PVD equipment vacuum pump system method for selecting main pump

Choose a good vacuum pump systems mainly to choose a good main pump, select the type of the main pump and pumping speed is important.

Method / Step 1:

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Determination of the main pump type

(1) Determine the type of main pump based on the limit pressure that the vacuum chamber needs to establish.Generally, the ultimate pressure of the main pump is selected to be half an order of magnitude lower than the ultimate pressure of the vacuum chamber.

(2) Select the main pump according to the working pressure required for the process production in the vacuum chamber. The working point of the main pump should be correctly selected, and within the working pressure range, the total amount of gas generated in the vacuum chamber process should be excluded.Therefore, the working pressure in the vacuum chamber must be within the optimum pumping pressure range of the main pump.

(3) The main pump is selected according to the volume of the vacuum chamber and the required pumping time. The volume of the vacuum chamber has an effect on the time the system draws to the ultimate vacuum.When the pumping time is required to be constant, if the vacuum chamber volume is larger, the pumping speed of the main pump is also larger.

(4) combined correctly vacuum , since the suction pump selectively, thereby sometimes choose one suction pump can not meet the requirements, requires several pump combination, complement each other to meet the pumping requirement.

(5) If the vacuum system strictly requires no oil, then a variety of oil-free pumps should be selected as the main pump. If the requirements are not strict, you can choose to have an oil pump, and then add some oil pollution measures, such as adding cold traps, baffles, oil traps, etc., can also achieve clean vacuum requirements.

(6) When selecting a vacuum pump, you should know the composition of the pumped gas and select the appropriate pump for the gas component to be pumped. If the gas contains corrosive gas, particulate dust, etc., you should consider a dry vacuum pump , a corrosion-resistant vacuum pump, etc. Or install auxiliary equipment on the inlet pipe of the pump.

(7) Select the main pump according to the economic indicators such as the initial investment and daily maintenance costs of the entire vacuum system.

Method / Step 2:

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Calculation of pumping speed of main pump

After the type of main pump is selected, the next step is to specifically determine the size specification of the main pump pumping speed. The main basis for determining the pumping speed of the main pump is the working vacuum of the pumped container and its maximum exhaust flow rate, as well as the volume of the pumped container and the required pumping time.

(1) Calculation of the exhaust flow rate in the vacuum chamber. In the normal process, the gas flow generated in the vacuum chamber should be pumped away by the main pump in time to ensure that the pressure in the vacuum chamber meets the requirements of the working vacuum. The air flow during the process can be calculated by the following formula

Q=Qg+Qn+Qf+Ql (Pa .m3 /S)

In the middle

Q---the total air flow generated in the vacuum chamber,

Qg--the amount of gas discharged from the material being smelted or treated during the process, Pa.m3/s;

Qn--the outlet flow rate of the refractory insulation material used in the vacuum chamber, Pa.m3/s;

Qf--the gas flow rate of the inner wall of the vacuum chamber and all the components exposed to vacuum, Pa.m3/s;

Ql-- the air flow that leaks into the vacuum chamber through the sealed joints of the vacuum outdoor atmosphere , Pa.m3/s,

The above quantities do not necessarily exist in different vacuum application equipment, which should be specifically considered according to the actual situation.

(2) Calculation of the effective pumping speed required for the pumped container. Let the maximum exhaust flow rate in the pumped container be Q (unit: Pa.m3/s), and the required working pressure is Pg (in Pa), according to the working pressure Pg and maximum exhaust flow required by the vacuum container. Q, the effective pumping speed of the main pump at the exit of the pumping vessel is available.

Sez=Q/Pg(m3/s)

If it is a low vacuum system, the effective pumping speed of the pump

Sez=Ql+ Qg+Qn/Pg

In the formula, the symbolic meaning is the same.

(3) Calculate the pumping speed S of the main pump. Since the line between the outlet of the vacuum chamber and the inlet of the main pump is not determined before the main pump is selected, the conductance C is unknown, and the pumping speed S of the main pump cannot be accurately calculated. The pumping speed of the main pump can be roughly calculated according to the following empirical formula, ie

S=KsSez(m3/s)

Where Ks is the pumping speed loss coefficient of the main pump at the outlet of the vacuum chamber. When the trap is not used in the pipeline between the main pump inlet and the outlet of the vacuum chamber, Ks=1.3~1.4; when using the trap, Take Ks=2~2.5. After the main pumping speed S is roughly calculated, the main pump corresponding to the rough value S is selected in the product range of the vacuum pump according to the S value, and the pumping speed of the rough selected main pump is Sp.

(4) Checking and determining the pumping speed of the main pump. After the main pumping speed S is roughly calculated, the main pump of the appropriate specification is roughly selected according to the S value.

According to the inlet size of the rough selected main pump, choose to determine the size of the connecting pipe and other components, and then determine the flow conductance C of the pipeline by the flow conductance formula, and then press the main pumping speed of the main pump.

Sp=SezC/C-Sez

If the difference between the pumping calculated by (3) and (2) the crude pumping speed is small, the rough-selected pump can be used as the main pump, otherwise it should be re-selected.

Precautions

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The key issue in the design of the vacuum pump system is to select the main pump of the vacuum pumping unit.