How to choose vacuum system

The so-called vacuum system refers to a device which can meet the requirements of certain vacuum degree by combining vacuum pump, vacuum gauge and various parts through pipes in a proper way.

The basic requirements of vacuum system are:

Obtain the required limiting vacuum and working vacuum in the device or working room. The limiting vacuum degree refers to the degree of vacuum achieved by the device when there is no leakage and deflation. The second working vacuum degree refers to the degree of vacuum that can be maintained when the device is vacuum treated. A large amount of gas is often released during vacuum treatment, and the working vacuum will be significantly lower than the limit vacuum. Limit vacuum degree and working vacuum degree have their own importance, they affect the quality of devices from different sides. For example, in the exhaust vacuum system with oxide cathode device, a large amount of gas will be discharged when the electrode is degassed and the cathode is decomposed. If the working vacuum is poor, the electrode may be oxidized and the cathode activation may be poor. However, when the device is finally sealed off, there is basically no leakage, and the vacuum degree in the device depends on the limit vacuum degree of the exhaust vacuum system.

The time required to obtain a certain working vacuum. This time depends on the pumping speed of the vacuum system. This time can be shortened by increasing the pumping speed. Large studios, such as aerospace simulation devices, obviously need large pumping speed. Sometimes, although the studio is not large, due to the serious leakage, the vacuum system also needs a large pumping speed, otherwise a large number of leakage will greatly reduce the vacuum degree, making the system unable to work.

There are suitable residual gas components in the device or working room. It has been proved that it is not enough to simply improve and guarantee the limiting vacuum and working vacuum, and there should be some requirements for the residual gas composition. For example, in order to keep the sensitivity of the vacuum camera device within one year, the oxygen content of the residual gas in the device should not be higher than 10-2Pa high pressure resistant vacuum switch tube, which should not contain hydrocarbon molecules, etc.

In addition, the vacuum system must be simple in structure, reliable in performance, convenient in operation and maintenance, and cheap in price. In order to meet these requirements, it is necessary to comprehensively consider the selection of pumps, the determination and arrangement of pipe size, assembly process, etc. At the same time, these basic requirements are often interrelated. For example, because the vacuum system always has micro leakage, the pump with large pumping speed can obtain better working vacuum than the pump with small pumping speed, but the pump with large pumping speed is expensive and consumes more water and electricity, so there is a problem of reasonable selection.

Generally speaking, for a vacuum system with better air tightness, if there is no regular air release and no steam source inside, a smaller pump with better ultimate vacuum degree can be used. However, if there is a lot of air leakage in the system, the method of increasing the pumping speed must be used to ensure its working vacuum degree. At this time, the pump with larger pumping speed should be used, and the pipeline flow guide should be increased as much as possible, that is, the short and thick pipeline should be selected.

The residual gas composition of vacuum system, if there is no other gas source in the system, mainly depends on the characteristics of the pump.

In order to achieve the best configuration, we should consider the following points when selecting the vacuum system:

1. Determine the working vacuum range

First of all, the vacuum degree required by each process must be checked and determined. Because each process has its suitable vacuum range, it must be carefully studied and determined.

2. Determine the limit vacuum

On the basis of determining the vacuum degree required by the process, check the limit vacuum degree of the vacuum pump system, because the limit vacuum degree of the system determines the best working vacuum degree of the system.

Generally speaking, the limiting vacuum degree of the system is 20% lower than the working vacuum degree of the system, and 50% lower than the limiting vacuum degree of the previous stage pump.

3. Selection and calculation of main vacuum pump

s=2.303v/tlog(p1/p2)

Among them, s is the pumping rate of vacuum pump, V is the volume of vacuum chamber, t is the time required to reach the required vacuum degree, P1 is the initial vacuum degree, P2 is the required vacuum degree. For example: v = 500L t = 30s P1 = 760torr P2 = 50torr, then: S = 2.303v/t log (P1 / P2) = 2.303×500/30xlog (760 / 50) = 35.4l/s, of course, the above formula is only the theoretical calculation results, and there are several variables not taken into account, such as pipeline flow resistance, leakage, flow resistance of filter, temperature of extracted gas, etc. In fact, the safety factor should also be taken into account.

4. Vacuum volume

Check and determine the time required to reach the required vacuum, the flow resistance and leakage of the vacuum pipeline.

After the required vacuum degree is reached, the air extraction rate required to maintain vacuum under certain process requirements is considered.

5. Types of extracted gas and air volume

Check and determine the type and volume of air extraction required by the process. Because if the kind of pumped gas reacts with the liquid in the pump, the pump system will be polluted. At the same time, it is necessary to determine the appropriate exhaust time and the amount of gas produced in the process of extraction.

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