Ultimate Guide to Selecting and Calculating Vacuum Pumps


Vacuum, in theory, refers to no substance inside the volume. (In reality, true vacuum does not exist.) Usually, any gas pressure lower than normal atmospheric pressure (101325 Pa) inside a container is called a vacuum state.

Vacuum Degree

Vacuum degree indicates the degree of gas sparsity under vacuum conditions, usually expressed in terms of pressure values.

In practical applications, there are two types of vacuum degrees: absolute vacuum and relative vacuum. The value read from the vacuum gauge is called the vacuum degree.

The vacuum degree value represents the actual system pressure value is lower than the atmospheric pressure value, and the value displayed on the gauge is also called the gauge pressure.

The industry also refers to it as ultimate relative pressure, i.e., Vacuum degree = atmospheric pressure – absolute pressure (the atmospheric pressure is generally taken as 101325Pa, the ultimate absolute pressure of the water ring vacuum pump is 3300Pa, and the ultimate absolute pressure of the rotary vane vacuum pump is about 10Pa).

Absolute Vacuum & Relative Vacuum

Ultimate Relative Pressure

Relative pressure refers to the degree of gas sparsity inside a container compared to atmospheric pressure. It represents the actual system pressure value that is lower than the atmospheric pressure value.

Since the air inside the container is pumped out, the internal pressure is always lower than the external pressure.

Therefore, when using relative pressure or gauge pressure to represent it, the value must be preceded by a negative sign to indicate that the internal pressure of the container is lower than the external pressure.

Ultimate Absolute Pressure

The Ultimate Absolute Pressure refers to the difference between the absolute pressure inside a container and the theoretical vacuum pressure (which has a pressure value of 0 Pa).

As a result of technical limitations, it is impossible to pump the internal pressure of a container down to the absolute vacuum value of 0 Pa.

Therefore, the vacuum value achieved by a vacuum pump is always higher than the theoretical vacuum value. When using absolute vacuum to express this value, there is no need for a negative sign.

For example, if the vacuum degree of a device is marked as 0.098 MPa, then in reality, it is -0.098 MPa.

Pumping Capacity

Pumping Capacity is a measurement factor of the pumping speed of a vacuum pump, usually expressed in units of L/s and m³/h.

It compensates for the leakage rate of the system. It is easy to understand why a vacuum pump with high pumping capacity can easily achieve the desired vacuum degree, while a vacuum pump with low pumping capacity may be slow or unable to reach the desired vacuum degree when pumping the same volume of container.

This is because it is impossible for the pipeline or container to completely prevent gas from leaking, and the high pumping capacity compensates for the decrease in vacuum due to leakage.

Therefore, a high pumping capacity vacuum pump can easily achieve the ideal vacuum degree.

It is recommended that, when calculating the theoretical pumping capacity, choose a vacuum pump with a higher pumping capacity if possible. The formula for calculating pumping capacity will be introduced below.

The conversion methods between Pa, KPa, MPa, mbar, bar, mmH2O, Psi are shown in the following table:

Conversion Table for Commonly Used Pressure Units in Laboratories


Selection of Vacuum Pumps

1. The required vacuum degree for process

The working pressure of the vacuum pump should meet the requirements of the process, and the selected vacuum degree should be half to one order of magnitude higher than that of the vacuum equipment. (For example, if the required vacuum degree in absolute pressure is 100 Pa, the vacuum degree of the vacuum pump selected should be at least 50-10 Pa.)

If the absolute pressure requirement is higher than 3300 Pa, a water ring vacuum pump should be given priority as the vacuum device. If the absolute pressure requirement is lower than 3300 Pa, a rotary vane vacuum pump or a vacuum pump with a higher vacuum level should be selected as the vacuum obtaining device.

2. The required pumping capacity for the process

The vacuum pump requires a pumping speed (that is, the ability of the vacuum pump to discharge gas, liquid, and solid substances under its working pressure), usually expressed in units of m³/h, L/s, and m³/min.

The specific calculation method can be calculated based on the following formula for selection. Of course, the selection of vacuum pumps is a comprehensive process involving related experience and other factors.


  • S – pumping speed of the vacuum pump (in L/s).
  • V – the volume of the vacuum chamber (in L).
  • t – the time required to reach the required vacuum degree (in s).
  • P1 – the initial pressure (in Pa).
  • P2 – the required pressure (in Pa).

3. Determining the composition of the object being pumped

First, it is necessary to determine whether the object being pumped is gas, liquid or particles.

If the gas being pumped contains impurities such as water vapor or a small amount of particulate matter and dust, a rotary vane vacuum pump should be selected with caution.

If a high vacuum degree is required, a filtering device should be added to filter out impurities before using a rotary vane vacuum pump.

Second, it is important to know whether the object being pumped is corrosive (acidic or alkaline, what is the pH value?). If the gas contains corrosive factors such as acids and bases or organic corrosion, it should be filtered or neutralized before selecting a rotary vane vacuum pump.

