Refrigeration & Air Conditioning Technology, 8e - 58
Section 1 Theory of Heat
of the system will also rise. For example, if the outside
ambient (surrounding) temperature rises to 105°F, the
condenser saturation temperature on our sample system
would rise to about 135°F.
11. The refrigerant is now 100% liquid at the saturated
temperature of 125°F. As the liquid continues along
the coil, the air continues to cool the liquid to below
the actual condensing temperature. The liquid may go
as much as 20°F below the condensing temperature of
125°F before it reaches the metering device. Any liquid
at a temperature below the condensing temperature of
125°F is called subcooled liquid. In this example, the
liquid is cooled to 105°F before it reaches the metering
device. The liquid now has 20°F (125°F 2 105°F) of
12. The liquid refrigerant reaches the metering device
through a pipe, usually copper, from the condenser. This
liquid line is often field installed and not insulated. Since
the temperature of the liquid line is warmer than the temperature of the surrounding air, keeping the liquid line
uninsulated allows some additional heat to be rejected
by the refrigerant into the surrounding air, which helps
increase the operating efficiency of the system. Since the
liquid line may be long, and depending on the distance
between the condenser and the metering device, the
amount of additional heat being rejected may be significant. Heat given up here is leaving the system, and that
is good. The refrigerant entering the metering device may
be as much as 20°F cooler than the condensing temperature of 125°F, so the liquid line entering the metering
device may be 105°F.
13. The refrigerant entering the metering device is 100%
subcooled liquid. In the short length of the metering
device's orifice (a pinhole about the size of a small sewing needle), the subcooled liquid is changed to a mixture of about 75% saturated liquid and 25% saturated
vapor. The ratio of liquid to vapor leaving the metering
device depends on both the system and the application.
The 25% vapor, known as flash gas, is used to cool the
remaining 75% of the liquid down to 40°F, the boiling
temperature of the evaporator. The cooling performed
by flash gas is a system loss because it cools the liquid
temperature down to the 40°F evaporating temperature. The cooling is therefore wasted because it is not
performed in the evaporator to lower the temperature
of the inside air and remove humidity. The only way to
minimize flash gas is to get the temperature of the subcooled liquid entering the metering device closer to the
The refrigerant has now completed one cycle and is
ready to go around again. It should be evident that a refrigerant does the same thing over and over, changing from a
liquid to a vapor in the evaporator and back to a liquid in
the condenser. The expansion device meters the flow to the
evaporator, and the compressor pumps the refrigerant out
of the evaporator.
The following list briefly summarizes the refrigeration
The evaporator absorbs heat into the system.
The compressor pumps the heat-laden vapor.
The condenser rejects heat from the system.
The expansion device meters the flow of refrigerant.
Previously, we have used water and R-22 as examples of
refrigerants. Although many products have the characteristics of a refrigerant, we will cover only a few here. Unit 9,
"Refrigerant and Oil Chemistry and Management-
Recovery, Recycling, Reclaiming, and Retrofitting," provides
more detailed information.
The following four refrigerants either can no longer
be manufactured or have phaseout dates in the near future:
R-12-Used primarily in medium- and high-temperature refrigeration applications. Manufacturing and
importing banned as of January 1, 1996.
R-22-Used primarily in residential, commercial, and
industrial air-conditioning applications and in some
commercial and industrial refrigeration. R-22 was
phased out in new equipment in 2010, and total production will be phased out in 2020.
R-500-Used primarily in older air-conditioning applications and some commercial refrigeration. Manufacturing and importing banned as of January 1,
R-502-Used primarily in low-temperature refrigeration
applications. Manufacturing and importing banned
as of January 1, 1996. It is an azeotropic refrigerant blend that has negligible temperature glide and
behaves like a pure compound.
The following are some of the more popular refrigerants, even
though some of them may be replaced in the near future with
lower GWP refrigerants. Note: At the time of this writing,
the Environmental Protection Agency (EPA) has proposed
rules for refrigerant substitutes. The rules involve a change
in the listing status for certain refrigerant substitutes under
the Significant New Alternatives Policy Program (SNAP).
Some of the refrigerants and refrigerant blends listed below
may be affected when this proposed rule becomes mandatory. Many refrigerants and refrigerant blends that have high
global warming potentials (GWPs) will no longer be allow
to be used even as retrofit refrigerants. These refrigerants
and blends will still be listed as follows because they exist in
HVACR equipment from past installations and retrofits. It
is important that service technicians understand what these
refrigerants and blends consist of for service purposes. It is