Differential pressure pre-cooling uses the pressure gradient of the air to force cold air to pass through the gaps in the product packaging box to quickly cool the product.
When pre-cooling in cold storage, the cold air in the storage can only contact the outside of the packaging box and is difficult to enter the internal gaps of the vegetables. Therefore, the cooling speed is slow and pre-cooling dead corners are easy to occur, making the pre-cooling uneven. Differential pressure pre-cooling is a pre-cooling method developed to make up for the shortcomings of cold storage pre-cooling. The cost of pre-cooling is similar to that of cold storage pre-cooling. It is best to pre-cool packaged vegetables using the differential pressure pre-cooling method.
During differential pressure pre-cooling, all vegetables in the vegetable box stack are required to be cooled to a certain temperature in a short period of time. The lower the air temperature in the cold storage, the greater the air flow through the vegetable box stack. , the faster the cooling speed of vegetables. However, if the temperature of the cold storage is too low, some vegetables may suffer from freezing or chilling damage, because the wind speed flowing through the vegetables during differential pressure pre-cooling is much greater than that of the cold storage pre-cooling, so the storage temperature of differential pressure pre-cooling is required to be higher. It is 1~2℃ higher than the cold storage pre-cooling temperature.
Advantages of differential pressure precooling:
(1) Fast precooling speed. Differential pressure precooling forces the cold air in the warehouse to enter the gaps between vegetables in the packaging box and directly contact each vegetable. To derive the heat of vegetables, the pre-cooling speed can be 2 to 6 times faster than cold storage pre-cooling, but it takes at least 2 times longer than water pre-cooling and vacuum pre-cooling. Generally, differential pressure pre-cooling of vegetables only takes 3 to 6 hours, which is only 1/2 to 1/6 of the pre-cooling time of cold storage. Vegetables harvested in the morning can be sold on the same day. The facility construction costs and pre-cooling costs are similar to those of cold storage pre-cooling.
(2) Uniform pre-cooling, differential pressure pre-cooling, and vegetable gaps are included as an integral part of this ventilation system. The vegetables are stacked on both sides of the differential pressure pre-cooling fan air duct as required. After the vegetable stacks are stacked, use tarpaulin to seal the top and sides of the vegetable stack air duct. When the differential pressure pre-cooling fan is turned on, a negative pressure is formed in the air duct. Pressure creates a pressure difference on both sides of the vegetable stack, forcing the cold air in the warehouse to enter the gaps between the vegetables, taking away the heat of the vegetables, and evenly cooling the vegetables in each vegetable box of the entire pre-cooling system.
(3) Low equipment cost. Only a differential pressure pre-cooling ventilation system needs to be added to the pre-cooling warehouse.
Disadvantages of differential pressure precooling:
(1) The one-time processing capacity of the same large pre-cooling warehouse is smaller than that of the cold storage pre-cooling, which is generally the pre-cooling processing capacity of the cold storage. 60 to 70, but due to its short pre-cooling time, the total processing capacity will not be reduced within a certain period of time.
(2) Strict requirements for packaging boxes. Pre-cooled packaging boxes after packaging must have sufficient ventilation area. Differential pressure pre-cooling packaging boxes generally use perforated plastic boxes or perforated cartons. In order to ensure the strength of the carton and sufficient ventilation area, the length-to-width ratio of the carton should not be greater than 2.5:1; the height-to-width ratio should not be greater than 2:1 and not less than 0.25:1. The opening area of ??Japanese cartons is generally 2 to 3 times the surface area of ??the carton, while Australia stipulates that it should not be less than 4 times the surface area of ??the carton. In addition, different types of vegetables have different optimal ventilation areas. In order to make the limited opening area more effective for ventilation, the number, shape, size, and position of the air holes must be scientifically calculated. Generally, there are 1 to 2 holes on the horizontal surface and 2 to 3 holes on the long surface. When stacking items upside down, the ventilation holes on the horizontal and long sides must be well aligned.
(3) During pre-cooling, strict requirements are placed on the stacking of vegetable boxes. During differential pressure pre-cooling, the vegetable boxes should be placed in the pre-cooling warehouse. Under the action of the fan, the air on both sides of the vegetable boxes A certain pressure difference is generated, which drives the cold air in the pre-cooling store to pass through the vegetable box stack at a certain flow rate to quickly cool the vegetables. In order to allow the cold air to enter each vegetable box evenly, it can effectively take away the heat of vegetables. In addition to equipment factors, the stacking of vegetables is also critical. Vegetable stacking requirements: Except for the gaps between the vegetables in the ventilation holes of the packaging boxes, there should be no gaps elsewhere to prevent wind from escaping and reducing wind pressure. Therefore, it is required that there should be no gaps between the vegetable boxes and the pre-cooling cover must be tightly covered.