GB2192231A - Centrifugal compressor control - Google Patents

Description

GB2192231A 1 SPECIFICATION diffuser throat with very large kinetic energy

loss. Since a vaneless diffuser has better off Movable ring for part-load control of vaned design performance than a vaned diffuser, be diffuser centrifugal compressor cause it does not suffer from incidence loses, 70 it is often chosen where there is considerable This invention relates to centrifugal turboma- off-design operation.

chines, and, more specifically, to diffuser Various techniques have been used to in structure for use in such devices. crease the range between the surge and In centrifugal turbornachines such as gas choke limits of a compressor. Guide vanes in compressors, the kinetic energy of the flowing 75 the inlet of the compressor have been em medium which is issuing at high speed from ployed to vary the flow direction and quantity the impeller is converted into pressure energy of entering gas. Movable diffuser vanes have and the efficiency and stability of the com- also been employed to permit alignment of pressor is dependent upon the means for con- the vanes with the changing flow direction as verting the kinetic energy into static pressure. 80 the flow rate changes.

One of the major problems arising in the use Variable speed compressors wherein the of centrifugal gas compressors operating at speed of the impeller is varied to allow for fixed speeds for refrigeration systems, where changes in flow rates have been used with the compression load varies over a wide some success in the art. These variable speed range, is flow stabilization through the com- 85 machines, however, are very complex and pressor. The compressor inlet, impeller, and thus expensive to build and operate. As a diffuser passage must be sized to provide for consequence they have not found wide gen the maximum volumetric flow rate desired. If, eral acceptance in the art and, in particular, however, capacity control over the machine is the refrigeration industry.

accomplished by varying the position of a 90 An even more successful approach towards series of adjustable guide vanes located at the improving both the efficiency and operating inlet of the machine, while the diffuser throat range of a centrifugal compressor is through area is fixed the mass rate of flow of refrige- the use of a variable width vaned diffuser. In rant delivered to the impeller is varied to meet this particular application, the diffuser contains the changing load demands made on the ma- 95 a movable wall that can be selectively posi chine. However, at maximum flow, the refrige- tioned in regard to a fixed wall to control the rant leaving the impeller may be more than flow of refrigerant therebetween. In the mov the fixed diffuser can handle, thus the flow able wall control the inlet guide vanes of the becomes choked at the diffuser throat. When compressor are used in a conventional manner there is a low volumetric flow rate through 100 to regulate the mass flow of refrigerant such a compressor, the diffuser may be too through the machine while the diffuser wall large and the flow becomes unstable. As the position is varied to prevent surging. The volumetric flow rate is decreased from a sta- movable wall vaned diffuser concept is used ble range, a range of slightly unstable flow is to maintain optimum incidence independent of entered. In this slightly unstable range, flow in 105 load conditions. Squeezing the flow by moving both the impeller and diffuser 'becomes sepathe variable wall inwards, thus reducing the rated from the wall along the entire length of width of the diffuser, will result in a more the flow passage and there appears to be a radial flow direction since width reduction in partial reversal of flow in the diffuser passage creases the radial velocity component (conser creating noises and lowering the compressor 110 vation of mass) but leaves the tangential com efficiency. Below this slightly unstable range, ponent unaffected (conservation of annular the compressor enters what is known as momentum).

surge, Wherein there are periodic complete In accordance with the present invention, a flow reversals in the diffuser passage, de- fixed vane diffuser is provided in combination stroying the efficiency of the machine. 115 with an axially movable sleeve ring.

Many high-performance centrifugal stages It is an object of this invention to provide a employ a fixed vane diffuser section to method and apparatus for varying the capacity achieve the kinetic energy conversion since a of a centrifugal compressor in order to pro vaned diffuser is more efficient at designed vide a large range of stable flow rates, incidence than a vaneless diffuser. The low 120 It is another object of this invention to pro flow limit corresponds to the onset of a surge vide a centrifugal gas compressor having or stall condition which occurs as the fluid means therein to stabilize the gas flow there flow from the impeller becomes more tangen- through at extremely low flow rates.

tial as the flow decreases. This produces a It is a further object of this invention to large flow angle and magnitude with respect 125 provide a centrifugal compressor in which the to the leading edge of the fixed diffuser compressor efficiency is optimized over a vanes, creating a violent instability. The high wide range of flow rates.

flow limit corresponds to a choke condition It is another object of this invention to caused as increasing fluid flow from the impel- match the flow angle to the impeller blade ler becomes more radial and finally chokes the 130 angle.

