Refrigeration & Air Conditioning Technology, 8e - 45

Unit 3 Refrigeration and Refrigerants
Figure 3.14 The water is boiling at 40°F because the pressure is
0.122 psia, or 0.248 in. Hg. The room air is 75°F and gives up heat to the
40°F coil.
0.122 psia OR
0.248 in. Hg
COIL (408F)
ROOM AIR
(758F)

AIR (558F)
FAN

WATER

reasons that will be discussed later. We used it in this example
because most people are familiar with its characteristics.
To explore how a real refrigeration system works, we will
use refrigerant-22 (R-22) in the following examples because
it is commonly used in residential air-conditioning. See
Figure 3.15 for the temperature/pressure relationship chart for
several refrigerants, including R-22. This chart is like that for
water but uses different temperature and pressure levels. Take
a moment to become familiar with the chart; observe that
temperature is in the left column, expressed in °F, and pressure
is to the right, expressed in psig. Find 40°F in the left column,
read to the right, and notice that the gauge reading is 68.5 psig
for R-22. What does this mean in usable terms? It means that
when R-22 liquid is at a vapor pressure of 68.5 psig, it will
boil at a temperature of 40°F. When air is passed over the coil,
it will cool just as in the example with water.
The pressure and temperature of a refrigerant will correspond when both liquid and vapor are present under two
conditions:
1. When the change of state (boiling or condensing) is
occurring, and
2. When the refrigerant is at equilibrium (i.e., no heat is
added or removed).
In both conditions 1 and 2, the refrigerant is said to
be saturated. When a refrigerant is saturated, both liquid
and vapor can exist simultaneously. When they do, both
the liquid and the vapor will have the same saturation
temperature. NOTE: The same temperature for the liquid and
vapor is not true of some of the newer blended refrigerants that
have a temperature glide. Temperature glide will be covered
in Unit 9, "Refrigerant and Oil Chemistry and Management-
Recovery, Recycling, Reclaiming, and Retrofitting". ●✔✔ The saturation temperature depends on the pressure of the liquid/
vapor mixture, called the saturation pressure. The higher
the pressure, the higher the saturation temperature of the
liquid and vapor mixture. The lower the pressure, the lower
the saturation temperature.
Suppose that a cylinder of R-22 is allowed to sit in a room
until it reaches the room temperature of 75°F. It will then be

45

in equilibrium because no outside forces are acting on it. The
cylinder and its partial liquid, partial vapor contents will now
be at the room temperature of 75°F. The temperature and
pressure chart indicates a pressure of 132 psig, Figure 3.15.
The temperature/pressure chart is also referred to as a saturation chart because it contains saturation temperatures for
different saturation pressures.
Suppose that the same cylinder of R-22 is moved into a
walk-in cooler and allowed to reach the room temperature
of 35°F and attain equilibrium. The cylinder will then reach
a new pressure of 61.5 psig because while it is cooling off to
35°F, the vapor inside the cylinder is reacting to the cooling
effect by partially condensing; therefore, the pressure drops.
If we move the cylinder (now at 35°F) back into the
warmer room (75°F) and allow it to warm up, the liquid
inside it reacts to the warming effect by boiling slightly and
creating vapor. Thus, the pressure gradually increases to
132 psig, which corresponds to 75°F.
If we move the cylinder (now at 75°F) into a room at
100°F, the liquid again responds to the temperature change
by slightly boiling and creating more vapor. As the liquid boils
and makes vapor, the pressure steadily increases (according
to the temperature/pressure chart) until it corresponds to the
liquid temperature. This continues until the contents of the
cylinder reach the pressure, 196 psig, corresponding to 100°F,
Figure 3.16, Figure 3.17, and Figure 3.18.
The vapor that was generated because of the increase in
temperature is referred to as vapor pressure. Vapor pressure
is the pressure exerted on a saturated liquid. Any time saturated vapor and liquid are together, vapor pressure is generated. Vapor pressure acts equally in all directions, and this
action is what the pressure gauge reads on a refrigeration
or air-conditioning system. As the temperature of the liquid/
vapor mixture increases, vapor pressure increases. As the
temperature of the liquid/vapor mixture decreases, vapor
pressure decreases.
Further study of the temperature/pressure chart shows
that when the pressure is lowered to atmospheric pressure,
R-22 boils at about 241°F.
Do not perform the following
exercises-allowing refrigerant to intentionally escape into the
atmosphere is against the law! We mention these examples here
for illustration purposes only.
If the valve on the cylinder of R-22 were opened slowly
and the vapor allowed to escape into the atmosphere, the
loss of vapor pressure would cause the liquid remaining in
the cylinder to boil and drop in temperature. Whenever any
liquid boils, heat is absorbed in the process, which causes a
cooling effect. The heat in this case came from the R-22 liquid in the cylinder. Soon the pressure in the cylinder would
be down to atmospheric pressure, and the cylinder would
frost over and become 241°F. (We are assuming that the
vapor valve at the top of the R-22 cylinder is large enough
to allow the R-22 vapor to escape freely. This way, the
vapor will leave the cylinder at the same rate the R-22 liquid
is boiling in the cylinder.)



Refrigeration & Air Conditioning Technology, 8e

Table of Contents for the Digital Edition of Refrigeration & Air Conditioning Technology, 8e

Contents
Refrigeration & Air Conditioning Technology, 8e - Cover1
Refrigeration & Air Conditioning Technology, 8e - Cover2
Refrigeration & Air Conditioning Technology, 8e - i
Refrigeration & Air Conditioning Technology, 8e - ii
Refrigeration & Air Conditioning Technology, 8e - iii
Refrigeration & Air Conditioning Technology, 8e - Contents
Refrigeration & Air Conditioning Technology, 8e - v
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