Thermodynamics

312 | ThermodynamicsTABLE 6–1Typical operating efficiencies ofsome refrigeration systems for afreezer temperature of 18°C andambient temperature of 32°CType ofCoefficientrefrigerationofsystemperformanceVapor-compression 1.3Absorptionrefrigeration 0.4Thermoelectricrefrigeration 0.1efficient, and they have found limited use in some specialized applications(Table 6–1).A household refrigerator is designed to maintain the freezer section at18°C (0°F) and the refrigerator section at 3°C (37°F). Lower freezer temperaturesincrease energy consumption without improving the storage life offrozen foods significantly. Different temperatures for the storage of specificfoods can be maintained in the refrigerator section by using special-purposecompartments.Practically all full-size refrigerators have a large air-tight drawer for leafyvegetables and fresh fruits to seal in moisture and to protect them from thedrying effect of cool air circulating in the refrigerator. A covered egg compartmentin the lid extends the life of eggs by slowing down the moisture lossfrom the eggs. It is common for refrigerators to have a special warmer compartmentfor butter in the door to maintain butter at spreading temperature.The compartment also isolates butter and prevents it from absorbing odorsand tastes from other food items. Some upscale models have a temperaturecontrolledmeat compartment maintained at 0.5°C (31°F), which keepsmeat at the lowest safe temperature without freezing it, and thus extendingits storage life. The more expensive models come with an automatic icemakerlocated in the freezer section that is connected to the water line, aswell as automatic ice and chilled-water dispensers. A typical icemaker canproduce 2 to 3 kg of ice per day and store 3 to 5 kg of ice in a removable icestorage container.Household refrigerators consume from about 90 to 600 W of electricalenergy when running and are designed to perform satisfactorily in environmentsat up to 43°C (110°F). Refrigerators run intermittently, as you mayhave noticed, running about 30 percent of the time under normal use in ahouse at 25°C (77°F).For specified external dimensions, a refrigerator is desired to have maximumfood storage volume, minimum energy consumption, and the lowest possiblecost to the consumer. The total food storage volume has been increasedover the years without an increase in the external dimensions by using thinnerbut more effective insulation and minimizing the space occupied by the compressorand the condenser. Switching from the fiber-glass insulation (thermalconductivity k 0.032–0.040 W/m · °C) to expanded-in-place urethane foaminsulation (k 0.019 W/m · °C) made it possible to reduce the wall thicknessof the refrigerator by almost half, from about 90 to 48 mm for the freezer sectionand from about 70 to 40 mm for the refrigerator section. The rigidity andbonding action of the foam also provide additional structural support. However,the entire shell of the refrigerator must be carefully sealed to prevent anywater leakage or moisture migration into the insulation since moisturedegrades the effectiveness of insulation.The size of the compressor and the other components of a refrigerationsystem are determined on the basis of the anticipated heat load (or refrigerationload), which is the rate of heat flow into the refrigerator. The heat loadconsists of the predictable part, such as heat transfer through the walls anddoor gaskets of the refrigerator, fan motors, and defrost heaters (Fig. 6–55),and the unpredictable part, which depends on the user habits such as openingthe door, making ice, and loading the refrigerator. The amount of energy