1、17.1CHAPTER 17HOUSEHOLD REFRIGERATORS AND FREEZERSPrimary Functions 17.1Cabinets. 17.2Refrigerating Systems 17.4Performance and Evaluation 17.9Safety Requirements 17.11Durability and Service 17.12HIS chapter covers design and construction of householdTrefrigerators and freezers, the most common of w
2、hich are illus-trated in Figure 1.PRIMARY FUNCTIONSProviding optimized conditions for preserving stored food is theprimary function of a refrigerator or freezer. Typically, this is doneby storing food at reduced temperature. Ice making is an essentialsecondary function in some markets. A related pro
3、duct, the winecooler, provides optimum temperatures for storing wine, at temper-atures from 7 to 13C. Wine coolers are often manufactured by thesame companies using the same technologies as refrigerators andfreezers. Dual-use products combining a wine cooler and a refrig-erator and/or freezer have a
4、lso been manufactured.Food PreservationTo preserve fresh food, a general storage temperature between 0and 4C is desirable. Higher or lower temperatures or a humidatmosphere are more suitable for storing certain foods; the sectionon Cabinets discusses special-purpose storage compartmentsdesigned to p
5、rovide these conditions. Food freezers and combina-tion refrigerator-freezers for long-term storage are designed to holdtemperatures near 18 to 15C and always below 13C duringsteady-state operation. In single-door refrigerators, the frozen foodspace is usually warmer than this and is not intended fo
6、r long-termstorage. Optimum conditions for food preservation are detailed inChapters 19 to 24 and 28 to 42.Special-Purpose CompartmentsSpecial-purpose compartments provide a more suitable environ-ment for storing specific foods. For example, some refrigeratorshave a meat storage compartment that can
7、 maintain storage temper-atures just above freezing and may include independent temperatureadjustment. Some models have a special compartment for fish,which is maintained at approximately 1C. High-humidity com-partments for storing leafy vegetables and fresh fruit are found inpractically all refrige
8、rators. These drawers or bins, located in thefresh-food compartment, are generally tight-fitting to protect vul-nerable foods from the desiccating effects of dry air circulating inthe general storage compartment. The dew point of this airapproaches the temperature of the evaporator surface. Because
9、formany refrigerators the general food storage compartment is cooledwith air from the freezer, the air dew point is below 18C. Thedesired conditions are maintained in the special storage compart-ments and drawers by (1) enclosing them to prevent air exchangewith the general storage area and (2) surr
10、ounding them with cold airto maintain the desired temperature.Maintaining desired fresh-food temperatures while avoidingexchange with excessively dry air can also be achieved in a fresh-food storage compartment cooled with a dedicated evaporator.Higher humidity levels can be maintained in such a com
11、partmentbecause of the higher evaporating temperature, and also by allowingmoisture collected on the evaporator to be transferred back into theair by running the evaporator fan during the compressor off-cycle.The preparation of this chapter is assigned to TC 8.9, Residential Refriger-ators and Food
12、Freezers.Fig. 1 Common Configurations of Contemporary Household Refrigerators and FreezersFig. 1 Common Configurations of Contemporary Household Refrigerators and Freezers17.2 2010 ASHRAE HandbookRefrigeration (SI)Such fresh-food compartments have been configured as all-refrigerators, or have been i
13、ntegrated with freezers in refrigerator-freezers with two evaporators, using one compressor or separatecompressors.Some refrigerators have special-purpose compartments for rapidchilling, freezing, or thawing of food. Unlike rapid thawing in ambi-ent air or in a microwave oven, rapid thawing using re
14、frigerated airmaintains acceptable food preservation temperatures at the foodssurface layer. All of these functions require high levels of heat trans-fer at the surface of the food, which is provided by enhanced airflowdelivered by a special-purpose fan.New developments in food preservation technolo
15、gy address fac-tors other than temperature and humidity that also affect food stor-age life. These factors include modified atmosphere (reducedoxygen level, increased carbon dioxide level), removal of chemicalssuch as ethylene that accelerate food spoilage, and using ozone bothto neutralize ethylene
16、 and other chemicals and to control bacteriaand other microbes. Although these technologies are not yet avail-able or are uncommon in residential refrigerators, they representareas for future development and improvement in the primary func-tion of food preservation. Separate products using ozone gen
17、erationand ethylene absorption have been developed and can be placedinside the refrigerator to enhance food preservation.Ice and Water ServiceThrough a variety of manual or automatic means, most unitsother than all-refrigerators provide ice. For manual operation, icetrays are usually placed in the f
