Water ring vacuum pump (water ring pump) is a kind of coarse vacuum pump, which consists of pump cover, pump body, disc, impeller, mechanical seal, motor and other parts. The seal of the pump adopts mechanical seal, and the mechanical seal is installed between the impeller and the pump body. The clearance between impeller and pump body is determined by mechanical seal. Water-ring vacuum pump was initially used as self-suction pump, and then gradually used in petroleum, chemical industry, machinery, mining, light industry, medicine and food and many other industrial sectors. In many industrial processes, such as vacuum filtration, vacuum water diversion, vacuum feeding, vacuum evaporation, vacuum concentration, vacuum moisture regain and vacuum degassing, water ring vacuum pump has been widely used.
A disc is installed on the pump cover, and the disc is equipped with suction, exhaust holes and flexible exhaust valve discs. The function of flexible valve discs is to discharge the gas before the exhaust outlet when the gas pressure between impeller blades reaches the exhaust pressure, thus reducing the power consumed by excessive gas pressure and reducing the rate consumption. The intake pipe and exhaust pipe are connected with the pump cavity through the suction and exhaust holes installed on the disc on the pump cover. The shaft is eccentrically installed in the pump body. The impeller is fixed on the shaft with a flat key. The gap between the two ends of the pump is adjusted by the cushion between the pump body and the disc on the pump cover. The gap between the impeller and the disc on the pump cover is between the disc and the pump body. The gap between the impeller end face and the disc on the pump cover determines the loss of gas and its limit pressure in the flow from the intake port to the exhaust port in the pump chamber.
Proper amount of water is used as working fluid in the pump body. When the impeller rotates clockwise, the water is thrown around by the impeller. Because of the centrifugal force, the water forms a closed ring which is approximately equal in thickness depending on the shape of the pump chamber. The inner surface of the upper part of the water ring is just tangent to the hub of the impeller, and the inner surface of the lower part of the water ring is just in contact with the top of the blade (in fact, the blade has a certain insertion depth in the water ring). At this time, a crescent-shaped space is formed between the hub and the water ring of the impeller, which is divided into several small cavities with equal number of blades by the impeller. If the starting point is 0 degrees above the impeller, the volume of the small chamber increases from small to large at 180 degrees before the impeller rotates, and is connected with the suction port on the end surface. At this time, the gas is absorbed, and at the end of the suction, the small chamber is isolated from the suction port; when the impeller continues to rotate, the small chamber becomes smaller and compresses the gas; when the small chamber and the exhaust port continue to rotate, the gas is compressed. When connected, the gas is discharged out of the pump.
Vacuum drying methods can be divided into two parts: boiling point and melting point.
Drying through boiling point:
During the drying process, evaporation of liquid water has two ways: 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 19.6kPa air 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.
Two, through melting point drying (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. 1/2 1/4, which controls the saturated vapor pressure of ice at corresponding temperatures, is often controlled. If the temperature is dry at 40 C, the operating pressure should be 2.7 ~ 6.7Pa.
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.
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