GB2232445A

GB2232445A - A multi-stage vacuum pump

Info

Publication number: GB2232445A
Application number: GB9010434A
Authority: GB
Inventors: Katharina Koterewa; Zwetko Zwetkow
Original assignee: Medizin und Labortechnik Leipzig VEB
Current assignee: Medizin und Labortechnik Leipzig VEB
Priority date: 1989-05-09
Filing date: 1990-05-09
Publication date: 1990-12-12
Also published as: FR2646881A1; GB9010434D0; DD283190A5; DE4001668A1

Description

1 A Multi-stage vacuum pump The invention relates to a multi-stage vacuum

pump for producing medium-high and high vacuum.

The multi-stage vacuum pump may be used in industry and in the laboratory in the same way, in which the preferred fields of application are vacuum drying, vacuum impregnation, thermal processing in a vacuum, gas extraction in a vacuum and pre-vacuum production for oil and mercury diffusion pumps.

A multi-stage vacuum pump is known, more particularly a rotary piston pump, comprising two vacuum stages. The suction side of the first stage is in connection with the outlet side of the second vacuum stage via a connecting line. The second suction stage is connected to the chamber to be evacuated (DD-WP 62638) via a suction connection.

The disadvantage of this solution is the relatively low suction capacity, determined by the one-sided arrangement of the suction connection on a pump stage.

Furthermore, a multi-stage vacuum pump is known, which essentially comprises a casing, a separating disc and a rotor system of two separate pump stages (EP-PS 0198936).

From DE-OS 3710782 a method and a device for pumping away vapours and/or vapour-containing mixtures and/or ' mixtures of gas and vapour or similar substances is known, in which essentially several pumps or pump stages are used, these being connected in series.

2 The disadvantage of these known solutions having multistage pumps and/or pump combinations is that the achievement, i.e. the suction capacity of the multistage pumps or pump combinations is decided solely by the suction capacity of the first stage. This is because the suction connection is only connected directly with the first stage and the subsequent stages ensure that the compression side of the first stage is unloaded and thus a higher final pressure drop is achieved.

All of the following stages therefore fail to have an effect on the suction capacity of the pumps or pump combinations. The mass to power ratio is therefore not satisfactory.

The aim of the invention lies in creating a multi-stage vacuum pump through which the suction capacity of multi-stage pumps or pump combinations is substantially increased, which is however effected with or without a comparatively small increase in the construction size and in the cost of materials and labour.

It is the object of the invention to develop a multistage vacuum pump, in which all of the operating chambers of the pump stages are used to increase the suction capacity, in which the drive energy required is either not increased or only insubstantially increased.

According to the invention there is provided a vacuum pump having two pump stages and a pump inlet, in which the compression side of the first stage is connected by a connection line to the suction side of the second stage, wherein the suction side of the first pump 3 stage is connected directly to the pump inlet by a f irst line and the suction side of the second pump stage is similarly connected by a second line, in which an overpressure valve is arranged.

In a preferred arrangement the cross sectional area of the connecting line, which connects the compression side of the f irst pump stage to the suction side of the second pump stage, is much smaller than the cross sectional area of the overpressure valve, and the cross sectional area of the suction connection is equal to or greater than the sum of the cross sectional areas of the lines. It is advisable for the compression side of the first pump stage to have an overpressure valve and also for the compression side of the second pump stage to have an overpressure valve and for the throughflow capacity of each valve to correspond to that of the suction line of the related stage. The pump stages may be roller pistons, rotary slide valves, cell or membrane pumps.

The invention is explained by an example of embodiment. The related drawings are as follows:

Figure 1: shows a schematic sectional view of a twostage vacuum pump in accordance with the invention; Figure 2: shows a suction capacity curve of a twostage vacuum pump in accordance with the invention in comparison to the prior art.

only those parts which are important to the essence of the invention are shown.

4 In Figure 1 the first pump stage 1 and the second pump stage 2 are depicted, suction connection 3 is connected direct to the suction sides C7 of the pump stages 1;2 by lines 4;5. The cross sectional area of suction connection 3 is at least as large as the sum of the cross sectional areas of the lines C5.

In line 5 is located an overpressure valve 8, which is open from about 800 Pa or less pressure difference.

The compression side 9 of the first pump stage 1 has an overpressure valve 10, which is so optimised that it corresponds to the suction capacity of the pump stage 2.

