HK1114655B - Improving centrifugal compressor performance by optimizing diffuser surge control and flow control device settings - Google Patents

Description

Centrifugal compressor performance improvement by optimizing diffuser surge control and flow control

Background

As long as the centrifugal compressor is present, surge control problems accompany it. Depending on the type of surge mechanism present in the compressor system, many different approaches have been taken to improve the operating range for surge (including head and flow). Compressor surge triggered by diffuser stall (stall) can be suppressed by variable diffuser geometry, while surge triggered by impeller stall can be eliminated by using variable geometry inlet guide vanes.

Given compressor operating conditions in terms of flow and pressure ratio can be achieved by an infinite number of combinations of inlet guide vane/variable diffuser geometry settings. These various realizations of the same operating state point have different compressor efficiencies.

There is an improved method of selecting specific combinations to improve efficiency while maintaining surge-free operation of the compressor, and it is a primary object of the present invention to respond to this need.

Disclosure of Invention

The foregoing objects and advantages have been readily attained according to the present invention.

The present invention provides a method for optimal inlet guide vane/variable geometry diffuser positioning by using multiple, preferably two or three, pressure measurements along the flow path, e.g., impeller inlet pressure, impeller outlet/diffuser inlet pressure and diffuser outlet pressure. The maximum available diffuser pressure recovery can be used to determine the onset of surge. These maximum pressure recovery values are a function of the variable geometry diffuser setting only and are independent of flow, head and inlet guide vane over most of the operating range. Furthermore, they can be determined very quickly experimentally by pressure measurement. During operation, the known maximum pressure recovery value can be compared to a maximum pressure recovery value determined by real-time pressure measurement, and a determination can be made based on the optimum setting of the diffuser. According to the present invention, it appears that for maximum efficient operation of the compressor, the position of the diffuser should be determined such that its pressure recovery value is close to its maximum value. This actually brings the surge close to the operating point, however, with careful control and safety factors, stable operation is possible.

In one aspect of the invention, a method is provided for controlling operation of a compressor having an inlet and an outlet, a variable geometry diffuser in communication with the outlet, and inlet guide vanes in communication with the inlet, the method comprising the steps of: determining a load parameter indicative of onset of surge; and independently controlling the variable geometry diffuser and at least one of compressor speed and inlet guide vanes based on the load parameter to provide compressor efficiency enhancement and stable operation.

In another aspect of the present invention, a method for controlling operation of a compressor having at least two controllable operating parameters affecting operational stability is provided, the method comprising the steps of: determining a load parameter indicative of onset of surge, an operational value of a load parameter being controllable by any of the at least two controllable operational parameters; and independently controlling at least one of the at least two controllable operating parameters based on the load parameter to operate at a desired efficiency rate within a stable operating region of the compressor.

Drawings

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, in which:

fig. 1 is a sectional view of a centrifugal compressor showing a corresponding structure of the present invention;

FIGS. 2 and 2a are perspective and cross-sectional views, respectively, of a variable geometry diffuser suitable for use in accordance with the present invention;

FIG. 3 illustrates the operating characteristics and surge region of a centrifugal compressor;

FIG. 4 illustrates the efficiency of the compressor system at different region points and shows the surge line for a fully open variable diffuser, and the maximum surge line when a variable diffuser is used;

FIG. 5 illustrates the correlation between diffuser pressure recovery parameters and efficiency;

FIG. 6 illustrates the correlation between diffuser pressure recovery parameters and flow rate and variable diffuser orientation;

FIG. 7 illustrates a correlation between a diffuser pressure recovery parameter and a variable diffuser orientation;

FIG. 8 illustrates the dependence of a diffuser pressure recovery parameter on variable diffuser orientation;

FIG. 9 illustrates the effect of IGV and diffuser orientation on the diffuser pressure recovery parameter; and

fig. 10 and 11 show the pressure rise of the compressor assembly at the same full load for two different IGV (inlet guide vane)/diffuser settings.

Detailed Description

The present invention relates to control of centrifugal compressors, and more particularly to a system and method of operating such a compressor in which performance is improved by independently controlling and balancing a variable geometry diffuser and at least one inlet guide vane, as well as compressor speed. The following description is made in terms of control of the diffuser and inlet guide vanes and this is the preferred embodiment, however, this is not a limitation on the broad scope of the invention.

