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Minimum Allowable Condensing Pressure

Hot Gas Defrost

When the surfaces of evaporator coils operate at temperature below 32 °F and also below the dew point temperature of the air, frost will form on the coil. To reduce frost penalty we have to remove it periodically by defrosting.

The most widely used method of defrosting is hot gas.

What is the minimum condensing pressure for hot gas defrosting?

Typically this pressure is set from 120 psig to 160 psig. I believe that proper hot gas defrosting can be done at condensing pressure as low as 80 psig - 100 psig.

During hot gas defrosting our evaporator coils are working as condensers. The process of

condensation includes 2 steps:

  • Desuperheating
  • condensation

Desuperheating is reduction of hot gas temperature from discharge temperature to condensing temperature. For screw compressors, majority of the heat is available from the condensation process (around 90 %). Desuperheating can contribute only up to 10 % of total available heat. But this proportion is right in the compressor room. In real life, if we are using that hot gas for defrosting, majority of the superheat would be lost in hot gas line. Therefore, the condensation usually gives us from 95 % to 100 % of total defrost heat. Many operating engineers make the mistake of assuming that higher condensing pressure will give them more heat of defrosting, because of higher discharge

temperature. This is a wrong assumption. Desuperheating can usually give us a very small portion of total heat requirement.

Why should we increase the condensing pressure? By lifting condensing pressure up we will increase hot gas supply into the coil. We can get the same result by adjusting the hot gas balancing valve at lower condensing pressure. To keep constant hot gas supply to the coil, constant pressure should be kept in hot gas line. Outlet pressure regulator will help us to keep hot gas line pressure at a constant level all year around.

What is the pressure inside the coil during defrosting? Usually, we have two devices to control the pressure in the coil and condensate removal from the coil. These devices are back pressure regulator (BPR) and liquid drainer. The evaporator coils equipped with BPR have BPR pressure inside the coils during defrosting. The evaporator coils equipped with liquid drainer have nearly hot gas pressure inside the coils during defrosting.

My experience have shown that minimum hot gas pressure for the coils with BPR can be 90 - 100 psig and the minimum hot gas pressure for the coils with liquid drainer can be 70 -80 psig. This means that hot gas defrosting is an imagined barrier to reducing condensing pressure.

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