GB2331574A

GB2331574A - Sensing device for screw compressor valve

Info

Publication number: GB2331574A
Application number: GB9724467A
Authority: GB
Inventors: Gethin Bermingham
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-11-20
Filing date: 1997-11-20
Publication date: 1999-05-26
Also published as: GB2331574A9; GB9724467D0

Abstract

A sleeve 18 contains one or more sensing elements 19 positioned along the axis of a capacity slide valve of a screw compressor. A piston 2 (or any part connected to it) of the capacity slide valve is fitted with one or more devices which generate a field which can be sensed by the elements 19, to determine the position of the slide valve. The field may be magnetic or electrical or may comprise nuclear particles or photons. There is no mechanical contact between the relatively movable parts of the valve or sensing device.

Description

2331574 1 SENSING DEVICE FOR SCREW COMPRESSOR LOAD This invention relates

to a device for monitoring the position of the capacity control slide valve as fitted within twin and mono screw gas compressors.

It is common practice to control the volume flow rate through a screw compressor by the use of a sliding member that forms part of the compression space wall. At full volume the sliding member commonly called a capacity slide valve is located fully towards the suction end of the compressor so that all the gas inducted into the compression space is compressed prior to discharge. In order to reduce the volume flow through the compressor the capacity slide valve is moved towards the discharge end of the compressor so that an opening is formed close to where the compression cycle begins.

The opening so formed acts as a vent path that allows a quantity of gas to be returned to the suction side of the compressor. This results in a smaller volume of gas being compressed and discharged.

An improvement often incorporated into the capacity slide valve is a shaped area at the end of the capacity slide valve in the area adjacent to the discharge port that will alter the effective size of the discharge port when the capacity slide valve is moved, This has the effect of 1 altering the size of the discharge port by an amount that will tend to alter the degree to which the gas is compressed prior to being discharged. This change in port size has the effect of maintaining the correct amount of compression for the amount of gas inducted resulting in a saving of energy when compared to a compressor that does not have this feature.

An additional benefit that can be incorporated into a compressor fitted with a variable size discharge port is the ability to alter the size of the discharge port without venting gas back to the suction side of the compressor. This is done by providing an adjustment to the surface against which the capacity slide valve stops when it is fully towards the suction end of the compressor. This feature allows the compression ratio of the compressor to be altered to suit the operating conditions of the system in which the compressor is installed. This feature is commonly known as Variable Vi where Vi is the term used to describe the ratio of the volume of the compression space as the suction port shuts relative to the volume of the compression space as the discharge port opens.

The feature known as Variable Vi may be provided with an adjustment device that allows the Vi to be altered manually or automatically to suit the operating conditions of the compressor.

It is common practice to control the position of the capacity slide valve 2 by means of a cylinder containing a piston which is positioned by the supply and venting of oil.

It is common practice to provide the compressor with a device that will indicate the position of the capacity slide valve. This device often takes the form of shaft containing a helix that passes into or through the piston which in tum is connected to the capacity slide valve.

As the piston is moved within the cylinder the shaft is forced to rotate about its own axis by a pin set into the piston. The rotational position of the shaft is therefore related to the axial position of the capacity slide valve. The shaft is often used to indicate the position of the capacity slide valve via electrical equipment positioned in such a way as to sense the rotational position of the shaft. This equipment is commonly fitted external to the compressor. The shaft and electrical equipment are subject to wear caused by friction and vibration. This wear requires repair which necessitates the stopping of the compressor.

In order to improve the reliablity of the compressor the shaft may be replaced by a fixed device that is able to produce an output that is directly proportional to the position of the capacity slide valve without the requirement for any movement of the sensing device. However there is normally a cost penalty incured with such a device.

3 According to the present invention there is provided a number of fixed sensing elements that together can output electrical signals that relate to the position of the capacity slide valve without the need for contact with the sensing element and without any mechanical movement of the sensing element being required. The use of more than one sensing element who's outputs are used together enables these elements to be selected for their low cost. As there is no significant contact or movement no wear wilf occur and the use of more than one sensing device working together there is a significant cost saving over other fixed or movable sensing elements.

A typical means of monitoring the position of the capacity slide valve will now be described by way of a drawing in which:- Figure 1, which is a sectional view of a typical rotary screw gas compressor fitted with a rotary shaft for the indication of the capacity slide valve position.

