The use of mechanical refrigeration fruit storage and preservation methods. In the insulation performance is good in the warehouse, the use of mechanical control of temperature, humidity and ventilation, in order to reduce the fruit life activities, to achieve the purpose of reducing rotten consumption, maintain fresh quality, extend the storage period. Although the history of human spontaneous use of low-temperature conditions to preserve food can be traced back to ancient times, such as China's "Poetry" on the "chiseling ice rushing, Na in the ling yin" records, the ancient Egyptian murals also have the use of ice storage instances. But the history of mechanical refrigeration began in the middle of the 18th century. 1755 Scottish Will Cullen (William Cullen) invented the first decompression of water evaporation refrigeration machine, but did not promote the application. The current widely used refrigeration machine is 1850 ~ 1875 years of successive invention of liquefiable gas compression refrigeration machine, absorption refrigeration machine, pre-compressed air expander and low-pressure water evaporation refrigeration machine.
Principle
In nature heat is always transferred from a hot object to a cold one. Cold storage is the use of artificial refrigeration will be the fruit of the heat released constantly transferred to the outside of the library, in order to stably maintain the low temperature state of the library. At present, the most widely used artificial refrigeration is the change of state refrigeration, that is, the use of low boiling point, low condensing point of the refrigerant, so that its vaporization to absorb a large amount of heat. General refrigerant for liquid ammonia, Freon 12 and 22. ammonia refrigerant boiling point -33.4 ° C, vaporization per kg of liquid ammonia absorbs heat 1367.28 Joule, high refrigeration efficiency, low price. However, it is irritating, harmful to human body when the concentration reaches 0.5%, more than 16% will cause an explosion, and water contact will corrode the metal; Freon 12 boiling point of -29.8 ℃, 167.43 joules per kilogram of heat absorption when vaporized, the latent heat of ammonia is lower than that of the refrigeration capacity of a small, but non-odorant, non-toxic, non-corrosive, non-combustion and explosion, is widely used. Freon 22 boiling point -40.80 ℃, high latent heat, refrigeration capacity, and has the advantages of Freon 12, but the price is higher, the application has certain limitations. Refrigerant in the closed mechanical refrigeration system, by evaporation vaporization, compression, condensation liquefaction and other processes, you can cycle back and forth (see chart). With the compressor and expansion valve as the boundary, this system can be divided into high-pressure area and low-pressure area. Liquid refrigerant in a high-pressure state through the expansion valve after entering the evaporator, the pressure decreases rapidly from liquid to vapor, and absorb the heat of the surrounding air, vaporized refrigerant is pumped back to the compressor, after the pressurized high-pressure vapour refrigerant is sent to the condenser, high-temperature, high-pressure vapour refrigerant in the condenser through the cooling of the external media (water, air) and into a high-pressure liquid back to the liquid storage device, and at the same time External media to take away its heat to the liquid refrigerant and then through the expansion valve into the evaporator to complete a cycle. So the cycle is repeated, the evaporator can be around the air (cold storage) in the heat transfer to the external environment (water or air), so that the air temperature inside the library down to a predetermined low temperature. The cooling capacity of the refrigeration system is expressed in terms of the amount of heat taken away by the cooling medium per hour, i.e.: kcal/hour.
Refrigeration equipment
The refrigeration system in the cold storage warehouse includes evaporator, compressor, condenser and the required fans and meters. Various equipment must be reasonably matched to meet the needs of the cold storage.
