Refrigerators

REFRIGERATORS, structures for cooling and storing perishable products. A refrigerator consists of: a) cooled premises or chambers where the products to be cooled are placed, b) a machine and apparatus room where cold is generated. In order for the preservative effect of artificial

- cold on perishable products to be constant in its dynamics, a continuous cold chain is necessary, which must include: 1) Refrigerators at the places of production of perishable products, 2) refrigeration devices for the transport of said products—refrigerator wagons and refrigerator ships, and 3) Refrigerators at the places of product consumption. With the presence of the above-mentioned continuous cold chain, perishable food products can be delivered from afar to places of consumption in a fresh state and stored for a long time without spoilage, which to a significant extent eliminates the possibility of spoiled products reaching the consumer, and consequently prevents a whole series of possible diseases. No less important is the significance of Refrigerators for food supply problems, since thanks to Refrigerators, it is possible to utilize very distant food bases for food supply. The material for building large Refrigerators is brick and reinforced concrete. This construction material does not possess sufficient ability to protect against the leakage of cold from the cooled premises; for this purpose, additional 'insulation' is applied. The best thermal insulator is cork, which can be applied either in the form of cork granules poured between two walls, or in the form of cork slabs pressed from cork granules, with various resinous substances serving as the binding material. Besides cork, insulating materials include 'shevelin' (sheets made of several layers of linen fibers with paper padding between the layers), 'morozin' (slabs of pressed hemp shives), wood shavings, straw, peat, slag, etc. The floors of refrigeration chambers are constructed over the insulation from concrete or cement, Mettlach tiles, or seamless mastic. Asphalt and wooden floors are undesirable for Refrigerators. The doors of refrigeration chambers are constructed of a wooden frame and sheathing, the space between which is also filled with insulating material with a lining of water- and air-impermeable paper. The doors must fit tightly to the frame, which is achieved by a rubber gasket and a special latch. The exterior painting of Refrigerators is done in light tones to reflect radiant heat. Inside the refrigeration chambers, the walls and ceiling are usually whitewashed with lime; sometimes panels of glazed tiles are installed in order to be able to wash the walls, especially in small Refrigerators, for example, in factory-kitchens or grocery stores. The roofs of Refrigerators are constructed of slate—fireproof and waterproof; tar paper and iron roofs are undesirable for refrigerators due to their strong heating from the sun's rays. Among the systems of refrigeration machines, thanks to the reliability and economy of operation, compression system machines have the widest distribution (Fig. 1). These machines of huge industrial centers can be

Figure 1. Diagram of an ammonia refrigeration installation of the compression system. They work with the help of volatile liquids, which either pass into a vaporous state or are again transformed from vapor into liquid. The transition of a liquid into vapor is accompanied by the removal of a large amount of heat from the cooled medium; this circumstance is used for cooling purposes. A refrigeration machine working by means of compressing the vapors of a volatile liquid has the following components: a) a compressor for compressing the vapors of the refrigerant; b) a condenser for transforming the compressed vapors of the refrigerant into a liquid state under the action of cooling water; c) a regulating valve for regulating the flow of the liquid refrigerant; d) a refrigerator-evaporator or cooler, to transform the refrigerant into vapor and obtain cooling from this (Fig. 2). Refrigeration chamber

Fig. 2. Diagram of internal chamber cooling by direct evaporation.

