US3415441A - Method and device for the infinitely variable capacity control of pistontype compressors - Google Patents

Description

Dec. 10, 1968 T. K. KEHLER 3,415,441

METHOD AND DEVICE FOR THE INFINITELY VARIABLE CAPACITY CONTROL OF PISTON-TYPE GOMPRESSORS Filed Feb. 6, 1967 United States Patent 0 ABSTRACT OF THE DISCLOSURE A device for the variable capacity control of piston-type compressors with a by-pass duct at the delivery end of the compressor with an adjustable throttle valve in the duct and a clearance chamber at the termination of the by-pass duct and an automatic check valve provided between the clearance chamber and the cylinder of the compressor and connecting the clearance chamber with the cylinder space of the compressor.

The invention relates to a method and a device for the infinitely variable capacity control of piston-type compressors comprising a compressor cylinder including suction valve means at the suction end and delivery valve means at the delivery end of the compressor.

According to a conventional method, the capacity of piston-type compressors is controlled by means of a by-pass pipe extending from the delivery end of the com-- pressor back to the suction end, wherein a manually or automatically, controled throttle valve is incorporated defining the amount of compressed medium flowing back to the suction end of the compressor. Although this control system is of plain design and comparatively reliable in operation, it is also uneconomical since the eifert spent on the compression of the medium expanded into the suction pipe it lost entirely.

These power losses which are of some consequence particularly with large-sized units are avoided by means of the so-called backflow or dynamic pressure regulation causing negligible losses, which is now widely used because of its inherent advantages. According to this method, the suction valves are controlled in synchronization with the stroke motions of the piston and kept open beyond the stroke reversal by means of lifting claws so that part of the sucked-in medium is again forced back into the suction pipe. However, this interference with the working valves of the compressor is not always desirable and requires great care in order to avoid premature wear of the valves. Likewise, another conventional method of control using clearance chambers calls for the provision of valves for the closing and opening of the clearance chambers, while it is necessary for the purpose of achieving infinitely variable control to synchronize the operation of these valves with the stroke motions of the compressor or else the volume of the clearance chamber must be infinitely variable which involves considerable structural expense.

It is the object of the invention to provide for the infinitely variable capacity control of piston-type compressors without the use of positively actuated valves in such a manner as to preserve the advantages of conventional methods while avoiding their shortcomings.

According to the invention as based on the conventional features referred to above, part of the compressed medium is branched off the delivery end of the compressor and delivered to a chamber, to be returned during the suction stroke of the compressor from the chamber to the cylinder space of the compressor. This method is essentially a P CC combination between the conventional by-pass control system and the method of controlling by means of clearance chambers; however, substantial power losses as inherent in by-pass control are largely avoided and on the other hand, there is no need for any positively actuated valves as are necessary both for the clearance chamber control and for the backfiow or dynamic pressure control.

For the performance of the method according to the invention a device is provided, comprising a by-pass duct originating at the delivery end of the compressor, an adjustable throttle valve incorporated in the by-pass duct, a clearance chamber wherein the said by-pass duct terminates, and an automatic check valve used as a suction valve provided between the clearance chamber and the cylinder of the compressor and connecting the clearance chamber with the cylinder space of the compressor. In addition to its inherent economic advantages the device according to the invention distinguishes itself by its plain design and outstanding dependability in. operation, while permitting infinitely variable capacity control ranging from maximum delivery to idling. Moreover, noninterference with the working valves of the compressor precludes premature wear of the said valves and assures smooth operation of the compressor.

Capacity control by means of the device according to the invention is achieved by the alteration of the crosssectional flow area of the throttle valve inserted in the by-pass duct. When the throttle valve is closed, the compressor operates at full load. As soon as the throttle valve is opened, part of the compressed medium flows into the clearance chamber, building up an intermediate pressure therein which is superior to the suction pressure, and flows during the suction stroke of the piston through the check valve of the clearance chamber back into the cylinder space, from where it is delivered back into the delivery pipe. As a result of the intermediate pressure built up in the clearance chamber, the quantity of the medium sucked in from the suction pipe diminishes or suction ceases completely, so that delivery is correspondingly reduced. The throttle valve may be adjusted by hand or automatically controlled as a function of any operational factor, such as final pressure. The most convenient size of the clearance chamber, of the cross-section of the bypass duct and the throttle valve depends on the design features of the compressor to be controlled and above all, on the desired control range. By appropriate adaptation, particularly of the size of the clearance chamber, to given operational conditions, loss in efficiency can be held to a very low level.