Increasing efficiency values have long been a goal sought by centrifugal compressor designers. There is of course also a need for stable wide range compressor operation. In many instances, these desired features are not mutually inclusive. According to the invention, these features are carefully balanced by applying a measurement criterion that correlates load to onset of surge conditions.

For centrifugal compressors, one particularly effective load parameter is the pressure ratio across the diffuser. The look-up table pressure measurements or approximations can be readily obtained during compressor operation and are close to the onset of surge. According to the invention, the operation of the compressor is controlled on the basis of the known correlation of the current value of the parameter and the value causing the surge, and this provides the precondition for an improved control.

According to the present invention, different flow control mechanisms provide different results depending on stability and efficiency. For the centrifugal compressor of the present invention, it has been found that it is most efficient at a particular operating point and when the diffuser is as open as possible without causing surge. By utilizing the maximum possible setting of the diffuser from an efficiency standpoint, it is allowed to maximize efficiency within a reasonable safety factor, while maintaining control over at least the safety factor away from surge, in which case the pressure rise across the diffuser is controlled based on the load parameter.

For example, if a given compressor is operating with the diffuser partially open and a request for a greater pressure rise is received from the compressor controller, the amount of lift required is evaluated to determine whether it can be met by opening the diffuser further. The diffuser typically needs to meet new operating conditions if possible. If the load parameter indicates that the requested lift may result in surge if performed by controlling the diffuser, then control is instead performed by an alternative mechanism, in this particular embodiment by controlling the inlet guide vanes. By prioritizing the mechanism and by controlling it independently of the others, maximum stable efficiency can be achieved.

As described above, a method of stability control is achieved through the use of variable diffuser geometry. In many cases the variable geometry configuration controls not only the compressor system, but also the flow rate. In the case of another flow control device (e.g., inlet guide vanes), there is a performance versus efficiency tradeoff for the combination of different settings.

The present invention is most preferably described as a variable diffuser geometry device of the pipe diffuser type. The performance, efficiency and some geometric sensitivities of this type of diffuser have been described. Previously, the most effective combination of diffuser/IGV settings was determined by using non-measured information of flow field or operating parameters in addition to the actual orientation of the diffuser/IGV (inlet guide vanes) and elaborating a simple optimization strategy. Based on some criteria, the result is a one-to-one dependence of IGV position with respect to diffuser orientation. This allows the surge line of the compressor to be tailored to a desired characteristic, but also allows for efficient operation at low IGV settings and pressure operating conditions.

Pressure recovery inside a variable geometry pipe diffuser has also been described. The data shows that an increase in the total pressure recovery coefficient is obtained with the diffuser throat open. According to the invention, and in accordance with these teachings, the optimum operating condition is to open the diffuser as far as possible to avoid surge.

The key to this optimization of the system is the understanding of the fundamental flow phenomena and the use of flow measurement criteria that enable accurate, consistent and reliable determination of the optimal position of the IGV and variable diffuser. According to the invention, a flow measurement standard is provided which shows the possibility of determining an optimum position for efficient compressor operation at high load points. In particular, in the case of compressors employing inlet guide vanes and a tube diffuser with variable throat geometry, a load parameter describing the pressure ratio across the diffuser can be expressed, giving a valuable determination as to where surge occurs. This in turn provides the premise for maximum efficiency of operation to be achieved. In fact, the present invention describes an efficient operating condition of the diffuser that avoids costly inferential mapping for all operating conditions (flow, pressure rise for all IGV/diffuser orientations combined). This is accomplished by employing high precision measurements that are placed in field applications and by measuring or estimating the compressor flow rate within the field.

In fig. 1, a compressor 10 of the present invention is shown. Important components from the inlet to the outlet are inlet guide vanes (IGV's) 12, typically composed of a plurality, preferably a set of seven, non-curved, flat vanes, a backward inclined compressor (11 main vanes, 11 splitter vanes) with twenty-two (22) vanes, a vaneless small space 14 to a duct diffuser 16, and a collector 18 of constant cross-sectional area. For example, the impeller 20 may be 15.852 inches in diameter with a vane outlet height of 0.642 inches. The exit angle may be about 50.0 degrees and at a rotational Mach number (U)_tip/a₀) At around 1.3 the speed of operation may be 9200 rpm. Of course, there are an unlimited number of examples of a suitable compressor.

