Refrigeration & Air Conditioning Technology, 8e - 36
Section 1 Theory of Heat
Figure 2.13 Electrical energy used in an electric motor is converted to
work to boost water pressure to force circulation.
WATER FLOW OUT
Force is expressed in pounds, and distance is expressed
in feet. For instance, when a 150-lb man climbs a flight of
stairs 100 ft high (about the height of a 10-story building), he
performs work. But how much? The amount of work in this
example is equivalent to the amount of work necessary to lift
this man the same height. We can calculate the work by using
the preceding formula.
Work 5 150 lb 3 100 ft
5 15,000 ft-lb
the fans, pumps, and compressors that ultimately move air,
water, and refrigerant. In Figure 2.13 an electric motor turns
a water pump to boost the water pressure from 20 to 60 psig.
This takes energy. The energy in this example is purchased
from the power company.
The preceding examples serve only as an introduction to
the concepts of chemical energy, heat energy, and electrical
energy. Each subject will be covered in detail later. For now, it
is important to realize that any system furnishing heating or
cooling uses energy.
2.11 PURCHASE OF ENERGY
Energy must be transferred from one owner to another and
accounted for. This energy is purchased as a fossil fuel or as
electric power. Energy purchased as a fossil fuel is normally
purchased by the unit. Natural gas is an example. Natural
gas flows through a meter that measures how many cubic feet
have passed during some time span, such as a month. Fuel
oil is normally sold by the gallon, coal by the ton. Electrical energy is sold by the kilowatt-hour or kWh. The amount
of heat each of these units contains is known, so a known
amount of heat is purchased. Natural gas, for instance, has a
heat content of about 1000 Btu/ft3; the heat content of fuel
oil #2 is about 139,000 Btu/gallon. There are numerous types
of coal available and the heat content can range from 14 million Btu/ton up to over 26 million Btu/ton.
Notice that no time frame has been considered. This example
can be accomplished by a healthy man in a few minutes. But
if the task were to be accomplished by a machine such as an
elevator, more information would be necessary. Do we want
to take seconds, minutes, or hours to do the job? The faster
the job is to be accomplished, the more power is required.
Power is the rate of doing work. An expression of power is
horsepower (hp). Many years ago it was determined that
an average horse could lift the equivalent of 33,000 lb to a
height of 1 ft in 1 min, which is the same as 33,000 ft-lb/min,
or 1 hp. This describes a rate of doing work because the time
factor has been added. Keep in mind that lifting 330 lb to a
height of 100 ft in 1 min or lifting 660 lb to a height of 50 ft
in 1 min will require the same amount of power. As a point
of reference, the blower motor in the average furnace can be
rated at 1/2 hp. See Figure 2.14 for a visual representation of
When the horsepower is compared with the man climbing
the stairs, the man would have to climb the 100 ft in less than
30 sec to equal 1 hp. That makes the task seem even harder.
A 1/2-hp motor could lift the man 100 ft in about 1 min if
only the man were lifted. The reason is that 15,000 ft-lb of
work is required. (Remember that 33,000 ft-lb of work in
1 min equals 1 hp.)
Figure 2.14 When a horse can lift 660 lb to a height of 50 ft in 1 min, it
has done the equivalent of 33,000 ft-lb of work in 1 min, or 1 hp.
2.12 ENERGY USED AS WORK
Energy purchased from electrical utilities is known as electric power. Power is the rate of doing work. Work can be
explained as a force moving an object in the direction of the
force; it is expressed in units called foot-pounds, or ft-lb. It is
expressed by this formula:
Work 5 Force 3 Distance