Pumps are connected in series; why is it better to use different types of pumps?

The first question should be about different types of vacuum pumps, right? Parallel connection allows for a greater pumping capacity, essentially following the principle of 1+1=2; series connection enables a higher vacuum level, but in this case it’s not 1+1=2. In practice, it is rare to use vacuum pumps of the same model in series – instead, a pump with a high pumping capacity is used as the primary pump, while a pump with a lower capacity is used as the secondary pump. Moreover, combining pumps of different types often results in a higher vacuum level and better pumping capacity, such as a water ring pump + rotary vane pump, a rotary vane pump + Roots pump, or a rotary vane pump + sublimation pump

Why use vacuum pumps of different types instead of two of the same type connected in series?

The same configuration can also be used; for example, both pumps can be rotary vane vacuum pumps. However, the flow rate of the pump in the earlier stage must be higher than that of the pump in the later stage. The power levels of the two pumps differ, with the pump in the earlier stage having a relatively higher power level – and by a significant margin. When using two vacuum pumps with the same power level or similar power levels, the flow rate of the pump in the earlier stage is much higher than that of the pump in the later stage, while the difference in power levels is not significant. Take a look at your two pumps – is the difference in their power levels small? The lower the power level, the less electricity is consumed.

Thank you for the reply! Let me understand the reasons in brief.

Vacuum pumps with different maximum vacuum levels have varying requirements regarding inlet pressure; therefore, they need to be used in series. During startup, the low-vacuum pump should be started first, followed by the high-vacuum pump, while the reverse order is applied during shutdown

Generally speaking, rotary vane pumps have a high gas handling capacity, but they cannot be started under atmospheric pressure. In contrast, common water ring pumps, screw pumps, and claw pumps can be started at atmospheric pressure; these are known as pre-pumps or roughing pumps. Of course, modern screw pumps and claw pumps are capable of achieving very high ultimate vacuums. When the required gas extraction volume is met, a single pump is sufficient; whereas the use of rotary vane pumps in series is mainly aimed at increasing the pumping speed and achieving even higher ultimate vacuums. As a pre-pump, for explosion-proof requirements, the gas flow rate is generally low. You may also see many liquid ring pumps with high pumping speeds used as pre-pumps, but when combined in series with rotary vane pumps, the performance of the liquid ring pumps deteriorates significantly, and their flow rate is not as high as stated in the specifications. And why not use a series connection of the same type, such as screw pump followed by another screw pump? Such a series configuration is pointless; the screw pump itself is capable of compressing gas to an atmospheric pressure of 1 bar. The subsequent screw pump is a vacuum pump, which is not a positive-pressure pump, so there is no practical benefit to connecting them in series. Common combination formats include, for example, 1) a single primary pump (screw, claw, or water ring), 2) one stage of Roots pump + primary pump, 3) two stages of Roots pump + primary pump. Gases have a compression ratio, and pumps with different designs only need to operate within their respective pressure ranges. A parallel pump serves to increase the pumping capacity by a multiple. Moreover, primary pumps are generally more expensive than Roots pumps; therefore, by using a Roots pump in series with a primary pump, it is possible to increase the pumping speed and achieve a higher ultimate vacuum.