Third, consider if the object being pumped will contaminate rubber or oil products. Different vacuum equipment should be selected for different pumped media. If the gas contains a large amount of vapor, particles, and corrosive gases, an appropriate auxiliary device should be installed on the inlet pipeline of the pump, such as a condenser, filter, etc. (specifically, contact our technical engineering staff).

Fourth, consider whether the noise, vibration, and appearance of the vacuum pump have an impact on the factory.

Fifth, as the saying goes, you get what you pay for. When purchasing a vacuum pump or vacuum equipment, priority should be given to the quality of the equipment, transportation costs, and maintenance and upkeep fees.

Pumping Speed and Configuration of Vacuum System

Different vacuum systems require different vacuum levels. Therefore, a set of vacuum units must be used to complete the process, by connecting vacuum pumps working in different pressure ranges in series.

The high vacuum pump achieves the required vacuum degree of the system, while the low vacuum pump directly discharges to the atmosphere.

Obviously, the simplest vacuum unit is a direct-venting vacuum pump. However, a high vacuum system generally requires a three-stage unit, and a medium vacuum system generally requires a two-stage unit.

It is difficult to create an effective high vacuum unit using just one high vacuum pump and one low vacuum pump. There are several reasons for this.

One reason is the continuity of flow.

High vacuum pumps have restrictions on the pressure they can withstand at the front stage. When the pre-stage pressure is higher than a certain pressure, the pump cannot work properly.

When the pre-stage pump reaches this critical pressure, the pumping speed may decrease, so the exhaust flow rate of the pre-stage pump may be lower than that of the main pump, which breaks the requirement for flow continuity and will inevitably cause the vacuum unit to not work properly.

However, if a medium vacuum pump is connected between the high and low vacuum pumps, it can play a role of bridging the gap, ensuring flow continuity, and all pumps can work in their optimal state. Roots pumps can work in the medium vacuum range and are the most suitable, so they are also called Roots booster pumps.

Due to its low compression ratio, it can be connected to a range of several Pa to several hundred Pa. When a three-stage high vacuum unit enters a higher vacuum level, since the exhaust flow rate of the main pump significantly decreases, only a small pre-stage pump is needed to maintain the continuity of pumping. This method is often adopted in actual applications, which can reduce the energy consumption of the unit.

Another reason why a high vacuum unit often requires a three-stage unit is the limitation of the suction pressure of the high vacuum pump. The pump has an initial working pressure, and traditional high vacuum pumps are in the range of several Pa. Therefore, the pre-stage pump must pre-pump to this pressure before the main pump can start working.

However, the pre-stage pump that directly vents to the atmosphere often takes a long time to pump to this pressure because as the pressure decreases, the pumping speed of the pump also decreases. Especially for vacuum units with periodic pumping, the time required to reach the working vacuum degree is important.

The longer the pre-pumping time, the longer it takes to enter the working pressure, so adding a medium vacuum pump in combination with a low vacuum pump can reach the pressure at which the main pump can operate in a shorter time, which can ensure the efficiency of the equipment usage.

Roots pumps and oil booster pumps can both be used as medium vacuum pumps. Molecular booster pumps have a very high compression ratio, which allows them to achieve a clean vacuum and excellent high vacuum performance.

They also have a super-strong pumping capacity in the medium vacuum range. This makes molecular booster pumps currently the only vacuum pump that combines medium and high vacuum performance. Therefore, it can be paired with a low vacuum pump to form a high vacuum unit with performance comparable to a three-stage unit.

Specifically, due to the high endurance of molecular booster pumps, the pre-stage pump can easily be in a high flow state; and the high suction pressure of the molecular booster pump reduces the pre-pumping burden of the pre-stage pump.

Molecular booster pumps can work in the range of 100-50Pa, and the pre-stage pump from atmosphere to this pressure basically follows the rule that the pressure drops by one order of magnitude every time unit passes. Therefore, the unit can have a high pumping efficiency.

Simplifying high vacuum units and removing Roots pumps is another advantage of molecular booster pumps. For larger high vacuum application equipment, the pre-pumping capability of the pre-stage pump can be appropriately strengthened to further shorten the pumping time.

Since the pre-pumping time is much shorter than the entire exhaust process, the pre-stage pump can also be used as the pre-pumping function of multiple devices, which is often very practical. This greatly simplifies the vacuum unit for large-scale applications.

In some medium vacuum applications, it is necessary to enter the range of 10-1Pa, which is often difficult to achieve with a two-stage Roots pump unit.

However, using a three-stage Roots pump unit connected in series can raise the vacuum level by one order of magnitude to reach 10-1Pa. Therefore, three-stage units are also commonly used in medium vacuum applications.

Since molecular booster pumps can achieve full pumping speed at 10-1Pa, they can also replace two-stage Roots pumps in a three-stage medium vacuum unit.

Generally speaking, Roots pumps that work continuously in the low-end pressure range of medium vacuum can be completely replaced by molecular booster pumps.

Conversely, Roots pumps that work continuously in the high-end pressure range of medium vacuum should be relatively less because pre-stage pumps often have strong pumping speed in this pressure range.

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Founder of MachineMFG

As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.

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