1 HIGH VACUUM PRODUCTION

Introduction

Ideally, a vacuum is some space which contains “Zero matter” but in real life it is almost impossible to achieve the state of ideal vacuum. So, there is another definition of vacuum which can be stated as – an environment where the air/gas pressure is less than the atmospheric pressure alternatively an enclosed space where density of air is less than that at atmospheric pressure is called vacuum. To create vacuum, we need pumps. Basically, pump is defined as a device which suck out the matter from one space region and discharge to another space region with the help of creating gradient between two regions.

Mechanical pumps

Mechanical pumps are the most basic vacuum pumps which are being used in semiconductor industry, research laboratories, food industries etc. some of the common examples are rotary vane pump, turbomolecular pump etc. Mechanical pumps can be classified into two broad categories- 1. Positive displacement pumps, 2. Momentum transfer pumps

1.      Positive displacement pumps (PDPs): Meaning of positive displacement can be understood as when the pump piston or rotor moves, it displaces the fluid or gas ahead of it. As we know partial vacuum may be created by expanding the volume of container. If in any chamber, compartment of vacuum can be closed off, subsequently exhausted and then expanded again and this cycle continues then we can evacuate the chamber indefinitely upto certain very less pressure without requiring infinite expansion of chamber volume. This the principle of positive displacement pumps. Very common example which uses this principle is manual water pump. These pumps use fluid or gas pressure to transmit power. These pumps have very close fittings among the components hence chances of occurring of leakages is very less. Pressure relief valves is provided in these pumps because these pumps require protection against over pressure if the resistant to flow becomes very large. PDPs are found in variety of areas such as chemical processing, food cleaning industry, liquid delivery, biotechnology etc. The popularity and versatility of the PDPs is due to their compact design, continuous flow regardless of differential pressure, high viscosity performance and ability to handle high differential pressure. Rotary pumps and roots pump are the classic examples of this category.

2.      Principle Displacement pumps

3.

4.      Fig.1: Principle of the positive displacement type pump.

5.      The examples of displacement type pumps which works on the same principle

6.      Rotary vane pump, Scroll pump etc.

7.

8.        Momentum transfer pumps: In the momentum transfer pumps, gas molecules to be evacuated from the chamber is accelerated out to the exhaust side which is usually backed by positive displacement pump to maintain the operating base pressure of the order of 1×10-2 for high vacuum pump. Based on the laws of dynamics matter flows differentially at different pressures. From atmospheric pressure to the mild vacuum, interaction of molecules among themselves dominates and they push neighboring molecules known as viscous flow. If the pressure is below the mild vacuum, distance between the molecule increases then molecule interact with the chamber more often than among themselves resulting in more effective molecular pumping than the positive displacement pumping. Two main example of momentum transfer pump are turbomolecular pump and diffusion pump. The main difference between these two pumps are that in diffusion pump gas molecules are blown out using jet of oils moving in ultrasonic speeds inside the main body of pump and in turbomolecular pumps uses high speed fans to evacuate chambers.

Principle of Momentum transfer pump

Fig.2: Principle of the momentum transfer type pump.

The examples of displacement type pumps which works on the same principle Diffusion Pump, Turbomolecular Pump etc.

Rotary Pump

Working: Rotary pump take some volume of gas and compress it so that the pressure of the taken volume of gas become slightly higher than the atmospheric pressure and then discharge it to the atmosphere. Rotary pump evacuate the chamber from 760 Torr to 1×10-4 Torr. Typical internal parts of rotary pumps are shown in fig. Chamber in which vacuum is to be done is connected to the inlet part. Once rotary pump is connected to the chamber and vane starts rotating and eventually it entraps large volume of gas molecules which are present inside the chamber and as the vane moves further, gas expands in crescent shaped volume and as the vane rotates further it compresses the entrapped gas below the exhaust valve until the pressure of the entrapped gas molecules exceeds the atmospheric pressure. Eventually the pressure exceeds atmospheric pressure and gas molecules are then expelled to the atmosphere. The minimum pressure limit is determined by the leakage of the gas around the seals used in the various parts of rotary pump. For a single stage rotary pump, it is around 50 mTorr and for the two-stage rotary pump it is 0.1mTorr. In two stage rotary vane pumps, vent of the first stage is connected to the inlet port of second stage. Adding more stages in this series of two stage has no practical benefits. Rotary pumps have a problem in working if there are condensable vapours present in the chamber. One of the most common vapor of this kind is water vapor. As the water vapours inside the chamber will be compressed inside the rotary pump, it will condense into liquid prior to the opening of the exhaust valve. This liquid dilutes the oil and further it will corrode the pump. In order to prevent this condensation problem, a gas ballast is provided with the pump. the gas ballast valve will introduce small amount of dry gas in the region where compression takes place. This introduction of dry gas will reduce the amount of compression that a condensable vapor undergoes and hence reduces the condensation problem. The oil that surrounds the stator serve as lubricant of moving part and also it seals the outlet valves from any leaks. Rotary pump oils must have low vapor pressure still be viscous enough to seal across the vanes of pump. It is also important to check oil level time to time and always be maintained at proper level.

