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908 CHAPTER 60 flow of ammonia from the generator to the condenser are interposed an analyser and a rectifier. Some water vapor will be boiled out along with the ammonia vapor in the generator. The analyzer and rectifier serve to remove this water vapor from-the ammonia vapor. In the analyzer, the ammonia passes through strong solution which is on its way from the absorber to'the generator. This reduces the temperature of the. generated vapor somewhat to condense water vapor and tiie resulting-heating of the strong solution expels some am monia vapor without additional heat input. The ammonia vapor then passes through the rectifier where the residual small amount of water vapor is condensed by atmospheric cooling and,drains to the generator by way of the analyzer. _ The ammonia vapor,' which is still' warm, passes on to the condenser where it is liquefied by air cooling. The condenser is provided with fins for this purpose. The ammgjiw thus liquefied flows into the evaporator through a liquid trap. Hydrogen gas enters the upper-portion of the evaporator and flows downward in parallel-flow to the downward flowing liquid ammonia. The effect of the placing of a hydrogen at-, mosphere above the liquid ammonia in the evaporator is to reduce the partial pressure of the ammonia vapor in accord ance with Dalton's law of partial pressures. While the total or gage pressure in the evaporator and the pressure in the condenser are the same, there is substantially pure ammonia in the space where condensation is taking place, and conse-, quently the vapor pressure of tire ammonia substantially equals the total pressure. Under Dalton's law, the total pressure of a gas mixture is equal to the sum of the partial pressures of the individual gases. Consequently in the.evaporator the partial ammonia vapor pressure is less than the total pressure by the value of partial pressure of the hydrogen. The lesser urnmnni* vapor pressure results in evaporation of tire ammonia with absorp tion of neat from the surroundings of the evaporator and the cooling of the interior of tire refrigerator. The cool heavy (strong) gas mixture of hydrogen and ammonia vapor formed in the evaporator leaves the bottom of the evaporator and passes downward through the gas heat exchanger to the absorber. Since tire weight of a gas b proportional to its molecular weight and the molecular weight of ammonia b 17 and the molecular weight of hydrogen b 2, it follows that the specific weight of the strong gas b greater than that of the weak gas. Thb difference in specific weigbte b alone sufficient to initiate and maintain circulation between the evaporator-and the absorber. The gas heat exchanger transfers heat from one gas to the other. Thb saves some cooling in the evaporator by precooiing the entering gas. In the absorber, a flow of weak solution (water weak in ammonia) comes In direct contact with the strong gas. Am monia. b absorbed by water, and the hydrogen, which b practically insoluble, passes upward from the top of tire ab sorber through the gas heat exchanger into the evaporator. The liquid and gas flows are counter-current in the absorber. Since perfect separation of gases b not possible, some am monia vapor passes with the hydrogen from the absorber to the evaporator and determines the low temperature limit in tiie evaporator. From the absorber the strong solution flows through the liquid heat exchanger to the analyzer and thence to the strong liquid chamber of the generator. Heat applied to thb chamber causes vapor to pass upward through the analyzer and to the condenser. The solution passes through an aperture in the generator partition into the weak liquid chamber. Heat applied ,to tins chamber, causes, vapor and-hquid to pass upward through.the small diameter pipe, as in-an. air Ti/l/'to 1965 Guide And-Datb Book the separation vessel. Laberated ammonia .vapor passes' through the analyzer and thence to the condenser. -The weak solution from the separation vessel flows through the liquid heat exchanger and to the absorber. The liquid heat exchanger precools the liquid entering the absorber and preheats the liquid entering the generator. Further precooiing of the weak solution b obtained in the finned air-cooled loop between the liquid heat exchanger and the absorber. Actual unit construction includes additional refinements which increase efficiency or provide operational features but the cycle described above b the fundamental cycle of ab sorption refrigerators. PART II: HOUSEHOLD FREEZERS The functions of the household freezer are to provide a storage space.at a temperature of 0 For below and a fast freez ing zone where limited quantities of fresh food may be frozen in a reasonably short time with minimum dbturbance of the stored-load temperature. These functions should be performed with the following objectives in view: 1. Minimum temperature variations due to cycling of the re-' frigerating unit: 2. Well cabinet for economical power consumption. , 3. External surface temperatures in line with household re frigerator practice. 