The compression side 9 of the f irst pump stage 1 is additionally connected by the line 13 to the suction side 7 of the second pump stage 2. The cross sectional area of the line 13 is much smaller than the cross sectional area of the overpressure valve 10, it is about 10 to 15% of the cross sectional area of the overpressure valve.

The mode of operation of the pump is as follows:

An electric motor (not shown) drives the rotors 14 and 15. Runners 16a; 16b and 17a; 17b slide in slits in the rotors, these being guided on the inner walls of the pump stages 1 and 2. Thus the inner chambers of the pump stages 1;2 are divided in each case into suction side 6;7 and compression side 9;11. From atmospheric pressure (101325 Pa) down to about 800 Pa or less both pump stages 1;2 are almost evenly 1 participating in the evacuation of the plant, i.e. the pump stage 2 operates with a slightly reduced power as compared to the pump stage 1, since the line 13 means a leak for the pump stage 2, which makes up about 10 to 15% of the power of the pump stage 1.

Acting against final pressure of the pump (less than 800 Pa) overpressure valve 8 closes the line 5. The pump then operates down to final pressure according to the known principle of conventional multi-stage vacuum pumps (Fig. 2).

In Figure 2 the evacuation process of a volume of 4 m3 with a conventional two-stage vacuum pump having 4 M3/h suction capacity (curve 1-2-3) is counterbalanced by a vacuum pump in accordance with the invention (curve 11211-2 -3). Here the advantages of the multi-stage pump in accordance with the invention are illustrated. At suction pressures of up to about 800 Pa both stages operate in parallel, i.e. the effective suction capacity corresponds approximately to the sum of the suction capacity of both stages:

SW = S1 + S 2 - (delta) S in which:

S, - suction capacity of the 1st stage - suction capacity of the 2nd stage (delta) S - losses based on the leaking of the line 13 SW - effective suction capacity of the pump in 6 accordance with the invention in the pressure range over the switching pressure.

By switching pressure is meant the pressure at which overpressure valve 10 closes (800 Pa or less).

The pump out time, which is inversely proportional to the suction capacity is:

T = 1 S in which T - pump out time S - suction capacity with the pump in accordance with the invention this is less than with the conventional pump or pump combination.

In Fig. 2 this relationship is shown together with an example having definite times.

Even when evacuating the volume under consideration to a f inal pressure PE below the switching pressure, the pump out time of the pump according to the invention (Vs) is less than the pump out time of a conventional pump (T11s):

TIS < VIS since the fairly high suction capacity in the upper pressure range SW of the pump in accordance with the invention is broken down as compared to the suction Z v is 7 capacity SI'W of the conventional pump S r W > sly W in a more detailed comparison of pump out time TI T's = T1 + T2; in which T.s - comparison pump out time of the pump in acordance with the invention T1 - pump out time for the curve portion 1-V-2 T 2 - pump out time for the curve portion 2-3.

The relationships shown make it clear that the vacuum pump in accordance with the invention makes it possible to use smaller pumps for the same tasks, in addition to general applications as compared to conventional solutions, and thus to save material, operating time 25 and energy.

8

Claims

1. A vacuum pump having two pump stages and a pump inlet, in which the compression side of the first stage is connected by a connection line to the suction side of the second stage, wherein the suction side of the first pump stage is connected directly to the pump inlet by a f irst line and the suction side of the second pump stage is similarly connected by a second line, in which an overpressure valve is arranged.

2. A vacuum pump according to claim 1, wherein the cross sectional area of the connecting line, which connects the compression side of the first pump stage to the suction side of the second pump stage, has a much smaller cross sectional area than the overpressure valve.

3. A vacuum pump according to claims 1 or 2, wherein the cross sectional area of the pump inlet is equal to or greater than the sum of the cross sectional areas of the first and second lines.

4. A vacuum pump according to any preceding claim,wherein the compression side of the first pump stage has a first overpressure valve and the compression side of the second pump stage has a second overpresure valve and the throughflow capacity of the first and second valves corresponds to that of the first and second lines respectively.

0 t 9 A vacuum pump according to any preceding claim, wherein the pump stages are roller piston, rotary slide valve, cell or membrane pumps.

5.

6. A vacuum pump substantially as described herein with reference to the drawings.

Pub.1shed 1990 at The Patent OffcE, State House. 6671 H16L Holborn. London WClR 4TR Further copies maybe obtained from The Patent Office. Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Maxy Cray, Kent, Con. 1187