Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

below recommended levels can increase the energy use by as much as

25 percent.

6.Avoid excessive ice build-upon the interior surfaces of the evaporator.

The ice layer on the surface acts as insulation and slows down heat

transfer from the freezer section to the refrigerant. The refrigerator

should be defrosted by manually turning off the temperature control

switch when the ice thickness exceeds a few millimeters.

Defrosting is done automatically in no-frost refrigerators by

supplying heat to the evaporator by a 300-W to 1000-W resistance

heater or by hot refrigerant gas, periodically for short periods. The

water is then drained to a pan outside where it is evaporated using the

heat dissipated by the condenser. The no-frost evaporators are basically

finned tubes subjected to air flow circulated by a fan. Practically all the

frost collects on fins, which are the coldest surfaces, leaving the

exposed surfaces of the freezer section and the frozen food frost-free.

7.Use the power-saver switchthat controls the heating coils and prevents

condensation on the outside surfaces in humid environments. The low-

wattage heaters are used to raise the temperature of the outer surfaces

of the refrigerator at critical locations above the dew point in order to

avoid water droplets forming on the surfaces and sliding down.

Condensation is most likely to occur in summer in hot and humid

climates in homes without air-conditioning. The moisture formation on

the surfaces is undesirable since it may cause the painted finish of the

outer surface to deteriorate and it may wet the kitchen floor. About

10 percent of the total energy consumed by the refrigerator can be

saved by turning this heater off and keeping it off unless there is visible

condensation on the outer surfaces.

8.Do not block the air flow passagesto and from the condenser coils of

the refrigerator. The heat dissipated by the condenser to the air is

carried away by air that enters through the bottom and sides of the

refrigerator and leaves through the top. Any blockage of this natural

convection air circulation path by large objects such as several cereal

boxes on top of the refrigerator will degrade the performance of the

condenser and thus the refrigerator (Fig. 6–57).

These and other commonsense conservation measures will result in a

reduction in the energy and maintenance costs of a refrigerator as well as an

extended trouble-free life of the device.

314 | Thermodynamics

Cabinet

Refrigerator

Warm

air

Cool

air

Coils

FIGURE 6–57

The condenser coils of a refrigerator
must be cleaned periodically, and the
airflow passages must not be blocked
to maintain high performance.

EXAMPLE 6–8 Malfunction of a Refrigerator Light Switch

The interior lighting of refrigerators is provided by incandescent lamps whose

switches are actuated by the opening of the refrigerator door. Consider a

refrigerator whose 40-W lightbulb remains on continuously as a result of a

malfunction of the switch (Fig. 6–58). If the refrigerator has a coefficient of

performance of 1.3 and the cost of electricity is 8 cents per kWh, determine

the increase in the energy consumption of the refrigerator and its cost per

year if the switch is not fixed.

Light bulb

40 W

FIGURE 6–58

Schematic for Example 6–8.