Major energy saving measure

In energy saving process many people are over focused on floating condensing pressure. Definitely, floating condensing pressure saves energy, but these savings are not significant.

Example.

Refrigeration plant has 2 screw compressors. Each compressor is 250 HP. Suction pressure is 3.5 psig. Evaporative condenser has water pump of 20 HP and fans of 30 HP.

Initially this plant was operated at fixed condensing pressure of 150 psig. Recently, operation of this plant has been changed to floating condensing pressure. Currently, condensing pressure floats from 120 psig to 130 psig. These changes have saved 58 HP or 43.25 KW of energy for mentioned refrigeration plant. However, action of switching operation from fixed to floating condensing pressure can be divided on 2 steps.

Step 1. Condensing pressure was lowered from 150 psig to 125 psig. This step saved 52 HP of compressor power. This is 90% of total energy savings of 58 HP.

Step 2. Floating of condensing pressure from 120 psig to 130 psig saved 20% of condenser fan power. This savings are not significant, because condenser fan power is small compare to total refrigeration plant power. This energy savings are 30 x 0.2 = 6 HP or 10% of total energy savings.

When can we use floating condensing pressure? Typically, condensing pressure can be floated when outside temperature is between 50 degF and 70 degF. When outside temperature is below 50 degF, majority of the refrigeration plants operate at minimum condensing pressure and there is no floating. When outside temperature is above 70 degF condensers operate at full capacity and there is no floating as well.

Unfortunately, real floating of condensing pressure can be applied for a limited period of time. It means that this energy saving measure will save less than 10% of total energy savings. However, lowering condensing pressure can save more than 90% of total energy savings and this is major energy saving measure. There are several barriers (hot gas defrosting, liquid injection oil cooling, oil carry over and etc.) to run refrigeration plant at low condensing pressure. However, every barrier has a solution.