PRODUCT HISTORY
The first rule in servicing a refrigeration system is to
determine if the problem is an electrical or mechanical
failure.
First, try to obtain the product’s history of operation
from the customer. This will help identify the source of
the problem.
Good facts from the cabinet user can help identify
whether the problem is electrical, within the refrigera-
tion system, or a “misapplication by the user”. Get the
history of operation and failure by asking these ques-
tions:
1 ) Were there any brown-outs or power outages that
they are aware of?
2) Is the cabinet on a dedicated circuit?
3) Has any other equipment in this area had opera-
tional problems?
4) When was the last time the cabinet’s operation was
confirmed as working properly?
5) When was a problem noticed?
6) How long has the equipment run without this prob-
lem? (Years? Weeks? Days? Hours?)
7) Was anything tried prior to your arrival?
The refrigeration system should only be entered if it is
absolutely necessary. It is critical that a clean, uncon-
taminated system be maintained.
If a system is unable to reach the proper operating
temperature, a test of the unit’s mechanical refrigera-
tion components is required.
COMPRESSOR EFFICIENCY TEST
To test the compressor, place compound gauge on the
compressor’s suction port.
While the compressor is running, close off the suction
line so that only the port and valve are part of the com-
pressor’s low side.
When the valve is closed and vacuum has started, time
how long it takes to pull the compressor’s low side to its
lowest possible vacuum.
Compressors used on Thermo Forma cabinets should
be capable of pulling at least 20 to 22 inches of vacuum
in less than 40 seconds.
Next, shut off the compressor and watch the gauge. A
one or two inch rise in pressure is acceptable, since a
small amount of freon may remain on the low side of the
compressor, after which the reading should stabilize.
If the pressure continues to rise, the discharge reeds in
the valve head have failed, allowing high pressure gas to
return to the compressor.
If the compressor pulls less than 20 inches, the suction
reeds have failed.
If the compressor takes longer than 40 seconds to pull
to its ultimate low vacuum, one or both cylinders are not
functioning as they should. Any reading less than these
will require replacement of the compressor.
ENTERING THE SYSTEM
Entering the system should only be done as a last
resort. Extreme care must be used no matter what the
reason for entering the system. Of course there are
times when it cannot be avoided, such as component or
compressor replacement, or a leak within the system.
The system must also be entered any time you need to
obtain the operating pressures. Again, use extreme cau-
tion to avoid any possible contamination.
Cabinets that use hermetic compressors typically do not
have valve ports, so Thermo Forma has designed
process stubs for both suction and discharge sides of
the system.
Line taps should only be used to obtain pressure read-
ings, and not for the reprocessing of the system. The
opening of a line tap is too restrictive for the pressure of
vacuum procedures.
On a hermetic compressor system, once you have
determined that reprocessing a system is required,
recover refrigerant and remove the line taps.
Install hand valves at the process stub ends. Hand
valves will be less restrictive to flow because of a larger
opening. They will also be easier to use during repair
procedures.
EVACUATION
Once the system has been cleaned and components
have been replaced, you are ready to initiate the final
servicing procedures necessary to achieve proper cabi-
net operation. Drier should be replaced prior to system
processing.
Pull an evacuation to approximately 50 microns.
CHARGING
You should use a charging cylinder to measure in the
correct amount of refrigerant. The charging methods
are:
1. Add the refrigerant to the system until you reach a
balanced pressure. This will give you an approxi-
mate static charge.
2. Weigh in the refrigerant using a scale calibrated in
ounces.
The cabinet’s operation is now ready to be tested. A
final check of the refrigeration lines should be made
before running the cabinet.
Be sure the refrigeration lines are not kinked or rub-
bing against each other.
38 MAINTENANCE & REPAIR
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