WD Catalog 2018

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1-5 G - -
Figure 1-8
In this illustration a pound
of water changes to vapor
when 970 BTU’s of heat
energy are added to it. This
change in water is called a
“change of state”. Note
that the temperature in the
middle picture and the one
on the right remains the
same, 212 degrees.
Heat Measurement
30
-1
40
4
50
10
60
16
70
21
80
27
90
32
100
38
110
43
120
49
130
54
140
60
150
66
160
71
170
77
180
82
190
88
200
93
210
99
220
104
230
110
240
116
250
121
260
127
270
132
280
138
290
143
300
149
310
154
320
160
330
166
340
171
350
177
F°
C°
F°
C°
Fahrenheit/Celsius Conversion
Heat Quantity
Another measurement is heat quantity, or how much heat there is. British
thermal units or BTU’s are the accepted unit for measuring heat quantity.
For example, at sea level one BTU of heat energy raises the temperature
of one pound of water one degree Fahrenheit. If we keep adding BTU’s to
that pound of water, we will get to the boiling point of 212 degrees. At that
point the temperature will normally stop going up even if we continue to
add heat (BTU’s).
If you want the pound of water to change to steam (from a liquid to a vapor),
you have to add a lot of BTU’s. In fact you would add 970 BTU’s of heat energy
before the entire pound of water would change to steam. You would only add
180 BTU’s to take that pound of water from 32 degrees to the boiling point of
212. Look at the three kettle drawings in Figure 1-8 for a moment. They show
what happens to water when heat quantity is added. Review the drawings in
this illustration from left to right.
212°
212°
212° 212°
1 POUND
OF WATER
1
BTU
WATER
BOILS
970
BTU’S
VAPOR
(STEAM) + = + =
Here is the interesting part. When you add 970 BTU’s to change the water to
steam, the temperature stays at 212 degrees. All the BTU’s of heat energy went
into the steam. It took 970 BTU’s of energy to cause the water to change. If you
were to cool the steam back to water again, the 970 BTU’s of heat energy would
be given up to the air. The important thing about the example and illustrations
in Figure 1-8, is the large amount of heat it takes to change a substance like
water from one state to another. In fact, this is one of the reasons HVAC
systems are able to handle heat effectively. To put it another way, when you can
control a “change of state” you can move a lot of heat.
The heavy duty HVAC systems you will be working on are designed to move
heat energy quickly. Engine coolant and refrigerant are used to carry heat
energy into or out of the cab. Both of these liquids are good at absorbing and
giving off BTU’s of heat in the vehicle cab to make us comfortable.
Figure 1-7