Turbomolecular Pump

Turbomolecular pumps are the extended version of one of the earlier high vacuum pump known as molecular drag pump. Basic cross-sectional view of Molecular Drag pump is shown below.

It can be seen that a spiral channel is machined into the stator and a flared disc as a rotor. These molecular drag pumps reported to be designed in early 1900’s had very low pumping speed and misaligned bearing designs were also existed which limited its practical applicability for relaizing and maintaining high vacuum. To overcome these issues there was changes in design and this is called as modern molecular drag pump.

In the construction part, very high strength aluminium alloy is used for making rotor and the shape is kept to be like inverted cup. Inside and outside surface of the rotor is machined to create spiral grooves which are aligned with the stator in a specific pattern to give pump action. Both the surfaces (inside and outside) of the rotor creates elongated pumping path. The shape ,size and tolerances of the machined grooves changes from the inlet part to the exhaust part in order to allow for multiple compression stages. To provide cooling to the pump, flush gas is used intentionally. Flush gas is also used for exhausting the compressed gas inside the pump. In general, molecular drag pumps have the compression ratio of 109:1, for nitrogen. Pumping action of the molecular drag pump is dependent upon the time which gas molecules stay at rotor and stator and also on the velocity with which gas molecules hit on the blades of rotor. So higher the molecular weight of gas to be pumped , lesser the pumping speed. Now a days there is another change in design that the arrangement of the rotor and stator is arranged like a turbine, so now this pump is called as Turbo molecular pump (TMP). Outside view and internal parts are shown in figure below.

Turbomolecular pumps cannot compresses the air directly into the atmosphere so these pumps always require a backing pump attached with exhaust of the turbomolecular pump to accomplish the final stage of compression. Critical backing pressure for turbomolecular pumps is 10 to 40 Torr which allows these TMPs to be backed by rotary pump. Chambers pumped by these TMPs may achieve base pressure of the order of 10^-6 Torr.

Roots pump

Roots pump are one of the example of mechanical vacuum pumps which do not have any oil or lubricating fluid. Internal diagram of typical Root’s pump with 4 stages required for creation of vacuum is shown below. It consists of two double lobe impellers shown in black (dumble shaped) which rotates in opposite directions with the pump housing. In the diagram below, upper down faced arrow shows gas inlet connected from chamber and lower down faced arrow is the outlet of roots pump. Position of the rotors in situation I and II is such that the volume intake increases from I to II. As both the rotors move further some part of the volume is now cut off from the inlet side can be seen in situation III. In situation IV, this much volume is opened up to the exhaust side. Now gas under fore vacuum pressure which is higher than the intake pressure, flows in. This inside flown gas compresses the gas coming from the inlet side. As the impellers rotates further, this volume of compressed gas is ejected at the exhaust part. This process is repeated twice in one full rotation of both the impellers.

Sometimes three lobe and four lobe rotors are used for higher pressure duty. As shown in figure both the impellers are exactly identical. They are dimensioned and arranged in such a manner that when they rotates, large enough part of the first impeller is a close fit to the surface of second impeller. The two impellers are dimensioned to be in close fit with pump housing also. When these two impeller rotates, they really never touches each other at any point of time and also they do not touch housing of the pump at any point of time. In both the cases (impellers with housing or within impellers) there is a clearance of 0.05 mm to 0.25 mm.

Since the isolation from inlet port to the outlet port is very narrow (only clearance part) there is back flow of gas which is being pumped from exhaust region to the inlet region so efficiency of compression is lesser as compared to oil sealed vacuum pumps. As far as pumping speed is concerned there is an extra advantage in roots pump owing to the zero rubbing contact. Due to zero friction which results from contacts, impellers can rotate in very fast sped (1000 – 4000 rpm) results in much higher pumping speed.

The efficiency of pump occurs when pump is operated at pressure 5x 10-2 Torr with compression ratio of 10. So these roots pump require a backing pump like rotary pump for better efficiency and pumping speed.

Operating pressure range of single stage roots pump is 10 to 10-5 mbar total pressure and it is very much sensitive to the fore pressure. 3 and 4 stage roots pump can lower down the pressure upto 10-6 Torr.

REFERENCES

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