GB2243928A

GB2243928A - Method and apparatus for controlling the delivery rate of a compressor

Info

Publication number: GB2243928A
Application number: GB9105457A
Authority: GB
Inventors: Dipl Ing Dr Bauer Friedrich
Original assignee: Hoerbiger Ventilwerke GmbH and Co KG
Current assignee: Hoerbiger Ventilwerke GmbH and Co KG
Priority date: 1990-03-16
Filing date: 1991-03-14
Publication date: 1991-11-13
Anticipated expiration: 2011-03-14
Also published as: ATA62990A; ITMI910670A1; ITMI910670A0; AT401088B; DE4105593A1; GB9105457D0; GB2243928B; IT1245188B

Abstract

In a method for the continuous control of the delivery rate of a reciprocating compressor by the occasional holding open of the suction valves by means of associated lifting devices (8), the magnitude of the lifting force depends on the drive power received by the drive motor (11) of the compressor (3). The lifting force may be limited depending on the drive power. The drive power received by the drive motor (11) of the compressor (3) is supplied to a controller (10) as a signal. The controller (10) receives a control signal as the reference value and the drive power of the drive motor (11) as an actual value. From the difference between these two signals, the controller (10) determines the lifting force. <IMAGE>

Description

i A method and a device for the continuous control of the delivery rate of

a reciDrocating compressor The present invention. relates to a method for the continuous control of the delivery rate of a reciprocating compressor by the occasional holding-open of the suction valves by means of a lifting device, which is actuated by a lifting force supplied by a pressure medium. Furthermore, the invention relates to a device for carrying out the method. with a compressor Installationt which comprises at least one cylinder with a lifting device for the suction valve. which is actuated by a lifting force applied by a pressure medium. The magnitude of the lifting force can be determined by a controller.

Control methods of this type and devices for carrying out the methods are already known in several constructions and described for example in AT-PS 187 616. In these methods known as 11return flow control". a part of the gas sucked in at the time of the compression stroke of the compressor is pushed back again into the suction line due to the suction valve which is held open positively. The suction valve is closed first, as soon as the return flow forces exerted on the closure member of the suction valve by the gas flowing back overcome the holding-open force applied from outside. The compression and consequently the conveyance of the medium begin, upon each compression stroke, only after the closure of the suction valve. The holding-open force is applied by a pressure medium in the form of a lifting pressure by way of t4e lifting device to the closure member of the suction valve. By varying the lifting pressure. the respective delivery rate can be chosen, varied and adapted to the respective requirement.

These controls operate substantially without power losses and allow an infinite variation of the delivery rate. Their control range is however limited at the bottom end due to the operation of the reciprocating compressor. With a certain throttled delivery rate, the return flow f orces that occur are no longer sufficient to close the suction valve against the lifting force. The compressor then passes abruptly into the idling state. In practice, the lower limit of the control range of such return flow controls lies approximately at 20 to 40% of the full delivery rate. The sudden shutting-off of the delivery is undesirabler since it may lead to considerable fluctuations in the pressure and in the compressed gas quantity in the pressure system supplied by the compressor and may cause adverse oscillations, which in turn place a pulsating load on the drive motor and in the case of an electrical drive lead to current surges in the mains supply.

In order to remedy this, it is known to limit the lifting force to a certain value, which is lower than the lifting force necessary for shutting off the delivery. This limit can be ascertained empirically in a relatively simple manner, or even calculated. However, it depends largely on the operating conditions of the compressor and on its design, in addition to the constructionr and above all on the nature of the medium delivered, in particular on its density. It follows from this that the maximum admissible lifting force can in each case be preset in an optimum manner solely for a certain type of gas and a certain suction pressure.

If the nature of the gas, for example its molecular weight. or the suction pressure vary during the operation, the shut-off barrier no longer functions. Since the necessary lifting force increases with the gas density. the limit value can always be coordinated solely with the gas with the lowest density. However, in the case of delivery of a gas of greater density, the full control range can no longer be utilised, because a lifting force higher than the preset limit value would be necessary for this.

1 1 1 In principle it is possible to measure the gas density in the suction state continuously during operation, and to adapt the limit value for the lifting force continuously to the measured values. However. determining the gas density is complicated and therefore uneconomical. The gas density depends on the intake pressure, the intake temperature and on the molecular weight of the gas. The Intake pressure and temperature are simple to measure, but the measurement of the molecular weight is substantially more complicated.

The additionally required linkage of the measured values determining the gas density leads to further complications. In practice, this measure is therefore only seldom used.