Evaporator (or cooler)
Mainly includes coils (basic surface) or coils with fins (extended surface) and fans. The main role is to cool the air, the fan is forced to air flow through the cooling coil injected into the temperature adjustment space. In order to match the needs of the best storage conditions for fruit, it can be equipped with humidification, heating, defrosting and air purification devices. There are two kinds of evaporator: ceiling type and floor type. In the old-style fruit cold storage, most of them are installed in the library on the wall or ceiling of the simple coil, air circulation by gravity. The result is that the ground is cold, the top is hot, resulting in uneven temperature inside the library, not conducive to fruit storage. Modern evaporator commonly used forced air circulation method to improve the heat exchange efficiency, so that the temperature inside the library is uniform. Evaporator refrigeration surface has dry evaporation coil and add antifreeze liquid spray surface. The liquid with antifreeze is cooled through the evaporating coil and leaks in the bottom container, while the air in the storehouse is forced to flow through the leakage of the liquid surface and get cooled, this evaporator has the characteristics of high cooling efficiency, high air humidity and no defrosting. The temperature of the library can be adjusted through the expansion valve, control the flow of refrigerant into the evaporating coil to change the surface temperature of the evaporating coil.
In order to prevent refrigerant leakage in the evaporator or valve, the evaporator can not be placed directly in the library, but will be placed in the coil in the brine (20% NaCl or CaCl2 solution), and then cooled brine into the installation of the library of the cooling tube group, and constantly circulating to reduce the temperature of the library. This way of library temperature is more stable, but aggravate the compressor load and increase the consumption of electricity.
Condenser
The heat absorbed by the refrigerant from the library is exchanged in the condenser to the external environment medium (water or air) to go by the high-pressure, high-temperature gaseous state into a high-pressure, low-temperature liquid state. The condenser has water-cooled and air-cooled two forms.
Compressor
Compressor volume reciprocating (piston), rotary and spiral three. Reciprocating type is the most common. Either liquid ammonia or Freon 12 or 22 can be used as the refrigerant. Piston displacement volume and operating speed to determine the refrigeration capacity, refrigeration capacity to the number of kilocalories per hour or standard tons of calculation, 1 ton refrigeration capacity is equal to 3025 kcal/hour, equivalent to the heat required to melt 1 ton of ice.
Structure
Mechanical refrigeration must have a good body structure. The key components of the library body is a good insulation layer and vapor barrier, directly affecting the refrigeration efficiency, mechanical equipment and library life and library management costs.
Heat insulation
With thermal insulation materials to reduce the heat source into the library of a heat barrier. Thermal insulation materials, thermal performance expressed in the thermal conductivity (1 square meter, 1 meter thick, inside and outside the temperature difference of 1 ℃, the number of kilocalories of heat conduction per hour). The inverse of the thermal conductivity of the thermal resistance, the smaller the coefficient of thermal conductivity, the greater the thermal resistance, indicating that the better its thermal insulation performance. Common thermal insulation materials and thermal insulation performance as Table 1.
Table 1 of the library body of the thermal insulation layer has a board, spraying, felt pads, filler and other types of spraying polyurethane (polyurethane) thermal insulation performance is the best. Most of the modern assembly cold storage is sandwich panel heat insulation layer, which consists of two layers of metal panels intervened between the foam polyurethane panels, or polystyrene foam is composed of. In addition, the inner surface of the sandwich panel insulation layer can also be used plywood.
Vapor barrier
In the outer measurement of the vapor barrier, the vapor barrier should not be ruptured, in the doorway, corners, walls and ceilings between the particular attention. High-temperature air saturated vapor pressure is higher than the low-temperature air, wet vapor from the high-pressure area to the low-pressure area. Therefore, the cold storage outside the hot air in the wet vapor is bound to pass through the heat insulation layer, to the cold storage of low-temperature air movement. When the wet steam in the heat insulation layer to move, it will encounter enough to make the wet steam condensation of the cooling zone, resulting in wet steam condensate gathered in the heat insulation layer, thus destroying the thermal insulation properties of the heat insulation layer, so that the thermal conductivity rises, to the library of the seepage of heat increase, so that the failure of the heat insulation layer. Therefore, the outer layer of the insulation layer must be equipped with a good vapor barrier, commonly used vapor barrier materials are aluminum foil, plastic film (≥ 6 microns), aluminum foil synthetic materials, asphalt and linoleum or asphalt and plastic film.
Cold storage management
Cold storage needs to be frequently adjusted storage temperature, humidity and ventilation.
