Thermodynamics

314 | ThermodynamicsCoolairWarmairRefrigeratorCoilsCabinetFIGURE 6–57The condenser coils of a refrigeratormust be cleaned periodically, and theairflow passages must not be blockedto maintain high performance.Light bulb40 Wbelow recommended levels can increase the energy use by as much as25 percent.6. Avoid excessive ice build-up on the interior surfaces of the evaporator.The ice layer on the surface acts as insulation and slows down heattransfer from the freezer section to the refrigerant. The refrigeratorshould be defrosted by manually turning off the temperature controlswitch when the ice thickness exceeds a few millimeters.Defrosting is done automatically in no-frost refrigerators bysupplying heat to the evaporator by a 300-W to 1000-W resistanceheater or by hot refrigerant gas, periodically for short periods. Thewater is then drained to a pan outside where it is evaporated using theheat dissipated by the condenser. The no-frost evaporators are basicallyfinned tubes subjected to air flow circulated by a fan. Practically all thefrost collects on fins, which are the coldest surfaces, leaving theexposed surfaces of the freezer section and the frozen food frost-free.7. Use the power-saver switch that controls the heating coils and preventscondensation on the outside surfaces in humid environments. The lowwattageheaters are used to raise the temperature of the outer surfacesof the refrigerator at critical locations above the dew point in order toavoid water droplets forming on the surfaces and sliding down.Condensation is most likely to occur in summer in hot and humidclimates in homes without air-conditioning. The moisture formation onthe surfaces is undesirable since it may cause the painted finish of theouter surface to deteriorate and it may wet the kitchen floor. About10 percent of the total energy consumed by the refrigerator can besaved by turning this heater off and keeping it off unless there is visiblecondensation on the outer surfaces.8. Do not block the air flow passages to and from the condenser coils ofthe refrigerator. The heat dissipated by the condenser to the air iscarried away by air that enters through the bottom and sides of therefrigerator and leaves through the top. Any blockage of this naturalconvection air circulation path by large objects such as several cerealboxes on top of the refrigerator will degrade the performance of thecondenser and thus the refrigerator (Fig. 6–57).These and other commonsense conservation measures will result in areduction in the energy and maintenance costs of a refrigerator as well as anextended trouble-free life of the device.EXAMPLE 6–8Malfunction of a Refrigerator Light SwitchFIGURE 6–58Schematic for Example 6–8.The interior lighting of refrigerators is provided by incandescent lamps whoseswitches are actuated by the opening of the refrigerator door. Consider arefrigerator whose 40-W lightbulb remains on continuously as a result of amalfunction of the switch (Fig. 6–58). If the refrigerator has a coefficient ofperformance of 1.3 and the cost of electricity is 8 cents per kWh, determinethe increase in the energy consumption of the refrigerator and its cost peryear if the switch is not fixed.