The compressor is typically run on a chiller system. The working gas (r134a) is drawn from the evaporator conduit, compressed, and then flows out into the condenser conduit.

Pressure measurements may be taken in the evaporator, condenser and adjacent plenum and connected to vaneless space 14 before the diffuser (see fig. 1). The pressure measurement inside the plenum can be used to obtain an approximation of the average pressure inside the vaneless space upstream of the diffuser inlet at minimum fluctuations and therefore more costly signal conditions or expensive measurement devices can be reduced.

The tube diffuser geometry comprises three (3) basic parts or sections (see fig. 2a) including a short constant area throat 22 (which may be, for example, 0.642 inch diameter), may have a first length or flow path section 24 that diverges, for example, by 4 degrees, and may also have a second length or flow path section 26 that diverges, for example, by 8 degrees, although it should be appreciated that these diameters and divergences are given by way of example only as an unlimited number of possibilities and that other configurations are still possible well within the broad scope of the present invention.

Fig. 2 and 2a show a perspective view and a cross-sectional view, respectively, of a preferred embodiment of a pipe diffuser geometry. As mentioned above, the tube diffuser also acts as a flow stabilization device. As shown in fig. 2 and 2a, a rotatable inner ring 27 is provided for adjusting the throat area of the diffuser in response to angular displacement relative to the outer ring portion 28. This rotation is used as a reference throughout the application as diffuser orientation.

It should be appreciated that the variable geometry diffuser illustrated in fig. 2 and 2a is one embodiment of such a configuration, which represents a non-limiting example, and that other types of controllable diffusers are within the broad scope of the present invention.

The following is an example of the analysis of the load parameter.

The present invention encompasses the use of load parameters when other compressor components or operating settings cause surge to occur. In this category, one example of another embodiment considers the instability of the impeller. As described above, load parameters associated with the onset of surge caused by impeller instability can be determined and used to control changes in operating conditions to maximize efficiency while maintaining stable operation.

To illustrate the effect of variable diffuser orientation on flow efficiency and stability, the surge line through a fully open diffuser using only IGV's as flow control was first determined (see line 3, fig. 3). In this figure, Pevaporator is the evaporator static pressure and Pcondensor is the condenser static pressure. Also shown is the surge line when using IGV full open and using only variable diffuser geometry orientation as flow control (see line 2, FIG. 3). Between these two lines is an operating region where compressor instability or surge may occur. Due to the fact that the compressor system surge initially occurs in the diffuser, the sensitivity of the surge region to different diffuser/IGV orientations was studied under total pressure operating conditions.

To identify the key physical phenomena and measurement criteria that describe the optimal control of the variable diffuser/IGV setting, ten (10) measurement conditions were selected and represented in the figure as numbers 3-13. The combination of high, medium and low flows at high, medium and low operating pressures specify nine (9) of these assay conditions (points 5-13). Eight (8) of these conditions (points 5-6 and 8-13) are within the potential instability region. For comparison with a surgeless flow point, one of the nine (9) combinations (high flow, low pressure, point 7) is selected to be outside the surge region and point 4 is selected at a very high flow point in the intermediate operating state, and therefore must be within the steady state operating state of the compressor. In each of these operating states, different combinations of variable geometry orientation/IGV position are determined and the performance point of the compressor is taken. This is shown in the cluster of points taken for each of the ten pressure rise/flow combinations (fig. 3).

Fig. 4 shows the corresponding efficiency points. As a hint, each of these points has a constant total pressure ratio, however, the effect of diffuser geometry orientation can now be estimated. As shown in the main part of the figure, each of these combined boxes is shaded to correspond to the diffuser geometry. It is clear from fig. 4 that when the diffuser is open, the efficiency increases up to the surge point (or fully open when within the stable outer profile).

The main objective is to determine a measurement criterion that gives the right information on the moment to reach maximum efficiency while avoiding surge.

The calibration study showed the pressure ratio across the diffuser. To make this determination, it is necessary to measure the pressure before and after the diffuser as previously described. As described above, in order to make the measurement easier and to obtain a measurement of lower fluctuating pressures at a more stable average, it is necessary to measure the pressure before the diffuser in the plenum adjacent to the vaneless space. While this plenum pressure measurement does describe the pressure in the vaneless space, it is only an estimate of the actual vaneless diffuser space pressure and is not very accurate. A plot of this ratio (Pcondenser/Pplenum) versus flow is shown in FIGS. 5 and 6.

