In recent years, the technique of producing high vacua has got great importance. Apart from its use in radio and X-ray equipment, it has helped us for investigating the behaviour of atoms and molecules under low pressure conditions. This has the reciprocal effect causing further improvements in exhaust pumps and high vacuum technique, in general. Today we can produce as low a pressure as 10-9 mm. Simultaneously with the development of these high vacuum pumps, the development of delicate gauges to measure the very low pressures produced by them has got priority.

#### Exhaust Pumps and Their Characteristics:

Before describing the pump, let us consider briefly the characteristics of a good vacuum pump which are:

i. The Exhaust Pressure:

In a vacuum pump, there is an inlet, fine or intake side, from which the gas (or vapour) from the vessel to be exhausted is taken into the pump, and an outlet or exhaust side, from which it is expelled out. The exhaust pressure is the pressure on the exhaust side of the pump. This may be atmospheric or much lower than it which varies from pump to pump.

The higher the vacuum desired to produce on the fine or the intake side of the pump, the smaller must be the exhaust pressure on its outlet side. For high vacuum the pressure is first reduced from atmospheric to a small fraction of it, say to 1 mm or so, using an ordinary pump called auxiliary.

This fore-vacuum or backing pressure is then reduced from 10-4 to 10-7 mm by a fine or high vacuum pump. For the purpose, the backing and the high-vacuum pumps are arranged in series so that the gas or vapour from the vessel to be exhausted is drawn in at the inlet of the latter and expelled at its outlet into the fore-vacuum of the former. It is finally expelled out into the atmosphere.

ii. The Degree of Attainable Vacuum:

By this we mean the lower limit of the pressure possible to obtain in the vessel, connected to the pump. This depends largely on the exhaust pressure. If it be very low, it may result in the passage of the gas or vapour in the reverse direction.

Theoretically, there is no lower limit to the attainable pressure in a diffusion-condensation pump, but, in a molecular pump, a definite limit is set by the constant ratio it bears to the exhaust pressure. The limit may be extended using connecting tubes of wide bores in between the vessel to be exhausted and the pump, as it largely minimises the resistance to the flow of the gas or the vapour from the former to the latter.

iii. The Speed of the Pump:

The speed of a pump may be defined as the relative rate of reduction of pressure in a given volume.

If p0 be the limiting value of the attainable pressure, with the help of a given pump, p, the pressure at an instant t in the vessel of volume V, connected to it, and S, the speed of the pump at this pressure, the rate of reduction of pressure in the vessel, i.e., dp/dt,, i.e., written as-

Therefore the speed of exhaust of a pump may be defined as the rate of change of volume of the gas or vapour in the vessel at any given instant, the measurement of volume being effected at the pressure attained by the pump at that instant.

Eqn. (25.8) says, that when the pump starts working, p is much greater than p0, so that p0/p is practically zero, and hence E is nearly equal to S. In the beginning, the pumping speed of the pump is practically equal to its intrinsic speed. But as p is progressively reduced and approaches p0, E decreases gradually and finally becomes zero when p = p0. Thus a pump loses all its pumping speed at the lowest attainable pressure.

As the pumping speed (E) depends not only on its intrinsic speed (S) and the lower limit of the attainable pressure (p0), but also on the resistance to its flow. It follows that the wide bores of the connecting tubes are very helpful.

#### Types of Pumps:

The classification of the different types of exhaust or vacuum pumps is given below:

#### The Common Air Pump:

The pump consists of a receiver plate connected to a cylinder C, through a tube bent twice at right angles, as shown in Fig. 25.1. The cylinder is fitted with a piston. Both the cylinder and the piston carry valves, V1 and V2, respectively, so that they open only upwards. The vessel F to be exhausted is placed over the receiver plate in the manner shown.

At the beginning the piston is moved up to the top of the cylinder, from its initial position, when the valve V2 remains closed due to atmospheric pressure on it, and the valve V1 is forced open for the pressure of the air or gas in the vessel. As a result it comes and collects in cylinder C. The piston is then moved down. The valve V1 now remains closed owing to the increased pressure on it.

The valve V2 is thrown open by the gas in the cylinder and the gas escapes out into the atmosphere. The operation is repeated a number of times. In each operation the gas is collected in the cylinder during the upward stroke of the piston and is forced out during the downward stroke. Hence after some time, good vacuum in produced in the vessel F.

This type of pump is unable to give a high vacuum, as the pressure of the residual gas or air in the vessel being unable to force the valve V1 open and get into the cylinder. Thus, complete vacuum cannot be created by this pump.

