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View Subscription Details: Letter ( 09/16/2010 ) - HTML Format

Operating Strategy of Cold Storage Facilities

 

To save energy, many cold storages try to keep the temperature in refrigerated rooms as high as possible. Is it a good energy saving idea to keep the highest possible temperature in the refrigerated room?

Example.

A cold storage facility has 2 small freezers and 2 large freezers. A small freezer has 2 evaporators in the penthouse. A large freezer has 4 evaporators in the penthouse. Temperature in these freezers is 0°F. Average outside temperature is 50°F. A refrigeration plant has several screw compressors with economizers. A condensing pressure is 130 psig. The suction pressure is 3 psig.

To minimize the refrigeration load, the temperature in all freezers is set to 0°F. Evaporators of these freezers will cycle to keep this temperature. Due to this cycling, an actual refrigeration load will have significant fluctuation. The capacity of a trimming compressor will change from 20% to 100%. Assume that this trimming compressor will have equal operating time at different capacities from 20% to 100%. At mentioned operating conditions, average energy loss for trimming compressor will be around 20 %. Assume that two compressors will be operated. Average losses per compressor will be 10%.

However, another operating strategy can be implemented. During the days (8 Hrs), one compressor will be operated at full capacity. Two large freezers will have 1 or 2 evaporators operated (to keep operating compressor at full load) and set point temperature for these freezers will be -5°F. Small freezers will operate 1 evaporator per freezer and set point temperature in these freezers will be 0°F. During the evenings and nights (16 Hrs), large freezers will operate 2 evaporators per freezer and small freezers will operate 1 evaporator per freezer. However, set point temperature for small freezers will change to -5°F. To handle this refrigeration load, 2 compressors will be operated at full capacities. During evenings and nights, temperature in the freezers will be lowered to -2°F. It means that average temperature in these freezers will be -1°F. Temperature difference between outside air (50°F) and freezer temperature will increase from 50°F to 51°F. It means that energy use will increase by 2 % and refrigeration plant efficiency will be reduced by 2 %. This efficiency is comparable to the efficiency of the refrigeration plant that has a compressor VFD for trimming compressor. Typically, compressor VFD use additional 3 - 3.5 % of energy. It means that average compressor efficiency will be reduced by 1.5 - 1.75 % for the plant with 2 operating compressors. However, typical operation (freezer temperature 0°F and trimming compressor load, unload) without compressor VFD will give the losses of 10 % per compressor.

I think that slight fluctuation of the freezer temperatures and operation of the screw compressors at full capacity will give us better efficiency of the refrigeration plant. Other benefits of this approach are;

- Less energy use during the day when energy costs are typically higher.

- Starts and stops of the compressors will be minimized and the life mechanical seals of these compressors will be prolonged.

 

 

 

 

 

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