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View Subscription Details: Letter ( 10/29/2012 ) - HTML Format

Summer Optimum Condensing Pressure

 

To maximize the energy efficiency of a refrigeration plant, it should be operated at optimum condensing pressure all year around. How to determine the optimum condensing pressure? How to implement the required condensing pressure? This newsletter will be focused on these 2 questions.

During summer operation, condensing pressure should go up and down based on fluctuation of the wet bulb temperature of the ambient air.

Example

Wet bulb temperature is 65 degF, optimum wet bulb approach is 10 degF. Optimum condensing temperature is 65 +10 = 75 degF. Corresponding condensing pressure is 125 psig.  During the night, wet bulb temperature dropped to 60 degF. Optimum condensing temperature will be 60 + 10 = 70 degF. Optimum condensing pressure is 115 psig. I took the optimum approach of 10 degF as an  example. Every refrigeration plant has its own optimum wet bulb approach and usually it is unchanged during summer operation, however it is a different story for winter operation. Operating engineers should have information about this approach for their plant. Make sure that readings of all the sensors (condensing pressure, wet bulb temperature and etc.) are correct otherwise the energy efficiency of the plant will suffer. To implement floating condensing pressure a refrigeration plant should have PLC with a wet bulb approach feature. Unfortunately, many refrigeration plants do not have a wet bulb approach feature. How to keep optimum condensing pressure for these plants? You have two options.

1. Keep optimum balance between the compressor and condenser capacities. This approach works well for the plants with steady refrigeration loads. Typically, cold storages have relatively steady loads and optimum balance can be kept. How can you do that? First, optimum balance between the compressor and condenser capacities should be determined. Then, this balance should be implemented.

Example. Refrigeration load is 100%. 4 compressors should be operated to handle this load. This plant has 3 condensers. Total condenser capacity is oversized. We determined that the optimum condensing pressure can be kept by 2 condensers. This means that 2 condensers should be operated when 4 compressor run.

When refrigeration load is 50%, 2 compressors should be operated. To keep the balance between compressor and condenser capacities, 1 condenser should be operated as well.

2. Loads of many refrigeration plants have significant fluctuation during production time. Different operating strategy should be chosen for these plants. Typically, wet bulb temperature changes gradually. It increases during the day and decreases during the night. To implement optimum condensing pressure, set points of the condensing pressure should be changed 3 - 4 times per day. Normally, I give my customers a table. From time to time, operating engineers check wet bulb temperature of ambient air and by using the mentioned table they can determine optimum condensing pressure for particular wet bulb temperature. It is very important to keep optimum condensing pressure all year round because this is major energy saving measure.

In the next newsletter, I will provide information on how to run a plant at condensing pressure below 100 psig.

 

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