A typical rotary shaft indication device is shown in Figures 1 in which a pin 1 is set into the piston 2. The pin 1 is positioned so that it will engage with a helical groove 3 set into a shaft 4.

The piston 2 is connected to the capacity slide valve 5. the capacity slide valve 5 forms part of the compression space 6. The compression space 6 is also formed by the fixed walls 7 and the rotors 8. Gas is 4 1 inducted through ports 9 and discharged through ports 10.

The shaft 4 is mounted into a bearing 11 that maintains the axial position of the shaft while allowing rotary motion to take place. When the piston 2 is forced to move by the action of oil pressure onto one or both sides of the piston 2 the pin 1 reacts against the groove 3 which in turn causes the shaft 4 to rotate by an amount that is dependant upon the angle of the helical groove 3 and the distance travelled by the piston 2.

Cams 12 fitted to the shaft 4 external to the cylinder 13 engage switches 14 that may be connected to control equipment that may be used to control the compressor and the oil supply to the cylinder 13. A potentiometer 15 may also be fitted to the end of the shaft 4 which may be used to provide an electrical output those voltage current or resistance is related to the position of the capacity slide valve 5.

A seal 16 is provided between the shaft 4 and the cylinder 13 so that oilis prevented from leaking to the outside of the cylinder 13.

An adjustable surface 17 may be provided to allow the size of the discharge port 10 to be altered to suit the operating conditions of the compressor.

An embodiment of the invention will now be described, by way of an 1 example only, with reference to the accompanying drawing in which Figure 2, which is a sectional view of a typical rotary screw compressor fitted with sensing elements and emitting devices according to the invention.

A sleeve 18 containing one or more sensing element 19 is positioned along an axis parallel to the axis of the capacity slide valve 5. The sleeve 18 may be mounted through the cylinder 13 so that an electrical supply and return signals may be provided to the sensing elements 19. The sleeve 18 may be fitted with a seal 20 that prevents oil or gas from leaking out of the cylinder 13.

The sleeve 18 may be closed at the end that terminates within the cylinder 13 so that gas and oil may not enter the sleeve 18.

The sleeve 18 may be made of any material such as metal or plastic or metal that is rigid enough to support the sensing elements and is sufficently temperature and chemically stable to withstand thermal effects and chemical attack by oil and the gas that the compressor is compressing. The strength and rigidity of the sleeve may be improved by being filled with a substance 23 able to withstand the temperatures that the sleeve 18 may experience.

Alternatively the substance 23 that is used to fill the sleeve 18 may be 6 1 of such a strength and stability that the sleeve 18 may be discarded. The piston 2 or any part connected to it such as the capacity slide valve 5 may be fitted with one or more devices 20 that generate a magnetic field that can be sensed by the sensing elements 19. 5 Alternatively the devices 20 may emit nuclear particles or photons or an electrical field that can be sensed by the sensing elements 19 that are sensitive to such particles or photons or fields.

The sleeve 18 may be connected to an enclosure 21 that may be used to house electrical equipment 22 that may be used to convert data obtained from the sensing elements 19 into a form of signal that may be used to indicate the position of the capacity slide valve 5.

The signal outputted may take any form such as a variable voltage or binary code or any other form that can be used to indicate the position or movement of the capacity slide valve 5.

As ther is more than one sensing element 19 used there may be an advantage in positioning the sensing elements 19 in such a way that the position of the capacity slide valve 5 may be more accurately calculated when it is in a position where its effect on the performace of the compressor is more critical. Such a position may be when the capacity slide valve 5 is close to the suction end of the compressor. At this position a small movement of the capacity slide valve 5 may have a larger effect upon the performance of the compressor than when the 7 capacity slide valve 5 is positioned towards the discharge end of the compressor.

There may also be an advantage in positioning the sensing elements 19 so that the position of the capacity slide valve 5 may be more accurately monitored when it is in contact with adjustable surface 17.

The sensing elements 19 may produce outputs that have the form of an onloff signal dependant upon the position of the devices 20.

Alternatively the sensing elements 19 may produce an output that varies dependant upon the proximity of the devices 20. This output may take any form such as voltage or current or inductance or any other form that may be usefully used to indicate the position of the capacity slide valve 5.

If a small number of sensing elements 19 are used then their positioning or output signals may be arranged to give a high accuracy over a proportion of their sensing area with a less accurate output over the remainder of their sensing area.

