Refrigeration & Air Conditioning Technology, 8e - 24

24

Section 1 Theory of Heat

a liquid. This is called condensing the vapor. Any heat
added at point 5 is sensible heat; it raises the vapor temperature above the boiling point, which is called superheating. Any water vapor with a temperature above the
boiling point of 212°F is superheated vapor. The concept
of superheat will be important in future studies. Note
that in the vapor state it takes only 0.5 Btu to heat the
water vapor (steam) 1°F, the same as when water was in
the ice (solid) state. SAFETY PRECAUTION: When examining these principles in practice, be careful because the water
and steam are well above body temperature, and you could be
seriously burned.●●

1.9 SPECIfIC HEaT
We now realize that different substances respond differently to heat. When 1 Btu of heat energy is added to 1 lb of
water, it changes the temperature 1°F. This holds true only
for water; other heated substances have different values. For
instance, we noted that adding 0.5  Btu of heat energy to
either ice or steam (water vapor) caused a 1°F rise per pound
while in these states. Ice and steam heated, or increased their
temperatures, at twice the rate of water. Adding 1 Btu would
cause a 2°F rise. This difference in heat rise is known as
specific heat.
Specific heat is the amount of heat necessary to raise the
temperature of 1 lb of a substance 1°F. Every substance has
a different specific heat. Note that the specific heat of water
is 1  Btu/lb/°F. See figure  1.15 for the specific heat of some
other substances.

1.10 SIZING HEaTING EQUIPMENT
Specific heat is significant because the amount of heat required
to change the temperatures of different substances is used
to size equipment. Recall the example of the house and furnace earlier in this unit. The following example shows how
this would be applied in practice. A manufacturing company
may need to buy a piece of heating equipment to heat steel
before it can be machined. The steel may be stored outside in
the cold at 0°F and need preheating before machining. The
desired metal temperature for the machining is 70°F. How
much heat must be added to the steel if the plant wants to
machine 1000 lb/h?
The steel is coming into the plant at a fixed rate of
1000  lb/h, and heat has to be added at a steady rate to
stay ahead of production. figure 1.15 gives a specific heat
of 0.116 Btu/lb/°F for steel. This means that 0.116 Btu of
heat energy must be added to 1 lb of steel to raise its temperature 1°F.
Q 5 Weight 3 Specific Heat 3 Temperature Difference
where Q 5 quantity of heat needed. Substituting in the
formula, we get:
Q 5 1000 lb/h 3 0.116 Btu/lb/°F 3 70°F
Q 5 8120 Btu/h required to heat the steel for machining.
The previous example has some known values and an
unknown value to be found. The known information is used
to find the unknown value with the help of the formula. The
formula can be used when adding heat or removing heat and
is often used in heat-load calculations for sizing both heating
and cooling equipment.

Figure 1.15 The specific heat table shows how much heat is required to raise the temperature of several different substances 1°F.
SUBSTANCE

SPECIFIC HEAT

SUBSTANCE

SPECIFIC HEAT

Air
Aluminum

Btu/lb/ºF
0.24 (Average)
0.214

Ice
Iron

Btu/lb/ºF
0.504
0.129

Ammonia
Beef, lean

0.525
0.77

Kerosene
Lead

0.50
0.031

Beets
Building Brick
Concrete
Copper

0.90
0.20
0.156
0.092

Marble
Porcelain
Pork, fresh
Seawater

0.21
0.18
0.68
0.94

Cucumbers

0.97

Shrimp

0.83

Eggs
Fish

0.76
0.76

Spinach
Steam

0.94
0.5

Flour
Gold

0.38
0.0312

Steel
Water

0.116
1.00



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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Refrigeration & Air Conditioning Technology, 8e - i
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