GB2367332A

GB2367332A - Multi-stage screw compressor driven by independent electric motors with electronic speed control

Info

Publication number: GB2367332A
Application number: GB0023456A
Authority: GB
Inventors: Philip Nichol; Lyndon Paul Fountain; Terrence Edward Coker
Original assignee: Compair UK Ltd
Current assignee: Compair UK Ltd
Priority date: 2000-09-25
Filing date: 2000-09-25
Publication date: 2002-04-03
Anticipated expiration: 2020-09-25
Also published as: WO2002025115A1; EP1387961A1; GB2367332B; EP1387961B1; GB0023456D0; DE60117821T2; US20040101411A1; ES2260285T3; AU2001290100A1; ATE319932T1; DE60117821D1

Abstract

Each compression stage 10,11 of a multi-stage screw compressor 30 is provided with its own independent variable speed drive motor 31,32, the speeds of which are controlled by an electronic controller 33 to provide a required gas flow delivery rate and pressure. The controller 33 processes signals received from a plurality of devices monitoring operating parameters of the compressor 30, with at least one of the monitoring devices monitoring the torque and the speed of the drive motors 31,32.

Description

2367332 IMPROVEMENTS IN MULTI-STAGE SCREW COMPRESSOR DRIVE ARRANGEMENTS

The invention relates to improvements in the 5 drive of a multi-stage screw compressor using independent electric motors with electronic speed control.

In a multi-stage screw compressor a single fixed speed driver is currently used to drive the individual 10 stages of the compressor simultaneously through a gearbox. Normally this requires a speed-increasing gear train, as the speed of the driver is considerably less than the drive required by the compressor stages. The speed of each stage has to be matched for best 15 efficiency and to share the work done by each stage. As the gear ratio has to be changed to effect a change in output volume, to enable a range of different output volumes to be provided from a common set of stages, a unique gear-set is needed for each nominal 20 output. When a range of different final del' L ivery pressures is required, this also necessitates, in many cases, a unique gear ratio for each operating pressure.

A set of compressor stages may be used, running 25 at different speeds, to give a rance of output air flows. To obtain an increase in air flow, the speed of all the stages must be increased. Due to the difference in performance characteristics of each stage, the increase in speed of each stage will not be 30 the same. In addition to this, the relative speed of the stages may need to be altered depending on the desired final stage delivery pressure or overall pressure ratio. The basic parameter that determines the relative speed of the stages is the work done in 35 each stage. To obtain the best effficiency, the work has to be balanced equally in each stage.

The result of this is that, for a given air flow rate and delivery pressure, a specific set of speeds for the various compression stages has to be determined. Having determined the speeds, the appropriate gears must be selected. This imposes a 5 further limitation. Due to the restriction imposed by the need to have whole numbers of gear teeth, the ideal ratio may not be possible.

A further consideration is that, for series produced machines, the performance of similar 10 compression stages will not be identical due to manufacturing tolerances giving rise to clearance variations. With fixed gear ratios there is no means of compensating for this variation, which may adversely affect the performance of the compressor as 15 the balance of work between the stages will be sub optimal. Furthermore, if a user wishes to use a compressor at a duty at a distance from the design point, the efficiency of the machine will be reduced or, in extreme cases, overheating of individual stages 20 may occur.

Another consideration is that, to provide capacity control of a multi-stage compressor, inlet throttling can only be used over a very narrow range of speeds as it effectively increases the pressure 25 ratio across the machine. This again leads to overhea--ing. For this reason multi-stage compressors are usually controlled by total closure of the inlet by a control valve. This provides very coarse pressure or flow control with poor efficiency.

30 Varying the speed of the drive motor has been used to control some machines to improve efficiency at part load. With a fixed ratio of speeds between the stages this leads to an imbalance of work between the stages which may limit the control range.

35 It is an object of the present invention to overcome these disadvantages.

According to the invention there is provided a multi-stage screw compressor comprising at least two stages of compression, each compressor stage comprising a pair of rotors driven to effect air compression; each compression stage being provided 5 with independent variable speed drive means; and a control unit for controlling the speeds of the independent motors, the control unit comprising processing means for processing signals generated by a plurality of devices monitoring operating parameters 10 of the compressor, and adjusting the speeds of the drive means to provide a required air flow rate and delivery pressure.

A preferred embodiment of the present invention will now be described, by way of example only, with 15 reference to the accompanying drawings in which; Fig. 1 is a schematic representation of the operation of a typical prior art screw compressor; and

20 Fig. 2 is a schematic representation of a screw compressor according to the present invention.

A typical prior art two-stage compressor 5 is shown in Fig. 1. Although a two-stage, oil-free 25 machine is shown for clarity, the principles are the same where more stages are involved or where the stages have oil or water injection.

Each of the two compressor stages 10, 11 consists of a pair of contra-rotating, helically cut fluted 30 rotors supported at each end in rolling bearings in a rigid casing. Each casing is attached to a single gearbox 12. The drive motor 13 is coupled to the input gear in the gearbox 12, which transfers drive to the stages 1-0, 11 via a pinion on the shafts 12a, 12b 35 of each stage 10, 11.

