A ROTARY DISPLACEMENT COMPRESSOR AND A METHOD FOR REGULATING A ROTARY DISPLACEMENT COMPRESSOR

Abstract: A rotary displacement compressor is provided with adjustable regulating means (22) determining the internal value ratio of the compressor. The regulating means (22) is adjusted in response to the sound level of the sound, sensed by sound sensing means (28) adjacent to the outlet port (20) of the compressor.

Description: AROTARYDISPLACEMENTCOMPRESSORANDAMETHODFOR REGULATINGAROTARYDISPLACEMENTCOMPRESSOR. BACKGROUNDOFTHEINVENTION The present invention in a first aspect relates to a rotary displacement compressor having adjustable regulating means determining the internal volume ratio of the compressor. In a second aspect of the invention, it relates to a method for regulating such a compressor. The internal compression of a compressor is independent of the pressure in the pressure channel and is for a certain working fluid depending only on the volume ratio of the compressor, i.e. the relation between the volume of a compression chamber at the moment it just has been closed off from the inlet port and the volume of a compression chamber at the moment just before it is opened towards the outlet port. Assuming a constant inlet pressure, a certain volume ratio thus results in a certain pressure in a compression chamber just before it is opened towards the outlet port, the end pressure of the compressor. It is desirable that the internal compression corresponds to the pressure in the pressure channel, so that the pressure in a compression chamber just before it opens towards the outlet equals the pressure in the pressure channel. If these pressures differ from each other, i.e. at over- or undercompression, a rapid flow of gas through the outlet port occurs each time a compression chamber opens towards it, whereby the pressures become equalized. The flow velocity during this short moment is much higher than the flow velocity of the working fluid when it is displaced out through the outlet port by the rotors, and the direction thereof can be to or from the pressure channel depending on if there is over- or undercompression. These flow pulses consume power and as a consequence the efficiency of the compressor will decrease. By these reasons there is always an effort to adapt the built-in volume ratio to the pressure in the pressure channel. In some cases this pressure, however, can vary, which under such condition makes it desir- able to correspondingly make it possible to vary the volume ratio. Since long it is therefore known to provide a compressor with devices regulating this, so called V--regulation. This is accomplished in that the position of the edge of the outlet port, which determines the moment of opening, can be varied in steps or continuously. By this the volume of a compression chamber at the moment of opening can be changed and therewith the volume ratio. In this way it can be achieved that the pressure in said compression chamber roughly equals the pressure in the pressure channel. Constructively this can be made in many ways, partly depending on which kind of rotary dis¬ placement compressor it relates to. On e.g. a rotary screw compressor having two cooperating rotors, a frequently used regulating device consists of an axially movable slide, displaceably mounted in guiding means parallel to the rotors. The slide has a surface facing the working space, which surface forms a part of the barrel w-all of the working space and complies with its shape. The end of the slide facing the high pressure end of the compressor is provided with an edge forming an edge of the oudet port. When the position of said edge is changed by displacement of the slide, the moment of opening of a compression chamber towards the out- let port will be changed and with this its volume at that moment. For adjusting the slide to a correct position, where neither under- nor overcompression prevails, it is known to have the slide position influenced by sensed operating parameters of the compressor. Examples of such devices are disclosed in SE 427063, SE 430709, DD 127 878, US 3 936239 and WO89/03482. The operating parameters sensed in the compressors disclosed in the above mentioned patent documents are either the electrical power consumption of the prime mover, the difference between the outlet pressure and the pressure in a compression chamber just before opening or the flow direction in a channel connecting the working space to the outlet channel. In the first alternative the slide is adjusted to a position where the power consumption is at its minimum, which corresponds to a minimum of losses in efficiency due to under- or overcompression. In the second alternative the pressure in the compression chamber affects the slide to move in a direction of larger outlet area, whereas the pressure in the pressure channel affects the slide to move in the opposite direction so that these pressures balance each other. In WO 89/03482 a channel connects the outlet channel to a working chamber just before it starts to open towards the outlet port. The flow direction of working fluid in this connection channel is indicative of whether the pressure in the working chamber is high er or lower than the pressure in the outlet channel. The regulating slide is adjusted to a position where the flow through the connection channel is at zero which means that said pressures are equal. These known ways of governing the V- -regulation entail various drawbacks. Using the power consumption as the governing par.ameter introduces a source of error in that fluctuations in the electricity supply network affects the sensed parameter. Furthermore, the power consumption as a function of the deviations of the end pressure in the compressor from the pressure in the pressure channel has a very flat characteristic, resulting in a poor accuracy, which allows the influence of said fluctuations to be relatively dominating. To use the pressure difference for governing the regulation has shown to be difficult to work in practice. The main reason for that is that sensing the end pressure in the compressor cannot be accomplished in a reliable way, since the sensed pressure fluctuates, and