18、reezing compartment in a stream ofair that is substantially below 0C or placed in contact with adirectly refrigerated evaporator surface.Automatic Ice Makers. Automatic ice-making equipment inhousehold refrigerators is common in the United States. Almost allU.S. automatic defrost refrigerators eithe
19、r include factory-installedautomatic ice makers or can accept field-installable ice makers.The ice maker mechanism is located in the freezer section of therefrigerator and requires attachment to a water line. Freezing rate isprimarily a function of system design. Water is frozen by refriger-ated air
20、 passing over the ice mold. Because the ice maker must sharethe available refrigeration capacity with the freezer and fresh-foodcompartments, ice production is usually limited by design to 2 to3 kg per 24 h. A rate of about 2 kg per 24 h, coupled with an ice stor-age container capacity of 3 to 5 kg,
21、 is adequate for most users.Basic functions of an ice maker include the following:1. Initiating ejection of ice as soon as the water is frozen. The needfor ejection is commonly determined by sensing mold tempera-ture or by elapsed time.2. Ejecting ice from the mold. Several designs free ice from the
22、mold with an electric heater and push it from the tray into an icestorage container. In other designs, water frozen in a plastic trayis ejected through twisting and rotation of the tray.3. Driving the ice maker is done in most designs by a gear motor,which operates the ice ejection mechanism and may
23、 also be usedto time the freezing cycle and the water-filling cycle and to oper-ate the stopping means.4. Filling the ice mold with a constant volume of water, regardless ofthe variation in line water pressure, is necessary to ensure uniform-sized ice cubes and prevent overfilling. This is done by t
24、iming asolenoid flow control valve or by using a solenoid-operated,fixed-volume slug valve.5. Stopping ice production is necessary when the ice storage con-tainer is full. This is accomplished by using a feeler-type icelevel control or a weight control.Many refrigerators include ice and water dispen
25、sers, generallymounted in one of the doors. Ice is fed to the dispenser dischargewith an auger that pushes ice in the storage bucket to the dispenserchute. Many of these units also can crush the ice prior to dispensingit. A self-closing flap is used to seal the opening when the dispenseris not in us
26、e. Water is chilled in the fresh-food compartment in a res-ervoir. Solenoid valves control flow of water to the dispenser.CABINETSGood cabinet design achieves the optimum balance ofMaximum food storage volume for floor area occupied by cabinetMaximum utility, performance, convenience, and reliabilit
27、yMinimum heat gainMinimum cost to consumerUse of SpaceThe fundamental factors in cabinet design are usable food stor-age capacity and external dimensions. Food storage volume hasincreased considerably without a corresponding increase in externalcabinet dimensions, by using thinner but more effective
28、 insulationand reducing the space occupied by the compressor and condensingunit.Methods of computing storage volume and shelf area are de-scribed in various countries standards e.g., Association of HomeAppliance Manufacturers (AHAM) Standard HRF-1 for the UnitedStates.Thermal LoadsThe total heat loa
29、d imposed on the refrigerating system comesfrom both external and internal heat sources. Relative values of thebasic or predictable components of the heat load (those independentof use) are shown in Figure 2. External heaters are used to controlmoisture condensation on cool external surfaces. The do
30、or gasketregions thermal loss includes conduction of heat through the gasketand through the cabinet and door portions of this region, as well assome infiltration. A large portion of the peak heat load may resultfrom door openings, food loading, and ice making, which are vari-able and unpredictable q
31、uantities dependent on customer use. Asthe beginning point for the thermal design of the cabinet, the signif-icant portions of the heat load are normally calculated and then con-firmed by test. The largest predictable heat load is heat passingthrough the cabinet walls.Fig. 2 Cabinet Cross Section Sh
32、owing TypicalContributions to Total Basic Heat LoadFig. 2 Cabinet Cross Section Showing Typical Contributions to Total Basic Heat LoadHousehold Refrigerators and Freezers 17.3InsulationPolyurethane foam insulation has been used in refrigerator-freezer applications for over 40 years, originally using
33、 CFC-11 anozone-depleting substance (ODS) as the blowing agent. Because ofthis ozone damage, the Montreal Protocol began curtailing its use in1994. Most U.S. manufacturers of refrigerators and freezers thenconverted to HCFC-141b as an interim blowing agent; those inmany other parts of the world move
