Page 184 - Boiler_Operators_Handbook,_Second_Edition

Refrigeration & AC 169

a vacuum which might result in ambient air containing
moisture entering the system should it be leaking. I be-
lieve I said elsewhere in this book that a system can be
tight under pressure but leak under a vacuum because a
piece of rust or a defect in construction can produce the
equivalent of a check valve at the leak site.
If you are pumping down the system for mainte-
nance, that requires opening the system, a separate inde-
pendent vacuum pump should be used to recover the re-
maining refrigerant and evacuate the system. A recovery
system should also be used to remove refrigerant from a
compressor, condenser, or receiver before opening them
for inspection or maintenance of the refrigerant side of
the system. Recall that evacuating a system for mainte-
nance and opening a system require you be licensed by
EPA to handle refrigerants.

Load Control Valves
When a refrigeration system serves multiple
loads control valves are installed in the liquid lines
Figure 5-28. Isolating valve with bellows seal to start and stop the flow of liquid refrigerant to each
load. These are normally solenoid controlled valves
actuated by a thermostat. Aboard ship we had evap-
orators in many spaces. Some were used all the time
such as ships stores, food freezers, refrigerators, and
other equipment in the galley. Then each deck in each
hold could have coils for refrigeration or freezing of the
cargo in that space. The Santa Louisa, my first ship as
a licensed ship’s engineer carried bananas back from
South America and we refrigerated all the holds that
contained those bananas. Facilities ashore such as food
distribution centers, large supermarkets, and the like
can have central refrigeration condensing units serving
freezers and refrigerated boxes.

Defrost Valves and Lines
In order to defrost evaporators used in freezing
applications some systems include a defrost valve and
piping or tubing that transfers refrigerant from the hot
gas lines to the inlet of the evaporator to melt ice accu-
mulating on the evaporator. A defrost valve is identical
to a load control valve with the exception of size (larger
Figure 5-29. Isolating valve with packing because it handles vapor) and location; a defrost valve is
always connected downstream of the throttling device.
operation of a compressor then permits a “pump down” The defrost valve admits the hot gas into the evaporator
of the system transferring almost all of the refrigerant so it has a higher temperature rating. When defrost con-
to storage in the receiver. Temporary overriding of the trols call for defrosting the load control valve is closed,
system controls can be used to lower the suction pres- any circulating fans within the freezer space are shut
sure and all elements between the King Valve and the down, and the defrost valve is opened to admit hot gas
compressor inlet until it is almost at atmospheric, zero to the evaporator. Once the ice is melted off normal op-
psig. When pumping down you should avoid pulling eration is restored.