# Vacuum pump selection principle and related calculation formula

Before selecting the type, we must make clear several basic concepts about vacuum pump.

Vacuum: the thinness of a gas in a vacuum state, usually expressed as vacuum. The value read from the vacuum meter is called vacuum degree. Vacuum degree value is to express the actual value of the system pressure lower than the atmospheric pressure, the value expressed from the table is also known as table pressure, the industry is also known as the limit relative pressure, namely: vacuum degree = atmospheric pressure – absolute pressure (atmospheric pressure is generally 101325Pa, water ring vacuum pump limit absolute pressure 3300Pa; The ultimate total pressure of rotary vacuum pump is about 10Pa.

Ultimate relative pressure: relative pressure is how much lower the measured internal pressure is than atmospheric pressure. The actual value of the system pressure is lower than the atmospheric pressure. The pressure inside the container is always lower than the pressure outside the container because the air inside is being pulled out. So when you write it in terms of relative pressure or surface pressure, you have to put a negative sign in front of it, which means that the pressure inside the container is lower than the external pressure.

Ultimate total pressure: total pressure is how much higher the measured internal pressure is than the theoretical vacuum (theoretical vacuum pressure value is 0Pa). It is compared to the theoretical value of the absolute vacuum pressure. Due to the limitations of the process, we could not pump the internal pressure to the value of absolute vacuum 0Pa in any case. Therefore, the vacuum value pumped by the vacuum pump is higher than the theoretical vacuum value. So when you’re dealing with an absolute vacuum, there’s no negative sign in front of it.

Air pumping: air pumping is a vacuum pump pumping speed measurement factor. The general units are expressed in terms of L/S and m3/h. Is the parameter that makes up for the leakage rate. It is not difficult to understand why a vacuum pump with a large volume of air is easy to pump the vacuum we need, while a vacuum pump with a small volume of air is slow or even unable to pump the vacuum we want. Because it is impossible for the pipeline or container to be absolutely airtight, and the large amount of air extraction makes up for the decrease of vacuum degree caused by the air leakage. Therefore, the ideal vacuum degree can be easily extracted with large amount of air. It is suggested that, in the case of calculating the theoretical pumping capacity, we try to select the vacuum pump with a higher pumping capacity. The specific calculation formula of the pumping amount will be introduced below.

After knowing the basic parameters of vacuum pump, such as vacuum degree, absolute pressure and relative pressure, we can enter the formal selection of vacuum pump.

1.The vacuum degree required by the process
The working pressure of vacuum pump should meet the requirements of process working pressure, and the vacuum degree should be half to one order of magnitude higher than that of vacuum equipment. (for example: vacuum process requires the vacuum degree of 100pa(absolute pressure), and the vacuum degree of vacuum pump should be at least 50pa-10pa). Generally, if the absolute pressure is required to be higher than 3300Pa, water ring vacuum pump is preferred as the vacuum device. If the absolute pressure is required to be lower than 3300Pa, water ring vacuum pump cannot be selected. Rotary plate vacuum pump or vacuum pump with higher vacuum level is selected as the vacuum obtaining device.

2. Exhaust volume (exhaust rate) required by the process
Vacuum pump requires the rate of air extraction (that is, the ability of vacuum pump to discharge gas, liquid and solid under its working pressure), generally m3/h, L/S and m3/min. The specific calculation method can refer to the following formula for self-calculation and selection. Of course, the selection of vacuum pump is a comprehensive process, involving relevant experience and other factors.

S = (V/t) by ln (P1 / P2)

Where, S is the vacuum pump pumping rate (L/ S)

V is the volume of vacuum chamber (L)

T is the time required to reach the required vacuum degree (s)

P1 is the initial pressure (Pa).

P2 is the pressure required.

Determine the composition of the object being drawn
First, whether the extracted object is gas, liquid or particle, if the extracted gas contains water vapor or a small amount of particles and dust and other impurities, carefully select the rotary vacuum pump, if the vacuum degree requirements are high, then filter device should be added to filter before using rotary vacuum pump as vacuum acquisition equipment.

Second, you should know whether the object is corroded (acidic or alkaline, what is the PH value?). , if contain acid and alkali corrosion or organic corrosion and other factors of the gas, should be filtered or neutralized before the selection of rotary vacuum pump.

Third, is the extracted object contaminated with rubber or oil? According to different pumping medium to choose the corresponding vacuum equipment, if the gas contains a lot of steam, particles, and corrosive gas, should be considered in the pump inlet pipe line installation of the corresponding auxiliary equipment, such as condenser, filter, etc. (specific contact our corresponding technical engineers).

Fourth, the vacuum pump noise, vibration, aesthetic impact on the factory.

As the saying goes, cheap is not good. When purchasing vacuum pump and vacuum equipment, the quality of the equipment, transportation and its maintenance and maintenance costs should be given priority.

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