Refrigeration & Air Conditioning Technology, 8e - 22

22

Section 1 Theory of Heat

Figure 1.12 The intensity of the radiant heat diminishes by the square
of the distance.

2 FT
1
4

THE AMOUNT OF
HEAT THAN WHEN
THE HAND IS HELD
AT 1 FT

1 FT

4 TIMES THE AMOUNT OF HEAT
WHEN THE HAND IS BROUGHT
TO 12 THE DISTANCE

1.7 SENSIBLE HEaT
Heat level or heat intensity can readily be measured when
it changes the temperature of a substance (remember the
example of changing 1 lb of water from 68°F to 69°F). This
change in the heat level can be measured with a thermometer.
When a change of temperature can be registered, we know
that the level of heat or heat intensity has changed; this is
called sensible heat.

1.8 LaTENT HEaT
Another type of heat is called latent or hidden heat. Heat
is added in this process, but no temperature rise occurs. An
example is heat added to water while it is boiling in an open
container. Once water is brought to the boiling point, adding more heat only makes it boil faster; it does not raise the
temperature, figure 1.13.
There are three other terms that are important to understand when referring to latent heat transfers: latent heat
of vaporization, latent heat of condensation, and latent
heat of fusion. Latent heat of vaporization is the amount of
heat energy, in Btu/lb, required to change a substance into a
vapor. For example, the latent heat of vaporization for water
at atmospheric conditions is 970.3 Btu/lb. For ease of calculation, we often round this value to 970 Btu/lb. This means
that if we have 1 lb of water at 212°F that we want to change
to 1 lb of steam at 212°F, we would need to add 970 Btu of
heat energy to the water.
Latent heat of condensation is the amount of heat
energy, in Btu/lb, required to change a vapor into a liquid.
This is, in effect, the opposite of latent heat of vaporization.
For example, the latent heat of condensation for steam at
atmospheric conditions is 970.3  Btu/lb. If we had 1  lb of
steam at 212°F that we wanted to change to 1 lb of water
at 212°F, we would need to remove 970 Btu of heat energy
from the steam.
Latent heat of fusion is the amount of heat energy needed
to change the state of a substance from a solid to a liquid or
from a liquid to a solid. The latent heat of fusion for water is
144 Btu/lb. If we had 1 lb of ice at 32°F under atmospheric
conditions, we would have to add 144 Btu of heat energy to
the ice in order to melt it. If, on the other hand, we had 1 lb of

Figure 1.13 Adding three times as much heat only causes the water to boil faster. The water does not increase in temperature.

2128F

2128F



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
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