It is the object of the invention to provide an improved control method of the aforementioned type, In which the control range can be largely utilised independently of the molecular weight of the' respective gas delivered, so that the control method can be used advantageously even when delivering gases having a different or varying molecular weight.

This object is achieved with the method according to the invention due to the fact that, at least in the range of the lowest deliver.

7 rate which can be achieved by the control, the magnitude of the lifting force is chosen depending on the drive power received by the drive motor of the compressor. The lifting force exerted by the lifting device is thus not determined, or at least not exclusively determined, according to the results of the compression or the delivery values of the gas achieved, such as suction or final pressure. flow quantity etc., but at least in the range with the highly throttled delivery rate also depends on the power received by the drive motor, for example an electric motor. The control thus takes place due to the determination or monitoring of the power used for driving the compressor system. The motor power Is simple to ascertain and evaluate as a control quantity. Since the influence of the gas properties on the motor power is furthermore substantially less than on the lifting force, a further essential advantage of the invention is extensive independence of the entire control process from the properties of the gas delivered.

In a further embodiment of the invention, the lifting force can he restricted depending on the drive power so that it is varied so as to reduce the delivery rate only to a value corresponding to a given minimum drive power. A lower limit of the drive power is thus determined. and if the drive power falls below said lower limit, no further variation, for example an increase, of the lifting f orce takes place.

Consequently ' the compressor is prevented in a simple manner from being switched suddenly to idling by the lifting device. The possible control range can be fully utilised in this way independently of the gas propertiest because the limit value of the lifting force is adapted automatically to the properties of the gas just conveyed.

In the case of highly variable pressures. even a suction pressure or final pressure compensation may be superimposed.

In a preferred embodiment of the method according to the invention. the lifting force within the entirecontrol range depends on a predetermined control signal formed as a reference value and also depends on the drive power received by the drive motor formed as an actual value,, the lift ng force'being increased In the case of the drive power lying above the reference value and reduced in the case of the drive power lying below the reference value. Consequently. when delivering gases with a different and continuous ly'varying molecular weight, not only the full utilisation of the control range is possible. but a largely 1 linear relationship results between the control signal and the resulting delivery rate over the entire control range. The at least approximately linear relationship simplifies the entire control sequence and the actuation both when determining the reference value manually as well as automatically.

A simple implementation of the method according to the invention may take place in this case due to the fact that the lifting force is determined by the difference between the control signal as a reference value and a signal corresponding to the drive power as an actual value. This variation is simple to implement, because only one device is necessary for forming the difference between the two signals. Furthermore, for limiting the lifting force, the control signal given as the reference value can be limited. due to which the lifting force is simultaneously prevented from exceeding a desired limit value.

A device for carrying out the method according to the invention consists in that preceding the lifting deviceis a pressure-limiting valve for limiting the lifting force, to which a limiting signal is supplied. which is similar to the drive power received by the drive motor of the compressor system. with this arrangement, the entire delivery of the compressor is prevented from being shut off r if a predetermined mini milm delivery rate is not reached.

In a further, preferred device for carrying out the method according to the invention, in which the magnitude of the lifting force is determined by a controller, the controller receives a predetermined control signal as the reference value and an Anput signal similar to the drive power received by the drive motor of the compressor system, as the actual value. From these two signals, the controller forms a lifting pressure signal, which is supplied to the individual lifting devices ot the compressor system. As an essential feature, this device is also based on the finding that the drive power received by the drive motor of the compressor Is largely independent of the gas properties. in particular of the molecular weight of the gas delivered. but has a linear relationship with the delivery rate. Thus, by using the received drive power for controlling the compressor system. a control curve extending at least approximately in a linear manner and furthermore extensively independent from the gas properties Is achieved. The shutting- off of the delivery, on falling below the lowest possible delivery rate, can In this case also be prevented due to the f act that a limit value corresponding to the lowest admissible delivery rate is provided for the reference value.

Within the scope of the invention, the controller may be preceded by a range switch with a branch for the control of an additional. overflow control, for example a bypass valve. so that on reaching the predetermined limit pressure the range switch controls the additional overflow control and actuates for example a bypass valve. In this way it i.ct possible to regulate the delivery rate of the compressor system down further than is possible solely with the return flow control.

Adjustable boosters may be incorporated in the lifting pressure lines leading to the individual lifting devices. The necessary lifting force may then be adjusted individually for each lifting device. In practice It is sufficient If only some of the lifting devices are provided with such a booster.