Temperature control
Fruit cold storage temperature is too high can not be good to inhibit the biochemical changes in the fruit and decay; too low and will cause cold or frost damage to the fruit. Fruit cold storage, a representative temperature measurement point of the temperature should be strictly controlled in the predetermined temperature range of ± 0.55 ℃. The temperature uniformity of the entire warehouse depends on the air supply method, the pile of goods and outdoor heat source leakage and other factors. The main factors affecting the stability of the temperature of the cold storage of fruit are: ① the temperature difference between the temperature of the fruit into the warehouse and the temperature of the warehouse is smaller, the more favorable to the stability of the temperature of the warehouse, cooling to the optimal storage temperature of the speed of the faster. Therefore, it should be from the picking time, transportation and pre-cooling measures to reduce the temperature difference; ② cold storage volume, storage capacity and refrigeration efficiency. Usually the design of the daily storage volume shall not exceed 1/10 of the storage capacity, otherwise slow cooling, large fluctuations in storage temperature. Harvest season should pay attention to improve the refrigeration efficiency of the evaporator, by increasing the evaporative area per unit volume of cold storage and will be several times the evaporator evaporation of refrigerant for forced circulation and other methods to improve the refrigeration efficiency; ③ fruit respiratory heat of the size of the direct impact of the library temperature. In different packaging and different stacking methods under the effect of respiratory heat dispersal is very different, in order to timely dispersal of respiratory heat, the library should have a sufficient amount of air circulation, and blow directly to all parts of the library. Between different fruits, the same fruit in different temperature conditions, respiratory heat release differences (Table 2). Fruit containers should be stacked into ventilated stacks, so that each container has more surface exposure to the flow of air, in time to take away the heat released by the fruit.
Humidity control
Maintaining high relative humidity in the library is an important condition to reduce water loss of fruit. Should be based on a variety of fruit storage requirements of the relative humidity range to be adjusted. Make the evaporator temperature as close as possible to the indoor temperature, you can maintain a higher relative humidity indoor, or make the evaporator on the cold surface of the water condensation is reduced to a minimum. If the reservoir body insulation performance is poor, there is heat leakage, aggravate the burden of the evaporator, so that the evaporator surface appears condensation or frost, and defrosting will make the relative humidity inside the library is rapidly reduced. Modern jacketed cold storage insulation performance is good, the ceiling and four walls for the refrigeration surface, the relative humidity inside the library to maintain can be close to 100%. In the case of low relative humidity, the library can be placed in the humidifier, that is, a very fine water droplets to increase the humidity in the library.
Ventilation
Fruit storage process in the library will release a large amount of carbon dioxide and ethylene, is not conducive to the storage of fruit, in order to prevent corrosion and short-term treatment of certain gases, such as sulfur dioxide, etc. should not be in the library for a long period of time, they should be discharged out of the library in a timely manner, for fresh air. Ventilation should be considered outside temperature, humidity and reduce the load of the freezer. The outside world air from the position in front of the evaporator fan into the library, both in a timely manner to replace the fresh air, but also conducive to increasing the humidity in the library. If the amount of incoming air is certain, the temperature inside the library can be smaller changes. Or the use of gas washing machine, water or spray brine to absorb carbon dioxide, sulfur dioxide and other gases.
Fruit cold storage conditions and control
In a certain temperature range, the respiratory heat released by the fruit will be enhanced with the increase in temperature (Table 3). Due to the variety of fruit, physiological and biochemical characteristics are different, or very different, and thus the requirements for storage conditions are also different. About the fruit optimal storage temperature, relative humidity, the highest freezing point, moisture content, specific heat and the possible period of cold storage and other storage conditions, such as Table 3. the recommended temperature in the table is the optimal fruit temperature for long-term storage. For short-term storage, the fruit temperature can be slightly higher. Water content and freezing point are approximate experimental values. Knowing the maximum freezing point can prevent frost damage to the fruit, and water content is the basis for calculating specific heat and latent heat of freezing. (Wang Wenhong Li Zhensan)
Table 2
Table 3
Table 3