Mechanical refrigeration of the hottest fruits and

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Mechanical refrigeration of fruits and vegetables

I. what is mechanical refrigeration

refrigeration is to transfer the heat in the warehouse to the outside with the help of the mechanical condensation system in the warehouse with good heat insulation performance, so as to reduce the temperature in the warehouse and keep it within the range conducive to the long-term storage of fruits and vegetables. The advantage of cold storage is that it is not affected by external environmental conditions and can maintain the temperature required by products all year round. The temperature, relative humidity and ventilation in the cold storage can be controlled and adjusted. However, the cold storage is a permanent building with high cost. Therefore, before building the cold storage, the selection of storage location and the design of the storage should be carefully considered. When the sample is stretched, the selection and installation of the condensation system and the capacity of the storage should be carefully considered. At the same time, the future development should also be noted. Although cold storage has many advantages, fruits and direct shear experiments are more. Fragile rocks, layout surfaces or fragile sandwich vegetables are living organisms, and the life of cold storage is still limited

II. Principle of mechanical refrigeration

fruits and vegetables have a lot of field heat and respiratory heat when they enter the cold storage. In addition, the heat leakage of the storage body, the field heat carried by the packaging box, and the heat load generated by lighting, machinery and personnel operation should be eliminated in order to maintain the low temperature in the cold storage. This process is completed through the change of the state of the refrigerant. The working principle of mechanical refrigeration is to use the heat absorption characteristics of the refrigerant when it changes from liquid state to gas state, so as to make it change each other in the closed refrigerator system, so as to reduce the temperature of fruits and vegetables in the warehouse and maintain the constant low temperature conditions, so as to delay the aging of fruits and vegetables, prolong the storage life and maintain the quality. At present, the approximate value of the elastic limit of materials without obvious linear segment in the apparent elastic limit stress-strain curve. The refrigerants used are ammonia and halogenated hydrocarbons or chlorofluorocarbons. Ammonia is a refrigerant commonly used in large-scale refrigeration equipment. It has low price, low boiling point temperature and large gasification latent heat. However, it is easy to damage human skin and mucous membrane when leaking. When containing water, it is easy to corrode metals. When ammonia is mixed with oil and air to a certain concentration, there is a risk of explosion and combustion, which should be avoided. Because of these shortcomings, freon has replaced most of the small and medium-sized refrigeration equipment in modern times. China is actively taking measures to eliminate R12, R22 and other commonly used refrigerants and carry out extensive research on CFCs substitutes

III. structure of cold storage

1. Heat insulation and damp proof course

in addition to good and firm warehouse frame buildings, cold storage shall also have heat insulation and damp proof course. The heat insulation layer can isolate the heat transfer inside and outside the cold storage and ensure the appropriate low temperature in the cold storage. The thermal insulation material should be small thermal conductivity, odorless, not easy to absorb moisture, light weight and low price. The designer shall design a reasonable thickness of thermal insulation layer according to the actual situation and specific conditions of the area where the cold storage is located, so as to ensure the effective and economic operation of the cold storage. The six sides of the cold storage are affected differently by the external temperature. If a roof is added above the thermal insulation layer on the top of the cold storage to form a buffer space, the thickness of the thermal insulation layer can be smaller; The wall surface exposed to the sun for a long time needs a larger thickness of the thermal insulation layer than the shaded wall. The ground temperature of cold storage buildings is also affected by the ground temperature, and the requirements for the thermal insulation layer can also be handled flexibly

damp proof course is another important part of the cold storage structure. In the absence of damp proof course, hot and cold air will meet in the insulation layer, and will condense into water droplets when reaching the dew point. After the insulation material is damp, the thermal insulation performance will be reduced. Generally, damp proof course can be added on both sides of the insulation layer, or only the outer damp proof course can be made. Commonly used asphalt, asphalt felt, plastic coating, plastic film or metal plate made of damp proof course, so that the service life of cold storage can be extended

2. Refrigeration system

the refrigeration system is the most important equipment in the cold storage, which is composed of evaporator, compressor, condenser, regulating valve, fan, duct and instrument. The refrigerant circulates in the sealing system, and the refrigerant supply and the number of times entering the evaporator are controlled according to the needs, so as to obtain the appropriate low temperature conditions in the cold storage. The size of the refrigeration system shall be selected according to the capacity of the cold storage and the required refrigeration capacity, that is, the evaporator, compressor and condenser shall match the heat to be removed from the cold storage to meet the cooling needs. The evaporator is installed in the cold storage, and the blower is used to blow the cooled air to all parts of the cold storage. The large cold storage often connects the evaporator with the air duct to extend the air supply distance, expand the distribution range of the cold air in the cold storage, and make the temperature drop more uniform. The compressor is the "heart" of the refrigeration system, which promotes the circulation of refrigerant in the system. Generally, the refrigeration capacity of the compressor in the medium-sized cold storage is about kcal/h. The designer will select it according to the specific conditions such as the capacity of the cold storage and the number of products. The function of the condenser is to remove the heat from the compressed gaseous refrigerant and condense it into liquid refrigerant. The condenser has air cooling, water cooling and air water combined cooling modes. Air cooling is only used in small cold storage equipment, while water-cooled condensers can be used in all forms of refrigeration systems. The refrigerating capacity of the refrigerating unit can be started or stopped by manual or automatic control system according to the monitoring of the temperature in the warehouse, so as to maintain the appropriate temperature for storing fruits and vegetables. At present, many refrigerators are equipped with microcomputer systems to monitor and record the temperature changes. When the refrigerant is vaporized in the evaporator, the temperature will reach below 0 ℃, contact with the wet air in the warehouse to make it saturated, and form ice on the outer wall of the evaporator. The ice layer is not conducive to heat conduction and affects the cooling effect. Therefore, in the cold storage management, the frost must be removed in time, that is, the so-called "frost flushing". Cold water can be used to spray the evaporator for frost flushing, or the heat absorbed refrigerant can be introduced into the outer coil of the evaporator for circulation to melt the frost

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