This is mathematically shown below:

Let V c.c. be the volume of the vessel F and the tube up to the bottom of C and v be that of the cylinder. During the first upward stroke, therefore, volume V of the gas expands to (V + v) c.c. Again since during the downward stroke of the piston, a volume v of the gas is swept out, the volume of the gas left behind is V c.c., i.e. V/(V + v) of the original volume (V + v) c.c. During the next upward stroke, this volume expands to (V + v) c.c. again. During the downward stroke, v c.c. is forced out, leaving behind V/(V + v) of the volume left after the first stroke, or V/(V + v) of V/(V + v) of the original volume (V + v) c.c., i.e. [V/V + v]3 of the original volume. f

In a similar way, after the third stroke, the volume of air or gas left behind will be [V/V + v]3 of the original volume, and therefore, after n strokes, the volume left behind will be [V/V + v]3 of the original volume (V + v).

Therefore, it is clear that this expression, [V/V + v]3 can never be zero, whatever be the value of n. In other words we can say that whatever the number of strokes given, there will always be some air or gas left behind in the vessel so there can be no perfect vacuum created inside it.

It is seen that the pressure can hardly be reduced below 1 cm of mercury column with the help of this pump. This is partly due to the inability of the gas to open the valve V1 and partly to leakage and the presence of moisture in the vessel or receiver to be exhausted. To get low pressures, therefore, other types of pumps are used. The Rotary Oil Pumps being the more suitable for the purpose.

#### Rotary Oil Pumps:

The pump was initially devised by Gaede.

These are of two different types, viz.:

(i) The rotary vane oil pump, and

(ii) The stationary vane oil pump.

The principle is for both types:

A massive cylindrical shaft or ‘rotor’ revolving eccentrically inside a hollow stout steel cylinder or ‘stator’, compressing the gas or vapour entering it, and finally ejecting it through a whole pump is kept immersed in oil, to serve to three-fold purposes-

(i) Providing automatic lubrication,

(ii) Preventing leakage of gas or vapour into the high vacuum created, and

(iii) Making for efficient cooling of the pump.

The main parts of the pump are shown in Fig. 25.2. Here C is the hollow, cylindrical steel chamber or ‘stator’ and S is the stout and massive cylindrical shaft or ‘rotor’, rotating eccentrically by means of an electric motor. Such that it is always in contact with the stator at some peripheral I point P.

A slot, cut diametrically, right across the rotor, carries two vanes, slid into it.

These are not only kept apart from each other, but also pressed against the walls of the stator by one or more springs in-between them. The space is thus between the stator and the rotor into two separate compartments.

On either side of P, the rotor and the stator thus remain in contact. The stator is provided with an inlet (I) and an outlet port O, the latter being fitted with spring-operated valve V. The whole pump is kept immersed in oil.

With the rotation of the rotor in the direction shown, the space between the rotor and the stator, on the inlet side, goes on increasing, but that between the rotor and the outlet side of the stator goes on decreasing. As a result the gas or vapour from the vessel connected to 1 is continually drawn into the former and that in the latter gets progressively compressed.

When its pressure becomes sufficiently high, it forces open valve V to escape out of the outlet O. The process is repeated itself until a pressure as low as 10-3 mm is produced in the vessel connected to the pump. A self-sealing oil-valve prevents the gas or vapour from being sucked back into the exhausted vessel, even when the pump stops to work.

It consists of a stout outer cylinder C, inside which is mounted eccentrically, a cylinder R, known as the rotor. This is shown in Fig. 25.3. S is a spring and V is the vane, which keeps the gas or air, already inside the cylinder apart from the fresh incoming gas or air. The outer cylinder is provided with an inlet tube i, connected to the vessel to be exhausted and an outlet tube O, which is provided with a valve, opening outwards.

In order to prevent any leakage, the whole pump is immersed in oil [Fig. 25.3(a)]. A special type of valve prevents the oil from getting into the vessel being exhausted, when the pump is stopped. The rotor is driven at an extremely high speed by a separate electric motor in the direction shown by the arrow heads.

Fig. 25.3(a) reveals the condition to start with, when the inlet tube is connected to the vessel to be exhausted and when the gas from the vessel has just been admitted into the space in between the cylinder and the rotor R. Fig. 25.3(b) shows the condition when the rotor starts to rotate eccentrically, and the gas is being compressed.

Fresh gas enters into the cylinder through the inlet tube i, behind the rotor and is kept apart from that already present by the vane V. Fig. 25.3(c) exhibits the process of compression while Fig. 25.3(d) shows the final stage of compression. At this stage due to an increased pressure of the gas in C, the valve at the mouth of the outlet tube O is forced open. As a result the gas is expelled out.

The gas behind the rotor is similarly compressed and forced out. The cycle is repeated, until a high vacuum is produced in the connected vessel. On an actual arrangement the pumping system consists of two such units, in series with each other.

They are mounted side by side and worked by the same motor. The first unit directly works from the atmosphere and the second works from the fore-vacuum created by it. The highest speed of working attainable is about 6 litres per minute and the achieved vacuum is about 10-3 mm.

If a vessel be connected to the outlet tube O, the gas will be compressed into it, and so the pump can also be used as a compression pump.