US3421685A - Compressor control - Google Patents

Description

Jan. 14, 1969 3, ANDERSON ET AL 3,421,685

COMPRESSOR CONTROL Filed Feb. 21. 1966 ATTORNEY.

United States Patent 2 Claims ABSTRACT OF THE DISCLOSURE An overload control for a constant speed compressor which is responsive to a decrease in compressor speed indicative of compressor overload to decrease the load thereon and therefore maintain the desired compressor operating speed.

This invention relates to the automatic control of a centrifugal gas compressor. More particularly, this invention relates to a control device for varying the capacity -of a centrifugal gas compressor. Still more particularly,

this invention relates to a capacity control device responsive to compressor shaft speed for a centrifugal compressor employed in a refrigeration machine.

In high tonnage refrigeration machines, a compressor is provided for compressing refrigerant flowing within the machine. The compressor is powered by a prime mover, for example, a turbine or an internal combustion engine. The compressed refrigerant is condensed and the liquid refrigerant formed thereby passed to an evaporator for chilling a relatively large quantity of water for flow in a closed circuit forming a part of an air conditioning plant. Since the temperature of this chilled water is one indication of the load imposed on the refrigeration machine, many plants utilize a control system which varies the output of the refrigeration machine in response to the chilled water temperature.

The automatic control system usually employs a chilled water sensor which provides a signal to a regulating circuit that varies the capacity of the refrigerant compressor by changing the position of the compressor suction throttling mechanism. Under certain circumstances, when the suction throttling mechanism, such as guide vanes, are opened in response to a signal from the chilled water sensor to increase refrigerant flow therethrough, it is possible to impose a compressor load great enough to slow or stall the prime mover.

It is, therefore, the chief object of this invention to provide a compressor control which will maintain compressor load on the prime mover within the power capacity thereof. It is a further object of this invention to provide a compressor overload control which can be incorporated into present compressor control systems. It is another object of this invention to provide a compressor overload control which is completely automatic and independent of the prime mover governing controls.

These objects are attained by providing a current generator or other speed responsive device on the shaft of the compressor or driver. The signal produced thereby is utilized in the compressor suction throttling control system. If the compressor load exceeds the power capability of the prime mover causing it to slow down, the signal produced by the current generator will override the chilled water sensor section of the suction throttling control system to maintain the compressor guide vanes or inlet dampers in a fixed position or close them an amount suflicient to produce a compressor loading within the power capability of the prime mover so that the desired constant speed will be maintained.

We will describe our control device as applied to a 3,421,685 Patented Jan. 14, 1969 turbine driven refrigeration machine for purposes of illustration only, it being understood that the invention is not limited thereto but it also applicable to a wide variety of gas compressor plants as it can be utilized in any centrifugal gas compressor, whether motor or turbine driven.

Other objects and features of this invention will be apparent upon a consideration of the ensuing specification and drawing in which:

The figure is a schematic diagram of the control system forming the subject of this invention.

Referring more particularly to the drawing there is shown a constant speed turbine 1 driving a centrifugal compressor 2. Inlet guide vanes 3 in the suction line leading to compressor 2 are positioned by a reversible shaded pole actuator motor 4 in response to the capacity-overload control 5. Actuator motor 4 has windings 4' and 4", windings 4' when electrically loaded causing the motor to rotate in a direction to open the guide vanes 3 while windings 4" when electrically loaded causing the motor to rotate in a direction to close the guide vanes. A chilled water sensor 6 provides an input signal to the control 5 in response to cooling load imposed on the refrigeration machine. A current generator 7 on the turbine drive shaft 8 provides an input signal to the control 5 through lines 9 and 10 in response to shaft speed.

Primary overload switch 11 and secondary overload switch 12 are actuated by electromagnetic force created by the generator current flowing through the switch windings 11' and 12' to prevent turbine overload. A signal current flows through lines 9 and 10 whenever the shaft 8 is rotated, the current strength being proportioned to the shaft speed.