34、d straight to cyclopentane. Useof HCFC-141b was phased out in 2003 in the United States, and inmost of the world. The three widely used blowing agents currentlyin use areCyclopentane, which has the lowest foam material cost, requireshigh capital cost for safety in foam process equipment, increasesre
35、frigerator energy use by about 4% compared to HCFC-141b,and can be difficult and expensive to implement in locations withvery tight volatile organic compound restrictions.HFC-134a, which has the next lowest foam material cost,requires high-pressure-rated metering and mixing equipment, andincreases r
36、efrigerator energy use by 8 to 10% compared to HCFC-141b. HFC-245fa, which has the highest foam material cost, increasesrefrigerator energy use by 0 to 2% compared to HCFC-141b,requires some revision to existing foam equipment, and retainsinsulating characteristics best over time.Recently, flat vacu
37、um-insulated panels (VIPs) have beendeveloped (Figure 3) to provide highly effective insulation valuesdown to 0.004 W/(mK). A vacuum-insulated panel consists of alow-thermal-conductance fill and an impermeable skin. Fine min-eral powders such as silicas, fiberglass, open-cell foam, and silicaaerogel
38、 have all been used as fillers. The fill has sufficient compres-sive strength to support atmospheric pressure and can act as a radi-ation barrier. The skin must be highly impermeable, to maintain thenecessary vacuum level over a long period of time. Getter materialsare sometimes included to absorb s
39、mall amounts of cumulativevapor leakage. The barrier skin provides a heat conduction pathfrom the warm to the cool side of the panel, commonly referred toas the edge effect, which must be minimized if a high overall insu-lation value is to be maintained. Metalized plastic films are suffi-ciently imp
40、ermeable while causing minimal edge effect. They havea finite permeability, so air gradually diffuses into the panel, degrad-ing performance over time and limiting the useful life. There is alsoa risk of puncture and immediate loss of vacuum. Depending onhow the vacuum panel is applied, the drastic
41、reduction in insulationvalue from loss of vacuum may result in condensation on the outsidewall of the cabinet, in addition to reduced energy efficiency. In com-mercial practice, vacuum-panel insulation is one of the least cost-effective options for improving efficiency, but, where thicker wallscanno
42、t be tolerated, they are a useful option for reaching specifiedminimum efficiency levels.External condensation of water vapor can be avoided by keep-ing exterior surfaces warmer than the ambient dew point. Conden-sation is most likely to occur around the hardware, on doormullions, along the edge of
43、door openings, and on any cold refrig-erant tubing that may be exposed outside the cabinet. In a 32Croom, no external surface temperature on the cabinet should bemore than 3 K below the room temperature. If it is necessary to raisethe exterior surface temperature to avoid sweating, this can be donee
44、ither by routing a loop of condenser tubing under the front flange ofthe cabinet outer shell or by locating low-wattage wires or ribbonheaters behind the critical surfaces. Most refrigerators that incorpo-rate electric heaters have power-saving electrical switches thatallow the user to deenergize th
45、e heaters when they are not needed.Some refrigerators with electric heaters use controls that adjustaverage heater wattage based on ambient conditions to provide nomore heat input than necessary.Temporary condensation on internal surfaces may occur withfrequent door openings, so the interior of the
46、general storage com-partment must be designed to avoid objectionable accumulation ordrippage.Figure 2 shows the design features of the throat section where thedoor meets the face of the cabinet. On products with metal liners,thermal breaker strips prevent metal-to-metal contact between innerand oute
47、r panels. Because the air gap between the breaker strip andthe door panel provides a low-resistance heat path to the door gas-ket, the clearance should be kept as small as possible and the breakerstrip as wide as practicable. When the inner liner is made of plasticrather than steel, there is no need
48、 for separate plastic breaker stripsbecause they are an integral part of the liner.Cabinet heat leakage can be reduced by using door gaskets withmore air cavities to reduce conduction or by using internal second-ary gaskets. Care must be taken not to exceed the maximum dooropening force as specified
49、 in safety standards; in the United States,this is specified in 16CFR1750.Structural supports, if necessary to support and position the foodcompartment liner from the outer shell of the cabinet, are usuallyconstructed of a combination of steel and plastics to provide ade-quate strength with maximum thermal insulation.Internal heat loads that must be overcome by the systems refrig-erating capacity are generated by periodic automatic defrosting, icemakers, lights, timers, fan motors used for air circulation, and heat-ers used to prevent undesirable internal cabinet swe
