Refrigeration & Air Conditioning Technology, 8e - 20

20

Section 1 Theory of Heat

is being used to express a relationship to the normal expected
temperature for the time of year or to the inside temperature.
Cold has no numerical value and is used by most people as a
basis of comparison. Cold is sometimes referred to as a "level
of heat absence."

1.4 CoNDUCTIoN
Heat transfer by conduction can be explained as the energy
actually traveling from one molecule to another. As a molecule moves faster, it causes the nearby molecules to do the
same. For example, if one end of a copper rod is placed in a
flame, in a short time the other end will get too hot to handle.
The heat travels up the rod from molecule to molecule, figure 1.8. Heat transfer by conduction is used in many applications. For example, heat is transferred by conduction from
the hot electric burner on the cookstove to the pan or pot
resting on it.
Heat does not conduct at the same rate in all materials. Copper, for instance, conducts heat at a different rate
than iron. Glass is a very poor conductor of heat. Touching a
wooden fence post or another piece of wood on a cold morning does not give the same sensation as touching a car fender
or another piece of steel. The steel feels much colder. Actually,
the steel is not colder; it just conducts heat out of the hand
faster, figure 1.9.
The different rates at which various materials conduct
heat have an interesting similarity to the conduction of electricity. As a rule, substances that are poor conductors of heat
are also poor conductors of electricity. For instance, copper is
one of the best conductors of electricity and heat, and glass is
one of the poorest conductors of both. Glass is actually used
as an insulator in some electrical applications.

Figure  1.8 The copper rod is held in the flame only for a short time
before heat is felt at the far end.

MOLECULES OF COPPER
COPPER ROD

FLAME

Figure 1.9 The car fender and the fence post are actually the same temperature, but the fender feels colder because the metal conducts heat
away from the hand faster than the wooden fence post.

OUTSIDE
TEMPERATURE
(108F)

(108F)

1.5 CoNVECTIoN
Heat transfer by convection is used to move heat from one
location to another by means of currents set up in a fluid
medium. The most common fluid mediums in the heating
and air-conditioning trades are air and water. Many large
buildings have a central heating plant that heats water
and pumps it throughout the building to the spaces to be
heated. Notice the similarity of the words convection and
convey ("to carry from one place to another"). Convection
can be classified as either forced or natural.
A gas-fired furnace is an example of forced convection.
Room air is drawn into the return of the furnace by the
blower. It then passes over the furnace heat exchanger, which
exchanges heat to the air from a gas flame. The air is then
forced into the ductwork and distributed to the various rooms
in the structure. figure 1.10 shows this process in which 70°F
room air is entering the furnace; 130°F air is leaving; and the
blower is creating the pressure difference to force the air into
the various rooms. The blower provides the forced convection.
Another example of heat transfer by convection occurs
when heated air rises naturally. This is referred to as natural
convection. When air is heated, it expands, and the warmer
air becomes less dense or lighter than the surrounding
unheated air. This principle is applied in many ways in the
air-conditioning industry. Sections of baseboard heaters are
an example. When the air near the floor is heated, it expands
and rises. This heated air is replaced by cooler air around
the heater, which sets up a natural convection current in the
room, figure 1.11.



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
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Refrigeration & Air Conditioning Technology, 8e - Contents
Refrigeration & Air Conditioning Technology, 8e - v
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