During the freeze-drying process, the pressure in the freeze-drying chamber used to be thought that the lower the better, but the current view does not think so, but should be controlled within a certain range.
The low pressure is of course favorable for the sublimation of the ice in the product. However, when the pressure is too low, it is unfavorable for heat transfer, the product is not easy to obtain heat, and the sublimation rate is reduced instead. Experiments show that: when the pressure of the freeze-drying chamber is lower than 0.1 mbar, the convective heat transfer of the gas is negligible;
when the pressure is higher than 0.1 mbar, the convective heat transfer of the gas increases significantly. At the same plate temperature, when the pressure is higher than 0.1 mbar, the product easily obtains heat, so the sublimation rate increases.
However, when the pressure is too high, the sublimation rate of ice in the product is slowed down, and the heat absorption drop of the product is reduced. As a result, the temperature of the product itself rises, and when it is higher than the eutectic temperature, the product will melt, resulting in freeze-drying failure.
The suitable pressure of the freeze-drying chamber is generally considered to be between 0.1 and 0.3 mbar. Within this pressure range, it is conducive to both heat transfer and sublimation. When it exceeds 0.3 mbar, the product may melt. At this time, a vacuum alarm signal should be issued to cut off the heating of the product, and even start the freezer to cool the freeze-drying chamber to protect the product from melting.
The pressure in the freeze-drying chamber is composed of the partial pressure of air and the partial pressure of water vapor, so a thermal vacuum gauge that can measure the full pressure should be used to measure the vacuum degree; it is not appropriate to use a compression vacuum gauge, which uses mercury as the medium. Compression vacuum gauges should not be used due to mercury vapor hazardous products.
1 gram of ice can produce about 10,000 liters of steam at a pressure of 0.1 mbar. In order to remove a large amount of water vapor, it is not enough to rely on a mechanical vacuum pump to remove it.
The condenser acts as a cooling so that a large amount of water vapor condenses on the cooling surface inside it, so the condenser actually acts as a water vapor pump. The heat released when a large amount of water vapor condenses can make the temperature of the condenser rise, which is a normal phenomenon.
However, due to the insufficient cooling capacity of the condenser freezer, the small surface of the condenser for adsorbing water vapor, or the generation of too much water vapor due to too much heat to the product, the temperature of the condenser will rise excessively. when this happens.
The water vapor pressure difference between the freeze-drying chamber and the condenser decreases, resulting in a decrease in the sublimation rate; at the same time, the partial pressure of the water vapor in the freeze-drying system increases, which deteriorates the vacuum degree, which in turn causes the sublimation rate to decrease. Slow, the product absorbs less heat, and the product temperature rises, causing the product to melt and freeze-drying to fail.
Therefore, in order to freeze-dry a good product, it is necessary to maintain a good and stable vacuum in the system. The condenser is always required to be below -40°C because the vapor pressure of ice at -40°C is around 0.1 mbar.
Application of freeze-drying Machine
Applicable to all kinds of vegetables, fruits, meat, all kinds of prepared food, health care products, etc.