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View Subscription Details: Letter ( 02/28/2017 ) - HTML Format

Calculation of Energy Savings


To make the right decision about investments in energy efficiency of the refrigeration plant, potential energy savings should be calculated correctly. Many people mention floating condensing pressure as a very good energy saving measure. I have found that this information is misleading. Floating condensing pressure control reduces condenser capacity when it is oversized compare to the current refrigeration load. Meaning that this measure saves condenser energy. Typically, condenser energy is 10 – 15% of total plant energy. This is not significant. Additionally, floating condensing pressure control will be off during summer and winter operation. During summer operation condensers will run at full capacity to keep the required condensing pressure. During winter operation condensing pressure will be kept at a minimum allowable level and the floating feature will be off as well.  

For example. I have found that when condenser pressure floats, around 20% of condenser energy can be saved. This means that energy savings will be 2 - 3% of the total plant energy use. However, these savings can be achieved during periods of moderate ambient temperature or 2,500 - 3,500 hours per year. This is around 30% of the total operating time. Meaning that average annual energy savings of floating condensing pressure will be 0.5 – 1%. This is not significant, but to provide these savings sophisticated PLC should be installed.

To make the right decision about investment in energy efficiency, two major steps should be done.

Step 1. Real potential energy savings should be calculated. When we lower condensing pressure, the compressors energy use will be reduced and energy will be saved. However, to achieve this lower condensing pressure additional condensers energy will be used. Real energy savings will be equal to the compressors energy savings minus additional energy use of the condensers.

Step 2. Real operating time should be calculated. For example.  Minimum allowablecondensing pressure was reduced from 120 psig to 80 psig. However, we cannot run a plant at 80 psig all year around. Typically, this pressure can be achieved when the ambient temperature is below 40 degF. This means that the plant can operate at this condensing pressure 3,000 - 4.000 hours per year.

Calculate potential energy savings yourself or ask a trusted consultant to help you with these calculations. 

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