Air is drawn through an air filter 14 and inlet control valve 15 into the inlet port of the first stage 10 where it is partially compressed. The partially compressed air from the first stage 10 passes to an intercooler 16, where its temperature is reduced before the air is passed to the inlet of the 5 second stage 11 for further compression. on leaving the second (or final) stage 11 the fully compressed air passes via a check valve 17 to an aftercooler 18 for further cooling, after which it is delivered to the user via air delivery outlet 19.

10 In this embodiment the intercooler 16 and aftercooler 18 are each cooled by ambient air being drawn over them by a motor driven fan 20. An alternative is to use water-cooled heat exchangers.

Fig. 2 shows a compressor 30 according to the 15 present invention. The essential operation is as described above, but differs from the prior art compressor 5 in that independent, variable speed motors 31, 32 drive each stage 10, 11 of the compressor 30 independently, with no mechanical link

20 between the individual motor driven stages 10, 11. The characteristics of the motors 31, 32 are matched to the corresponding compressor stages 10, 11.

The speed of the motors 31, 32 is controlled by an electronic controller 33. The ba S4 -C control 25 parameter is the required final air delivery pressure or delivery air flow rate. The speed at which each of the stages 10, 11 is driven is increased to give a greater air flow or is reduced to give a lesser air flow. The maximum rotary speeds are limited to predetermined levels based on mechanical considerations. The minimum speeds are either pre-determined or are determined by measuring the delivery temperatures of each stage 10, 11. As the speed of the rotors in any stage slows down, the stage becomes less efficient 35 causing the temperature to rise. When this reaches a pre-set maximum value, the compressor 30 is stopped or unloaded via an inlet valve 15.

To maintain optimum efficiency under all conditions, the speeds of the individual compressor stages 10, 11 are varied to compensate for a variety of factors. These factors include altitude, 5 barometric pressure, ambient temperature and coolant temperature, blocking of filters and wear. Manufacturing variations in the compressor stages 10, 11 are also compensated for.

This control is achieved by continuously 10 measuring air delivery pressures and temperatures from each stage 10, 11, as well as the input torque and speed to each stage 10, 11. Appropriate measuring devices are used to measure these parameters and transmit signals to the electronic controller 33. The 15 motors 31, 32 may have feedback loops directly to the controller 33. The controller 33 processes the signals and sets the speed of the stages 10, 11 to achieve the desired delivery air flow and pressure. Then, using the measurements previously described, the 20 controller 33 makes small adjustments to the stage speeds to minimise power consumption, balance the work evenly between the various stages and maintain safe operating temperatures.

Although the description above only refers to air

25 compressors, it should be understood that this invention can also be used for compressors for other gasses.

Claims

CLAIMS:

1. A multi-stage screw compressor comprising at least two stages of compression, each compressor stage 5 comprising a pair of rotors driven to effect gas compression; each compression stage being provided with independent variable speed drive means; and a control unit for controlling the speeds of the independent motors, the control unit comprising 10 processing means for processing signals generated by a plurality of devices monitoring operating parameters of the comoressor, and adjusting the speeds of the drive means to provide a required gas flow delivery rate and pressure.

2. A screw comoressor as claimed in claim 1 in which monitoring devices monitor the delivery temperatures of the gas at ea--h compressor stage.

20

3. A screw comoressor as claimed in any one of the preceding claims in which at least one monitoring device monitors:he ambient temperature.

4. A screw compressor as claimed in any one of the 25 preceding claims further comprising cooling means provided between adjacent stages, wherein at least one monitoring device monitors the temperature of the gas after passing through the cooling means.

30 S. A screw comcressor as claimed in any one of the preceding claims in which at least one monitoring device monitors --he delivery pressure of the gas at each compression stage.

35 6. A screw comcressor as claimed in any one of the preceding claims in which at least one monitoring device mon_:iors:he tora--,e and speed of each drive means.

7. A screw compressor as hereinbefore described with reference to and as shown in Fig. 2 of the accompanying drawings.

is 50: N9798: TAB: AC: FURNDOCS Amended claims have been filed as follows CLAIMS:

1. A multi-stage screw compressor comprising at least two stages of compression, each compressor stage S comprising a pair of rotors driven to effect gas compression; each compression stage being provided with independent variable speed drive means; and a control unit for controlling the speeds of the independent drive means, the control unit comprising 10 processing means for processing signals generated by a plurality of devices monitoring operating parameters of the compressor, and adjusting the speeds of the drive means to provide a required gas flow delivery rate and pressure, wherein at least one of said is monitoring devices monitors the torque and speed of each drive means.

2. A screw compressor as claimed in claim 1 in which monitoring devices monitor the delivery temperatures 20 of the gas at each compressor stage.

3. A screw compressor as claimed in any one of the preceding claims in which at least one monitoring device monitors the ambient temperature.

2S 4. A screw compressor as claimed in any one of the preceding claims further comprising cooling means provided between adjacent stages, wherein at least one monitoring device monitors the temperature of the gas 30 after passing through the cooling means.

5. A screw compressor as claimed in any one of the preceding claims in which at least one monitoring device monitors the delivery pressure of the gas at 3S each compression stage.

6. A screw compressor as hereinbefore described with reference to and as shown in Fig. 2 of the accompanying drawings.

: 209798: TAB: AC: FURNDOCS