312 | Thermodynamics
efficient, and they have found limited use in some specialized applications
(Table 6–1).
A household refrigerator is designed to maintain the freezer section at
18 °C (0°F) and the refrigerator section at 3°C (37°F). Lower freezer tem-
peratures increase energy consumption without improving the storage life of
frozen foods significantly. Different temperatures for the storage of specific
foods can be maintained in the refrigerator section by using special-purpose
compartments.
Practically all full-size refrigerators have a large air-tightdrawer for leafy
vegetables and fresh fruits to seal in moisture and to protect them from the
drying effect of cool air circulating in the refrigerator. A covered egg com-
partmentin the lid extends the life of eggs by slowing down the moisture loss
from the eggs. It is common for refrigerators to have a special warmer com-
partment for butterin the door to maintain butter at spreading temperature.
The compartment also isolates butter and prevents it from absorbing odors
and tastesfrom other food items. Some upscale models have a temperature-
controlled meat compartment maintained at 0.5°C (31°F), which keeps
meat at the lowest safe temperature without freezing it, and thus extending
its storage life. The more expensive models come with an automatic ice-
makerlocated in the freezer section that is connected to the water line, as
well as automatic ice and chilled-water dispensers. A typical icemaker can
produce 2 to 3 kg of ice per day and store 3 to 5 kg of ice in a removable ice
storage container.
Household refrigerators consume from about 90 to 600 W of electrical
energy when running and are designed to perform satisfactorily in environ-
ments at up to 43°C (110°F). Refrigerators run intermittently, as you may
have noticed, running about 30 percent of the time under normal use in a
house at 25°C (77°F).
For specified external dimensions, a refrigerator is desired to have maxi-
mumfood storage volume,minimumenergy consumption, and the lowestpos-
sible cost to the consumer. The total food storage volume has been increased
over the years without an increase in the external dimensions by using thinner
but more effective insulation and minimizing the space occupied by the com-
pressor and the condenser. Switching from the fiber-glass insulation (thermal
conductivity k0.032–0.040 W/m · °C) to expanded-in-place urethane foam
insulation (k0.019 W/m · °C) made it possible to reduce the wall thickness
of the refrigerator by almost half, from about 90 to 48 mm for the freezer sec-
tion and from about 70 to 40 mm for the refrigerator section. The rigidity and
bonding action of the foam also provide additional structural support. How-
ever, the entire shell of the refrigerator must be carefully sealed to prevent any
water leakage or moisture migration into the insulation since moisture
degrades the effectiveness of insulation.
The size of the compressor and the other components of a refrigeration
system are determined on the basis of the anticipated heat load (or refrigera-
tion load), which is the rate of heat flow into the refrigerator. The heat load
consists of the predictable part, such as heat transfer through the walls and
door gaskets of the refrigerator, fan motors, and defrost heaters (Fig. 6–55),
and the unpredictable part, which depends on the user habits such as open-
ing the door, making ice, and loading the refrigerator. The amount of energyTABLE 6–1Typical operating efficiencies of
some refrigeration systems for a
freezer temperature of 18°C and
ambient temperature of 32°C
Type of Coefficient
refrigeration of
system performanceVapor-compression 1.3
Absorption
refrigeration 0.4
Thermoelectric
refrigeration 0.1