4. Ease erf access to all parts of refrigerated space. 5. Efficient, reliable and quiet refrigeration system. 6. Sturdy construction including adequate moisture vapor sealing. ' 7. Durable and attractive appearance with low odor levels ooih mdered in the choice of internal materials. CABINETS Modern chest type freezers are of all-steel construction with synthetic enameled exteriors. The exterior shell b stiffened at' the base by tiie bottom pan, which supports the insulation/ and a sub-base or angle iron frame. The top of the shell b strengthened by a rigid tie with'the interior compartment' Thb rigid tie may be in the form of a wood frame with a decorative breaker strip of rubber or plastic,. or plastic laminates may be used for both the decorative and struc tural functions. Baked enamel on steel has received prefer ence for the construction of interior compartments with aluminum also in general use. Steel and copper' tubing are both in general use for evaporator circuits. Interior com partments are generally supported by the bottom pan of the outer shell oh blocks of rigid insulation. Lid construction generally consists of an inwardly flanged sheet'steel exterior carrying a plastic laminate inner panel and a balloon-type rub ber gasket. Counterbalancing springs and a lid latch with pro visions for locking arc standard equipment. Upright freezers have followed the pattern of household refrigerators or the larger reach-in refrigerators. Interior locker doors may be used to reduce air spillage and to form convenient loading shelves when the rniin door b opened. The question .of air spillage from upright' cabinets has re ceived considerable attention. Laboratory tests comparing chest and,upright types indicate no. significant difference,'ih operating,costs"with" a reasonable number of door openings per day. When the opening frequency, b'high, as in a com mercial dispensing cabinet, the frost' accumulation in the upright type may become objectionable, but in household use thb objection has not been experienced. Since a household freezer may be operated in damp' loca tions such as a cellar or may be usedin a modern kitchen, it b important that exterior surface temperatures be kept high enough to avoid objectionable condensation., The surface .Household Refrigerators and Food Freezers 909 - adjacent to the breaker strips b usually the critical area' in thb respect. A minimum cross-eectional area of materials of low conductivity consistent with durability must be .used Air circulation across the surface of the breaker strips should-be retarded as much as possible. Thb should be ac complished by restriction, rather than by isolation of the air, as the latter method will lead to a troublesome frost condition Surface temperature depressions of 4 F deg below ambient to a 70 F room should be acceptable. To avoid condensation ih thb area a low-wattage insulated heater wire may be attached to the inside surface of the shell near the top or near the front in the case of-upright design. A similar heater may be. used to tiie door when necessary. Where condensing unit design permits, a portion of the condenser tubing may be attached to the inside surface of the shell to elevate these surface tem peratures. Center mullions in multiple lid cabinets are fre quently a source of condensation. It b advisable to provide for free air circulation under the cabinet to avoid condensation uiuler the bottom pan. Insulation thickness of 3 to 4 to. b used on the average, except in the lid, where considerably less insulation b usually provided. Insulation has been predominantly of the glass fiber and mineral wool types to batt form. The so-called high density form of these insulations which offers slightly better insulation than standard grades has been extensively used. Recent changes to manufacture of these* fibrous insulations have' produced finer fibers with more insulating value per dollar of cost. Silica aerogel, certain expanded plastics and special gases replacing the air in magi insulation offer inter esting possibilities for increasing the storage space by reducing 'tiie insulation volume with materials of exceptionally high insulating value. Since a substantia] portion of the heat leak- :age of a cabinet occurs through the gaskets, breaker strips and mechanical supports, it b difficult to realize completely tire gain to insulating effect indicated by the low conductivity factors of these materials. - A high resistance to moisture vapor penetration through the outer shell of a freezer cabinet b of the utmost importance for satisfactory, performance during the Tong operating life normally expected. While the accumulation of moisture' to-the insulation by the process of diffusion can be controlled-by mediocre sealing of the shell, the influx of mobtureTaden air due to the small changes to internal air pressure which occur with cycling of the refrigerating unit (and the less frequent