considerable pressure pulses are generated each time the means hmiting a compression chamber passes the sensing point. It will therefore be practically impossible to use this way for reaching the balanced position where neither under- nor overcompression prevails. SUMMARY OF THE INVENTION The object of the present invention therefore is to find a better way for governing the adjust¬ ment of the built-in volume ratio. This has according to a first aspect of the invention been achieved by providing a rotary screw compressor of the mtroductionally specified kind with sound sensing means adjacent to the outlet port of the compressor, means registering the sound level of the sound sensed by said sound sensing means, and governing means connected to said registering means and controlling said regulating means in response to the registered sound level of said sound. In a second aspect of the invention a method for regulating the internal volume ratio of the compressor includes the steps of sensing the sound by sound sensing means adjacent to the outlet port of the compressor, registering the sound level of said sound and adjusting the regulating means in response to the registered sound level of said sound. The invention is based on the insight that the above -discussed flow pulses, which occur when a working chamber starts to open towards the outlet, create a sound, and that the sound level thereof is depending on how large these pulses are. The more the end pressure deviates from the delivery pressure, the more powerful are these pulses and the higher is the sound level of the sound thus created. Measuring the sound level adjacent to the outlet port thus delivers information in this respect. When the sound level is at minimum this means that the internal volume ratio matches the delivery pressure. A change in the delivery pressure then results in that the sound level increases, indicating over- or undercompression and that the regulating means should be adjusted accordingly. Although the invention can be applied to various types of rotary displacement compressors having either continuous V--regulation or stepwise regulation through lift. valves, it is particu¬ larly suitable for rotary screw compressors, having an axially adjustable slide valve for regulating the internal volume ratio. The invention can advantageously be carried out by using a micro-processor for governing the regulating means in response to the signals from the sound sensing means. Other advantageous embodiments of the invention are specified in the dependent claims. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The invention will be further explained through the following detailed description of a preferred embodiment thereof and with reference to the accompanying drawing showing a schematic section through a compressor according to the invention. The compressor illustrated in the figure is of the rotary screw type having a pair of inter- meshing rotors 12 (of which only one can be seen in the figure). The surrounding casing 10 has the shape of two intersecting cylinders, each cylinder housing one of the rotors 12. The rotors 12 are provided with helical lobes and intermediate grooves forming chevron-shaped working chambers. The working fluid is supplied to the working space from an inlet channel 14 through an inlet port 18 and leaves the compressor through an outlet port 20 to an outlet channel 16. Along one of the intersecting lines of the cylinders .an axially movable regulating slide 22 is provided. The regulating slide 22 forms a part of the casing limiting the working space and has a front edge 24 forming an edge of the outlet port 20. The axial position of the edge 24 determines the moment when a closed working chamber starts to open towards the outlet port 20. The more to the right in the figure this edge 24 is located the smaller is the volume of the working chamber at the opening moment. By means of the regulating slide 22 the internal volume rate thus can be regulated when axially adjusting the same. The compressor is provided with means through which the regulating slide 22 is moved to a posi¬ tion where the internal volume ratio is such that the end pressure in the working chamber equals the pressure in the outlet channel 16. These means include a sound sensing device 26 provided in the outlet channel 16 adjacent to the outlet port 20. The sound level sensed by the sound sensing device is registered by a registering unit 28. By an actuating device 34 the regulating slide 22 is adjusted to a position, where the registered sound level is at minimum. Although this could be accomplished manually, the preferred embodiment shown in the figure is provided with means for automatically performing this operation. These means include a micro-processor 30 and a governing unit 32 governing the regulating slide 22. Signals indicating the sound level registered by the registering device are received by the micro-processor 30. These signals are stored in the micro-processor 30 and each signal is compared with the preceedingly received signal. If the difference between the sound levels indicated by these two signals exceeds a predetermined value an output signal goes from the micro-proessor 30 to the governing unit 32 for initiating movement of the regulating slide 22. In the memory of the micro-processor 30 also is stored information about in which direction the regulating slide 22 was moved during the last proceeding adjustment. This information is received as a feed-back signal from the governing unit 32 to the micro-processor 30. If the signal from the registering unit 28 indicates a higher sound level than the proceeding signal the governing unit 32 initiates adjustment of the regulating slide 22 in a direction opposite the preceeding adjustment. When a signal from the registering unit 28 indicates a lower sound level than the preceeding signal the governing unit 32 initiates adjustment of the regulating slide 22 in the same direction as the preceeding adjustment. And when a signal from the registering unit 28 indicates a sound level which is equal to that of the preceeding signal there will be no adjusting movement. In practice equal here means that