The cooling effect occurs in the evaporator coil when the liquid refrigerant evaporates there, taking the heat necessary for its vaporization from the surrounding medium—air or a non-freezing solution of table salt and other salts. Therefore, the evaporator is called a refrigerator-cooler. The evaporated refrigerant, after its use in the evaporator coil, is again sucked in by the compressor, compressed in it, condensed-liquefied in the condenser, and after 'throttling' in the regulating valve, again enters the evaporator in liquid form to produce the necessary cold during its evaporation. Compression machines thus form a closed system in which this circular process continuously occurs. In this case, the refrigerant is theoretically not consumed anywhere, but in practice, one has to reckon with its insignificant leakage through existing invisible openings in the system. Methods of cold transfer. Lowering the temperature of the air in the cooled room can be achieved either by cooling the air inside the chamber itself or by introducing it after preliminary cooling from the outside. Based on these features, internal cooling and external cooling are distinguished. Internal cooling, with the compression system of machines, is performed either by direct evaporation of the refrigerant in tubular coils or by the circulation of non-freezing cooled brine through them. Tubular coils are located under the ceiling or near the walls of the chamber so that natural air circulation is formed due to the difference in specific gravity. However, with this method, air circulation is weak, which affects the uniformity of the temperature in the chamber. External cooling is carried out by means of special air coolers, usually placed outside the cooled chambers. This system is called air or blower cooling, since in it, air from the chamber is blown by a fan through an air cooler, after passing through which it is forced back. Cooling by brine circulation—brine cooling—consists of transferring cold to the cooled room by means of brine, which must first be cooled in the evaporator due to the evaporation of the refrigerant in it (Fig. 3). With the usual content of salts (NaCl, CaCl₂, MgCl₂) in the solution, about 25% by weight, it is quite possible to lower its temperature to -20° and lower, and then transfer the obtained reserve of cold in the brine to the cooled room with the help of a pump. The brine, giving up its cold while passing through the coils, heats up by 2-3° and returns back to the evaporator for its repeated cooling. Refrigerators must be equipped with the necessary measuring instruments: thermometers, thermographs, hygrometers, psychrometers, anemometers, etc., which make it possible to conduct thorough and constant observation of storage conditions. The results of storing perishable products in Refrigerators are influenced by the following factors: a) type of product, b) its preparation and condition, c) air temperature in the chamber and its uniformity, d) air humidity in the chamber, e) air circulation, the amount of its exchange inside the chamber, f) ventilation, g) purity of the air in the chamber, h) keeping the chamber clean—sanitary condition of it, i) placement of the product in the chamber, j) storage period of the product, k) preparation of the product for release. A significant influence on the storability of the product is exerted by the moisture content in the product. The general rule is—lower moisture content in

Fig. 3. Diagram of internal chamber cooling by brine circulation.

