GB2261745A - Pneumatic restrictor - Google Patents

Abstract

A pneumatic restrictor, in particular for installation on the intake side of electro-pneumatic converters, is suggested, characterised by a flow channel (1) of rectangular cross-section which is composed of a plurality of short, straight channel sections (2) which are arranged in the flow direction in such a manner that the middle axes of two successive channel sections (2) are at an angle of more than 10 DEG and the flow-inside walls (3) of the channel section meet at the same angle with sharp edges. <IMAGE>

Description

PNEUMATIC RESTRICTOR The invention relates to a device for controlled pressure reduction of a flowing pneumatic medium by way of multiple changes in the flow direction whilst substantially maintaining the flow cross-section.

In many applications or cicumstances, the pressure in a compressed-air system needs to be reduced. For example, the usual pressure of 1.4 to 10 bar for loading pneumatic control elements is much too high for electro-pnematic converters when used in the same system. Therefore, this pressure has to be reduced to a specified lower value.

The pressure reduction of a flowing pneumatic medium is achieved, for example, by the insertion of diaphragms or apertures having a defined circular passage cross-section which is considerably smaller than the respective flow crosssection in front of and behind the diaphragm. The achievable pressure drop is then inversely proportional to the fourth power of the diameter (#p"sl/d4).

Devices of this type operate satisfactorily so long as the diaphragm cross-sections have a diameter of more than about 0.6 mm. Smaller diameters, for example such as those required in pneumatic restrictors of electro-pneumatic converters, present considerable problems. Such diameters are in the range of between about 0.2 and 0.3 mm. They have to be produced to very narrow tolerances, because deviations in diameter affect the pressure drop at the diaphragm to the power of four such that constant and reproducable flow-through ratios are not obtained.

In additon, the diaphragms have to be neatly finished and deburred, which is difficult and perfect results may often only be achieved by very elaborate processes.

Finally, extremely small orifices of this type are sensitive in operation to particles carried in the pneumatic medium. In spite of providing sieves on the intake side, frequently minute quantities of oil, water, dust, rust or soot or a mixture of same are deposited on the diaphragm, causing an increase in the pressure drop, which in turn can cause considerable faults in the electro-pneumatic transformer at the output side. Additionally, moisture or humidity carried in the pneumatic medium can cause corrosion of the diaphragm, whereby the aperture cross-section is uncontrollably widened, again resulting in faults.

Capillary pipes having a diameter in the region of 1 mm have been proposed to alleviate the disadvantages of diaphragms having a very small diameter. However, it has been found that a very long capillary pipe is required to obtain the same pressure drop as with a diaphragm of, for example, 0.25 mm diameter, so that this solution is in most cases unsuitable as a substitute for a compact and very space-saving diaphragm, as it occupies too much space.

An alternative for diaphragms with very small diameter are restrictors which have been designed for hydraulic media and wherein the flowing medium is ducted through a restrictor path in which the straight section interchanges with circular sections and in which the medium is also alternately ducted through central bores in the circular sections of a first channel plane into a second and back into the first channel plane (see page 6/7 of Technical Hydraulic Handbook of the Lee Company Technical Center, 2 Pattipaug Road, P. O. Box 424, Westbrook, Conneticut 06498-0424 USA). For the same reasons, these restrictor paths, called disco Jet", like the capillary pipes, are to replace diaphragms having very small diameters and have, at the same pressure drop, a 4 to 8 times larger flow cross-section relative to the diaphragms.However, it is a disadvantage that the "Visco Jets" are very expensive to manufacture and thus unsuitable as pneumatic restrictors, at least for electro-pneumatic converters.

It is, therefore, an object of the present invention to provide a pneumatic restrictor which replaces diaphragms having very small diameters, which does not occupy an unacceptable amount of space and which can be manufactured at acceptable cost.

Surprisingly, it has been found that this object can be achieved by means of a device which is characterised by a flow channel of rectangular cross-section, composed of a plurality of short, straight channel sections, which are arranged in the flow direction in such a way that the middle axes of two successive channel sections are at an angle of more than 100 and that the flow-inside walls of the channel sections meet at the same angle with sharp edges.

The successive channel sections are preferably at a respective angle of approximatedly 900 and are placed in one and the same plane. The device is particularly easily manufactured with a base portion in which the channel is arranged as a cross-sectionally U-shaped recess, and a plane cover portion, which is connected to the base portion as a wall which seals the channel.

