US 3493167 A
Description (OCR text may contain errors)
Feb. 3, 1970 G J. AKERHIELM ETAL 3,493,157
COMPRESSOR CONTROL Filed Aug. 16, 1967 INVENTORS. GEORGE J. AKERHIIELM. BY JAMES F. WAKE.
I United States Patent "ice US. Cl. 230-4 3 Claims ABSTRACT OF THE DISCLOSURE A control system for regulating centrifugal compressor capacity by varying compressor speed to effect a first range of reduction in capacity and thereafter maintaining compressor speed constant and varying compressor guide vane adjustment to effect an additional capacity reduction.
BACKGROUND OF THE INVENTION In many centrifugal gas compressor installations, such as refrigeration systems where the compressor is employed to pump refrigerant in the gaseous state from low side pressure to high side pressure, a variable load on the compressor is encountered. In order to maintain compressor efliciency and prevent compressor surge, it has been the practice to vary compressor capacity. This is accomplished by varying the position of inlet guide vanes and/or diffuser vanes or by varying compressor speed. At high compressor outputs, the most efficient way of varying capacity is to vary compressor speed. However, if compressor speed is dropped below a certain point, compressor surge may occur. To avoid this problem movable guide vanes are utilized to regulate compressor capacity, resulting in a compressor having a wide stable operating range, at the expense of compressor efficiency.
Another problem encountered in compressor operation is the danger of overloading the driver under extreme operating conditions. For example, in a refrigeration system employing a centrifugal compressor the load on the compressor at start-up can exceed the design operational load to an extent which could damage the driver.
SUMMARY OF THE INVENTION This invention relates to a compressor control system to vary compressor output according to the load thereon by utilizing a load controller to provide a signal proportional to the load which is effective to alter the set point of a constant speed governor over a range of high loads and which is effective at a preselected medium loading to maintain compressor speed constant While activating a compressor guide vane controller so as to vary compressor capacity by the variation of inlet guide vanes over a range of low compressor loads.
BRIEF DESCRIPTION OF THE DRAWING The figure is a schematic diagram of a control system for a turbine driven centrifugal refrigeration system.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, there is illustrated a steam turbine 3 driving a centrifugal refrigerant compressor 5, which supplies refrigerant to condenser 7, expansion valve 9 and water chiller 11. From water chiller 1-1, the refrigerant is returned to the compressor inlet which has adjustable guide vanes 13 therein. The turbine is supplied with motive fluid through line 15 which is regulated by governor valve 17 to control turbine speed. The governor valve 17 is comprised of a positive displacement pump 19 operably associated with the turbine shaft so as to 3,493,167 Patented Feb. 3, 1970 provide fluid under pressure, proportional to turbine speed, to a diaphragm 21, which, through linkages 23, controls the positioning of the valve disc 25 to regulate the motive fluid supplied to the turbine so as to maintain a constant speed thereof. The initial setting of valve 17 is obtained by the adjustment of a needle valve 27 which bleeds a portion of the pumped fluid from the governor valve circuit so as to limit the pressure obtainable therein.
