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View Subscription Details: Letter ( 05/05/2008 ) - HTML Format

The goal

 

What is our goal in energy saving?

For many years, we were focused on PLCs, VFDs and design of refrigeration plants. Certainly these issues are important for energy saving, but they are the foundation of efficient operation. However, the efficient operation of refrigeration plants is the final goal.

Two steps should be done for efficient operation.

Step 1. Determination of optimum (the best) set points and optimum operating strategies.

Step 2. Implementation of optimum set points and optimum operating strategies.

Determination of optimum set points and optimum operating strategies is the most difficult, nonetheless, a necessary task. Very often we just guess about these points and strategies because of limited knowledge of this issue. How can we reach the goal, if we do not know what it is?

Usually, there are two ways to determine optimum set points and optimum operating strategies.

1.     Do extensive research on your refrigeration plant. This is a long and expensive way to determine the best set points and the best operating strategies. Thousands of tests should be done at different refrigeration loads and different ambient conditions. This research can take a few years, without a guarantee of success.

2.     Hire somebody, who has done similar research on other refrigeration plants. Findings of previous research can be applied for your refrigeration plant. I think that this is the best way to go, because you will save time and money.

 

Major energy savings will come from optimization of condensing pressure.

 

Winter optimum condensing pressure.

Our speculation of winter optimum condensing pressure is usually correct. The lower condensing pressure that we have, the better the efficiency of the refrigeration plant is. During winter operation, the optimum condensing pressure is well below 100 psig. Efficiency of refrigeration plant will improve, if it operates at condensing pressure closer to optimum condensing pressure. However, very often we are wrong in implementation of better condensing pressure.

Usually, hot gas defrosting is the barrier to lower condensing pressure. I think that it is an imagined barrier. Some people do hot gas defrosting at 150 psig, other people do hot gas defrosting at 120 psig. I think that more than 99% of evaporating coils can be defrosted at condensing pressure lower than 100 psig. A precise adjustment should be done for hot gas defrosting at low condensing pressure.

 

Summer optimum condensing pressure.

During summer operation, condensing pressure should be at optimum level. At optimum condensing pressure, total power consumption of compressors and condensers is the minimum. Condensing temperature(pressure) should float and should follow the wet bulb temperature of ambient air.

Optimization of refrigeration plant operation is the best way to save energy in industrial refrigeration. This approach can give you 10-20 times better return on your investment. You can do this optimization yourself or you can hire somebody. You have a choice.

 

 

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