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

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Servicing Practices
• If the temperature reaches below 0°C, the saturated moisture
(water) within the expansion device can freeze and subsequently
stop the refrigerant flow.
• As the expansion valve warms, due to the lack of refrigerant, the
ice melts and moisture returns to the expansion valve and once
more generates an intermittent cooling.
• Whether or not freezing actually occurs depends primarily upon
the amount of moisture (water) and size of the ice particles
formed.
Be aware of the risk of corrosion and its impacts
In addition to possible freezing, another serious problem – namely
corrosion – can occur within the system due to the presence of
moisture. Corrosion can create serious problems because often
its effects are not apparent until serious damage has occurred.
For example, moisture in the form of water alone can cause rust
after a period of time. However, moisture plus the refrigerant create
much more corrosion problems. Refrigerant such as R12 containing
chlorine will slowly hydrolyse with water and form hydrochloric
acids. This acid greatly increases the corrosion of metals.
The corrosion processes can be characterised as follows:
• Heat increases the rate of corrosion due to acids because at
higher temperatures the acid-forming process is accelerated.
This acid, of course, attacks all the materials it contacts, the rate
of corrosion of the individual materials being determined by their
corrosion-resistant qualities. Steel will generally corrode at lower
moisture levels than copper or brass.
• Compressor lubricant presents another problem caused by
moisture, particularly in the case of polyol ester (POE) and poly
alkyl glycol (PAG) lubricants, used with hydrofluorocarbon (HFC)
refrigerants. In fact, these types of lubricant have an affinity for
moisture and will absorb it rapidly if left open to the atmosphere.
Mineral lubricants do not mix with water in the same range as
polyol ester lubricants.
• Water changed into acid emulsifies with lubricants, the two
forming an intimate mixture of exceedingly fine globules. This
effect is called “sludging” of the oil and greatly reduces its
lubricating ability. Corrosion becomes troublesome from the
operating standpoint when the metallic surface is eaten away
and a solid, detachable product is formed. This formation is
commonly known as “sludge”. Sludge exists as slimy liquids, fine
powders, granular solids or sticky solids and causes a variety
of problems. They can plug fine strainers, expansion valves and
capillary tubes. And because they usually contain acids they
corrode whatever they cling to, accelerating damage.
Eliminating moisture problems
To eliminate moisture problems it is necessary to take precautions
and actions, which will ensure a moisture-free system. It is important
to change the filter drier frequently. The most effective way to
eliminate moisture from a system is through the use of a high vacuum
pump to create a vacuum deep enough to evaporate and remove
this moisture. The recommended level of evacuation is of 1 millibar
absolute (100 Pa) to achieve the evacuation of moisture. This level
of vacuum must be maintained for 10 minutes without the help of a
vacuum pump.
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