The control system is provided with a temperature controller 29 which is responsive to the temperature of the chilled water leaving chiller 11 as evidenced by chilled water sensor 31 to vary control air pressure from a suitable source (not shown) so as to provide control air pressure between 3 p.s.i. and 15 psi to the control system. Under high load conditions evidenced by a control pressure range between 15 p.s.i. and 9 p.s.i., the capacity of the compressor is varied by changing the speed of the turbine. This is accomplished by a secondary bleed valve 33 which is responsive to control pressure transmitted thereto to change the governor valve set point as evidenced by bleed valve 27 by bleeding governor valve pressure therefrom. Secondary bleed valve 33 is sized so as to be wide open at 9 psi. at which point the pressure in the governor valve circuit is such as to provide a bottom limit to compressor speed. When the system capacity is being varied by changing turbine speed and thus the pumping rate of the compressor, it is also subjected to an overload control comprised of an electropneumatic transducer 35 responsive to turbine speed as evidenced by an electrical signal produced by a signal generator 37 on the turbine drive shaft which transmits a pneumatic signal to a differential controller 39' including upper and lower bellows elements 38 and 40. As may be noted in the drawing lower bellows 40 communicates with the output of the transducer 35. System control pressure from the temperature controller 29 is provided to the upper bellows element 38 of the differential controller 39. As long as the two signals provided to the differential controller are equal, av constant 9 p.s.i. signal is transmitted through a three-way solenoid valve 41 to guide vane regulator 43 which is suflicient to maintain the vanes wide open. If the signal from the transducer 35 drops below that from temperature controller 29, it indicates that the turbine is overloaded. Differential controller 39 senses the difference in the signals being applied to the bellows elements and automatically throttles the signal to the guide vane regulator 43 by transmitting a reduced pressure signal to the regulator, partially closing the vanes, unloading the compressor and permitting the turbine to return to the desired speed. During high load operation, control pressure from temperature controller 29 is also utilized to hold switch 45 open so as to maintain solenoid operated three-way valve 41 in a position to communicate'differential controller 39 with guide vane regulator 43.
Once the signal from temperature controller 29 drops below 9 psi, it is ineffective to further vary the governor set point. As such, below that point, the governor will act to maintain turbine speed constant at the level dictated by a 9 psi. signal. A control pressure below 9 psi. will allow switch 45 to close, activating three-way solenoid valve 41 so as to communicate control pressure directly to guide vane regulator 43. The low range system capacity, therefore, is varied solely by the positioning of the guide vanes 13 under the influence of control pressure from temperature controller 29 below the 9 p.s.i. level.
As can be seen from the above, applicants compressor control provides for economical operation of a wide range of compressor loads and includes a provision to prevent driver overload at extreme high load conditions such as start-up.
While we have described a preferred embodiment of g the invention, it is to be understood the invention is not limited thereto.
1. A control system for a gas compressor having a variable speed driver comprising:
load sensing means operable in response to compressor loading to provide a signal proportional thereto; means for varying the speed of the compressor in response to the signal from said load sensing means; speed sensing means operable in response to the speed of the compressor to provide a signal proportional thereto;
means associated with the inlet of the compressor to vary the capacity thereof; and
differential control means operable in response to the difference between the signal from said load sensing means and the signal from said speed sensing means indicative of an overload on the driver to actuate said compressor inlet means to vary compressor capacity to prevent overload thereof.
2. A control system for a gas compressor according to claim 1 wherein the variable speed driver comprises a steam turbine,
said load sensing means including a pneumatic control operable in response to compressor loading to provide a variable pressure air signal,
said means for varying compressor speed including a steam governor valve, operable in response to air pressure from said pneumatic control to vary the quantity of steam supplied to the turbine,
said speed sensing means including a signal generator,
operably associated with the compressor to provide an electric current proportional to compressor speed, said speed sensing means further including an electro-pneumatic transducer for providing a variable pressure air signal in response to the variations in electric current from said signal generator.
3. A control system for a gas compressor according to claim 1 wherein said means for varying the speed of the compressor is nonresponsive to a range of signals from said load sensing means indicative of low compressor loading, the control system further including means, responsive to a range of signals from said load sensing means indicative of low compressor loading to bypass said speed sensing means and said differential control means to eifect control of said compressor inlet means directly by said load sensing means.
References Cited UNITED STATES PATENTS 1,381,513 6/1921 Smith l0312 2,385,664 9/1945 Warner 230-1 14 2,397,443 3/ 1946 Statham et al 10316 2,401,910 6/1946 Condit 2307 2,632,307 3/1953 Massey et al. 23011 2,941,120 6/1960 Harmon 10335 3,226,011 12/1965 Gustafson 2302 3,248,043 4/1966 Taplin 230-115 WILLIAM L. FREEH, Primary Examiner US. Cl. X.R. 230--11