Refrigeration & Air Conditioning Technology, 8e - 23

Unit 1

23

Heat, Temperature, and Pressure

water at 32°F under atmospheric conditions, we would have
to remove 144 Btu of heat energy from the water in order to
freeze it.
The following example describes the sensible heat and
latent heat characteristics of 1 lb of water at standard atmospheric pressure from 0°F through the temperature range to
above the boiling point. In the chart in figure  1.14 notice
that temperature is plotted on the left margin, and heat content is plotted along the bottom of the chart. The heat content scale on the bottom of the diagram indicates the amount
of heat added during the processes being described, not the
actual heat content of the substance. We can see that as heat
is added, the temperature will rise except during the latent- or
hidden-heat processes.
The following statements should help you to understand
the chart:
1. Water is in the form of ice at point 1 where the example
starts. Point 1 is not absolute zero. It is 0°F and is used as
a starting point. Since no heat has been added to the ice
at point 1, we can see that the heat content scale at that
point is zero.
2. Heat added from point 1 to point 2 is sensible heat.
This is a registered rise in temperature. Since it took
16  Btu of heat energy to raise the temperature of the
ice from 0°F to 32°F, we can conclude that it takes only
0.5 Btu of heat energy to raise the temperature of 1 lb
of ice 1°F.
3. When point 2 is reached, the ice is at its highest temperature of 32°F. This means that if more heat is added, it will

be latent heat and will start to melt the ice but not raise
its temperature. Adding 144 Btu of heat will change the
1 lb of ice to 1 lb of water. Removing any heat will cool
the ice below 32°F.
4. When point 3 is reached, the substance is now 100%
water. Adding more heat causes a rise in temperature.
(This is sensible heat.) Removal of any heat at point 3
results in some of the water changing back to ice. This
is described as removing latent heat because there is no
change in temperature.
5. Heat added from point 3 to point 4 is sensible heat; when
point 4 is reached, 180 Btu of heat will have been added
from point 3. The water has been heated from 32°F to
212°F, which is a 180°F rise. Referring back to the definition of the Btu, you will see that it takes 1 Btu of heat
energy to raise the temperature of 1 lb of water 1°F. So,
a 180°F temperature rise will require 180  Btu of heat
energy.
6. Point 4 represents the 100% saturated liquid point. The
water is saturated with heat to the point that the addition of any more heat will cause the water to boil and
start changing to a vapor (steam). If any heat is removed
from the water at this point, it will simply cool to a
temperature that is lower than 212°F. Adding 970 Btu
makes the 1 lb of liquid boil to point 5 and become a
vapor.
7. Point 5 represents the 100% saturated vapor point. The
water is now in the vapor state. Heat removed would be
latent heat and would change some of the vapor back to

Figure 1.14 The heat/temperature graph for 1 lb of water at atmospheric pressure explains how water responds to heat. An increase in sensible heat
causes a rise in temperature. An increase in latent heat causes a change of state, for example, from solid ice to liquid water.
DEGREES
8C
8F

SENSIBLE HEAT

121.1 250

970 Btu LATENT HEAT

100.0 212
93.3 200

4

65.5 150

144 Btu
LATENT HEAT

5

180 Btu SENSIBLE HEAT

37.8 100
16 Btu SENSIBLE HEAT
10.0
0.0

50
32

-17.8

0

1

0

2

3

16

160
100

200

340
300

1310
400

500

600
700
800
900
HEAT CONTENT (Btu)

1000

1100

1200

1300

1400



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
Refrigeration & Air Conditioning Technology, 8e - vi
Refrigeration & Air Conditioning Technology, 8e - vii
Refrigeration & Air Conditioning Technology, 8e - viii
Refrigeration & Air Conditioning Technology, 8e - ix
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