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Steam jet vacuum pump is a kind of vacuum equipment with large pumping capacity, wide working range, simple structure and no relative moving parts. It is mainly used in degassing, decolorizing, drying, deodorizing, distillation, refrigeration and transportation. In particular, it has no selectivity to the inhaled gas, and it shows excellent advantages compared with mechanical vacuum pump (oil pump) in food production, petrochemical industry, grease processing, polyester production, material concentration with organic solvent, etc.

1、 Working principle of steam jet vacuum pump

Steam jet vacuum pump is to form vacuum by using the gas dynamic principle of conversion between static pressure energy and kinetic energy during fluid flow. When the steam with certain pressure passes through the throat diameter of Laval nozzle, the static pressure energy is completely converted into kinetic energy and reaches supersonic velocity. At the same time, a vacuum is formed at the nozzle outlet. Under the effect of pressure difference, the extracted gas is pumped into the suction chamber and mixed with the super sonic steam. At the same time, it enters the venturi tube and is discharged from the diffuser of the venturi at subsonic speed, At the same time, the velocity of the mixed gas gradually decreases, and the pressure increases, and then it is discharged from the exhaust outlet. If several jet pumps are used in series, a condenser is added between the pump and the pump to condense the steam, and a higher vacuum can be obtained. The whole steam jet vacuum pump consists of several stages of pump body and condenser. The pump bodies at all levels are composed of nozzle, inlet and diffuser. The nozzle can be single or multiple. The nozzle is generally made of stainless steel. Other parts such as suction chamber and diffuser can be made of stainless steel, cast iron and carbon steel.

2、 Main factors affecting the capacity of vacuum pump

1. Working steam and its dryness

The low steam pressure and pressure fluctuation have great influence on the capacity of vacuum pump, so the steam pressure should not be lower than the required working pressure. However, the structural design of the vacuum pump used has been finalized. If the steam pressure is increased too much, the pumping capacity and vacuum degree will not be increased. In addition, to ensure that the steam pressure supplied by the boiler is stable, it is better to use a boiler to provide working steam for the steam jet pump, so that the steam pressure will not fluctuate and the performance of the vacuum pump will be stable. ,

The dryness of steam also has a great impact on the performance of vacuum pump, in which water content will cause vacuum fluctuation, and if water content is too much, it will even be unable to pump vacuum. The usual method is to install a steam separator in front of the drum to obtain the working steam with higher dryness, and to effectively insulate the steam pipeline. In order to obtain the best working benefit, the working steam for jet vacuum pump should be superheated steam of 5 ℃ ~ 10 ℃. Especially for the fifth stage pump, the quality of steam is very high. If there is a small amount of water, it may cause ice jam in the nozzle, which will cause the vacuum degree of the fifth stage ejector to drop or not work.

2. Requirements for circulating cooling water

If the cooling water supply is insufficient, the condenser will heat up, the sound of air flow will increase, the vacuum degree will drop rapidly, and even the steam will return to the suction pipe. For the intercooled condenser, the water supply pressure should be 0.2MPa, and the water supply should be slightly larger than the actual consumption, so that the fluctuation of water consumption will be more stable. In addition, in order to avoid vacuum fluctuation caused by unstable water supply, it is better to use a separate circulating water system to supply water to the condenser of vacuum pump.

If the cooling water temperature is too high, the capacity of vacuum pump will decrease, and sometimes even can't pump vacuum, which is generally less than 32 ℃. In addition, the higher the cooling water temperature, the more steam consumption. The influence of cooling water quality on the capacity of vacuum pump is also a factor that can not be ignored. If the water quality is poor and the hardness is high, the condenser will be fouling or even blocked, which will seriously affect the heat exchange performance and make the steam difficult to condense, thus affecting the vacuum degree. Therefore, the circulating cooling water should be pure softened water.