Hot gas
supply
To
provide efficient and safe hot gas defrosting of evaporator coils, 3 steps
should be done properly:
1.
Hot gas
supply
2.
Hot gas
condensation
3.
Ammonia condensate
draining
Many people are concerned about pressure drop in hot gas
line and undersupply of hot gas for adequate defrosting. My research has shown
that very often hot gas is oversupplied to the defrosting coil. Why is it not
good idea to oversupply hot gas?
-
Safety issue.
I found that hot gas balancing
valves of many evaporator coils are wide open. Sometimes evaporator coils are
designed without hot gas balancing valves, but they have additional small hot
gas pilot valve. Pilot valve will open first and gradually increase pressure in
the coil up to 30-40 psig and then main hot gas valve will
open. Even at this approach main hot gas valve has pressure difference (between
inlet and outlet) around 100 psig
or higher. This is
significant pressure difference and wide open main hot gas valve will create
very high velocity of hot gas supply into the coil. This velocity is not an
issue for properly operated evaporator coil. However, some malfunctions in plant
operation can lead to coil damage.
1.
If for some reason coil was not pumped out properly, entering high velocity hot
gas can lead to water hammer in evaporator coil and this coil can be
damaged.
2.
Properly designed hot gas line has liquid drainer. In real life, this drainer
can be plugged by dirt, rust and etc. Liquid ammonia will collect in hot gas
line. As soon as main hot gas valve opens, liquid ammonia from hot gas line will
fly into the coil and this coil can be damaged as well.
Gradual supply of hot gas will prevent these
catastrophic events. In worst case scenario, evaporator coil will not defrost
properly, but it will not be damaged.
-
Efficiency issue.
Many people believe that higher supply of hot gas will
lead to faster defrosting. I do not agree with this conclusion. Majority of low
temperature evaporator coils are bottom feed overfed coils. Oversupply of hot
gas will create a lot of ammonia condensate. It is not easy task to drain this
condensate from that coil because it should be pushed out through small
orifices. I found that poor ammonia condensate draining is a major reason for
poor hot gas defrosting. Sometimes oversupply of hot gas can create a lot of
blow by gas which will go to the suction line and it will create a significant
parasitic load.
Safety and efficiency of refrigeration plants can be
improved by balanced supply of hot gas into the evaporator coils during
defrosting cycle.