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Chapter 4 Heating Systems | Up
Utilitiesman Basic Volume 02 - Manual for electric, plumbing, water and other utilities | Next
Heat Transfer |
heat to warm a room. Another example tells us that IO
pounds of water at 80°F will melt more ice in a given
length of time than 1 pound of water at 100oF. The
former has more heat, but the latter has a higher
temperature. Temperature is the measurement of heat
intensity in degrees Fahrenheit or Celsius. Therefore,
temperature measurements can be made by using a
glass thermometer calibrated either in degrees
Fahrenheit or Celsius. The generally accepted way of
s t a t i n g m e a s u r e m e n t s o f t e m p e r a t u r e i n
English-speaking countries is in degrees Fahrenheit.
The thermometer measures the degree of sensible
heat of different bodies. The thermometer can make a
comparison only between the temperature of a body
and some definitely known temperature such as the
melting point of ice or the boiling point of water.
Figure 4-1 shows a comparison of the scales of
Fahrenheit and Celsius thermometers. It also shows
the marking of the freezing and boiling points of pure
water at sea level. The range of the Fahrenheit
thermometer between the freezing point and the
boiling point is 180° (32° to 212° = 180°). On the
Celsius thermometer, the range is 100° (0° to 100° =
100°) from the freezing point to the boiling point.
Figure 4-1.—Comparison of Fahrenheit and Celsius
thermometers.
To convert Fahrenheit readings to Celsius:
(°F - 32°) ÷ 1.8 = °C
To convert Celsius readings to Fahrenheit:
(°C x 1.8) + 32° = °F
The heat that can be measured by a thermometer
and sensed or felt is referred to as "sensible heat." An
example of sensible heat is presented by placing a
small vessel of cold water over a gas flame and putting
a thermometer in the water. Upon observation, you
note that the thermometer indicates a rise in
temperature. Also, if you place your finger in the water
several times, you will feel (or sense) the change in
temperature that has taken place.
The unit of measurement for a given quantity of
heat is the British thermal unit, abbreviated and
commonly known as Btu. One Btu is the amount of
heat needed to change the temperature of 1 pound of
water 1° Fahrenheit at sea level. If one Btu is added to
1 pound at 50°F, the temperature of that pound of water
will be raised to 51°F.
All substances above absolute zero contain heat.
There is heat even in ice, and its melting point is fixed
at 32°F. Because of a fundamental law of nature, when
ice at 32°F melts into water at 32°F, a change of state
takes place. The ice (solid) has turned into water
(liquid). A certain amount of heat is required during
this change of state. This heat is known as latent heat.
Latent heat is the amount of heat required to change the
state of a substance without a measurable change in
temperature.
There are other types of heat that you will
encounter in heating. These are as follows:
Specific heat–The ratio between the quantity of
heat required to raise 1 pound of any substance
1oF and the amount of heat required to raise the
temperature of 1 pound of water 1oF.
Superheat–The amount of heat added to a
substance above its boiling point.
Total heat–Is the sum of sensible heat plus latent
heat.
We previously mentioned absolute zero. But, what
is absolute zero? Scientists have determined that when
the temperature of a substance has been reduced
to -460°F that all the heat has been removed from a
substance. At this point all the molecules cease to have
motion. Absolute zero is the lowest temperature
obtainable. Heat is present in all substances when the
temperature is above absolute zero.
4-2
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