Manual for Refrigeration Servicing Technicians - UNEP - Division of ...

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Servicing Practices
Moisture
Moisture causes several operating problems in RAC systems and
understanding the basis of these problems is important. Basically,
moisture can be classified as visible and invisible. “Visible” moisture
is high concentration of water and can be seen with the eye, and is
in liquid form. Occasionally, liquid water is found in systems but this
is rather unusual. “Invisible” moisture is water in low concentrations
and cannot be seen with the eye, since it is essentially dissolved in
the refrigerant. It is important to remember that moisture can easily
get into a system but it is difficult to get out.
The main situations and activities that can result in moisture entering the
system are:
• when there is a leak in part of the system that experiences subatmospheric
pressure (thus, drawing air into the system)
• during servicing and repair when the system is opened and
exposed to the air and evacuation is not carried out properly
• when filters or lubricant are exchanged
• during charging with refrigerant and transfer hoses have not
been purged properly.
The technician should be particularly aware of the possible introduction of
moisture when carrying out these activities.
There will always be some moisture within any refrigerating system,
and complete elimination is almost impossible. However, at very
low concentrations the moisture is unlikely to cause any significant
problem. On the other hand, if the moisture is present in higher
concentrations, then a series of problems can arise. The “tolerable”
concentration of moisture differs between refrigerants, oil types,
operating temperatures and compressor designs. Nevertheless, in
almost all systems – except for ammonia systems – the maximum
concentration of moisture should not exceed around 10 ppm.
Above these concentrations, the moisture can have negative effects
on the system, such as reactions with the oil, causing the unit to
malfunction and accelerating burn-out of hermetic compressors.
Recognising the presence of moisture
The presence of moisture within the system can be recognised by certain
observations:
• The system will stop due to low suction pressure, and proceed
to warm up. Since it is the formation of solid ice within the
expansion device that has caused the blockage, the warming
will result in the disappearance of the ice and thus the unit will
work properly again. However, the process will occur again as
the ice once again forms at the expansion device.
• Decreasing pressure, where the suction pressure steadily decreases
over several hours – even to a vacuum. Then pressure suddenly
becomes normal again. This abnormal cycle will keep repeating.
• If, during system shutdown, one warms the refrigerant control
with a safe resistance heater (hot pad) or radiant heat bulb, the
ice will melt. Should the system then begin to work properly,
there is definitely moisture in the refrigerant.
These symptoms are explained by the following:
• The solubility of water in refrigerant reduces with temperature,
so as the refrigerant passes through the expansion device, the
dissolved moisture may become saturated water.
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