Select a good vacuum pump system is to choose the main pump, the choice of the main pump type and pumping speed is very important
Determination of main pump type
(1) determine the type of main pump according to the ultimate pressure required by the vacuum chamber. In general, the selection of the limit pressure of the main pump is half to an order of magnitude lower than the limit pressure of the vacuum chamber.
(2) select the main pump according to the working pressure required by the vacuum chamber for process production. The working point of the main pump should be correctly selected. Within the working pressure range, it should be able to eliminate all the gas produced in the process of vacuum chamber. Therefore, the working pressure in the vacuum chamber must be within the range of the optimal pumping speed pressure of the main pump.
(3) select the main pump according to the volume of the vacuum chamber and the required extraction time. The size of the vacuum chamber has an effect on the time to the limit vacuum. When the pumping time is certain, if the vacuum chamber volume is larger, the pumping speed of the main pump is also larger.
(4) correct combination of vacuum pump, because the vacuum pump selectively pumping air, so sometimes choose a pump can not meet the requirements of pumping air, need several pump combination, mutual supplement to meet the requirements of pumping air.
(5) if the vacuum system strictly requires no oil, it should choose all kinds of non-oil pump as the main pump. If the requirements are not strict, you can choose the oil pump, and then add some anti-oil pollution measures, such as cold trap, baffle, oil trap, can also meet the requirements of clean vacuum.
(6) when choosing a vacuum pump, the gas components to be extracted should be known, and the corresponding pump should be selected according to the gas components to be extracted. If the gas contains corrosive gas, particle dust, etc., then should consider the choice of dry vacuum pump, corrosion resistant vacuum pump, etc. Or install auxiliary equipment in the inlet pipe of the pump.
(7) select the main pump according to the initial investment, daily maintenance cost and other economic indicators of the whole vacuum system.
Calculation of pumping speed of main pump
After the type of the main pump is selected, the next is to determine the specific size of the main pump pumping speed specifications. The main pump pumping speed is determined based on the working vacuum degree and the maximum exhaust flow of the pump, as well as the volume of the pump and the required pumping time.
(1) calculation of exhaust flow in vacuum chamber. In the normal process, the gas flow generated in the vacuum chamber should be removed by the main pump in time to ensure that the pressure in the vacuum chamber meets the requirements of working vacuum degree. The gas flow in the process can be calculated by the following formula
Q = Qg + Qn + + Ql Qf (Pa. M after/S)
Type in the
Q– the total flow of air generated in the vacuum chamber,
Qg– flow rate of gas emitted by materials being smelted or treated in the process, Pa. M ^ 3 /s;
Qn– air flow rate of refractory insulation materials used in vacuum chamber, Pa. M ^ 3 /s;
Qf– gas flow rate, Pa.
Ql– air flow rate of the vacuum outdoor atmosphere leaking into the vacuum chamber through the sealing connections, Pa. M ^ 3 /s,
The above quantities may not exist in different vacuum application equipment, which should be considered according to the actual situation.
(2) calculation of the effective pumping speed required by the pumped container. Let the maximum exhaust flow in the pumped container be Q(unit: Pa. M ^ 3 /s), and the required working pressure be Pg(unit: Pa), then the effective pumping rate Sez of the main pump at the outlet of the pumped container can be obtained according to the working pressure Pg and the maximum exhaust flow Q required by the vacuum container
Sez = Q/Pg after (m/s)
In the case of 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) rough calculation of the pumping speed of the main pump S. Since the pipeline between the outlet of the vacuum chamber and the inlet of the main pump was not determined before the selection of the main pump, its flow guide C was unknown, and the pumping speed S of the main pump could not be accurately calculated. According to the following empirical formula rough out the pumping speed of the main pump, namely
S = KsSez after (m/S)
Where Ks is the loss coefficient of pumping speed of the main pump at the outlet of the vacuum chamber. When the collector is not used in the pipeline between the inlet of the main pump and the outlet of the vacuum chamber, Ks=1.3~1.4. When traps are used, Ks=2~2.5. After rough calculation of the pumping speed of the main pump S, select the main pump that conforms to the rough calculation value S from the vacuum pump series according to the value of S, and set the crude pumping speed of the main pump as Sp.
(4) check and determine the pumping speed of the main pump. After rough calculation of the pumping speed of the main pump S, select the main pump with appropriate specifications according to the value of S.
According to the rough selection of the inlet size of the main pump, choose to determine the size of the connecting pipeline and other components, and then calculate the flow guide C of the pipeline by the flow guide formula, and then press the following formula to actualize the pumping speed of the main pump
Sp = SezC/C – Sez
By (3) calculate Sp if with (2) rough calculation of the pumping speed difference is very small, the coarse selection of the pump as the main pump, otherwise should be selected again.
Matters needing attention
The key problem of vacuum pump system design is to select the main pump of vacuum pump group.