In multi-stage compression. the device according to the invention may be further developed due to the fact that an intermediate pressure controller is provided for controlling the second and/or a higher compression stage of the compression system, which controller adapts the lifting force to the respective intezmediate pressure and the relationships in the respective compression stage.

In the following description, the invention is described in detail with reference to embodiments illustrated in the drawings. Figure 1 shows a diagram, in which the delivery rate and the motor power corresponding thereto, which depend on the lifting pressure supplying the lifting force, are illustrated in the case of different gas properties. Figure 2 is a diagrammatic circuit diagram of a compressor system, which is controlled according to the method of the invention, and Figures 3 and 4 are each circuit diagrams of embodiments of the control device, by way of example.

In Figure 1, the lifting pressure p in bars is plotted on the x-axis and the delivery rate Q in per cent is plotted on the y-axis. In additionf parallel to the y-axis. the drive power N in per cent of the motor power at the time of full delivery. is illustrated. The two curves 1 and 2 show the course of the delivery rate Q depending on the lifting pressure p when delivering gases with different properties. The curve 1 illustrates for example the course in the case of a gas with a molecular weight M = 2.7, and the curve 2 in the case of a gas with a molecular weight M = 6.2. This illustration also shows that the drive power N increases in an approximately linear manner with the delivery ratep the idling power in the case of zero delivery amounting for example to 33% of the full drive power.

- a - From the shapes of the curves 1 and 2 it can be- seen that an Increase in the lifting pressure p results in a continuous reduction of the delivery rate Q. This is true up to a point A on the curve 1 and a point B on the curve 2. If the lifting pressure is increased only slightly beyond the points A or B respectively, the delivery rate imtely jumps to zero. The flow forces exerted on the valve closure members by the gas flowing back through the auction valves are no longer sufficient to close the auction valves against the lifting force applied thereto. In -order to avoid this sudden shutting-of f of the delivery rate, which not only causes surges in the gas circuit but also causes current surges in the electrical mains. the control range must be restricted to the continuously developing part of the curves 1 and 2. This was achieved hitherto by limiting the lifting pressure p to a fixed value, which lies just below the pressure of 2.8 bars, which is associated with the point A. in Figure 1, this limit pressure is shown as a broken line and is designated by the reference pl.

The straight line representing the limit pressure pl intersects the curve 2 at the point C. which corresponds to.a delivery rate of approximately 70%. Iff while the limit pressure is set to pl, a gas of a higher molecular weight is delivered, for example according to the curve 2, then the continuous control is restricted to the region of the curve 2 lying above the point C. The control is thus reduced to a region between 100 and about 70%. In order to utilise the entire possible control range into the vicinity of the point B, the lifting pressure limit must be adjusted to a higher limit value p2, in the embodiment for example to a pressure of approximately 3.55 bars. When delivering gases having a fluctuating molecular weight, the molecular weight must therefore be measured constantly and the lifting pressure limit must be adapted automatically to the molecular weight measured, j 1 corresponding to the existing physical relationship. This is extremely complicated, and the instruments for determining the molecular weight are expensive.

The present Invention is based on the f act that the drive power "of the' motor driving the compressor system,in accordance with. the compression work. is independent of. the molecular weight of the gas, but has a linear relationship with the delivery rate. Using this findingr according to the invention the lifting pressure Is limited depending on the drive power. Inthe embodiment according to Figure 1 for le, the lifting pressure is Limited to the value which correspmds to a delivery rate of approximately 33%. With this one adjustment, the limitation for the two types of gas takes place just before the points A and B on the curves 1 and 2 respectively. In Flgim 1, these two limiting points are designated by the references A, and B'. This limitation is valid for all gases and molecular weights between the two curves 1 and 2 and also outside the latter. without the molecular weights having to be measured. In all cases. the entire control range available between 33 and 100% can be utilised.

In one variation of the method according to the invention, the virtually linear relationship between the delivery rate Q and the drive power N is utilised for simplifying and facilitating the control. By specifying the reference value for the drive power N as a control signal, a linear relationship between this control signal and the delivery rate Q achieved can be attained. In this case, the lifting pressure is deter so that the actual drive power is used as an actual signal, and so that by. means of a control device this actual signal 13 kept as close as possible to the given control signal or reference value. In this embodiment, the limitation of the control takes place by limiting the control signal to a drive power N which corresponds to approximately 33% of maximum delivery rate - The compressor system 3 illustrated in Figure 2 consists of a compressor with four cylinders 4. 5r 6 and 7 acting on two sides. The two cylinders 4 and 5 jointly form the first compression stage. the cylinder 6 is the second stage and the cylinder 7 having a somewhat smgller construction form the third compression stage. Associated with each cylinder 4 to 7 on each cylinder side is a lifting device 8, which acts on a suction valve which is only diagrammatically. Thus. each cylinder 4 to 7 is provided with two lifting devices 8. The lifting devices 8 are connected to a controller 10 by way of lifting pressure lines 9 associated therewith.

