characteristic is used in refrigeration. The heat of the
air, of the lining of the refrigerator, and of the food to
be preserved is transferred to a colder substance, called
the refrigerant.
Three methods by which heat may be transferred
from a warmer substance to a colder substance are
conduction, convection, and radiation. These
principles are explained in chapter 4 of this TRAMAN.
SPECIFIC HEAT
SPECIFIC HEAT is the ratio between the quantity
of heat required to change the temperature of 1 pound
of any substance 1°F, as compared to the quantity of
heat required to change 1 pound of water 1°F. Specific
heat is equal to the number of Btu required to raise the
temperature of 1 pound of a substance 1oF. For
example, the specific heat of milk is .92, which means
that 92 Btu will be needed to raise 100 pounds of milk
1oF. The specific heat of water is 1, by adoption as a
standard, and specific heat of another substance (solid,
liquid, or gas) is determined experimentally by
comparing it to water. Specific heat also expresses the
heat-holding capacity of a substance compared to that
of water.
A key RULE to remember is that .5 Btu of heat is
required to raise 1 pound of ice 1oF when the
temperature is below 32°F; and .5 Btu of heat is
required to raise 1 pound of steam 1°F above the
temperature of 212°F.
SENSIBLE HEAT
Heat that is added to, or subtracted from, a
substance that changes its temperature but not its
physical state is called SENSIBLE HEAT. It is the
heat that can be indicated on a thermometer. This is the
heat human senses also can react to, at least within
certain ranges. For example, if a person put their finger
into a cup of water, the senses readily tell that person
whether it is cold, cool, tepid, hot, or very hot. Sensible
heat is applied to a solid, a liquid, or a gas/vapor as
indicated on a thermometer. The term sensible heat
does not apply to the process of conversion from one
physical state to another.
LATENT HEAT
LATENT HEAT, or hidden heat, is the term used
for the heat absorbed or given off by a substance while
it is changing its physical state. When this occurs, the
heat given off or absorbed does NOT cause a
temperature change in the substance. In other words,
sensible heat is the term for heat that affects the
temperature of things; latent heat is the term for heat
that affects the physical state of things.
To understand the concept of latent heat, you must
realize that many substances may exist as solids, as
liquids, or as gases, depending primarily upon the
temperatures and pressure to which they are subjected.
To change a solid to a liquid or a liquid to a gas, ADD
HEAT; to change a gas to a liquid or a liquid to a solid,
REMOVE HEAT. Suppose you take an uncovered pan
of cold water and put it over a burner. The sensible heat
of the water increases and so does the temperature. As
you continue adding heat to the water in the pan, the
temperature of the water continues to rise until it
reaches 212°F. What is happening? The water is now
absorbing its latent heat and is changing from a liquid
to a vapor. The heat required to change a liquid to a gas
(or, the heat that must be removed from a gas to
condense it to a liquid) without any change in
temperature is known as the LATENT HEAT OF
VAPORIZATION.
Now suppose you take another pan of cold water
and put it in a place where the temperature is below
32°F. The water gradually loses heat to its
surroundings, and the temperature of the water drops to
32°F until all the water has changed to ice. While the
water is changing to ice, however, it is still losing heat
to its surroundings. The heat that must be removed
from a substance to change it from a liquid to a solid
(or, the heat which must be added to a solid to change it
to a liquid) without change in temperature is called the
LATENT HEAT OF FUSION. Note the amount of
heat required to cause a change of state (or the amount
of heat given off when a substance changes its state)
varies according to the pressure under which the
process takes place. Figure 6-2 shows the relationship
between sensible heat and latent heat for one substance
– water at atmospheric pressure.
To raise the
temperature of 1 pound of ice from 0°F to 32°F, you
must add 16 Btu. To change the pound of ice at 32°F to
a pound of water at 32°F, you add 144 Btu (latent heat
of fusion). There is no change in temperature while the
ice is melting. After the ice is melted, however, the
temperature of the water is raised when more heat is
applied. When 180 Btu are added, the water boils. To
change a pound of water at 212°F to a pound of steam at
212°F, you must add 970 Btu (latent heat of
vaporization). After the water is converted to steam at
212°F, the application of additional heat causes a rise
in the temperature of the steam. When you add 44 Btu
6-3
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