The front pump of roots pump is composed of roots pump vacuum unit. The selection of the pumping ratio between roots pump and front pump is very important. Because the compression ratio of Robo pump is very limited. This compression ratio determines the capacity of the previous pump, because the compression ratio is the inverse ratio of the pumping speed of the two pumps. Generally, the compression ratio is within 2-10. In addition, we should consider the requirements of pre pumping time. Vacuum equipment is used as auxiliary equipment in all kinds of mechanical equipment. If the normal working time is much longer than the pre pumping time, a smaller front stage pump can be selected (the compression ratio can be larger). If the vacuum is larger and it is required to pre pump quickly, a larger front stage pump will be used. Sometimes, in order to give full play to the efficiency of the equipment, a larger pre pumping pump is installed in front of the unit, which is stopped after normal operation, and maintained by a smaller pre pump, so as to save power.
The requirements for the ultimate pressure of Roots vacuum pump unit are also necessary to be considered when selecting the front stage pump. The high requirements for the ultimate vacuum degree (up to 1 × 10-3pa ~ 1 × 10-2Pa pressure) generally select the two-stage rotary vane oil seal mechanical pump or two-stage sliding Yan mechanical pump as the front stage pump. The mechanical pump with single-stage oil seal can be used as the front pump when the limit vacuum is between (1 × 10-2Pa ~ 1 × 10-1pa), and the reciprocating vacuum pump or liquid ring vacuum pump can be used as the front pump when the limit vacuum is between 133 ~ 1333pa.
When the reciprocating or liquid ring rough vacuum pump is selected as the front pump of Roots vacuum pump, the speed of the front pump is generally 1/2-1/4 of that of Roots vacuum pump, which can not be too large, otherwise the compression ratio will be too large, resulting in high exhaust temperature of Roots vacuum pump and can not work normally. When selecting the pre pump, it is necessary to consider whether there is corrosive medium in the pumped gas, whether there is condensable gas, whether there is a large amount or a small amount. If there is corrosive gas, oil seal mechanical pump can not be selected. Oil seal mechanical pump is not suitable to pump out the gas containing a large amount of condensable steam, otherwise it will make the sealing liquid emulsify quickly. When pumping out the gas containing a small amount of condensable steam, oil seal mechanical pump with air ballast device must also be used as the front pump.
In order to obtain a high pumping rate at a low inlet pressure (1-100pa), a Roots vacuum pump can be used as a three-stage or four-stage unit composed of a front stage pump of another roots vacuum pump. At this time, the pumping speed ratio between roots vacuum pumps is generally between 2 and 5.
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