The compressor system 3 is driven by a common drive motor 11. 1 t is an electric motorr which is connected by way of a drive shaft 12 to the compressor system 3 and is supplied with power by way of an electrical supply line 13 from alternating-current mains 14. Incorporated in the supply line 13 is an output meter 15. for example a wattmeter, which continuously measures the drive power received by the drive motor 11 and transmits it by way of a measuring line 16 to the controller 10.

In Figure 2, a switching station 18 is also shown, in which - either manually by the operator or automatically by a control or regulating device - a signal corresponding to the desired delivery rate of the compressor system 3 is produced, which sigml is supplied by way of a control lead 21 to the controller 10. ' The controller 10 compares the control signal supplied as a reference value with the actual value supplied thereto by way of the measuring lead 16 from the wattmeter 15. If the actual value varies from the reference value,, the controller varies the lifting pressures for the purpose of bringing the actual value 1 closer to the reference value. or alte=tively it limits -the lifdx)g pressures depending on the motor output. The lifting pressure is supplied by way of the lifting pressure lines 9 to the lifting devices 8 at the suction valves of the individual cylinders.4 - 7.

in the circuit diagram illustrated in Figure 3. the drive power of the drive motor (not shown) of the compressor system Is used as a basin for limiting the lifting force. The two cylinders 4 and 5,, which are connected to each other by way of an Indicated Intermediate cooler 22, respectively form a first and a second compression stage.

The suction valves of both stages are each provided with a lifting device 8, to which the liftinq pressure is supplied by way of lifting pressure lines 9. The lifting pressure is supplied by way of pres sure- limiting valves 23p which precede each lifting pressure line 9. Leading to each pressure limiting valve 23 are two control lines, a control signal lead 24 and a power signal lead 25. A control signal corresponding to the volumetric flow delivered by the compressor system is supplied by way of the control signal lead 24, whereas the power signal lead 25 supplies a signal which corresponds to the driving power received by the drive motor of the compressor system. The pressure-limiting valves 23 respectively transmit the smaller signale due to which the compressor system is prevented from being switched accidentally to idling in the case of greater throttling of the delivery.

It is also apparent from Figure 3 that incorporated in the control signal lead 24 Is an intermediate pressure regulator 26r which by way of a lead 27 takes into consideration the intermediate pressure in the intermediate cooler 22 between the two compression stages. Boosters 28 are Incorporated in the signal leads 25 adjoining the measuring lead 16, by way of which a signal similar to the drive power is supplied. An individual booster 28 may precede each pressure-limiting valve 23. It is thus possible to adjust the lifting pressure separately for each individual lifting device 8, which may possibly be necessary if the individual compression stages or compressor sides work with different effective piston surfaces.

In this circuit, the control signal supplied by the process control system by way of the lead 21, which determines the desired delivery rate of the compressor (reference value is converted directly into the lifting pressure and supplied by way of the signal leads 24 and the pressure-limiting valves 23 to the lifting devices 8 of the Individual cylinders 4, 5. The Intermediate pressure regulator 26 in this case takes Into consideration and compensates for the different relationships in the second compression stage. As soon as the lifting pressure caused by the control signal exceeds the pressure which is supplied to the pressure-limiting valves 23 by way of the power signal leads 25, the pressure-limiting valves 23 block the control signal leads 24 and supply to the lifting devices 8 the constant pressure regulated by the boosters 28. Thus the lifting pressure is limited to a value which can be selected freely and adjusted simply.

and which cannot be exceeded.

With this measure. with the correct adjustment of the limiting pressure on the one hand,, the delivery of the cylinders 4, 5 is reliably prevented from being completely shut off by accident. on the other hand, due to the adaptation of the limit valueof the lifting pressure to the drive power received from the compressor.

the limit value is adapted to the properties of the delivered clasr and in particular to the molecular weight, so that for each molecular weight, the full control range can be utilised.

The embodiment of the control device illustrated in Figure 4 differs from the circuit according to Figure 3 substantially due to the fact that the control signal is not converted directly into the lifting pressurer but Is used as a reference. value for the drive power of the driving machine driving the cylinders 4, 5. In a manner similar to the circuit illustrated in Figure 2. the control signal, f or example a quantitative signal, Is supplied by the control station designated by the reference 18 In Figure 2r by way of the lead 21 to the controller 10.

