Chapter 6 Introduction to Refrigerants
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temperatures and remain harmless to refrigerants and
equipment.
Refrigeration lubricant must be able to travel
freely through all parts of the system. The lubricant’s
ability to do this is determined by several factors.
These include the type of refrigerant used, the operat-
ing temperatures in the system, and the properties of
the lubricant. A good refrigeration lubricant has low
wax content, high thermal and chemical stability, a low
pour point, and low viscosity.
Wax Content
Many refrigeration lubricants contain paraffin wax.
When wax precipitates out of a lubricant at low tem-
peratures, it is called wax separation. Wax that sepa-
rates from a refrigerant-lubricant mixture can plug
refrigerant control orifices and clog a refrigeration sys-
tem. Lubricants that have low wax content are desir-
able because they reduce the problems associated with
wax separation.
A floc test is a test that determines how easily wax
separates out of a mixture of refrigerant and lubricant.
This test is used on mineral oils and the refrigerants
that are miscible (mixable) with mineral oil. Examples
of these refrigerants include R-11, R-12, and R-22. The
test is conducted by mixing 10% refrigerant with 90%
oil. The mixture is sealed in a glass tube, and then it
is cooled slowly until a flocculent (cloudy) precipitate
of wax appears. The highest temperature at which this
occurs is recorded as the floc point. Using a refrigerant
with the proper floc point will help avoid wax separa-
tion at the lowest temperature in the system.
Pro Tip
Floc Point
Synthetic lubricants do not contain wax and there-
fore have no fl oc point.
Thermal Stability and Flash Point
Thermal stability is a lubricant’s ability to remain
stable in high heat areas. Lubricants with lower ther-
mal stability tend to form hard carbon deposits at hot
spots in the compressor, such as valves or discharge
ports. A lubricant’s chemical stability is its ability to
not react on a chemical level with refrigerants or other
substances found in a refrigeration system. If a lubri-
cant is not chemically stable, it can break down eas-
ily and form corrosive solutions that can damage the
refrigeration system.
The temperature at which the vapors from a
lubricant surface ignite is the lubricant’s flash point.
In many hermetic units, refrigeration lubricants come
in direct contact with the hot motor windings in the
Floc Point
Synthetic lubricants do not contain wax and there -
f ore have no fl oc p oint.
compressor, which makes having a high flash point
important. The lower the flash point of a lubricant, the
more likely it is that the lubricant will ignite when it is
exposed to high temperatures.
Viscosity
Viscosity is a measure of a liquid’s resistance to
flow. A fluid with high viscosity resists flowing, while
a fluid with low viscosity flows more easily. The vis-
cosity of a lubricant changes with the temperature.
Lubricants at very low temperatures may not pour and
can become a plastic solid. The lowest temperature at
which a lubricant will flow is called the pour point.
Use lubricants that have a pour point appropriate for
the operating temperatures inside the refrigeration
system.
Domestic freezers with refrigerant temperatures
as low as 0°F to 5°F (–18°C to –15°C) require a lubricant
with a pour point of –20°F (–29°C). For commercial food
freezers with refrigerant temperatures as low as –50°F
(–46°C), a pour point of –60°F (–51°C) is desirable.
Dielectric Properties
The amount of moisture in a refrigeration lubricant
can be measured by its resistance to the flow of a cur-
rent of electricity through it without breaking down.
This is known as its dielectric strength. A refrigera-
tion lubricant with acceptable moisture content should
have a minimum dielectric value of 25,000 volts.
Refrigeration Lubricant Additives
To improve the performance of a lubricant, many
manufacturers add certain chemicals. These are
designed to inhibit foaming or the formation of sludge.
Lubricants that contain moisture or air can form sludge
or varnish. This may cause damage to the unit.
Refrigeration lubricants sometimes include a very
small amount of antifoam inhibitor to reduce foaming.
Compressor parts are sometimes given a phosphating
treatment to improve lubrication.
6.7.2 Types of Refrigeration Lubricant
There are a variety of refrigeration lubricants avail-
able, including mineral oils (MO), polyol ester lubricants
(POE), alkylbenzene lubricants (AB), and polyalkyl-
ene glycol lubricants (PAG). The type of lubricant used
must match the type of refrigerant used. Most new
azeotropic mixtures and single HFC refrigerants use
polyol ester lubricants. Traditional CFC refrigerants
require mineral oil. Typically, different types of lubri-
cant should not be used within the same system.
Mineral oils are not miscible with most HFC
refrigerants. Polyol ester (POE), alkylbenzene (AB),