When both liquid and vapor are present in a closed container, the mixture is said
to be saturated. Under these conditions, the pressure in the container will be
equal throughout the container. This pressure is known as the vapor pressure.
Vapor pressure has a definite and unchangeable value at a specified temperature
for a pure refrigerant. (Refrigerants that are mixtures have a range, called a glide,
instead of a fixed pressure-temperature relationship.) For example, the vapor
pressure of R-22 at 70°F is 136.12 pounds per square inch absolute (psia). This
is the same as 121.42 pounds per square inch gauge (psig). The vapor pressure,
or the pressure of the saturated vapor and liquid, will increase as the temperature
is increased. The saturated vapor pressure is known for a wide range of
liquid/vapor combinations. As long as both liquid and vapor are present, this
pressure will exist. This is true if the R-22 is in a cylinder, as is the refrigerant
in Figure 3-4, or in a receiver, storage tank, coil, or elsewhere. The amounts of
liquid and vapor make no difference as long as both liquid and vapor are present.
A cylinder can be nearly full or nearly empty—the pressure will be the same. Figure 3-4
shows that the vapor pressure at a given temperature is the same regardless of the relative
amounts of liquid and vapor.