Different vacuum systems require different vacuum degrees. Therefore, it is often necessary to complete by a set of vacuum units, that is, the vacuum pumps working in different pressure ranges are connected in series.
The vacuum pump on the high vacuum side can reach the vacuum degree required by the system, while the vacuum pump on the low vacuum side is directly discharged to the atmosphere. Obviously, the vacuum unit is a vacuum pump directly discharging the atmosphere. However, the high vacuum system generally needs three-stage units, and the medium vacuum system generally needs two-stage units.
It is difficult for a high vacuum pump and a low vacuum pump to form an effective high vacuum unit. There are several reasons for this. Flow continuity is one of them. The high vacuum pump has the limitation of pressure resistance of the front stage, that is, if the front stage is higher than a certain pressure, the pump cannot work normally. When the current stage pump reaches this critical pressure, the pumping speed will often decrease, so the exhaust flow of the former stage pump may be less than the exhaust flow of the main pump, and the inconsistent flow will destroy the requirements of flow continuity, which will inevitably cause the vacuum unit not to work normally. However, if another middle vacuum pump is connected between the high and low vacuum pumps, it can play a connecting role, with continuous flow, and each pump can work in the state. Roots pump can work in the middle vacuum range, which is suitable, so it is also called roots booster pump. Because of its low compression ratio, it can connect several PA to several hundred PA. When the three-stage high vacuum unit enters into a higher vacuum degree, because the exhaust flow of the main pump is significantly reduced, at this time, only a small front pump can maintain the continuity of pumping, which is often used in practice, so as to reduce the energy consumption of the unit.
Another reason why high vacuum units often need three-stage units is due to the limitation of suction pressure of high vacuum pump. All pumps have initial working pressure, and traditional high vacuum pumps are in the range of several PA. Therefore, the primary pump must be pre pumped to this pressure to start working. However, it often takes a long time for the front stage pump of direct exhaust to pump to this pressure, because the pumping speed of the pump is decreasing as the pressure decreases, especially for the vacuum unit with periodic pumping, there is a requirement for the time to reach the working vacuum degree. The longer the pre pumping time is, the longer the time to enter the working vacuum degree is. Therefore, adding a middle vacuum pump to cooperate with the front stage low vacuum pump can A short time to reach the main pump can work pressure, so that the system can enter the working pressure as soon as possible, to ensure the efficiency of the equipment.
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