the difference between the indicated sound levels is below a certain value. If the compressor is working at correct internal volume ratio any change in the delivery pressure will result in over- or undercompression. By the sound level registration and the means described above this will start an adjustment of the regulating slide 22 until the sound level reaches its minimum again, and the compressor will continue to work at the new internal volume ratio adapted to the changed delivery pressure. It is desirable to avoid that the system also reacts to minor fluctuations in the sound level, due to changes in the delivery pressure which are so small that there is no practical need to compensate for them. Therefore the micro-processor 30 is so programmed that the difference between an indicated sound level and the preceeding indication has to exceed a certain minimum in order to create a signal ordering adjustment. In the description above difference thus means a difference above this minimum difference, and equal means that the difference is below this minimum. However, such a system would not be able to react to a slow but substantial change in delivery pressure, since the difference between each two consecutive sound level indications in this case could be below that minimum, even if the signals represent a considerable change when added. The micro¬ processor thus has to have ability to handle such a situation. Therefore the feed-back signal to the micro-processor from the governing unit 32 indicates whether this is activated for adjustment of the regulating slide 22 or if it is in-active. If the signal indicates in-activation the micro-processor 30 does not only store the latest preceeding sound level indication, but also the lowest sound level indication since the last adjusting movement. The comparing function of the micro-processor in this situation compares the incoming sound level indicating signal with both the latest preceeding sound level indication and said lowest sound level indication and react to initiate adjustment movement of the regulating slide 22 if any of these comparisons shows a difference above the predetermined minimum difference. By the means described above the compressor will all the time be adapted to the delivery pressure by changing the internal volume ratio in response to the changes in the sensed sound level. The actuating device 34 preferably is a hydraulic piston, frequently used for slide valve adjustment in rotary screw machines, in which case the governing unit 32 takes the form of valves, which close or open by order from the signals from the micro-processor 30. Actuation by electrical means or mechanically e.g. using a worm and pinion of course also are possibilities obvious to those skilled in the art. Since applications of the different types of actuating means are well-known in the field .and since they form no essential part of the invention a description of their function is supposed to be superfluous. In the described embodiment the adjustable regulating means are constituted by a displace- able slide valve, which is suitable when continuous regulation is desired. The invention, however, can as well be employed when the regulation is effectuated stepwise e.g. by means of lift valves, the opening and closing of which is controlled by the sensed sound level.

Claims: CLAIMS 1. A rotary displacement compressor having adjustable regulating means (22) determining the internal volume ratio of the compressor, c h a r a c t e r i z e d by sound sensing means (26) adjacent to the outlet port (20) of the compressor, means (28, 30) registering the sound level of the sound sensed by said sound sensning means, and governing means (32) connected to said registering means (30) and controlling said regulating means (22) in response to the registered sound level of said sound. 2. A compressor according to claim 1 being of the rotary screw type having a pair of intermeshing helical rotors (12) forming working chambers, wherein said regulating means (22) include an axially displaceable slide valve determining the moment when a closed working chamber starts to communicate with the outlet port (20) of the compressor. 3. A compressor according to claim 2, wherein said registering means (28, 30) and governing means (32) are so connected to the regulating means (22) that the regulating means (22) are adjusted to a position where said sound level is substantially at minimum. 4. A compressor according to claim 3, wherein said registering means (28, 30) include a micro-processor (30) for storing, comparing and processing the value of said registered sound level. 5. A method for regulating the internal volume ratio of a rotary displacement compressor having adjustable regulating means (22) determining the internal volume ratio of the compressor, c h a r a c t e r i z e d b y - sensing the sound by sound sensing means adjacent to the outlet port (20) of the compressor, - registering the sound level of said sound, and - adjusting said regulating means (22) in response to the registered sound level of said sound. 6. A method according to claim 5, wherein the working medium is compressed by a pair of intermeshing helical rotors (12) forming working chambers, the internal volume ratio is regulated by axially displacing a slide valve (22) determining the moment when a closed working chamber starts to communicate with the outlet (20) of the compressor, said slide valve (22) forming said adjustable regulating means. 7. A method according to claim 6, wherein said slide valve is adjusted to a position where said sound level is substantially at minimum. 8. A method according to any of claims 5 to 7, wherein adjustment of said regulating means (22) is controlled by a micro-processor (30), the input of which includes signals repre¬ senting the registered sound level of said sound, which signals are stored, compared and processed for attaining an output including signals effectuating adjustment of the regulating means (22).

Inventors: LUNDIN STIG, SALETTI HAAKAN, LUNDIN, STIG, SALETTI, HAAKAN
Applicants: SVENSKA ROTOR MASKINER AB
Application year: 1992
Main classification: F04C18
IPC codes: F01C1, F04C18, F04C28, F04C29