Considering the operation of this control system under normal operating conditions, current from the current generator 7 flowing through windings 11' and 12 will produce electromagnetic force suificient to maintain primary overload switch 11 closed and secondary overload switch 12 open against the action of springs 19 and 20 respectively. Terminals 14 and 15 of guide vane actuator switch 16 are connected to lines 13 of a conventional power supply. Current flow from terminal 14 across contactor 16' through line 17 will energize windings 4 of actuator 4 to drive the guide vanes in a direction so that flow to the compressor through the vanes is less restricted, while current flow from terminal 15 across contactor 16' through line 18 to actuator motor windings 4" will cause the actuator 4 to drive the guide vanes in a direction so that flow to the compressor through the vanes is more restricted. Chilled water sensor 6 will actuate switch 16 in response to the demand on the refrigeration system to maintain the proper guide vane position therefor.

Under abnormal conditions when an excessive load is imposed on the machine causing the turbine shaft speed to decrease, the current produced by current generator 7 will decrease. Current through the windings of primary overload switch 11 will be decreased to a point where the force of spring 19 will exceed the electromagnetic force produced by current flow through windings 11', opening switch 11 thereby preventing current flow to terminal 14 to prevent further opening of the guide vanes irrespective of the position of the switch 16. If turbine shaft speed continues to decrease, a still lower current will be provided to the windings of switches 11 and 12 by current generator 7. The electromagnetic force holding switch 12 open will decrease to a point where the force of spring 20 is suflicient to close secondary overload switch 12 causing current to flow through line 18 regardless of the position of switch 16 causing the guide vane actuator 4 to close the guide vanes to further reduce compressor load. When the turbine shaft reaches normal operating speed, the signal produced by the current generator 7 will open switch 12 and close switch 11 in sequence thereby allowing chilled water sensor 6 to control refrigeration machine operation via switch 16.

It is to be understood that under normal operating conditions, the turbine governor will maintain turbine speed constant irrespective of guide vane position. The turbine has sufiicient power to handle the compressor even when the guide vanes are fully opened in response to normal operating conditions. The overload control is utilized under certain abnormal conditions such as start up when the guide vanes would be positioned wide open in response to the chilled water sensor signal and the dense charge of refrigerant provided to the compressor would result in an excessive compressor load which would cause the turbine to slow down or stall. By utilizing the overload control system of our invention, the compressor load on the turbine will never exceed the power capabilities of the turbine. As an excessive load is imposed, the current generator signal communicated to the control will override the signal from the chilled water sensor to provide a guide vane setting which will produce a compressor load equal to but not greater than the maximum turbine output. This will prevent turbine slow down or stall.

While we have described a preferred embodiment of our invention, it is to be understood that the invention is not limited thereto since it may be otherwise embodied within the scope of the following claims.

We claim:

compressor having suction throttling means operable in response to a first control signal to vary compressor output according to the load therein, comprising a current generator operably associated with the shaft of said compressor to provide a second control signal proportional to compressor shaft speed, and, control means responsive to a signal current drop of a first magnitude from said current generator indicative of a predetermined compressor overload causing a decrease in compressor speed to override said first control signal to prevent further compressor overload by preventing further operation of the suction throttling means.

2. An overload control according to claim 1 wherein said control means is responsive to a drop in the second control signal current of a second magnitude to effect operation of the suction throttling means in a closing direction.

References Cited UNITED STATES PATENTS 2,305,810 12/1942 Muller 230-114 2,949,735 8/1960 Stefueza 230114 1,222,352 4/1917 Banner 2301 15 1,281,216 10/1918 Schellens 2301 15 2,339,150 1/1944 Codrington 230-114 2,376,142 5/ 1945 Hoffman 2304 2,425,607 8/ 1947 Edwards 23011 2,478,423 8/1949 Ponomareff 230- 2,983,111 5/1961 Miner 230115 3,292,846 12/1966 Harper 2301 15 ROBERT M. WALKER, Primary Examiner.

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