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How to choose a backing pump for a roots pump?

Author: BOWAH

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For the roots pump, the selection of the pumping speed ratio between the roots pump and the backing pump is very important, because the compression ratio of the roots pump is very limited. This compression ratio determines the capacity of the backing pump because the compression ratio is inversely proportional to the pumping speed of the two pumps. Generally, the compression ratio is selected within 2~10.

In addition, the requirements for pre-pumping time should be considered as the vacuum system is used as auxiliary equipment in various mechanical equipment. If the normal working time is much longer than the pre-pumping time, a smaller backing pump can be selected (the compression ratio can be larger). If the vacuum chamber is large and requires pre-pump quickly, use a larger backing pump.

Sometimes in order to make better use of the efficiency of the vacuum system, a larger pre-pump is installed in front of the unit, and it is stopped after normal operation, and a smaller backing pump is used to maintain it. This is done to save power.

The requirements for the ultimate pressure of the vacuum system must also be considered when selecting the backing pump. If requirements for the ultimate vacuum degree are relatively high (up to 1×10-3Pa~1×10-2Pa ), a two-stage rotary vane pump, oil-sealed mechanical pump, or a two-stage slide valve mechanical pump is used as a backing pump.

If the ultimate vacuum degree is required to be between 1×10-2Pa~1×10-1Pa), a single-stage oil-sealed mechanical pump can be selected as the backing pump. A liquid ring vacuum pump can be used as the backing pump if the ultimate vacuum degree is required to be between 133 and 1333Pa. When the liquid ring vacuum pump is used as the backing pump of the roots vacuum pump, the pumping speed of the backing pump is generally 1/2-1/4 of that of the roots vacuum pump. The exhaust temperature of the vacuum pump is too high to work normally.


When selecting a backing pump, it is also necessary to consider whether there is a corrosive medium in the pumped gas, whether it contains condensable gas, and whether it is a large amount or a trace amount. If it contains corrosive gas, the oil-sealed mechanical pump cannot be used. Oil-sealed mechanical pumps are not suitable for pumping gas containing a large amount of condensable steam, otherwise, the sealing liquid will be quickly emulsified. When pumping gas containing trace condensable steam, an oil-sealed mechanical pump with a gas ballast device must also be used. The pump acts as a backing pump.

In order to obtain a high pumping rate at the low inlet pressure (1~100Pa), a Roots vacuum pump can be used as a three-stage or four-stage unit composed of a Roots vacuum pump as the backing pump of another Roots vacuum pump. At this time, the Roots vacuum pump The pumping speed ratio between them is generally between 2 and 5.

The limit vacuum of a single-stage water ring pump is not high, but its structure is simple and easy to maintain. It is generally used for low vacuum and high pumping speed. For example, a single-stage water ring pump can be used for a large number of negative pressure operations. The two-stage water ring pump can improve the ultimate vacuum of the vacuum unit, so the two-stage water ring pump is widely used in the vacuum unit.


The ultimate vacuum of a roots pump is low, especially when it is combined with a water ring pump, the scope of use is limited, and the ultimate vacuum of the entire unit may be lower, but if two or more Roots pumps are used Combining the pump in series with the water ring pump can greatly improve the ultimate vacuum of the unit. Therefore, in this type, it is usually seen that two booster pumps are connected in series and then a two-stage water ring pump is used as a backing pump to form a unit. However, the water ring pump is used as a backing pump, and its ultimate vacuum is the saturated vapor pressure of water at this temperature. Especially in hot weather, the water temperature is relatively high, and the ultimate vacuum drops sharply, which affects the ultimate vacuum of the vacuum system.


How to choose a backing pump for a roots pump?
This compression ratio determines the capacity of the backing pump because the compression ratio is inversely proportional to the pumping speed of the two pumps. Generally, the compression ratio is selected within 2~10.
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