How to calculate the vacuum degree of vacuum pump

The commonly used vacuum units are PA, kPa, MPa, atmospheric pressure, kg (kgf / cm2), mmHg, mbar, bar, psi, etc.
Before selecting the type, we must make clear several basic concepts about vacuum pump.
Vacuum degree: the thinness degree of gas in vacuum state, usually expressed as vacuum degree. The value read from the vacuum gauge is called vacuum degree. The value of vacuum degree indicates that the actual value of the system pressure is lower than the atmospheric pressure. The value expressed from the table is also called the table pressure, which is also called the limit relative pressure in the industry, that is: vacuum degree = atmospheric pressure - pressure (the atmospheric pressure is generally 101325pa, the limit pressure of water ring vacuum pump is 3300pa; the limit pressure of rotary vane vacuum pump is about 10Pa)
Limit relative pressure: the relative pressure is how much lower the measured internal pressure is than "atmospheric pressure". It shows that the actual value of system pressure is lower than that of atmospheric pressure. Because the air inside the container is pumped, the pressure inside is always lower than the pressure outside the container. Therefore, when expressed in terms of relative pressure or gauge pressure, the value must be preceded by a minus sign, indicating that the internal pressure of the container is lower than the external pressure.
Limit pressure: the pressure is how much higher the measured internal pressure is than the theoretical vacuum (the theoretical vacuum pressure value is 0Pa). The object of comparison is the vacuum pressure value of theoretical state. Due to the limitation of technology, we can not pump the internal pressure to the value of 0 PA, so the vacuum value of vacuum pump is higher than the theoretical vacuum value. So when you use vacuum, there is no minus sign in front of the value.
Pumping capacity: pumping capacity is a measure of the pumping speed of a vacuum pump. General units are L / s and m3 / h. It is a parameter to make up the air leakage rate. It's not hard to understand why it's easy for a vacuum pump with a large volume of air to pump the vacuum degree we need, but a vacuum pump with a small volume of air can't even pump the vacuum degree we want? Because it's impossible for a pipeline or a container to keep air tight all the time, and a large volume of air can make up for the factors that lead to the decrease of the vacuum degree caused by the air leak, so, a large volume of air It is easy to get the ideal vacuum value. It is suggested that, in the case of calculating the theoretical pumping capacity, we try to choose a vacuum pump with a higher pumping capacity. The specific calculation formula of air extraction will be introduced below.
How to calculate the vacuum degree of vacuum pump:
After knowing the basic parameters of vacuum degree, pressure and relative pressure, we can enter the formal selection of vacuum pump.
1. 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 higher than that of vacuum equipment by half to one order of magnitude. (for example, the vacuum degree of 100 Pa (pressure) is required for the vacuum process, and the vacuum degree of the selected vacuum pump shall be at least 50 pa-10 PA). Generally, if the pressure is required to be higher than 3300pa, the water ring vacuum pump is preferred as the vacuum device. If the pressure is required to be lower than 3300pa, the water ring vacuum pump cannot be selected, and the rotary vane vacuum pump or higher vacuum level vacuum pump is selected as the vacuum obtaining device.
2. Air extraction required by the process (extraction rate:
Vacuum pump requires air extraction rate (that is, the ability of vacuum pump to discharge gas, liquid and solid under its working pressure), general unit:
m3/h，L/S,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
The integration process involves relevant experience and other factors.
S=(V/t)×ln(P1/P2)
Where: S is the pumping rate of vacuum pump (L/s)
V is the volume of vacuum chamber (L)
T time required to reach the required vacuum (s)
P1 is the initial pressure (PA)
P2 is the required pressure (PA)
3. Determine the composition of the object to be extracted
*. whether the object to be pumped is gas, liquid or particle. If the gas to be pumped contains water vapor or a small amount of particles and dust and other impurities, the rotary vane vacuum pump should be carefully selected. If the vacuum degree is required to be high, a filter device should be added to filter before using the rotary vane vacuum pump as a vacuum obtaining equipment.
Second, it is necessary to know whether the object is corroded (acid or alkaline, what is the pH value?), if it contains acid-base corrosion or organic corrosion and other factors
The gas should be filtered or neutralized to select the rotary vane vacuum pump.
Third, is the object to be pumped contaminated with rubber or oil? For different media to be pumped, corresponding vacuum equipment should be selected. If there is a large amount of steam, particles and corrosive gas in the gas, it should be considered to install corresponding auxiliary equipment on the air inlet pipeline of the pump, such as condenser, filter, etc. (specific technical engineers).
Fourth, whether the noise, vibration and beauty of the vacuum pump affect the factory.
Fifth, as the saying goes, cheap is not good. When purchasing vacuum pump and vacuum equipment, priority should also be given to the quality of equipment, transportation, maintenance and repair costs, etc.

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