Other advantageous embodiments of the scope of the invention are described in claims 4 to 9.

Further details will be explained in more detail, based on the exemplary embodiment illustrated in Figures 1 and 2.

Fig. 1 illustrates the base portion seen from the top; Fig. 2 illustrates a cross-section through the base portion.

Fig. 1 illustrates an enlarged base portion 4 in which a flow channel 1 is provided as a U-shaped recess. The channel 1 comprises a plurality of channel sections 2, which are arranged in the flow direction so that the middle axes of two successive channel sections are at an angle of approximately 90 . The flow-inside walls 3, which meet with sharp edges or corners are arranged at the same angle. Fig. 1 further illustrates bores 6 in the base portion 4 for the accommodation of bolts or the like, and the beginning of the channel 7 and the end of the channel 8, in which respect connecting bores are to be provided either in the base portion 4 or in the cover 5 (not illustrated). If a small pressure drop is to be achieved by means of a device having a specified flow channel, then it is sufficient to provide a further connecting bore, for example at a location 9.The position of the connecting bore is easily determined, as the pressure drop is directly proportional to the length of the channel.

Fig. 2 illustrates how the channel sections 2 are incorporated in the base portion 4, and how the U-shaped recesses become, by way of a plane cover portion 5 (illustrated by broken lines), a flow channel which is enclosed on all sides. The vertical wall distance A is used as a governing reference dimension. This wall distance can be selected to be at least about 3 to 5 times greater than the diameter of a diaphragm of the same value. For electropneumatic converters, the inventive devices are dimensioned approximately 30 by 30 by 5 mm and can directly replace conventional diaphragms as far as space requirements are concerned.

In an exemplary embodiment, a diaphragm of 0.25 mm diameter intended for a discharge of approximately 100 l/h, could be replaced by a device of the present invention for which a cross-sectionally square channel with a wall distance A of 0.7 mm and 80 respectively right angled bent channel sections and an overall length of the middle flow path of 200 mm had been chosen. The construction volume (breadth, length, height) was 11.2 x 9.45 x 3 mm3.

Tests involving variation of the wall distance A have shown that the pressure drop is inversely proportional to the power 4.7 of the wall distance A, i.e. the wall distance enters more strongly into the calculation than the diaphragm diameter, in which case the effect on the pressure drop is inversely proportional to only the fourth power of the diameter. This requires certain care in maintaining the wall distance, but it is not at all as critical as when maintaining the diameter of the diaphragms as the desired pressure drop is precisely adjustable by way of shortening the channel length via connecting bores between the beginning and the end of the channel.

Claims (10)

1. Device for controlled reduction of pressure of a flowing pneumatic medium by multiple deviation of the flow direction with substantially constant flow cross-section, said device having a flow channel (1) of generally rectangular crosssection which comprises a plurality of short, straight channel sections (2) which are arranged in the flow direction in such a manner that the middle axes of two successive channel sections (2) are at an angle of more than 100 and the flowinside walls (3) of the channel sections (2) meet at the same angle with sharp edges.

2. Device according to claim 1, wherein two successive channel sections form an angle of approximately 900 between them.

3. Device according to claim 1 or 2, wherein the channel sections are located in one plane.

4. Device according to claim 3, wherein a plate-shaped base portion (4), in which the channel (1) is arranged as a crosssectionally U-shaped recess, and a plane cover portion (5) which is connected as a channel-closing wall to the base portion (4).

5. Device according to one of claims 1 to 4, wherein the channel (1) is of square cross-section.

6. Device according to one of claims 1 to 5, wherein a middle flow path of the channel sections (2) is 1 to 3 times as long as the vertical distance (A) between the lateral walls of the channel (1).

7. Device according to claim 6, wherein the middle flow path is between 50 and 300 times as long as the vertical distance (A) between the lateral walls of the channel.

8. Device according to one of claims 1 to 7, wherein the length of the flow channel (1) and/or the number of changes in direction is matched to the desired pressure drop.

9. Device according to one of claims 1 to 8, wherein the flow channel (1) has one or more connecting locations (9) between the inlet and the output aperture (7, 8) for setting a reduced pressure drop.

10. Device substantially as hereinbefore described with reference to the accompanying drawings.