Chapter 6 Introduction to Refrigerants
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The thermodynamic properties of a refrigerant
under saturated conditions can be shown numerically
in a pressure-enthalpy table. In addition to showing the
pressure and temperature of a refrigerant, a pressure-
enthalpy table shows the volume of one pound of
vapor and the density of the liquid refrigerant for any
given temperature. The enthalpy, or heat content, of the
refrigerant is also shown. An example of a pressure-
enthalpy table is given in Figure 6-11. By subtract-
ing the liquid heat content value from the vapor heat
content value, you can calculate latent heat. This is the
amount of heat that is required to change a substance
from one phase into the other. To use a pressure-
enthalpy table, find the temperature being investigated
in the left-hand column, and move across the columns
horizontally to find the pressure, volume, density, and
enthalpy at the given temperature.
6.4.4 Pressure-Enthalpy Diagrams
The thermodynamic properties of a refrigerant
that are numerically shown in a pressure-enthalpy
table can be visually graphed in a pressure-enthalpy
diagram. Although a pressure-enthalpy diagram
includes the same information contained in a pressure-
enthalpy table, it can be very confusing when you first
try to interpret the different types of lines and data
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0
–10
–20
–30
–40
–10 0 10 20 30 40 50 60 70 80 90 100 120 140 160 180
R-502
R-22
R-500
R-134a
R-12
Temperature
(°F)
Pressure (psig)
Refrigerant Pressure-Temperature Curves
Goodheart-Willcox Publisher
Figure 6-9. This curve demonstrates the principle of the combined gas law that states pressure and temperature both rise
and fall in relation to each other. It also shows the varying pressure characteristics of different refrigerants. Given the same
temperature, R-134a and R-12 operate at a lower pressure than R-22 or R-502.
R-134a Pressure-Temperature Chart
Temperature (°F) Pressure (psig)
–40 14.8*
–35 12.5*
–30 9.8*
–25 6.9*
–20 3.7*
–15 0.1*
–10 1.9
–5 4.1
0 6.5
5 9.1
10 11.9
15 15.0
20 18.4
25 22.1
30 26.1
35 30.4
40 35.0
*Pressures below atmospheric pressure are specified using in. Hg vacuum
Goodheart-Willcox Publisher
Figure 6-10. Like pressure-temperature curves, pressure-
temperature charts can be used to fi nd the temperature of
a refrigerant if the pressure is known. Typically, pressure-
temperature charts from manufacturers include data for more
than just one refrigerant.