Addtime： 2020-07-16 Browse times： 2722
Before selecting the type, we must make clear several basic concepts about vacuum pump.
Vacuum degree: the degree of rarefaction of a gas in a vacuum state, usually expressed by vacuum. The value read from the vacuum meter is called vacuum degree. The value of vacuum degree indicates that the actual value of system pressure is lower than the atmospheric pressure. The value shown on the table is also called the gauge pressure, which is also known as the limit relative pressure in the industry, that is: vacuum degree = atmospheric pressure absolute pressure (atmospheric pressure is generally 101325pa, the ultimate absolute pressure of water ring vacuum pump is 3300pa; the limit absolute pressure of vane vacuum pump is about 10Pa.
Limit relative pressure: the relative pressure is how much lower the measured internal pressure is than the "atmospheric pressure". It shows that the actual value of system pressure is lower than that of atmospheric pressure. As the air inside the container is pumped, the pressure inside is always lower than that outside the container. Therefore, when expressed in terms of relative pressure or gauge pressure, the value must be preceded by a negative sign, indicating that the internal pressure of the container is lower than the external pressure.
Limit absolute pressure: the absolute pressure is how much higher the measured internal pressure is than "theoretical vacuum (theoretical vacuum pressure value is 0Pa)". It compares the absolute vacuum pressure of the theoretical state. Due to the limitation of technology, we can't pump the internal pressure to the absolute vacuum value of 0Pa in any case. Therefore, the vacuum value pumped by the vacuum pump is higher than the theoretical vacuum value. So when expressed in absolute vacuum, there is no negative sign in front of the value.
Pumping capacity: pumping capacity is a measure factor of vacuum pump pumping speed. General units are expressed in L / s and m3 / h. It is the parameter to make up the air leakage rate. It is not difficult to understand why a vacuum pump with large pumping capacity can easily pump the vacuum degree we need when pumping a container with the same volume, while a vacuum pump with a small pumping capacity is very slow and even unable to reach the vacuum degree we want? Because the pipeline or container can never be absolutely free of air leakage, and the large volume of pumping can make up for the factor of vacuum drop caused by air leakage It is easy to get the ideal vacuum value of the atmospheric quantity. It is suggested that we should choose the vacuum pump with higher pumping capacity as far as possible when the theoretical pumping capacity is calculated. The specific calculation formula of pumping capacity will be introduced below.
After knowing the basic parameters of vacuum degree, absolute pressure and relative pressure, we can enter the formal selection of vacuum pump.
Vacuum degree required by 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: the vacuum process requires a 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 higher than 3300pa, the water ring vacuum pump is preferred as the vacuum device. If the absolute pressure requirement is lower than 3300pa, the water ring vacuum pump cannot be selected, and the rotary vane vacuum pump or the vacuum pump with higher vacuum level is selected as the vacuum acquisition device.
2. Air extraction rate required by the process
The pumping rate required by the vacuum pump (that is, the ability of the vacuum pump to discharge gas, liquid and solid under its working pressure) is generally in 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 process, involving 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 is the time required to reach the required vacuum degree (s)
P1 is the initial pressure (PA)
P2 is the required pressure (PA)
3 determine the composition of the extracted object
First, whether the object to be pumped is gas, liquid or particles. If the extracted gas contains water vapor or a small amount of particles and dust and other impurities, the rotary vane vacuum pump should be selected carefully. If the vacuum degree is required to be higher, the rotary vane vacuum pump should be added for filtration before the rotary vane vacuum pump can be used as the vacuum acquisition equipment.
Second, to know whether the object is corroded (acidic or alkaline, what is the pH value?), if there are acid-base corrosion or organic corrosion and other factors of gas, should be filtered or neutralized before selecting rotary vane vacuum pump.
Third, whether the extracted object pollutes the rubber or oil? According to the different extracted medium, the corresponding vacuum equipment should be selected. If the gas contains a large amount of steam, particles and corrosive gas, the corresponding auxiliary equipment, such as condenser and filter, should be installed on the air inlet pipeline of the pump (contact our corresponding technical and engineering personnel for details).
Fourth, whether the noise, vibration and beauty of the vacuum pump have an impact on 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 the equipment, transportation, repair and maintenance costs, etc.
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