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Roots vacuum pump unit performance index of selection

On the surface, the pumping speed and zero flow compression ratio of roots pump are two distinct concepts, which seem to be two performance indicators that have no relationship at all. But in fact, they are not only performance indicators of the pumping capacity of roots pump, but also closely related to each other. Zero flux compression ratio compared with the pumping speed can reflect more roots pump design and manufacturing quality, and more on the test and application has incomparable advantages, so we think it is at zero flow compression ratio as a roots pump suction capability evaluation index.


Generally, the roots pump must be matched with the front pump to work, so its pumping speed is closely related to the front pump. Roots pump and liquid ring pump, rotary vane vacuum pump, slide valve pump, piston pump.. The vacuum unit can be composed of various types of vacuum units, and various proportions can be adopted according to different requirements. In this way, the vacuum unit can have different ultimate pressure and pumping speed. Therefore, the so-called lutz pump pumping speed is actually the pumping speed of the vacuum unit. It depends to a large extent on the type and size of the front pump, that is, the maximum pressure and pumping speed of the front pump. In "the roots vacuum pump product quality classification standard of" although rules on the backing pump model, but due to the former stage pump (rotary vane pump, plunger pump) specified in the standard, geometric pumping speed of the pump (1. 1 ~ 1. 2) the name of the pumping speed, thus pumping speed measurement error that alone reached 2%.

Rotary Piston Vacuum Pump.jpg

Inevitably, the change of ambient temperature will also have an impact on the measurement of the pumping speed, especially when the vacuum degree is high, which indirectly affects the pumping speed by affecting the vacuum degree of the front-stage pump. "Roots vacuum pump product quality grading standards specified in the assessment of the pumping speed must be with" the roots vacuum pump "standard recommended in the former stage pump, for what ZJ had the following - 300 and its roots pump problem is not big, but for what ZJ had - 600 and more than its roots pump is big problem.


The performance index that truly reflects the pumping capacity and design and manufacturing quality of roots pump itself is zero flow compression ratio, which is the ratio of front stage pressure to inlet pressure at zero flow rate. The compression ratio reflects how much gas the roots pump returns from the outlet to the inlet, that is, the pumping efficiency. The maximum of zero flow compression ratio Komax can be used to measure the pumping capacity of roots pump.


Zero flux compression ratio from the roots pump itself, which is from the design point of view, it depends on the roots pump rotor profile design, processing and pump assembly, that is depends on the rotor and rotor, the rotor and the pump body, rotor and clearance between the side cover. Good rotor type line can make the rotor maintain uniformity in the process of meshing clearance, so that when we are likely to ensure continuous operation under the condition of maximum allowable differential pressure, selection of smaller clearance, in order to make Komax has great value. The roughness of the rotor surface will also result in loss of compression ratio at low pressure, because some of the gas adsorbed on the high pressure side of the rotor surface will be released when transferred to the low pressure side.

Roots and Screw Pump System.jpg

From the test point of view, the zero flow compression ratio depends on the pressure and the type of gas at the front stage. The standard has specified that the test gas is a non-condensable gas, generally referred to as air, and therefore the test of zero flow compression ratio is completely dependent on the pressure at the front stage. The test of zero flow compression ratio generally starts from 10Pa(front stage pressure), so the limit pressure of the front stage pump is not high, even the single-stage rotary disc (slide valve) pump can meet the requirements. There is no strict requirement for the pumping speed of the front stage pump, which is usually within the range of the ratio between the roots pump and the front stage pump. For large pumping speed roots pump, this ratio can be obtained more, because the former stage pump to test the size of the pumping speed affect only forward on the piping test enclosure of aeration, has no effect on zero flow compression ratio.


Zero flux compression ratio is one of the biggest advantages, to arrive at the zero discharge of roots pump compression ratio, a former level pressure curve, can according to the matching of the backing pump pumping speed, inlet pressure curve, the approximate calculation of roots pump and backing pump pumping speed of the unit, so that the factory can need according to customer's production process, through the calculation to select the optimal proportion of the roots vacuum pump units. Roots vacuum pump unit pumping speed (approximate)


In conclusion, the pumping speed and zero flow compression ratio of roots pump are both performance indicators related to the pumping capacity, so only one of them needs to be evaluated. Compared with the two, the zero flow compression ratio has incomparable advantages:


1. It can truly reflect the design and manufacturing quality of roots pump itself.


2. The influence of the front pump on it is small.


3. For large - speed roots pump , it does not require complex front - stage pump system.


4. The pump speed of various matching  roots pump  (unit) can be calculated to determine the optimal design scheme of the roots pump unit required by the user. Therefore, it can be said without hesitation that the zero-flow compression ratio should be chosen as the evaluation index of the suction capacity of the roz rotary vane vacuum pump.

ZJP roots pump.jpg


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