GB2029513A

GB2029513A - Improvements in or relating to control systems for compressors

Info

Publication number: GB2029513A
Application number: GB7836388A
Authority: GB
Original assignee: Cox and Wright Ltd
Current assignee: Cox and Wright Ltd
Priority date: 1978-09-11
Filing date: 1978-09-11
Publication date: 1980-03-19
Also published as: GB2029513B

Abstract

In a control system for compressors 11 operating in bank a stop signal to the motor of any compressor is inhibited if the compressor oil temperature is insufficient to prevent emulsification of the oil. A stop signal generated by a high pressure sensor 15 is applied to one compressor at a time by a sequencing switch 17 via a respective gate 23 and delay means 27, the gate 23 additionally receiving a signal from an oil temperature sensor 29 of the respective compressor. The stop signal applied to any compressor is also applied to a driver 18 of the sequencing switch 17 to step the switch on to the next compressor. A second sequencing switch 22 driven by the same driver 18 applies a start signal to the starter 13 of one compressor at a time, the start signal being generated by an autostart 26 circuit responsive to signals from a low pressure sensor 16 and the start/stop circuits of all the compressors. The start signal applied to any compressor is propagated to the next compressor via respective delay means 27 and a gate 28 that receives additionally the signal from the low pressure sensor 16 whereby the start signal is inhibited if the low pressure sensor signal is not present. <IMAGE>

Description

SPECIFICATION Improvements in or relating to control systems for compressors This invention relates to control systems for compressors.

To supply the total compressed air requirement of a large user it is common for a number of compressors to be installed to operate in bank. Such schemes have certain advantages over the provision of a single large compressor, in that, with a fluctuating air demand, one or more machines can be offloaded when the demand falls and, also, if one machine fails or needs to be taken down for service, the others will still provide an air supply. A control system is then needed that will respond to the pressure in the air system being supplied and automatically shut-down a machine when the pressure rises to a maximum or start up a machine when the pressure falls to a minimum.

However, in the case of rotary oil-sealed vane type compressors starting and stopping of a machine brings a risk of emulsification of the oil in the compressor, and for this reason known systems for the control of such compressors are designed to keep all the machines running continuously and to merely off-load a machine that is not required at any time instead of stopping it. This need to keep machines running even though they are not usefully working results in waste of power, unnecessary heat generation and unnecessary noise. It is therefore an object of this invention to achieve a compressor control system that enables vane compressors not currently required to be stopped instead of merely offloaded.

According to the invention, each compressor motor has a start/stop circuit in which stop signals are inhibited if the compressor oil temperature, sensed by a respective oil temperature sensor, is inadequate to prevent emulsification. Preferably, a start signal is propagated from one compressor to the next so long as a signal demanding increased output is maintained. Preferably also, a stepping circuit operates to transfer a persisting or repeat stop signal to the next compressor when one compressor has received a stop signal. Delay circuits may be incorporated to prevent unnecessary starting or stopping of compressors in response to transient pressure variations.

One system in accordance with the invention will now be described by way of example with reference to the accompanying drawings.

Figure 1 shows three compressors 11 arranged for operation in bank to supply a common air system that includes a compressed air receiver 1 2. Each compressor has a respective starter 1 3 for its driving electric motor and all three starters are under the control of a common controller 14. Two pressure switches 15, 1 6 on the air receiver 1 2 respectively signal to the controller 14 when predetermined maximum and minimum air pressures are reached in the system being supplied. The controller also receives three signals from respective oil temperature sensors in the three compressors.

Figure 2 illustrates the arrangement of the controller 14. A stepping switch 1 7 has a driver 18 and two banks of fixed contacts 19, 20 traversed by respective moving contacts 21, 22, the number of contacts in each bank 19, 20 being equal to the number of compressors. The moving contacts 21, 22 receive, respectively, the signal from the high pressure switch 1 5 and an 'autostart' signal from an automatic start unit 26. Each fixed contact of the bank 21 is able to deliver a 'stop' signal to the starter 1 3 of its respective compressor via a respective AND gate 23, delay unit 24 and output device 25. Each fixed contact of the bank 22 is able to deliver the 'autostart' signal to the starter 1 3 of the respective compressor via the respective output device 25.The automatic start unit 26 receives the signal from the low pressure switch 1 6 and also three signals from the outputs of the three output devices 25.

