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Vacuum drying methods of liquid ring vacuum pump are various and traditional. Generally speaking, it can be divided into two ways: boiling point and melting point.
Drying by Boiling Point
In the drying process, there are two ways of liquid water vaporization: axial evaporation and boiling. Water vaporizes much faster when boiling than when evaporating. Evaporation of water into steam can take place at any temperature. Water boiling into steam can only be carried out at a specific temperature, but when the pressure is reduced, the boiling point of water decreases. For example, at 19.6 kPa pressure, the boiling point of water can be reduced to 60 degrees C. Vacuum dryer is to provide heat source in the vacuum state, through heat conduction, heat radiation and other heat transfer modes to supply enough heat of moisture in the material, so that evaporation and boiling simultaneously, and accelerate the speed of vaporization. At the same time, the vaporized vapor is pumped out quickly by vacuum, and a negative pressure state is formed around the material. A large humidity gradient is formed between the inner and outer layers of the material and between the surface and the surrounding medium, which speeds up the vaporization speed and achieves the purpose of rapid drying.
Vacuum drying process is affected by heating mode, heating temperature, vacuum degree, coolant temperature, material type, initial temperature and pressure. Usually, there are three ways of heating: heat conduction, heat radiation and combination of the two.
Drying by melting point (also called freeze drying)
In the drying process, wet materials (or solutions) are frozen to solid state at low temperature (-10-50 C), and then sublimated directly to gaseous state under high vacuum (130-0.1Pa), which is also called sublimation drying.
The necessary condition for this vacuum drying is that the pressure of the drying process should be lower than the saturated vapor pressure of the ice at the operating temperature. The saturated vapor pressure of ice is usually controlled at 1/2-1/4 of the corresponding temperature. The operating pressure should be 2.7-6.7Pa if it is dried at 40 C.
Wet materials can also be frozen by themselves without pre-freezing, but by vaporization and heat absorption of water at high vacuum. This kind of freezing energy consumption is small, but it will cause loss of foam or splash on liquid materials, and it is not easy to obtain porous homogeneous dry matter.
Generally, heat is supplied by heating medium through the wall of drying chamber. Therefore, it is necessary to supply heat of wet materials to ensure a certain drying rate and avoid ice melting.
During the drying process, the sublimation temperature is generally – 35 – 5 C, and the extracted water can be frozen and aggregated on the condenser or directly discharged by the vacuum pump. If the heat required for sublimation is supplied directly by the dried material, in this case, the temperature of the material decreases rapidly, so that the vapor pressure of the ice is very low and the sublimation rate decreases.
2SK Liquid Ring Vacuum Pump
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