product gives more assurance of its preservation. The state of the product's surface influences its preservation: the crust of chilled meat, the skin of fruits, the scales of fish, by representing a protective layer against the introduction of microbes, creates more favorable conditions for storing the product. Furthermore, the degree of the product's soundness is of great importance. Only a product that is well-prepared, fresh, and healthy can be stored for a long time and without significant changes in a refrigerator. To obtain the best preservation of a product, thermal preparation is also required, namely the lowering of its temperature—cooling or freezing; in this case, preliminary cooling should be performed immediately after its preparation (meat after slaughter and dressing, fish after the catch, fruits after picking from the tree, milk after milking, etc.) in order to prevent, from the very first moment, the development of processes leading to the spoilage of products. Products, depending on their type and kind and the storage period, are subjected to either cooling or freezing. By cooling is meant the lowering of the temperature of a product to the beginning of the freezing of its liquid parts; by freezing—cooling until its liquid parts transition into a solid state. Cooling and freezing can be performed in air and in liquids. Due to the low heat capacity of air, a significant amount of time is required for cooling and freezing products under these conditions. For cooling and freezing products in liquids, in water, or in brine, much less time is required due to the greater heat capacity and better thermal conductivity of liquids. Methods of rapid freezing are applied most often to fish, but can also be applied to meat. Among the methods of rapid freezing, the Ottesen method deserves attention, which consists of the product being immersed in a brine of NaCl at a temperature from -15° to -20°; the Zarotchenzeff method, consisting of irrigating the product with atomized brine of a low temperature. In this case, the product being frozen is either subjected to the direct action of the atomized brine (direct contact) or is first placed in an impermeable package. In the Khristodulo rapid freezer, meat placed in special boxes-containers is cooled by brine sprinkling. The disadvantages arising from the contact of brine with meat in contact methods are eliminated here by the containers. Recently, a method of rapid cooling of products by means of the evaporation of liquid carbon dioxide has been developed. In this case, products for cooling are either immersed in it or sprayed with it. The CO₂ vapors formed during the process are sucked out by the compressor of the refrigerating machine. The application of this method has not yet emerged from the experimental stage. Each type of product requires a specific temperature and air humidity in the chambers that is most favorable for it. For the limits of such optimal temperatures and degrees of humidity for various products, see Ice, Glacier. Fluctuations in temperature during the day are possible without special consequences for the produce within the range of 1° to 2° in the direction of an increase or decrease. The usual air humidity in the chambers fluctuates from 70% to 90% depending on the type of product and the storage temperature. The air exchange in the chamber, depending on the type of product, amounts to 4 to 20 volumes per hour, with fresh outside air being cooled and dried as it passes between the cooling surfaces and then introduced into the chambers. The purity of the air in the chambers is sometimes disturbed by the entry of products in a state of spoilage; this entails the formation of bad odors, which affects the quality of other products entering the chambers. For deodorization, besides the immediate removal of defective products and increased ventilation, ozonation of the air in the chambers is often used. Keeping the chambers clean. Cleanliness in the chambers and corridors of the refrigerator must be maintained by thorough and daily cleaning and periodically performed disinfection. During a general cleaning of the refrigerator, floors are cleaned with scrapers and stiff brushes and washed with strong lye. Walls, ceilings, and air ducts are washed with a 5% solution of ferrous sulfate, after which whitewashing is performed with a freshly prepared strong solution of slaked lime. Furthermore, it is necessary to conduct a constant fight against rodents. The loading of products into the chambers is performed depending on their type: in a suspended state, in bulk, in stacks, or in rows with spacers for air circulation. Chambers should be loaded only with homogeneous products. Regardless of the method of loading, products must not come into direct contact with the walls or the floor, in order to thereby prevent the contamination of goods and allow cold air to circulate. Loading standards for products per unit area, including aisles (about 10%), average from 400 to 800 kg/m² for storage. For cooling and freezing—from 150 to 200 kg/m², depending on the type of product and the height of the chamber. The duration of storage of products in a refrigerator is determined partly by the technical possibility of storage and partly by the state of the product upon its arrival. The approximate duration of storage of products at procurement refrigerators in production areas fluctuates from 7 to 15 days; for distribution refrigerators in consumption centers—from 1 to 5 months. The maximum periods of preservation of products in a refrigerator, given compliance with optimal cold storage conditions, depend on the type of product, fluctuating within the range from a few days for fresh berries to several years for frozen meat. Before the release of certain products from the refrigerator, preliminary preparation is performed. Thus, frozen meat, in order to obtain a product of satisfactory quality, must be subjected to thawing—defrosting—before its release; to eliminate the sweating of eggs, gradual warming of them over 1–2 days is necessary; to prevent the moistening of fruits when temperature conditions change, gradual warming of them is also required. All these operations take place in a special room equipped with heating devices, which is known as a defroster. The delivery of perishable food products from producing regions to consumption centers must take place under such conditions that from the moment of receiving the product until its transfer to distribution refrigerators or the consumer, it is constantly and without interruption under the influence of cold. By cold transport is meant not only the actual transportation of various perishable products but also those devices thanks to which the possibility of these refrigerated transports is ensured. Railway cold transport includes: special cars with ice-salt cooling, ice supply stations for them, and special pre-cooling stations for fruits and some other products. Cold transport is subdivided into: horse-drawn, railway, water, and air. For horse-drawn transport of frozen products, insulation of the body alone is sufficient, given that the reserve of cold in the products themselves is sufficient for a duration of travel not exceeding two days. If products that are not sufficiently cooled are transported, then to maintain the temperature, cooling by ice alone or ice with salt is used by means of installing special pockets. In automotive cold transport, it is possible to install machine cooling as well, given the availability of motive power. Railway cold transport for the carriage of perishable products has special cars, which must, regardless of fluctuations in the outside air temperature, maintain a constant temperature inside, determined by the type of product being transported. Cars of the indicated type form isothermal rolling stock (Fig. 4). Existing types

Figure 4. Isothermal car: 1—ice tanks; 2—hooks for hanging carcasses.