For starting purposes the compressors are arranged in cyclical sequence, in that a 'start' signal from the output device 25 of the No. 1 compressor may be propagated, via a respective delay unit 27 and AND gate 28, to the output device 25 that signals the starter of No. 2 compressor, a 'start' signal from the output device of No. 2 compressor may similarly be propagated to the starter of No. 3 compressor, and a start signal from the output device of No. 3 compressor may likewise be propagated to the starter of No. 1 compressor.When the low pressure switch 1 6 calls for increased output, the signal from the automatic starting unit 26 is applied directly by the switch bank 20 to one of the compressor starters 1 3. The respective delay unit 27 delays the propagation of the 'start' signal to the next compressor for a selected period, say 1 to 1+ minutes.Then the delay unit output is applied to the respective AND gate 28 which also receives on a second input the signal from the low pressure switch 1 6. If the increased output demand has been satisfied there will no longer be a signal from the switch 1 6 and the AND gate 28 will therefore not pass the 'start' signal, but if the demand has not been satisfied a switch signal will be present on the second input of the gate 28 and the 'start' signal will be passed to start the next compressor; and so on, to the third compressor. The delay units thus ensure that additional compressors are not started unnecessarily.

When the high pressure switch 15 signals that the upper pressure limit has been reached, the switch bank 1 9 passes the 'stop' signal to whichever compressor corresponds to the particular fixed contact that the movable contact 21 is currently engaging; except that the 'stop' signal has first to pass the respective AND gate 23, and then is delayed by, say, 1-13 minutes by the respective delay unit 24, before the compressor is actually stopped. The AND gate 23 receives on a second input the signal from the oil temperature sensor 29 of the respective compressor which temperature sensor senses the difference in temperature between the oil in the compressor and the air in the compressor inlet.If the oil temperature is inadequate to prevent emulsification no signal is received by the gate 23 on its second input and consequently the 'stop' signal cannot pass the gate; the compressor goes on running until an adequate oil temperature has been attained.

The delay unit 24 ensures that the compressor is not stopped unnecessarily by a transientsurge in pressure.

When a 'stop' signal is passed to the starter of a compressor it is also applied to the switch driver 1 8 via an OR gate 30. The switch driver then steps the switch contacts 21, 22 on to the next fixed contact in each bank so that if the signal from the high pressure switch 1 5 persists or reappears the 'stop' signal is applied to the control circuit of the next compressor.

An especially advantageous feature of this control system is that in the case of a plant having compressors of different capacities the controller will 'look' for the compressor most suitable to supply the current air demand.

Thus, there may be, say, two large compressors that run during the day and a small one for reduced demand during the night, in which case the controller will at night 'hunt' round until it finds the small compressor. This control system is also suitable for air supply installations having a main compressor and a smaller standby compressor; the standby compressor will start automatically if the main machine should fail.

Claims

1. A control system for compressors operating in bank, wherein each compressor motor has a start/stop circuit in which stop signals are inhibited if the compressor oil temperature, sensed by a respective oil temperature sensor, is inadequate to prevent emulsification.

2. A system according to claim 1, comprising means to propagate a start or stop signal cyclically from one compressor to another so long as a signal demanding increased output or reduced output, resectively, persists.

3. A system according to claim 2, wherein delay means are provided which delay propagation of a start or stop signal from one compressor to the next.

4. A system according to claim 1 or claim 2 or claim 3, comprising stop signal sequence switching means which receives a signal from a high pressure sensor and applies it as a stop signal to one compressor at a time via respective gate means and delay means, said gate means also receiving a signal from the oil temperature sensor of the respective compressor.

5. A system according to claim 4, wherein a stop signal applied to any compressor is also applied to driver means of the sequence switching means to cause the sequence switching means to step on to the next compressor.

6. A system according to any one of the preceding claims, comprising start signal sequence switching means which receives a signal from an autostart circuit and applies it as a start signal to one compressor at a time, and wherein a start signal applied to any one compressor is also applied to the next compressor in sequence via respective delay means and gate means, said gate means receiving in addition a signal from a low pressure sensor whereby the start signal is inhibited if the low pressure sensor signal is not present.

7. A system according to claim 6, wherein the autostart circuit is responsive to signals from the low pressure sensor and from all the start-stop circuits of all the compressors.

8. A system according to claims 4 and 6, wherein the stop signal sequencing means and the start signal sequencing means are driven by common driver means.

9. A control system for compressors operating in bank, substantially as described with reference to the accompanying drawings.