2 GB2192231A 2 It is an additional object of this invention to the evaporator 35 flows through compressor provide a centrifugal compressor having a dif- suction line 11 back to the compressor 32 fuser with a movable sleeve ring for extending under the control of compressor inlet guide the stable operating range of the compressor vanes 12. The gaseous refrigerant entering the by postponing rotating stall in the vaned difcompressor 32 and discharged from the com fuser. pressor 32 to outlet scroll 47 and through the It is still a further object of this invention to compressor discharge line 37 to complete the allow twisted diffuser vanes in combination refrigeration cycle. This refrigeration cycle is with the movable sleeve ring. continuously repeated during normal operation It is a yet still further object of this inven- 75 of the refrigeration system 31.

tion to provide a centrifugal compressor that Also, as shown in Figure 1, the centrifugal eliminates flow losses due to leakage through compressor 32 of the refrigeration system 1 the clearance between the vanes and the slots includes an electric motor 25 for driving the in a movable wall, and eliminates contact and compressor 32 which is under the control of sticking between the movable wall and the 80 the control system 33. Allso, it may be seen vanes. that the compressor inlet guide vanes 12 are The various features of novelty which char- opened and closed by a guide vane actuator acterize the invention are pointed out with 14 controlled by the control system 33. Fur particularity in the claims annexed to and ther, movable sleeve ring 47 and 42 con- forming a part of this specification. For a bet- 85 trolled by control vanes 50 and 51 and actua ter understanding of the invention, its operat- tor 53.

ing advantages and specific objects attained The control system 33 includes a compres by its use, reference should be had to the sor motor starter 22, a system interface accompanying drawings and descriptive matter board 16, and a processor board 17. Also, a in which there is illustrated and described a 90 temperature sensor 13 for sensing the tem preferred embodiment of the invention. perature of the heat transfer fluid leaving the Other objects and advantages of the present evaporator 35 through the tubing 10, is con invention will be apparent from the following nected by electrical lines 20 directly to the detailed description in conjunction with the ac- processor board 17.

companying drawings, forming a part of this 95 Preferably, the temperature sensor 13 is a specification, and in which reference numerals temperature responsive resistance device such shown in the drawings designate like or corre- as a thermistor having its sensing portion lo sponding parts throughout the-same, and in cated in the heat transfer fluid leaving the eva which; porator 35 with its resistance monitored by Figure 1 is a schematic illustration of a cen- 100 the processor board 17. Of course, as will be trifugal compression refrigeration system with readily apparent to one of ordinary skill in the a movable ring of the present invention; and art to which the present invention pertains, Figure 2 is an elevational view, partly in the temperature sensor 13 may be any of a section-of a portion of the centrifugal com- variety of temperature sensors suitable for pressor shown in Figure 1, incorporating the 105 generating a signal indicative of the tempera teachings of the present invention and show- ture of the heat transfer fluid leaving the eva ing the diffuser sleeve ring thereof in the open porator 35 and for supplying this generated position. signal to the processor board 17.

Referring to Figure 1, a centrifugal vapor The processor board 17 may be any device compression refrigeration system 31 is shown 110 or combination of devices, for receiving a plu having a control system 33 for operating the rality of input signals, for processing the re refrigeration system 31. As shown in Figure ceived input signals according to prepro 1, the refrigeration system 81 includes a com- grammed procedures, and for producing de pressor 32, a condenser 34, an evaporator sired output control signals in response to the 35, and an expansion device 36. In operation, 115 received and processed input signals. For compressed gaseous refrigerant is discharged example, the processor board 17 may com from the compressor 32 through compressor prise a microcomputer, such as a model 8031 discharge line 37 to the condenser 34 wherein microcomputer available from Intel Corporation the gaseous refrigerant is condensed by rela- which has a place of business at Santa Clara, 55- tively cool condensing water flowing through 120 California.

tubing 35 in the condenser 34. The con- The system interface board 16 includes a densed liquid refrigerant from the condenser plurality of switching devices for controlling passes through refrigerant line 39 and expan- the flow of electrical power from a power sion device 36 to the evaporator 35. The supply (not shown), through the system inter- liquid refrigerant in the evaporator 35 is 125 face board 16 to the guide vane actuator 14 evaporated to cool a heat transfer fluid, such the control valve actuator 53, and the motor as water, flowing through tubing 10 in the 25 for driving the compressor 32. Each of the evaporator 35. This cool heat transfer fluid is switching devices may be a model SC-140 used to cool a building or is used for other triac available from General Electric Company such purposes. The gaseous refrigerant from 130 which has a place of business at Auburn, New 3 GB2192231A 3 York. However, as will be readily apparent to tors 53 may be any devices suitable for mov one of ordinary skill in the art to which the ing the movable rings 41 and 42 in an axially present invention pertains, switches other than direction in the diffuser passage 46, which triac switches may be used as the switching also houses fixed guide vanes 48.

