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View Subscription Details: Letter ( 07/10/2007 ) - HTML Format

Efficient refrigeration plant

 

Many people mistakenly believe that their refrigeration plants are efficient. Sometimes wrong benchmarking creates illusion of good efficiency.

Recently, I talked to chief engineer of cold storage. He told me that according to benchmarking his refrigeration plant is very efficient. I asked him about minimum allowable condensing pressure. They have 110 psig because of poor hot gas defrosting at lower condensing pressure. Unfortunately, he did not know that every refrigeration coil can be defrosted at condensing pressure lower than 100 psig. Usually, every 1 psig of lower condensing pressure will save a few thousand of dollars per year.

It is easy to do hot gas defrosting at 150 psig head pressure. It is not easy to do defrosting at 100 psig. I know just a few people who can set up a refrigeration plant for hot gas defrosting at head pressure lower than 100 psig. I believe that many evaporator coils can be defrosted at 70-80 psig of head pressure (this is not setting of coil back pressure regulator). However, low condensing pressure requires precise setting of hot gas supply, back pressure regulator and etc.

As I mentioned above, benchmarking sometimes can give wrong information about efficiency of refrigeration plant. Usually, benchmark for cold storages is power consumption per volume unit of refrigerated space. However, this benchmark does not evaluate the temperature of incoming product. Several loads of warm product will require significant energy consumption. Product activity (inbound and outbound) can also have a significant influence on power consumption of cold storage refrigeration plant.

I believe that correct estimation of refrigeration plant efficiency can be done by comparing current operation and current set points with optimum operation and optimum set points. The closer current set points to optimum set points, the better efficiency of refrigeration plant.

Example. Two refrigeration plants have optimum head pressure of 100 psig. First plant operates at 150 psig, second plant operates at 120 psig. Obviously, first plant is more efficient than second one.

Certainly, it is not easy to determine the optimum set points and optimum operating strategies. However, experienced consultant can help you with this issue and you will have correct information about efficiency of your refrigeration plant.

 

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