Chapter 6 Introduction to Refrigerants
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Entropy (Btu/lb°R)
Saturated
vapor line
Volume
(ft3/lb)
Temperature (°F)
Quality lines indicate
what percentage is vapor
Saturated
liquid line
Pressure
(psia)
Temperature
(°F)
Enthalpy
(Btu/lb)
Saturation
curve
Pressure-Enthalpy Diagram Values
Enthalpy (Btu/lb)
Pressure
(psia)
Goodheart-Willcox Publisher
Figure 6-12. This simplifi ed pressure-enthalpy diagram identifi es the values represented by each type of line in the diagram.
into vapor before it enters the compressor. The tem-
perature of the refrigerant in the evaporator remains
steady at 35°F as it changes phase. Once the refrigerant
has completely vaporized, which is indicated by the
intersection of Line C–D and the saturated vapor line,
it continues to absorb heat until it reaches the com-
pressor at a temperature of 68°F. This additional heat
added to raise the vapor’s temperature is referred to as
superheat.
In the compressor, between Points C and B, the
temperature increases from 68°F to 183°F, the pressure
increases from 45.1 psia (30.1 psig) to 213.6 psia (198.6
psig), and the enthalpy increases from 179 Btu/lb to
198 Btu/lb. This is an example of adiabatic compres-
sion. Because the vapor’s volume is decreased so rap-
idly in the compressor, the heat of compression is not
lost to surrounding materials, causing the refrigerant’s
pressure, temperature, and heat content to increase.
The refrigerant vapor then leaves the compressor and
enters the condenser at Point B. Between Points B and A,
the refrigerant loses heat to the air or water surround-
ing the condenser and changes from a vapor back into
a liquid. By the time it reaches the metering device at
Point A, the refrigerant has dropped in temperature
to 120°F and is completely liquid since it has crossed
the saturated liquid line. Upon entering the metering
device, the refrigerant’s pressure drops from 213.6 psia
(198.6 psig) to 45.1 psia (30.1 psig), and the cycle begins
once more.
Coeffi cient of Performance
Pressure-enthalpy tables and diagrams can be
used to calculate a refrigerant’s coefficient of per-
formance. Coefficient of performance (COP) is the
ratio of refrigeration effect to the heat of compression.
Refrigerants with higher coefficients of performance
are more efficient than refrigerants with lower coeffi-
cients of performance. By calculating the coefficient of
performance of different refrigerants that can be used
in a system, a technician can determine which refrig-
erant would be most effective, assuming other factors,
such as the size of the compressor, are equal.
Formula for Coeffi cient of Performance
COP =
To calculate coefficient of performance, start by
calculating the refrigeration effect. Using the pressure-
enthalpy diagram of R-134a in Figure 6-13 as an exam-
ple, subtract the heat of the refrigerant entering the
evaporator (116 Btu/lb) from the heat of the refrigerant
entering the compressor (179 Btu/lb) to get a refrigera-
tion effect of 63 Btu/lb.
Solution
Refrigeration effect = Compressor heat – evaporator heat
Refrigeration effect = 179 Btu/lb – 116 Btu/lb
Refrigeration effect = 63 Btu/lb
Refrigeration effect
Heat of compression