The vacuum pumps used in vacuum system include diffusion pump, mechanical pump, rotary vane pump, roots increasing pump and dry pump. In order to obtain better vacuum effect and pumping rate, vacuum pump combination unit is usually used.

Industrial common combination: mechanical pump + diffusion pump, mechanical pump + molecular pump

Structure diagram of rotary vane pump:

Working process:

Gas enters the rotor and stator from the inlet

The eccentric rotor compresses air and delivers it to the outlet

The gas accumulates to a certain pressure at the outlet and ejects to the atmosphere

Scope and characteristics of work:

Atmosphere to 10-3 torr

Durable, high price

Because of the stator of the pump. The rotor is immersed in oil, and oil enters the container every week, and there is pollution

It is required that the mechanical pump oil has low saturated vapor pressure, certain lubricity, viscosity and high stability.

Schematic diagram of rotary vane mechanical pump

Generally, the mechanical pump is used to pump vacuum in a rough way, so it can not make the system vacuum too high, which will cause serious oil return flow back to the mechanical pump. As long as it reaches the working range of high vacuum pump, it can be used. Oil level control inhales many dust and pollution problems. It can also add a dust removal device and a filter device oil to the pump entrance, emulsify the gas and steam. It can add a gas device or a condenser in the roots pump. The temperature of the inhaled gas is too high, for example, it is higher than 50 ℃, and the air-cooled Roots pump should be used to cool the gas. Check the temperature of each part of the pump frequently and adjust the cooling water quantity in time.

Application of gas town:

The gas extracted by the pump usually contains a certain amount of condensable gas. If the partial pressure of the condensable gas exceeds the saturated vapor pressure at the pump temperature, they will condense and mix with the pump oil. With the accumulation of condensate, the performance of the pump oil will become poor and even emulsified. We can use gas town to prevent condensation of steam, thus avoiding oil pollution. The concrete method is to fill the dry air or nitrogen at room temperature into the compression chamber of the pump body from the gas ballast hole, so as to play a dilution role, so that the compressed steam partial pressure can be kept under the saturated steam pressure at the pump temperature, so the steam can not be compressed and other gases can be discharged out of the pump. Although the gas town can discharge steam, it also reduces the pumping speed and limit vacuum of the pump. Therefore, it is usually necessary to open the valve to discharge the steam, and then close the valve for a while, and then the pump limit vacuum can be achieved after a while. When the pump oil is polluted by a small amount of condensate, it can be purified from the pump oil through the gas town: close the inlet valve, open the gas town valve, and the pump oil can be recovered after a single time of operation.

Working principle of roots pump

Roots pump is divided into three types: low vacuum roots pump, medium vacuum roots pump (supercharger pump), high vacuum roots pump (dry pump).

The power required by the compressor gas of roots pump is proportional to the pressure difference. Once the gas pressure difference is too high, the pump may overload, which causes the motor winding to burn.

Roots pump features:

Fast start, fast work, insensitive to dust and steam in the gas, rotor does not need oil lubrication, provides a relatively clean working environment, small driving power, small mechanical wear, compact structure, small size, low operation and maintenance costs

Cooling problem of roots pump rotor:

If the pump works continuously for a long time, the temperature of the rotor will rise. In the range of 10 Torr, the heat conduction ability of the extracted gas will become very poor. The heat dissipation of the rotor is mainly radiated to the pump cavity and taken away. This will cause a large temperature difference between the rotor and the pump cavity. The thermal expansion of the rotor may lead to the reduction or even disappearance of the gap between the rotors and the dry pump cavity, and the rotor may be stuck, Even damage.

Generally, there are two ways to cool the rotor: 1. Add water cooler at the air outlet, and the exhaust return gas turns into low-temperature gas through the heat exchanger, which can cool the rotor through the clearance return flow; 2; The cooling fluid enters into the rotor through the neck hole of the spindle, and then the rotor is cooled by circulation. The amount of oil is adjusted according to the size to achieve a reasonable cooling effect, and the oil is cooled by circulating water (the biggest defect of the internal cooling method is that there are bubbles in the rotor, which will cause unbalanced operation of the rotor and damage the pump).

In order to achieve the maximum cooling effect, it is necessary to:

1. The backflow aeration is completed quickly, the blocking effect of cooler is small, and the diversion is large.

2. The heat exchange of the cooler to the gas should be sufficient, so the exchange area should be as large as possible.

But the two are contradictory, so we must consider comprehensively the law of gas flow state and the law of heat exchange to optimize the selection.

Of course, more factors need to be considered: rotor speed, pressure difference, gas temperature, cooling water factor, pump cavity volume, etc.

Common problems:

High oil temperature:

The cooling is not good, the oil level of the box is too high, the filter is blocked, the oil circuit is blocked, and the working fluid does not meet the requirements

Turbine does not run:

The oil level of the box is too low, the filter is blocked, and the inlet of the hydraulic pump leaks

Oil leakage at shaft end:

The oil seal is damaged, the oil seal is not installed properly, and the coupling produces rare effect (add a wind shield)

Rotor stuck or damaged:

There is something wrong with the cooling water

Sudden stop of pump:

The cooling water problem and the failure of the front stage pump lead to the high outlet pressure of roots pump and overload of roots pump

Vacuum degree Jiangdu

Front stage pump failure or poor performance, pipeline leakage

Noise in pump:

Due to the wear of machine parts, large abrasive particles in the production gas enter into the pump body. Because the narrow gap between the rotor and the pump cavity is very small (generally 0.1-0.2mm), and the wear of the pump during long-term operation, the rotor may rub and make abnormal noise.

Generally, the pump should be operated once a year for major overhaul: wear of gear and bearing, inspection of sealing device, replacement of sealing ring, inspection of rotor corrosion, rotor scaling, pump cavity corrosion and scaling.

Source [vacuum net]