devices. 70 According to the present invention, as The switching devices on the system inter- shown in Figure 2, movable ring 41 is sup face board 16 are controlled in response to ported for movement within recess 28 and control signals received by the switching de- diffuser passage 16 between an open position vices from the processor board 17. In this and a full throttling position. In the full throttl- manner, the guide vane actuator 14, the con- 75 ing position, movable sleeve ring 41 throttles trol valve actuator 53, operating control valves vapor flow through diffuser passage 16, and, and 51, and the motor 25 driving the preferably, in the open position, the movable compressor 32 are controlled by the proces- sleeve ring allows an unrestricted flow of va sor board 17. por through the diffuser passage. Bias means The guide vane actuator 14 and control 80 19 is provided between movable sleeve ring valve actuator 53 may be any devices suitable 41 and piston 63 for urging movable sleeve for driving the guide vanes 12 toward either ring 41 toward the full open position. Prefera there fully open or fully closed position or bly, the bias means 19 includes resilient opening or closing control valves 50 and 51 means such as a spring positioned within re in response to electrical power signals re- 85 cess 28. Control valve 51 generally mounted ceived via electrical lines 21 or 23, respec- on casing 67 provides a pressure force on tively. For example, the guide vane actuator piston 63 in a direction opposite the force of 14 and control valve actuators 53 may be an bias means 19 to move movable ring 41 to electric motor, such as a model MC-351 mo- ward the full throttling position. More specifi- tor available from the Barber-Coleman Com- 90 cally, as movable sleeve ring 41 moves for pany having a place of business in Rockford, ward, from left to right as viewed in the Illinois, for driving the guide vanes 12 and drawings, it is controlled by fluid pressure in control valves 50 and 51 toward either their conduit 65 which is controlled by control fully open or fully closed position depending valve 51, which in turn is opened or closed on which one of two switching devices on the 95 by valve actuator 53. The movable sleeve ring system interface board 16 is actuated in re- 41 is positioned in close proximity with the sponse to control signals received by the fixed guide vanes 48 to minimize flow losses switching devices from the processor board and give higher compressor efficienty at full 17. The guide vane actuator 14 and control load, valve actuators 53 may be controlled to drive 100 While this invention has been described the guide vanes 14 and control valves 50 and with reference to a particular embodiment dis 51 toward their fully open or fully closed posi- closed herein, it is not confined to the details tion according to any one of a variety of con- set forth herein and this application is in trol schemes designed to control the capacity tended to cover any modifications or changes of the refrigeration system 31 to match the 105 as may come within the scope of the inven load placed on the refrigeration system 31, tion.

and to match the flow angle with the vane

Claims (3)

1. angle at the vanes diffuser inlet. Preferably, CLAIMS valves 50 and 51 are

   operated in response to 1. In a centrifugal compressor having a cas the position of guide vanes 12, sensed by 110 ing with an impeller rotatably mounted therein sensing means 55. Sensing means 55 may be to move a working fluid from an inlet through limit switcher or a control linkage actuated by an annular diffuser passage to an outlet scroll, the guide vane in response to the movement the improvement comprising:

   of the guide vane 12 to predetermined posi- a fixed wall defining a portion of the annular tions, indicating restricted flow through comdiffuser passage; pressor 32. The compressor motor starter 22 a plurality of fixed vanes located in the an is a device for supplying electrical power from nular diffuser passage and secured to said the power supply 23 to the electric motor 25 fixed wall; of the compressor 2 to start up and run the a movable ring means located upstream of motor 25. For example, the compressor motor 120 said plurality of fixed vanes, mounted for starter 22 may be a conventional wyedelta (Y- movement with respect to said fixed wall ) contactor type motor starter. Of course, as wherein said movable ring means throttles va will be readily apparent to one of ordinary skill por flow through the diffuser passage from a in the art to_which the present invention per- full flow to a full throttle flow.

   tains, -the compressor motor starter 22 may 125
2. 2. The diffuser means of claim 1 wherein be any one of a variety of systems for sup- said movable ring means is positioned imme plying electrical power from the power supply diately downstream of the impeller. 1 23 to the electric motor 25 of the compressor
3. 3. A method of controlling flow of a fluid 32 to start and run the motor 25. through an annular diffuser passage at the The control valves 50 and 51, and actua- 130 outlet of an impeller of a centrifugal compres- GB2192231A sor comprising the steps of: fixing a plurality of guide vanes in a fixed position in the annular diffuser passage; and movably positioning a movable ring in the annular diffuser passage wherein the flow angle of the fluid flowing through the annular diffuser passage is matched with the flow angle at the fixed vanes.

   Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd. Dd 8991685, 1988. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.

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