pressure changes due to door openings) b independent of the area of the opening. In actual practice the pressure difference may be continually equalized by some venting arrangement such' as,the lid or door gasket, but the design must be such that the moisture entering such a vent will be deposited on a surface where it can be readily removed during defrosting. The process will work effectively only if the shell b-sealed so that its resistance to air flow b much greater than the resist ance of the venting means. Joints to the external shellare sealed with various forms of wax, tars, and plastisols. The effectiveness of sealing has been effectively checked by both air pressure and vacuum methods. The problem of locating the condensing unit has been treated to three ways. The sizes of cabinets generally have a shallow food compartment which allows the condensing unit to be located under the bottom insulation. Thb arrange ment offers a more accessible interior than the deeper com partments, but the ratio of floor area to usable volume b high. The lowest ratio of floor area to usable volume b offered by the design which locates the compressor under a step to the food compartment Thb arrangement, which may require more costly fabrication, offers a convenient fast-freezing zone if the bottom as well as the vertical surfaces of the shallow portion of'the'compartment b refrigerated. In the'larger cabinets the condensing unit may be contained to ah enclosure added to one end of the insulated cabinet Thb design usually results to waste volume above the condensing unit, and in some cases, this has .been arranged as a food'thawing zone or a storage compartment for wrapping materials.' Equipment for the convenience of the user include such items as: wire shelves, baskets and partitions, interior light, temperature indicator,-and warning devices, all of which may be factory inutulkd or sold as accessories. REFRIGERATING SYSTEMS -The evaporator in the majority of current chest type freez ers consists of tubing bonded to the exterior surface of the'food compartment. Generally, the tubing b placed only on the vertical walls, except where it b desired to form a fast-freezing surface by refrigerating a portion of the bottom or a shelf. A common form of thin type of evaporator uses copper or steel tubing which b held against the exterior of the compartment wall by various types of clips or clamps. A mastic with good thermal conduction or tar may be usedas a filleting material. Aluminum food compartments and aluminum tubing may be joined in arimtimate mechanical and thermal bond by the use ' of a special brazing process^ The expensive equipment required for thb process as well'as subsequent surface treatment of in' tenor surfaces' has limited its use. The use of aluminum evaporator tubing introduces a special problem where con nections are made to tubes of another material. Thb problem may be solved by the use of special pre-welded joints which are brazed in place by. the customary processes. Wide vari ations are found to the choice of tube spacing and length. The factors of cost and pressure drop in the evaporator tubing require a short tube length on necessarily large centers, whereas the factors of temperature drop from tube to wall and-in the wall between tubes call for a long tube lengttoand small center distances. Chest type freezer evaporators are also formed' from refrigerated plates mounted vertically, parallel to the food compartment walls, or as dividers between BPctiohs- These plates may be of tube and plate type, or the brazed double-wall type with, integral refrigerant passages^ Upright freezer evaporators usually consist of horizontal re frigerated plates which act as shelves. In some cases these shelves are of an open construction of expanded metal or wire bonded to the refrigerant tubing. Automatic defrosting freezers are now available to.which the evaporator surface b generally shielded from view within tiie refrigerated space and air circulation b accomplished by forced convection with a small fan. The evaporator surface is periodically defrosted by heat from hot refrigerant gas or direct electric heaters. The defrost period is normally only a few minutes and b automatically initiated by a timer switch; In the selection of condensing unitfor thb type of cooling it must be recognized that the heat load for a given freezer cabinet'will be raised to a considerable degree by adding s' forced convection as a'factor in the heat leak plus the fan motor heat if the motor b located within the refrigerated space. CONDENSING UNITS Experience in the household refrigerator industry has indi cated a preference for the use of hermetically sealed systems with capillary tube expansion device for freezers where re liability b of paramount importance. Ease of replacement should be considered. The low suction pressures involved to thb application re quire good compressor construction with high volumetric effi ciency. Compressor motor loads will be relatively light in