Further details and advantages of the invention will appear from the following description of embodiments of the invention with reference to the accompanying drawing in which:

FIG. 1 is a schematic view of an embodiment of the invention showing the control of a compressor, and

FIG. 2 is an axial cross-sectional view of another embodiment of the invention.

In both embodiments of the invention the compressor comprises a cylinder 1 and a piston 2 defining a cylinder space 3. On the side of the cylinder 2 valve chambers 4 and 5 are provided, of which the valve chamber 4 receives the suction valve 6 and communicates with the suction pipe 7, whereas the valve chamber 5 serves to receive the delivery valve 8 and leads to the delivery pipe 9. At its front end the cylinder 1 is closed by means of a cover 10.

As shown in FIG. 1, a clearance chamber 11 is provided on top of the cover 10 and communicates with the cylinder space 3 via a check valve 12 located in the cover 10, the said check valve permitting suction from the clearance chamber 11 into the cylinder space 3. Furthermore, a by-pass pipe 13 originates at the valve chamber 5 at the delivery end of the compressor, an adjustable throttle valve 14 being inserted in the by-pass pipe which leads to the clearance chamber 11.

Again, in the embodiment of the invention illustrated in FIG. 2 a by-pass pipe 13 with a throttle valve 14 originates at the valve chamber 5, a cooler being additionally inserted in the by-pass pipe 13. As difierent from the embodiment shown in FIG. 1, the by-pass pipe 13 does not terminate in a separate clearance chamber but in the valve chamber 4, which is part of the clearance chamber 11. In this instance, the clearance chamber 11 is composed of the valve chamber 4 and of a cylinder 17 communicating with same via a pipe 16, the piston 18 of said cylinder being displaceably by a handwheel by means of a spindle 19, so that the volume of the clearance chamber 11 can thereby be altered. Towards the suction pipe 7 the clearance chamber 11 and the valve chamber 4 are limited by means of an automatic check valve 21 opening in the same direction as the suction valve 6. Likewise, an automatic check valve 22 opening in the same direction as the delivery valve 8 is provided between the delivery pipe 9 and the valve chamber 5.

The method according to the invention and the operation of the control device according to the invention will be understood best with reference to the schematic view shown in FIG. 1. The medium to be compressed is sucked from the suction pipe 7 through the valve chamber 4 and via the suction valve 6 into the cylinder space 3 where it is compressed by means of the piston 2, passing through the delivery valve 8 into the valve chamber 5 and the delivery pipe 9. When the throttle valve 14 is closed, the whole of the compressed medium is evacuated through the delivery pipe 9 so that the compressor will operate at full load. If, however, the throttle valve 14 is partly opened, part of the compressed medium flows through the by-pass pipe 13 and via the throttle valve 14 into the clearance chamber 11 from where it is sucked back via the check valve 12 into the cylinder space 3 during the suction stroke of the piston.

Due to the backfiow of part of the compressed medium the amount of medium sucked in from the suction pipe 7 diminishes, as a result of which the quantity delivered by the compressor to the delivery pipe 9 is reduced accordingly. Since the pressure prevailing in the clearance chamber 11 is higher than that in the suction pipe 7 and in the valve chamber 4, the check valve 12 acting like a suction valve will open earlier during the suction stroke of the piston 2 than the suction valve 6, so that the medium is first sucked in from the clearance chamber 11 until such time when the same pressure prevails in the latter as in the valve chamber 4. By altering the cross-sectional flow area of the throttle valve 14 the amount of medium flowing back into the clearance chamber 11, and consequently, the delivery of the compressor can be varied. Thus it is possible, by the infinitely variable adjustment of the throttle valve 14 to vary the delivery infinitely from full load to zero.