One notable aspect of the Pcondenser/Pplenum determination of standard values is that a narrowly defined area (maximum efficiency) can now be determined where surge is defined. For example, at 40% of the design flow rate (or a 0.4 flow coefficient), there is only a 7% difference between Pcondenser/Pplenm at full diffuser opening (1.34 at PtA) and Pcondenser/Pplenm at the diffuser closed position (1.2 at PtB). As expected, the more open the diffuser throat, the more diffusion and the higher efficiency (fig. 6).

At this point, a curve fit illustrates that the bottom surge line (line 3, FIG. 6) can be determined at run-time and can be used as an upper limit for the diffuser parameters. This can effectively be a conservative control. To further increase the system efficiency, some more information is needed.

Since there is no total collapse in the Pcondenser/Pplenum measurement standard for surge (FIG. 6), it is determined that not all physical phenomena of the problem have been accounted for. It is also necessary to incorporate the correct orientation of the diffuser geometry into the analysis. To do so, Pcondenser/Pplenum was plotted as a curve relative to diffuser orientation (FIG. 7).

It can be seen that surge falls on a single line (line 2 in fig. 7). For any given diffuser orientation, there is a defined curve representing the maximum possible diffuser pressure rise that can be achieved. To further illustrate this collapse, only the maximum efficiency points over the eight test points are plotted along the two surge lines (fig. 3) from the surge region of fig. 7. This is essentially a subset of the data represented in fig. 7 and defines the upper limit of diffuser pressure recovery (line 21, fig. 8).

It is now clearly defined when maximum efficiency (or surge) will occur and a set of control schemes can be easily made to control for maximum efficiency and using this curve based on the current diffuser orientation. Because the current value of Pcondensor/Pplenum approaches the maximum value of Pcondensor/Pplenum (with an added safety factor) for a given diffuser orientation, the system can be stopped and surge avoided for maximum efficiency. The control curve can be determined by a minimum number of test points (4-8) along any surge line. Likewise, a minimum number of measurements are necessary to optimize the system (i.e., shroud side plenum pressure, evaporator pressure and diffuser orientation).

It is also important to note that the IGV orientation is not specifically used to define this curve in any way, and that the surge criteria are primarily determined by diffuser orientation. A weak function of Pcondenser/Pplenum on IGV localization is shown in FIG. 9. FIG. 9 is a contour plot of the data shown in FIG. 8 with the IGV position taken as the third dimension. The vertical profile in fig. 9 shows that the value of Pcondenser/Pplenum is relatively constant at surge for different positions, independent and independent of IGV positioning.

Previous data analysis demonstrated the application of Pcondenser/Pplenum measurement criteria to determine the optimal operating combination of diffuser and IGV settings in the likely surge region (shown in FIG. 3) to achieve maximum efficiency using diffuser orientation. To show the physics and reasons for the operation of this measurement standard, the following graphical description of the pressure rise through the compressor/diffuser system will be used (fig. 10).

Two cases of operating at the same pressure are described, one with a fully open diffuser and the other with a diffuser in either closed position. The fully open diffuser has a maximum static recovery coefficient. This is described by the greater lift in the diffuser pressure recovery in the fully open diffuser case (fig. 10 and 11). Therefore, to produce operating conditions at the same pressure, the compressor in the case of a fully open diffuser must be operated at a lower pressure rise (FIG. 11), for example, positioned closer to the IGV. This means that there is now more pre-swirl of the inlet for the fully open diffuser case than for either closed case and this will have a detrimental effect on operating efficiency.

Because the system losses are controllable in the diffuser region when the diffuser is closed to a greater extent, and the pressure recovery coefficient is less than half of that in the fully open condition for closed diffusers, the previously described smaller loss in efficiency due to more inlet preswirl in the compressor region is more than offset by increased losses in the diffuser.

The conclusion is that the more the diffuser is opened, the more efficient the system can be. Likewise, because the stability (surge) characteristics of the system are controllable by the flow in the diffuser, both maximum efficiency and surge occur at almost the same point. Thus, the diffuser performance (P) is described_condenser/P_denum) The measurement criteria of (a) naturally becomes a good measure of both stability and system efficiency.

