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What are the methods for the freeze dryer to perform strong pressure control?

August 10, 2022

The freeze dryer is to freeze the material containing a large amount of moisture in advance into a solid, and then sublime the water vapor directly from the solid under a certain vacuum condition. The substance itself is stored in the ice shelf when it is frozen, and the volume remains unchanged after drying.

 

The loose porous ice absorbs heat during sublimation, which causes the temperature of the product itself to drop and slows down the sublimation speed. In order to increase the sublimation speed and shorten the drying time, the product is processed. Heat properly. The whole drying process of the equipment is carried out at a lower temperature.

Freeze dryer is an indispensable technology in daily life. It not only prolongs the shelf life of food, but also provides the possibility to expand the scale of enterprise operation and increase the sales coverage area to a certain extent. Promote the development of my country's logistics industry.

 

When using instruments for food processing, it is necessary to correctly select the processing equipment according to the different properties of the food. At the same time, equipment research and development enterprises should also increase investment to improve the functionality of equipment, so that the functions can be more subdivided, and more food processing can be realized.

Although the current freeze dryer has met the production requirements, it still has the following shortcomings in terms of economic benefits, product quality, and production operations.

During the freeze-drying process of the product, the vacuum degree of its environment has a direct impact on the length of the freeze-drying cycle. If the vacuum is too low, the sublimation rate will decrease significantly; if the vacuum is too high, the freeze-drying rate will also decrease significantly due to poor heat convection conduction.

Therefore, various approaches must be taken to control the pressure to achieve the best conditions required for product drying. The main methods of pressure control are: ① the control method of the septum valve; ② the control method of the water vapor condenser; ③ the control method of the small butterfly valve; ④ the aeration method.

1. The separation valve control method refers to the purpose of pressure control by adjusting the cross-sectional area of ​​the passage between the drying box and the water vapor condenser to increase the partial pressure of water vapor in the drying box.

2. The water vapor condenser control method refers to the operation method of increasing the temperature of the surface of the water vapor condenser to reduce the condensation capacity, resulting in an increase in the partial pressure of water vapor in the drying box, so as to achieve the purpose of pressure control.

3. The small butterfly valve control method refers to the purpose of controlling the pressure in the box by intermittently opening or closing the valve between the water vapor condenser and the vacuum pump. The small butterfly valve control method controls the opening and closing of the valve by the signal sent from the control system.

 

When the pressure is higher than the set value (that is, the vacuum degree is too low), the valve is opened; when the pressure is lower than the set value (that is, the vacuum degree is too low) high), close the valve.

 

However, if the temperature of the condenser is very low at this time, the temperature of the product shelf is also very low. Due to the slowing down of the sublimation speed, the partial pressure of water vapor continues to decrease. This method cannot increase the indoor pressure, so the purpose of adjusting the pressure cannot be achieved.

4. The aeration method refers to the operation method of introducing sterile air or nitrogen into the drying box by controlling the air intake through the micro-adjustment valve.

This method is commonly used in actual production, but there are also certain defects:

①Due to the introduction of mostly non-condensable gas (air or nitrogen), the existence of this gas reduces the effective condensation area of ​​the condenser, which makes the condenser prone to ice blockage, and the introduction of nitrogen will also increase the cost.

②Because the gas is introduced from the outside, the cleanliness of the gas directly determines the quality of the product, which is easy to cause pollution and cause the product to have hidden dangers in quality and safety.

③ With the alternation of air aeration and high vacuum, the products with lighter texture and less loading are constantly reciprocating up and down. During this process, the friction between the product and the bottle wall makes the formed product into powder. Being taken away not only affects the loading and content of the product, but also affects the appearance of the product.

④Due to the direct introduction of gas from the filling room, normal fumigation and disinfection work cannot be performed in the clean area, and there are two identical freeze dryers that cannot speed up, thus hindering the production progress. Although the above methods can achieve the purpose of pressure control, they all have various drawbacks.

 

The degree of vacuum is directly determined by the vacuum pump, so it is recommended to start with the vacuum pump, study the variable frequency vacuum pump, and then automatically control the switch and operating speed of the pump through the indoor pressure, so as to fundamentally solve the problem of pressure control. At the same time, due to the power of the vacuum pump Smaller, you can also save energy.

 

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