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What is the optimized suction pressure?
It is common knowledge in the industry that raising suction pressure improves compressor efficiency. Typical improvement might be 1% to 2% per degree increase in saturated suction pressure.

Some designers suggest gradually increasing suction pressure when all space temperatures are satisfied and evaporator coils in low load mode. However, this is right only for the coils with variable frequency drives (VFD). The majority of modern refrigeration plants have single speed evaporator fans.

To operate the plant efficiently, the total, not just the compressor power, must be minimized. The fans require an amount of power that is not negligible. Perhaps the power required by the evaporator fans constitutes 10-15% of the total full load power. However, when those fans continue to draw a constant power while the compressor unloads, they may require 30% or more of the total power. They not only use electricity, but all energy used by the motors must be removed from the space as refrigeration load. As a rule of thumb, for every 4HP we add in the refrigerated space, we add 2.5HP in the engine room in additional compressor and condenser motor requirements.

Our research shows that refrigeration plants with single speed evaporator fans operate more efficiently at optimized suction pressure regardless of the refrigeration loads. Every plant has its own optimum T.D. (temperature difference). Major factor determining T.D. (the temperature difference between the coil temperature and the room air temperature) is a ratio between fan motor power and basic evaporator rating for this coil.

From our research, optimum T.D. can vary from 5 degreeF to 20 degreeF.

In real life T.D. can be increased by the follow factors:

  1. Suction pressure losses.
  2. Frost on the coil.
  3. Actual fan power usage. Real power usage can increase up to 25% due to increase of cold air density.
  4. A static pressure penalty.

During low refrigeration load, we suggest to switch off several evaporators or let them cycle.

Properly chosen T.D. can significantly improve efficiency of the refrigeration plants, especially for the plans with high ratio between fan power and basic evaporator rating.

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