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

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Refrigerants
Lubricants
Today, almost as many new lubricants are on the market as there are
refrigerants. Compressor manufacturers always specify oil type and
fill each model of compressor accordingly. One of the most common
mistakes whilst servicing is not checking which is the appropriate
lubricant for the serviced system; this could cause damage to
the system due to non-compatibility with refrigerant and system
components. In hermetic systems, the lubricant is in intimate contact
with the electrical motor windings. The oil must therefore provide
good, material compatibility and have high thermal stability properties.
Although the majority of the lubricant remains in the compressor, a
small amount will be circulated into the rest of the refrigerant circuit.
The lubricant must be able to resist both the high temperatures at
the compressor discharge valves and the low temperatures at the
expansion device. It must be sufficiently soluble with the refrigerant
itself in order for it to be returned back to the compressor, so that
over time, it does not become starved of oil, which could lead to
mechanical failure. There properties are discussed next.
Properties
The properties of a good refrigeration lubricant are:
• Low wax content. Separation of wax from the refrigeration oil
mixture may plug refrigerant control orifices
• Good thermal stability. It should not form hard carbon deposits
and spots in the compressor, such as in the valves of the
discharge port
• Good chemical stability. There should be little or no chemical reaction
with the refrigerant or materials normally found in systems
• Low pour point. This is the ability of the oil to remain in a fluid
state at the lowest temperature in the system
• Good miscibility and solubility. Good miscibility ensures that the oil
will be returned to the compressor, although a too high solubility
may result in lubricant being washed off the moving parts
• Low viscosity index. This is the ability of the lubricant to maintain
good oiling properties at high temperatures and good fluidity at low
temperatures and to provide a good lubricating film at all times.
Categories
Basically there are six main categories of refrigeration lubricants:
• mineral oils (MO)
• alkyl benzene oils (AB)
• polyol ester oils (POE)
• poly alpha olefin oils (PAO)
• poly alkyl glycol oils (PAG)
Traditionally, CFC refrigerants have been used with mineral and
alkyl benzene oils for the lubrication of compressors. This is now
undergoing change, with the introduction of HFC refrigerants, which
are immiscible with the traditional mineral oils, and need the use of
synthetic oils for miscibility and oil return.
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