[Refrigerated] railcars can be subdivided into the following groups: a) ventilation cars without cooling devices, but with air blowing through grilles in the end walls from the movement of the train. They are intended almost exclusively for autumn transport of fruits, when the unevenness of temperature does not have a significant influence due to the relatively low temperature of the outside air; b) Ice-cooled cars with lattice pockets are cooled only by ice, therefore the temperature inside them cannot be lower than +2°. Cars of this type have found application mainly for the transport of Siberian butter, as well as eggs, milk, and other products that do not require low temperatures; c) Ice-cooled cars with tanks are cooled by a mixture of ice and salt; the temperature inside them can be brought down to -3°. Ice-cooled cars of this type are intended for the transport of those products which require low temperatures for their preservation, such as meat, fish, etc. Besides railway transport, the transport of products by water in special refrigerated steamships has also become widespread. In view of the existence of a vast extent of river waterways in the USSR, river refrigerated barges must have great importance in the transport of perishable products. Refrigerated barges have special refrigeration chambers, properly insulated and cooled by a mechanical refrigeration installation. In recent years, several marine refrigerated steamships intended for the export of perishable products have been built here in the USSR. Recently, the use of so-called dry ice, used both for storing products and for transport, has been entering refrigeration practice. Dry ice is solid CO₂. The material for obtaining solid carbonic acid, dry ice, is carbonic acid obtained from the combustion of coke in an excess of air, from fermentation processes, and from the calcination of limestone. The production of dry ice (solid carbonic acid) consists of the following processes: a) obtaining pure gaseous CO₂, b) liquefaction of CO₂, c) lowering the pressure of liquid carbonic acid with partial obtaining of solid CO₂ in the form of snow, d) pressing the snow to obtain dry ice. The obtained solid CO₂ in the form of snow is compressed under a pressure of 30-50 atm in hydraulic presses into dry ice. Dry ice, depending on the method of manufacture, has a specific gravity of 1.1 to 1.4. For 1 kg of carbonic acid, 0.3-0.4 kg of dry ice is obtained. Dry ice has a number of advantages compared to ordinary ice, namely: a) it has a temperature of about -80°, b) its refrigeration capacity is almost twice the capacity of normal ice, c) pure dry ice does not produce any liquid when melting, but evaporates directly (whence it received the name "dry ice"), d) carbon dioxide acts at a sufficient concentration as an antiseptic agent: it retards the development of microbes. Thanks to the indicated qualities, dry ice has become widely used in the refrigeration business. Dry ice also has a number of the following advantages over ordinary ice in refrigerated transport: 1) the temperature inside the ice-cooled car when cooled with dry ice is much lower and more uniform, 2) one charge of "dry ice" (about 1,300 kg) is quite sufficient to maintain a uniform temperature for 6-10 days, which reduces the time the cargo spends in transit. Dry ice should receive wide application in the refrigeration business. Its high cost strongly hinders its wide distribution, which is why it has not yet received wide distribution in Western Europe, but in the USA the production of dry ice is very widespread. In the USSR, the first dry ice plant was built in Fili (near Moscow). In view of the enormous advantages of dry ice, its wide application in the USSR is undoubted. For the storage of perishable products, refrigeration installations are equipped in: factory-kitchens, canteens, premises for the manufacture of "ice cream," meat-fish, dairy, fruit-vegetable, and other stores, houses.

Figure 5. Air-blowing cooling system.

and workers' apartments. Refrigeration installations in factory-kitchens are usually arranged according to the type of the above-mentioned procurement and distribution refrigerators with storage chambers equipped with pipe and air cooling (Fig. 5). Premises for the manufacture of "ice cream" [are equipped with] pipe cooling. In stores and canteens, partly mechanical and partly ice cooling is used (see Ice, Glacier). At workers' houses, ice-houses of the Canadian type are built. Apartments are served by ice cabinets.