Choosing the ratio of roots pump to front vacuum pump is the key to make roots vacuum unit. Because roots vacuum unit compression ratio is very limited. This compression ratio determines the capacity of the front vacuum pump because the compression ratio is the inverse of the vacuum pump speed. The general compression ratio is chosen between 2 and 10. Pre-pumping requirements should also be taken into account. Vacuum equipment is the auxiliary equipment of various mechanical equipment. If the uptime is much greater than the pre-vacuum time, a smaller pre-vacuum can be selected (the compression ratio can be larger), if the vacuum degree is larger. Sometimes in order to better play the efficiency of the equipment, the unit is equipped with a larger pre-pump, after normal operation to stop it, and use a smaller pre-pump for maintenance, in order to save energy.
The extreme pressure of unit roots vacuum requirements, also need to consider the vacuum pump selection, at the beginning of the end of a vacuum to demand higher (1 x 1 x 10-10-3 pa ~ 2 pa pressure) generally choose double rotary oil seal mechanical pump or double sliding yan mechanical pump as the early stage of the vacuum pump. The ultimate vacuum (1 x 10-2 pa pa ~ 1 x 10-1) can choose single stage oil seal mechanical pump as the vacuum pump, the ultimate vacuum of 133 ~ 1333 pa can choose reciprocating vacuum pump or liquid ring vacuum pump as the pump.
When choosing reciprocating or original oil ring vacuum pump and the same root vacuum pump, pump pumping speed is usually the root vacuum pump 1/2-1/4 before, not too big, otherwise will make the compression ratio is too large, resulting in high exhaust temperature root vacuum pump, can not work normally. It is necessary to consider whether there is corrosive medium in the pumped gas, whether there is condensable gas, and whether there is a large trace amount. If contain corrosive gas. Cannot choose seal mechanical vacuum pump, seal mechanical vacuum pump is not suitable for pumping out containing a large number of compressible steam gas, will close the liquid emulsification quickly otherwise, when the gas removed contains tracking compressible gas, it is necessary to use seal mechanical vacuum pump containing gas to consider the universality of the pump equipment.
In order to achieve a higher extraction rate at a lower inlet pressure (1-100pa), a roots vacuum pump can be used as a third or fourth stage device consisting of a first stage vacuum pump of another roots vacuum pump. At this point, the extraction rate of roots vacuum unit is generally between 2 and 5.