Refrigeration & Air Conditioning Technology, 8e - 21

Unit 1

21

Heat, Temperature, and Pressure

Figure  1.10 Air from the room enters the blower at 70°F. The blower
forces the air across the hot heat exchanger and out into the structure
at 130°F.

Figure 1.11 Natural convection occurs when heated air rises and cool
air takes its place.

FLUE
WINDOW
OUTSIDE (108F)

NATURAL
CONVECTION
WARM AIR OUT
COOL AIR IN

1308F SUPPLY
AIR TO
STRUCTURE

DETAIL
WARM AIR EXPANDS AND
RISES BECAUSE IT IS NOW
LIGHTER. THE RISING
EFFECT DRAWS COOLER
AIR INTO THE BOTTOM.

HEAT
EXCHANGER

GAS FLAME
FOR HEAT

HOT WATER
BASEBOARD
HEAT

DECORATIVE
METAL
PROTECTIVE
COVER
COOL AIR FROM FLOOR
INTO THE BOTTOM

BLOWER

708F RETURN
AIR FROM
ROOM

1.6 RaDIaTIoN
Heat transfer by radiation can best be explained by using
the sun as an example of the heat source. The sun is approximately 93 million miles from earth's surface, yet we can feel
its intensity. Heat transferred by radiation travels through
space without heating the space and is absorbed by the first

solid object that it encounters. Radiation is the only type
of heat transfer that can travel through a vacuum, such as
space, because it is not dependent on matter as a medium
of heat transfer. Convection and conduction require some
form of matter, like air or water, to be the transmitting
medium. The earth does not experience the total heat of
the sun because heat transferred by radiation diminishes
by the inverse of the square of the distance traveled. In
practical terms, this means that every time the distance is
doubled, the heat intensity decreases by a factor of 4. If you
hold your hand close to a light bulb, for example, you feel
the heat's intensity, but if you move your hand twice the
distance away, you feel only one-fourth of the heat intensity, figure 1.12. Keep in mind that, because of the inversesquare-of-the-distance rule, radiant heat does not transfer
the actual temperature or heat quantity value. If it did, the
earth would be as hot as the sun.
Electric heaters that glow red hot are practical examples
of radiant heat. The red-hot electric heater coil radiates heat
into the room. It does not heat the air, but it warms the solid
objects that the heat rays encounter. Any heater that glows
has the same effect.



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
Refrigeration & Air Conditioning Technology, 8e - Cover2
Refrigeration & Air Conditioning Technology, 8e - i
Refrigeration & Air Conditioning Technology, 8e - ii
Refrigeration & Air Conditioning Technology, 8e - iii
Refrigeration & Air Conditioning Technology, 8e - Contents
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