Furthermore,, the controller also receives a signal corresponding to the drive power, by way of the measuring lead 16. The controller 10 converts the two signals into a lifting pressure signal, which is supplied by way of the lifting pressure line 9 and the adjoining branches to the lifting devices 8 of the individual cylinders 4.. 5. In this embodiment also. the cylinder 4 f orms the first compression stage and the cylinder 5 the second compression stage. Incorporated between the two stages is an intermediate pressure regulator 26. which by way of the line 27 compensates for the changed ratios in the second stage.

Incorporated in the respective lifting pressure lines 9 are boosters 28r in order to be able to adapt thelifting pressure individually to the requirements of the respective cylinder side and of the associated lifting device 8. For this purpose, it may also suffice it a booster 28 precedes only one lifting device 8 in each compression stage. Finally, incorporated in the control lead 21 in front of the controller - 10 is a range switch 29 f rom which a branch 30 leaves, by which an additional control device can be controlled, f or example for regulating the delivery rate outside the control range of the return flow control at the time of a smaller delivery rate. This may he a by-pass control for example. the by-pass valve - 14 being controlled by the signals supplied by the branch 30.

With the signal processing by the circuit according to Figure 4, a largely linear relationship automatically results between the control signal supplied by way of the control line '21 'and the delivery rate. This allows a substantial simplification of the control of the compressor system. irrespective of whether the latter is carried out manually or automatically. f or example using a process computer.

1

Claims

1. A method. f or the continuous control of the delivery rate of a reciprocating compressor by the occasional holding-open of the suction valves by means of a lifting device which is actuated by a lifting force supplied by a pressure. mediumt method being dmracterised in that the magnitude of the lif ting f orce at least In the range of the smallest. delivery rate able to be achieved by the control, is determined depending on the drive power received by the drive motor (11) of the compressor (3).

2. A method according to Claim 1. characterised in that the lifting force is limited depending on the drive power so that it is varied so as to reduce the delivery rate only to a value corresponding to a predetermined minimum drive power.

3. A method according to Claim 1. characterised in that the lifting force within the entire control range depends on a given control signal formed as a reference value and also depends on the drive power - received b_y the drive motor formed as an actual value, the lifting force being increased in the case of a drive power lying above the reference value and reduced in the case of a drive power lying below the reference value.

4. A method according to Claim 3, characterised in that the lifting force 15 determined by the difference between the control signal an a reference value and a signal corresponding to the drive power as an actual value.

5. A method according to any one of Claims 1 to 4, characterised in that, so as to limit the lifting force, the control signal supplied as the reference value is limited.

6. _ A device for carrying out the method according to one of Claims 1 to 5. with a compressor system, which comprises at least one cylinder with a lifting device for the auction valve, said lifting device being actuated by a lifting force applied by a pressure medium,, characterised in that preceding the lifting device (9) is a pressure-limiting valve (23) for 1 limiting the lifting forcep which receives a linLiting signal similar to the drive power received by the drive motor (11) of the compressor system (3).

7. A device for carrying out the method according to one of Claims 1 to 5. with a compressor system. which comprises at least one cylinder with a lifting device for the suction valve, which is actuated by a lifting force applied by a pressure medium. the magnitude of said lifting force being determdned by a controller, characterised in that the controller (10) receives a predetermined control signal as the reference value and an input signal similar to the drive power received by the drive motor (11) of the compressor system (3) as the actual value.

8. A device according to Claim 7. characterised in that f or the reference value, a limit value corresponding to the lowest admissible delivery rate is provided.

9. A device according to Claim 7 or 8,, characterised in that preceding the controller (10) is a range switch (29) with a branch (30) for controlling an additional overflow controlt for example a by-pass valve.

A device according to any one of Claims 6 to 9, characterised in that an adjustable booster (28) Is Incorporated In at least one part of the lifting pressure lines (9) leading to a lifting device (8); 1 11. A device according to any one of Claims 6 to 10, characterized in that in the case Of multi-stage compression, an intermediate pressure regulator (26) is provided for regulating the second andlor a higher compression stage of the compressor system, which said regulator adapts the lifting force to the respective intermediate pressure.

v Published 1991 at The Patent Office. Concept House. Cardiff F;Load. Newport. Gwent NP9 I RH. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point. Cwnifelinfach. Cross Keys, Newport. NPI. 7HZ. Printed by Multiplex techniques ltd. St Mary Cray, Kent.