The foregoing describes in detail methodology and metrics that can be used to optimize a centrifugal compressor system that utilizes variable diffuser geometry for inlet flow control and diffuser control system stability. The measurement criteria is the pressure ratio across the diffuser and diffuser orientation. For any given diffuser orientation, there may be a maximum obtainable pressure recovery value to stabilize operation. This is quite similar to the maximum pressure recovery coefficient before separation in a conventional parallel-wall diffuser. In centrifugal compressor systems, this split flow is injected into the system flow field and creates a variable and unstable flow.

Assuming that diffuser efficiency increases when the diffuser is open and that maximum opening of the diffuser can be determined by diffuser stability (stall and surge), the pressure recovery value can naturally be used as a predictor of both surge and maximum efficiency.

The above data shows that it is possible to use a measured pressure ratio across the diffuser to define the operating conditions. The measured pressures before and after the diffuser are measured under high pressure conditions, i.e. inside the adjacent chamber into the vaneless diffuser for upstream values and inside the condenser for downstream values. This is done to reduce the effect of transient conditions on the measured pressure.

There is a maximum achievable pressure recovery value for any given diffuser orientation, regardless of the inlet guide vane setting. The control scheme can be designed to ensure that the diffuser operates as open as possible (maximum efficiency) but never exceeds the maximum pressure recovery values (stall and surge).

The present invention may be embodied in other forms or carried out in other ways without departing from the spirit and essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are therefore intended to be embraced therein.

Claims (7)

1. A method for controlling operation of a compressor having an inlet and an outlet, a variable geometry diffuser in communication with the outlet, and inlet guide vanes in communication with the inlet, said method comprising the steps of:

determining a load parameter indicative of onset of surge; and

independently controlling the variable geometry diffuser and at least one of compressor speed and inlet guide vanes based on the load parameter to provide compressor efficiency enhancement and stable operation,

wherein the controlling step comprises:

determining from the parameter whether the desired change in compressor operation would result in surge if the variable geometry diffuser is operating;

when the load parameter indicates that surge is not to be induced, implementing the desired change by controlling a variable geometry diffuser; and

the desired change is implemented by controlling at least one of compressor speed and the guide vanes when the load parameter indicates that surge is to be induced.

2. The method of claim 1, wherein the load parameter comprises a pressure ratio across a variable geometry diffuser.

3. The method of claim 1, wherein the controlling step comprises independently controlling a variable geometry diffuser and inlet guide vanes.

4. A compressor system, comprising:

a compressor having an inlet and an outlet, a variable geometry diffuser in communication with the outlet, and inlet guide vanes in communication with the inlet; and

a controller programmed with information corresponding to a load parameter indicative of onset of surge and adapted to independently control the variable geometry diffuser and at least one of compressor speed and inlet guide vanes based on the load parameter to provide compressor efficiency improvement and stable operation,

wherein the controller is programmed to:

determining from the parameter whether the desired change in compressor operation would result in surge if the variable geometry diffuser is operating;

when the load parameter indicates that surge is not to be induced, implementing the desired change by controlling a variable geometry diffuser; and

the desired change is implemented by controlling at least one of compressor speed and the guide vanes when the load parameter indicates that surge is to be induced.

5. The system as claimed in claim 4, wherein the controller is programmed with information corresponding to a pressure ratio across the variable geometry diffuser as the load parameter.

6. The system of claim 4, wherein the controlling step comprises independently controlling the variable geometry diffuser and the inlet guide vanes.

7. A method for controlling operation of a compressor having at least two controllable operating parameters affecting operational stability, the two controllable operating parameters being a variable geometry diffuser and at least one of compressor speed and inlet guide vanes, the method comprising the steps of:

determining a load parameter indicative of onset of surge, by controlling any of the at least two controllable operating parameters, any of the at least two controllable operating parameters affecting an operating value of the load parameter, thereby enabling the operating value of the load parameter to be controlled; and

independently controlling at least one of the at least two controllable operating parameters based on the load parameter to operate at a desired efficiency rate within a stable operating region of the compressor,

wherein the controlling step comprises:

determining from the load parameter whether a desired change in compressor operation would result in surge if the variable geometry diffuser is operating;

when the load parameter indicates that surge is not to be induced, implementing the desired change by controlling a variable geometry diffuser; and

the desired change is implemented by controlling at least one of compressor speed and the guide vanes when the load parameter indicates that surge is to be induced.