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Chapter 6 Refrigeration | Up
Utilitiesman Basic Volume 02 - Manual for electric, plumbing, water and other utilities | Next
Specific Heat |
Figure 6-1.—The three states of matter.
effect on substances or bodies. When a body or
substance is stated to be cold, the heat that it contains is
less concentrated or less intense than the heat in some
warmer body or substance used for comparison.
UNITS OF HEAT
In the theory of heat, the speed of the molecules
indicates the temperature or intensity of heat, while the
number of molecules of a substance indicates the
quantity of heat.
The intensity and quantity of heat may be
explained in the following simple way. The water in a
quart jar and in a 10-gallon container may have the
same intensity or temperature, but the quantity of heat
required to raise these amounts of water to a higher
uniform temperature (from their present uniform
temperature) will differ greatly. The 10 gallons of
water will absorb a greater amount of heat than the
quart jar of water.
The amount of heat added to, or subtracted from, a
body can best be measured by the rise or fall in
temperature of a known weight of a substance. The
standard unit of heat measure is the amount of heat
necessary to raise the temperature of 1 pound of water
1°F at sea level when the water temperature is between
32°F and 212°F. Conversely, it is also the amount of
heat that must be extracted to lower by 1oF the
temperature of a pound of water between the same
temperature limits. This unit of heat is called a British
thermal unit (Btu). The Btu's equivalent in the metric
system is the calorie, which is the amount of heat
required to raise one gram of water 1o Celsius.
Suppose that the temperature of 2 pounds of water
was raised from 35°F to 165°F. To find the number of
Btu required to increase the temperature, subtract 35
from 165. This equals a 130° temperature rise for 1
pound of water. Since 2 pounds of water were heated,
multiply 130 by 2, which equals 260 Btu required to
raise 2 pounds of water from 35°F to 165°F.
MEASUREMENT OF HEAT
The usual means of measuring temperature is a
thermometer. It measures the degree or intensity of
heat and usually consists of a glass tube with a bulb at
the lower portion of the tube that contains mercury,
colored alcohol, or a volatile liquid. The nature of
these liquids causes them to rise or fall uniformly in the
hollow tube with each degree in temperature change.
Thermometers are used to calibrate the controls of
refrigeration.
The two most common thermometer
scales are the Fahrenheit and the Celsius.
On the Fahrenheit scale, there is a difference of
180° between freezing (32°) and the boiling point
(212°) of water. On the Celsius scale, you have only
100° difference between the same points (0° freezing
and 100° boiling point).
Of course, a Celsius reading can be converted to a
Fahrenheit reading, or vice versa. This can be
expressed in terms of the following formula:
F = (C x 1.8) + 32
To change Fahrenheit to a Celsius reading, the
terms of the formula are as follows:
C = (F-32) ÷ 1.8
TRANSFER OF HEAT
Heat flows from a substance of higher temperature
to bodies of lower temperature in the same manner that
water flows down a hill, and like water, it can be raised
again to a higher level so that it may repeat its cycle.
When two substances of different temperatures are
brought in contact with each other, the heat will
immediately flow from the warmer substance to the
colder substance.
The greater the difference in
temperature between the two substances, the faster the
heat flow. As the temperature of the substances tends
to equalize, the flow of heat slows and stops
completely when the temperatures are equalized. This
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