The devices 20 may produce a field or emission that extends over some distance. This distance of effect may be combined with the sensing range of the sensing elements 19 so that a relatively small number of devices 20 and a relatively small number of sensing elements 19 may produce an output that covers a large proportion of

8 1 travel of the capacity slide valve 5.

The devices 20 may be positioned with a spacing that is slightly greater or slightly smaller than the spacing of the sensing elements. This relative spacing may be used to improve the accuracy of the outputted signal by a system often known as a vernier scale. This scale may often be seen on mechanical measuring instruments. By the use of such a system a relatively small number of devices 20 and a relatively small number of sensing elements 19 may produce an output that is accurate and that may also cover a relatively large range of travel of the capacity slide valve 5.

Thus, embodiments of the invention may be produced cheaply and provide an advantage in function, cost or reliability.

Further modifications or improvements may be incorporated without departing from the spirit or scope of the invention herein intended.

9

Claims

A method of indicating the position of a capacity slide valve within a rotary screw compressor by the use of more than one sensing element giving an electrical output that changes in relation to the position of one or more magnetic or electrical field emitting devices that move with the capacity slide valve of a rotary screw compressor without the requirement for direct mechanical contact between the capacity slide valve and the sensing elements.
2. An arrangement as claimed in claim 1 whereby the sensing elements are located within a pressure tight compartment.
3. An arrangement as claimed in claim 1 whereby the alignment of the sensing elements is parallel to the axis of the capacity slide valve.
An arrangement as claimed in claim 2 whereby the pressure tight compartment is open to the outside of the pressure boundary of the rotary screw compressor.
5. An arrangement as claimed in claim 1 whereby the outputs of the sensing elements are of an onloff nature relative to the position of the emitting device or devices.
6. An arrangement as claimed in claim 1 whereby the outputs of the sensing elements are of a nature that changes gradually in relation to the position of the emitting device or devices.
7. An arraignment as claimed in claims 5 and ( or) 6 whereby the outputs of the sensing elements may be combined to give a resulting output that increases the accuracy of the indicated position of the capacity slide valve.
8. An arrangement as claimed in any of the other claims whereby the positioning of the sensing elements will allow the position of the capacity slide valve to be known more accurately at certain positions within the rotary screw compressor.
9. An arrangement as claimed in any of the other claims whereby the emitting devices are wholly within the pressure boundary.
10. An arrangement as claimed in any of the above claims whereby the serial positioning of the emitting devices will allow the position of the capacity slide valve to be known more accurately at certain positions within the rotary screw compressor.

h

10. An arrangement as claimed in any of the other claims whereby the positioning of the emitting devices will allow the position of the capacity slide valve to be known more accurately at certain positions within the rotary screw compressor.

Amendments to the claims have been filed as follows CLAIMS 1. A method of knowing the position of a capacity slide valve within a rotary screw compressor to a usable accuracy by the use of more than one sensing element arranged in series to give an electrical output or outputs that change in relation to the position of one or more magnetic or electrical field emitting devices arranged in series that move with the capacity slide valve of a rotary screw compressor without the requirement for direct mechanical contact between the capacity slide valve and the sensing elements.

2. An arrangement as claimed in claim 1 whereby the sensing elements are located within a pressure tight compartment.

3. An arrangement as claimed in claim 1 whereby the sensing elements are positioned parallel to the axis of the capacity slide valve.

4. An arrangement as claimed in claim 2 whereby the pressure tight compartment is open to the outside of the pressure boundary of the rotary screw compressor.

5. An arrangement as claimed in claim 1 whereby the outputs of the sensing elements are of an onloff nature relative to the position of the emitting device or devices.

6. An arrangement as claimed in claim 1 whereby the outputs of the sensing elements are of a nature that changes gradually in relation to the position of the emitting devices.

7. An arraignment as claimed in claims 5 and 6 whereby the outputs of the sensing elements may be combined in series to give a resulting output that increases the accuracy of the known position of the capacity slide valve.

8. An arrangement as claimed in any of the above claims whereby the serial positioning of the sensing elements will allow the position of the capacity slide valve to be known more accurately at certain positions within the rotary screw compressor.

9. An arrangement as claimed in any of the other claims whereby the emitting devices are wholly within the pressure boundary.