The ratio between the pre-pump and the main pump in a vacuum system

It’s best to ask the manufacturer for accurate information; experience is crucial

Although the answer is not exact, thank you still for your participation!

1) These so-called 1:1 and 2:1 are quite general terms, right! None of it is based on calculations; it comes from experience, and what is mentioned is merely their nominal pumping speed. 2) Such as the Roots pump of ZJ300, with a pumping speed of 300 L/S, or 1080 m3/h ； The liquid ring pumps are 2BV6 161-0, with a pumping capacity of 500 m3/h; and 2BE1 202-0, with a speed of 970 rpm and a pumping capacity of 750 m3/h. All of these values represent their designed pumping capacity or the maximum pumping capacity under standard conditions. The commonly mentioned 1:1 and 2:1 both refer to this ratio of pumping speeds! 3) Vacuum units developed based on this approach generally work without many problems, as there is a wealth of experience to support them. However, when the cooling water temperature is high in summer, problems such as overheating and jamming of the lower Roots pump can still occur sometimes. 4) As a manufacturer, or rather for a more advanced technical solution, detailed calculations are necessary, taking all extreme operating conditions into account! 5) At the pressure point where the main pump and the pre-pump meet, the actual pumping speed is often far from these given values. For example, with a ZJ300 Roots pump paired with 2BV6161-0, the actual pumping volume may be over 700 m3/h; when paired with 2BE1202-0 at 970 rpm, the pumping volume could be over 800 m3/h. At the same time, the inlet pressure is fixed, and the pressures at their junction points are also different! 6) In fact, the 1:1 ratio also contradicts the idea of using a Roots pump as a pressure-boosting pump; since it boosts pressure, and given that the mass flow rate remains constant, the volume of the gas must decrease (although the temperature rises). Isn’t it unreasonable to also include a pre-pump with a 1:1 volume ratio? This term is just a simple name. 7) Rotary vane pumps can be used in combination with screws or other dry-type pumps; similarly, the pressure is just lower, while the compression ratio of rotary vane pumps can be higher!

The analysis is indeed reasonable! The reason for using larger pre-pump models is to **reduce** the pumping time; for example, if 2BE 202 and 2BV6110 are chosen as the pre-pumps, their efficiency will be much higher. The key question is whether a Roots pump can actually increase the vacuum level further, given that the pumping speed of the preceding pump is almost the same as that of the Roots pump Is it still necessary to use a larger rotary vane pump, such as one with a capacity of 600 L/S? The water ring pump can achieve the required vacuum level, but it’s important to consider potential leaks in the system; during hot summer days, the vacuum level drops. Therefore, choosing a rotary vane pump with a capacity of 300 L/S is also a way to address such situations

The pumping speed definitely won’t be the same, because there’s also pressure!