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After the harvest of fruits and vegetables, life is not over, there is still respiration, that is, to absorb oxygen, carbon dioxide and water. In the process of metabolism, a large amount of respiratory heat is produced, so that nutrients are continuously consumed and a large amount of water is lost. As a result, fruits and vegetables wither, change color and soften, and reduce vitamins, resulting in deterioration and loss of commodity value. The only way to weaken the respiratory intensity of fruits and vegetables is to quickly reduce the temperature of fruits and vegetables to reduce their respiratory heat. Low temperature can not only inhibit the growth and reproduction of microorganisms, but also inhibit the activity of enzymes in food, delay the change of chemical composition in fruits and vegetables, and maintain the nutritional value and color, aroma and taste quality of food. The quality of fruits and vegetables can be improved by proper treatment after picking. Vacuum pre-cooling is considered to be an effective method to slow down the ripening of fruits and vegetables after picking and to ensure food quality.

Features of vacuum pre-cooling:
① The cooling speed of fruits and vegetables is fast, generally only 20~30min, if the cold storage is used for cooling, it takes 10~12h, and only the fruits and vegetables harvested in the morning can be processed, while the vacuum pre-cooling is not limited by the collection time;
The treatment time is short, the water loss is small, and the fruits and vegetables will not be deformed locally;
Vacuum pre-cooled fruits and vegetables keep fresh for a long time, suitable for long-term storage and long-distance cold transportation.
   1. Vacuum precooling principle:The boiling point of water at atmospheric pressure is 100 ° C. If water is in a vacuum environment, its boiling point will decrease with the decrease of pressure. When the pressure is reduced to 2334.6Pa, the water at a temperature of 20 ° C can boil. If the pressure continues to decrease 608.2Pa, the boiling point of water is approximately 0°C. The basic principle of vacuum pre-cooling is to place fruits and vegetables in a vacuum container and vacuum. When the pressure reaches 2660~2000pa, the water on the surface of fruits and vegetables begins to evaporate, and the water evaporation increases with the pressure drop. During the evaporation process, the heat of vaporization required by the water is provided by the fruit and vegetable, which is itself cooled. Usually in about 20min can be cold to about 3 ℃. At this time, the water lost by fruits and vegetables only accounts for 1% ~ 3% of its quality, which does not affect its freshness.
The easier the water evaporates in the process of decompression, the higher the cooling effect. The rate at which water evaporates depends on the ratio of volume to surface area. It also has a certain relationship with wilt structure, tissue density, wax, etc. The speed at which water moves from the body to the surface also affects the evaporation of water. Therefore, the speed of vacuum pre-cooling of different fruits and vegetables is different.
Vegetables can be divided into three categories according to the speed of vacuum pre-cooling:
A. when the vegetables are cooled to 0 ℃, it takes about 10~20min. Almost all of them are leafy vegetables, with large surface area ratio, soft tissue, and its structure is conducive to the evaporation of water.
B. class of vegetables cooling slower, such as soybeans. The surface area ratio is small and the tissue is hard. Strawberries and cauliflower also belong to this category, and it takes a long time to cool in vacuum.
c. vegetables including cucumbers, tomatoes, etc., the worst effect in vacuum pre-cooling. This kind of vegetable surface area ratio is small, the epidermis is thick, the tissue is dense, thus the water section hair difficult. Potatoes, carrots and other vegetables are not suitable for vacuum cooling.
   2. Vacuum precooling device Vacuum precooling deviceThere are three main components, namely the vacuum chamber, water trap and vacuum pumping unit.
(1) The suitable working pressure range for vacuum pre-cooling and fresh-keeping of the vacuum pumping unit is 600~700pa, which is limited to the temperature at which fruits and vegetables are not subject to frostbite. Under this working pressure, vacuum pre-cooling and preservation of vacuum pumps or vacuum units are: rotary vane vacuum pump and slide valve vacuum pump;-two-stage water ring pump unit;-rotary vane vacuum pump unit;-slide valve pump unit. Oil seal type mechanical vacuum pump is not suitable for pumping gas containing a large amount of water vapor, because the water vapor will be dissolved in the mechanical pump oil, so that the oil emulsification, the performance of the oil deterioration, affecting the performance of the vacuum pump. -The two-stage water ring pump unit can pump water and gas, but it pumps a large amount of water and gas, and the unit is quite large and not applicable. Therefore, it is usually necessary to configure a heat exchanger to prey on a large amount of fruits and vegetables to evaporate water and gas, so as to reduce the water and gas entering the pump as much as possible. The gas load of the vacuum unit consists of four parts, namely, the atmospheric pressure gas contained in the vacuum chamber and the heat exchanger volume, the air leakage of the device, the gas contained in the material, and the water vapor that the heat exchanger fails to capture. Usually, only the gas contained in the vacuum chamber and the heat exchanger is considered in the rough calculation, which can be ignored.
(2) heat exchanger The heat exchanger is used to condense the water vapor evaporated from the material. Itself is equivalent to a heat exchanger, the tube side of the refrigerant, shell side of the air and water vapor. The shell material is stainless steel, and the evaporator is made of copper tube. There is a thermal insulation layer outside the shell. When designing the heat exchanger, it is necessary to consider the large flow conductance when the gas passes through; at the same time, it is necessary to consider the efficiency of capturing water vapor. In general design, the water vapor capture efficiency should be above 95%. The pressure in the heat exchanger is half to an order of magnitude lower than the operating pressure, and the temperature of the heat exchanger is determined by this value.
The cooling consumption of heat exchanger includes three parts:
① Cooling consumption Q1 of evaporator material in heat exchanger;
Heat conduction of evaporator structure support material and radiant heat loss of evaporator Q2;
③ Cold consumption Q3 of water vapor condensation from material evaporation;
When determining the cold consumption of the heat exchanger, the pre-cooling cold consumption Q1 Q2 and the working cold consumption Q2 Q3 should be compared, and the larger value of the two should be taken. Single-stage compressor can be selected for the refrigerator, and R22 is selected for the refrigerant.