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.