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Modern Refrigeration and Air Conditioning
6.4.2 Pressure-Temperature Curves
A pressure-temperature curve shows how a refrig-
erant’s temperature and pressure both rise and fall in
relation to each other, Figure 6-9. These curves illus-
trate the combined gas law principle that temperature
and pressure are directly related. This means that as
one rises or falls, the other does the same. Note that
a pressure-temperature curve shows the traits of a
refrigerant under saturated conditions, meaning the
refrigerant’s temperature is equal to its boiling point at
the given pressure. As a result, a technician can use a
pressure-temperature curve to determine at what temper-
ature a refrigerant evaporates under any given pressure.
Pressure-temperature curves are used during
service to determine if a unit is operating at the
correct temperature or pressure, although a pressure-
temperature chart may be used as well, Figure 6-10. A
technician can use a gauge to measure the pressure in
an evaporator and then use a pressure-temperature
chart to find the temperature of the refrigerant inside
the component. For example, the vapor pressure inside
the evaporator of a system using R-134a is measured to
be 30.4 psig. Using the pressure-temperature chart in
Figure 6-10, a technician can determine that the tem-
perature of the refrigerant in the evaporator is 35°F.
The temperature listed in a pressure-temperature chart
is always the temperature of the refrigerant, not the
temperature of the tubing.
The temperature of the tubing is known as skin
temperature. In most HVACR service operations, it is
impractical to actually measure the refrigerant tem-
perature. This would require placing a probe inside
the refrigerant tubing to accurately measure the refrig-
erant temperature at various locations throughout the
system. By measuring the tubing temperature, the ser-
vice technician can use the following general guide-
lines to approximate the refrigerant temperature:
• The temperature of the refrigerant in the evaporator
is about 8°F to 12°F (4°C to 7°C) colder than the
evaporator when the compressor is running.
• The temperature of the refrigerant in the evaporator
is the same as the evaporator temperature when
the compressor is not running.
• The temperature of the refrigerant in an air-cooled
condenser is approximately 30°F to 35°F (17°C to
19°C) warmer than the ambient temperature.
• The temperature of the refrigerant in a water-
cooled condenser is approximately 20°F (11°C)
warmer than the water temperature at the drain
outlet.
• The temperature of the refrigerant in the condenser
will be about the same as that of the cooling
medium (air- or water-cooled) after the unit has
been shut off for 15 to 30 minutes.
6.4.3 Pressure-Enthalpy Tables
Pressure-enthalpy tables are useful resources that
help the service technician diagnose problems prior
to replacing components. By listing actual operat-
ing temperatures and pressures, a pressure-enthalpy
table, like a pressure-temperature chart, helps a tech-
nician determine if a unit is operating correctly. Most
manufacturers include a typical operating pressure-
enthalpy table for a unit. By taking a few temperature
and pressure measurements, a technician can compare
these measurements to the recommended cycle. This
will help narrow the troubleshooting of the unit to a
specific component of the system.
Adapted from ANSI/ASHRAE Standard 34–2010
Figure 6-7. Table showing the ASHRAE Standard 34 toxicity
and flammability classifications.
Toxicity and Flammability Ratings
Low Toxicity High Toxicity
No flammability
identified
A1 B1
Low flammability
A2L B2L
A2 B2
High flammability A3 B3
Adapted from ANSI/ASHRAE Standard 34–2010
Figure 6-8. Safety classifications for some popular
refrigerants. Note that most refrigerants have the lowest
toxicity and flammability ratings to help ensure technician
safety when working with these refrigerants.
ASHRAE Refrigerant Safety Classifications
Refrigerant Number Toxicity Flammability
R-11 A 1
R-12 A 1
R-22 A 1
R-123 B 1
R-134a A 1
R-401A A 1
R-404A A 1
R-406A
A 2
R-407C A 1
R-410A A 1
R-500 A 1
R-502 A 1
R-507A A 1
R-717 B 2L
R-744 A 1