Such power losses as occur with this type of control are comparatively unimportant, since the compressed medium flowing back into the clearance chamber 11 is not expanded to the level of the suction pressure in the suction pipe 7, so that the compression ratio is correspondingly reduced and a lesser effort will be required for the drive. In the event of idling, and provided the clearance chamber is of appropriate size, it is even possible for the pressure prevailing in the cylinder space 3 to approximately equal the full-load pressure prevailing in the delivery pipe 9 also during the suction stroke of the piston, so that practically no compression work is being performed and only throttle losses occur.

Basically, the device shown in FIG. 2 works in the same manner. However, with the throttle valve 14 open, the medium flowing back from the delivery end of the compressor through the bypass pipe 13 is-not returned to a separate clearance chamber but to the valve cham ber 4 from where it is again sucked into the cylinder chamber 3 by means of the suction valve 6. The check valve 21 prevents complete expansion of the returned medium into the suction pipe 7. The cylinder 17 with piston 18 adjoining the valve chamber 4 permits infinitely variable adjustment of the volume of the clearance chamber 11 defined by the valve chamber 4, the pipe 16 and the cylinder 17. This is an advantage insofar as the size of the clearance chamber can be varied so as to suit any given regulating range of the compressor.

For control operations close to full load a relatively small clearance chamber is an advantage, since a comparatively high pressure build-up is then possible within a short period of time even with the throttle valve only slightly open, whereas for idling, the clearance chamber should be as large as possible so as to receive the whole of the medium compressed during the piston stroke. By simple adjustment of the piston 18 of the cylinder 17 it is possible to adapt the volume of the clearance chamber 11 to any given regulating range, so that power losses during regulation can be reduced to a considerable extent. The piston 18 is either adjusted manually by means of the handwheel 20 or automatically by any appropriate adjusting device.

The check valve 22 provided between the valve chamber 5 and the delivery pipe 9 as shown in FIG. 2 serves to prevent an excessive amount of compressed medium to flow from the delivery pipe 9 through the by-pass pipe 13 and the clearance chamber 11 into the cylinder space 3, filling the same completely when the throttle valve 14 is wide open. In that case, the compressor would always have to work against full feed pressure. The cooler 15 incorporated in the by-pass pipe 13 serves to cool the overflowing pressure medium so as to avoid overheating of the compressor, particularly when delivery has been severely curtailed. The same result might also be achieved by incorporating the cooler in the delivery pipe of the compressor and the by-pass pipe 13 would branch 01f the said delivery pipe downstream of the cooler.

A number of variants of the embodiments hereabove described are possible within the scope of the present invention. For example, the clearance chamber can be defined by a suction chamber located in the cylinder-head in lieu of a valve chamber provided on the side of the cylinder of the compressor, or else part of the suction pipe might be used as a clearance chamber, in which case the said suction pipe would have to be closed by a valve corresponding to the check valve 21 shown in FIG. 2. Moreover, provision might be made for the gradual alteration of the size of the clearance chamber, such as for example, by the provision of one or several connectable and disconnectable clearance pockets. However, as a rule, a single clearance chamber of constant size, adapted to meet the requirements of given operational conditions, will be found to be sufficient.

I claim:

1. A device for the infinitely variable capacity control of piston-type compressors having a compressor cylinder including suction valve means at the suction end and delivery valve means at the delivery end of the compressor, comprising a by-pass duct originating at the delivery end of the compressor, an adjustable throttle valve incorporated in the by-pass duct, a clearance chamber wherein the said by-pass duct terminates, and an automatic check valve acting like a suction valve provided between the clearance chamber and the cylinder of the compressor and connecting the clearance chamber with the cylinder space of the compressor, the by-pass duct terminating in the clearance chamber branches off the delivery pipe downstream of the delivery valve of the compressor.

2. A device as claimed in claim 1, wherein an additional automatic check valve is provided incorporated in the delivery pipe of the compressor downstream of the branching off of the by-pass duct and opening in the same direction as the delivery valve of the compressor.

References Cited UNITED STATES PATENTS 7/1936 Heller 230--21 6/1966 Bargmann 230-22 1/ 1967 Leitgeb 23021 5 DONLEY J. STOCKING, Primary Examiner.

WARREN J. KRAUSS, Assistant Examiner.

U8. C1. X.R.

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