting. My research has shown that very often hot gas is oversupplied to the defrosting coil. Why is it not good idea to oversupply hot gas?

- Safety issue.

I found that hot gas balancing valves of many evaporator coils are wide open. Sometimes evaporator coils are designed without hot gas balancing valves, but they have additional small hot gas pilot valve. Pilot valve will open first and gradually increase pressure in the coil up to 30-40 psig and then main hot gas valve will open. Even at this approach main hot gas valve has pressure difference (between inlet and outlet) around 100 psig or higher. This is significant pressure difference and wide open main hot gas valve will create very high velocity of hot gas supply into the coil. This velocity is not an issue for properly operated evaporator coil. However, some malfunctions in plant operation can lead to coil damage.

1. If for some reason coil was not pumped out properly, entering high velocity hot gas can lead to water hammer in evaporator coil and this coil can be damaged.

2. Properly designed hot gas line has liquid drainer. In real life, this drainer can be plugged by dirt, rust and etc. Liquid ammonia will collect in hot gas line. As soon as main hot gas valve opens, liquid ammonia from hot gas line will fly into the coil and this coil can be damaged as well.

Gradual supply of hot gas will prevent these catastrophic events. In worst case scenario, evaporator coil will not defrost properly, but it will not be damaged.

 

- Efficiency issue.

Many people believe that higher supply of hot gas will lead to faster defrosting. I do not agree with this conclusion. Majority of low temperature evaporator coils are bottom feed overfed coils. Oversupply of hot gas will create a lot of ammonia condensate. It is not easy task to drain this condensate from that coil because it should be pushed out through small orifices. I found that poor ammonia condensate draining is a major reason for poor hot gas defrosting. Sometimes oversupply of hot gas can create a lot of blow by gas which will go to the suction line and it will create a significant parasitic load.

Safety and efficiency of refrigeration plants can be improved by balanced supply of hot gas into the evaporator coils during defrosting cycle.

 

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