Rotating Stall in Centrifugal Compressors
Introduction When dealing with centrifugal compressor applications, terms such as surge and stall are often used. While compressor surging is generally an understood phenomenon, the concept of rotating stall is often harder to explain and understand. This paper will provide a technical explanation of rotating stall and show how rotating stall can be minimized or prevented. Rotating stall is not a design or manufacturing defect, but an aerodynamic fact of life when dealing with centrifugal compressors. Although rotating stall does not adversely affect the reliability of the rotating parts of the compressor, it does change the operating characteristics of the chiller package. Additional noise and vibration are generated when operating in rotating stall. Depending on the severity and duration of operation in this condition, the vibration generated can fatigue system piping, sometimes leading to line breakage. The chances of the chiller operating in rotating stall during normal operation can be reduced or eliminated if care is taken during the selection process and if the system is operated properly once installed. Making a selection based on knowledge of how the system operates and training of operating personnel can make these two things happen. Selections should be carefully reviewed to allow for adequate turndown, based on the specific application requirements. Operating personnel should be educated about YORK chiller operation and allow the entering condenser water temperature to drop and track the outdoor wet bulb temperature. Operating this way provides the greatest energy savings for the chiller system and the best cooling tower and chiller operation. In multiple chiller installations, chillers should be run at higher loads with some chillers cycled off, rather than running multiple chillers at low load.
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otating stall is an aerodynamic disturbance that occurs in centrifugal compressors at reduced flow (reduced load) and/or increased head (increased temperature lift).
Impeller and vaned diffuser stalls occur at flows and heads that are very near the surge point. Centrifugal compressors do not operate for any length of time with impeller stall or vaned diffuser stall because even a small flow or head variation will shift the compressor There are three kinds of rotating stall: impeller stall, into a surge condition and out of stall. vaned diffuser stall, and vaneless diffuser stall. Which stall occurs, at what flows and at what heads, depends Although vaneless diffuser stall occurs at conditions on the compressor geometry, the position of the near the surge point, it may, depending on application compressor’s inlet prerotation vanes (PRV), and the and compressor characteristics begin to occur at conimpeller tip speed. ditions that are farther away from the surge point. Cenpage 1
trifugal compressors may operate for long periods of Several impeller blades usually stall at the same time time with vaneless diffuser stall. so that multiple stalls rotate around the impeller. When too many blades stall, or too much of the flow reRotating stall in a vaneless diffuser generates a char- verses in the stalled passages, all impeller flow stops. acteristic noise and vibration whose frequency is close When this happens, the higher pressure in the conto the impeller rotating frequency. This noise and vi- denser forces refrigerant to flow backwards, from the bration can be minimized when operating at reduced condenser, thru the diffuser, thru the impeller, to the load by allowing reduced condenser water tempera- evaporator. This complete reversal of flow, from the ture. condenser to the evaporator, is called a “surge”. Stall noise and vibration can also be minimized by se- Rotating stall in an impeller is sometimes called “inlecting a compressor whose full-load operating point cipient surge” because impeller stall occurs quite close is far removed from the surge point. to the surge point on a compressor performance map. Once an impeller begins to stall, only a small decrease Impeller Stall in flow or increase in head will stall the impeller completely, and cause the compressor to surge. In an impeller, rotating stall begins when the flow stream passing around one of the impeller blades separates Vaned Diffuser Stall from the back of the blade. This happens when the angle-of-attack of the flow approaching the blade be- Rotating stall in vaned diffusers also occurs near the comes so large that the blade “stalls” in the same way surge point and acts the same way as impeller stall. that an airfoil stalls. When one diffuser vane stalls, the stalled flow in the passage behind the vane causes an adjacent vane to The flow angle increases as the flow decreases, so the stall. This ends the stall of the first vane. Multiple stalls large angle-of-attack that stalls the blade occurs when rotate from vane to vane the flow is low. When the inlet PRVs are partly closed, the flow angle entering the impeller is reduced. This Vaneless Diffuser Stall reduces the tendency of the impeller blades to stall when the flow is reduced. Rotating stall in a vaneless diffuser is quite different from the other two kinds of stall. Vaneless diffuser stall The tendency of the blades to stall increases as the can begin some distance from the surge point, and impeller outlet pressure increases. At border-line flow typically occurs when the compressor PRVs are partly angles, stall only occurs when the impeller outlet pres- closed. Vaneless diffusers have no airfoil blades or sure is high; i.e., when the compressor head is high. vanes that can be “stalled”. The flow separation behind a stalled blade reduces the volume of flow in the passage behind the blade. It may even cause some of the flow to reverse itself and flow back out of the passage. This reduced (possibly reversed) flow increases the angle of the flow at an adjacent blade, causing the adjacent blade to stall. The stall of the second blade lowers the flow angle at the first blade, thereby restoring normal unstalled flow around the first blade. Thus the stall moves from blade to blade around the impeller. The stall is said to “rotate” around the impeller, hence the name “rotating stall”.
Instead, recirculating “eddies” form in a vaneless diffuser when the flow is reduced and/or the head is increased. These eddies are called “stall cells” because they affect the diffuser flow in much the same way as stalled diffuser vanes do. When an eddy forms in a vaneless diffuser, the altered flow on one side of the eddy causes the eddy to move sideways. The eddy moves (rotates) around the diffuser similar to the way a vane stall rotates around a vaned diffuser. Several eddies usually form and rotate around a vaneless diffuser at the same time. If the flow
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is further reduced, or the head further increased, the a vaneless diffuser design. A key advantage of a eddies become larger. If the eddies become too large, vaneless diffuser is the ability to accept a wide range of inlet flow conditions. the flow stops, and the compressor surges. Vaneless Diffuser verses Vaned Diffuser As discussed previously, the diffusing passages down-
Thus, YORK compressors with vaneless diffusers maintain high efficiency over a broad flow range. With no obstacles in the flow, vaneless diffus-
Compressor Performance Map
100
Temperature Lift %
PRV 100% 50%
25%
0%
0 Figure 1
Evaporator Cooling Load %
100
ers are not contributors to noise generation over most of their range, providing relatively quiet chiller operation over a wide operating range. The exception is The compressor diffuser is used to decelerate the high when the machine operates in rotating stall. Although velocity flow leaving the impeller, causing a static pres- compressor noise may increase when rotating stall sure rise -the purpose of the compressor. occurs, the compressor continues to be able to maintain its pressure rise with high efficiency. The YORK YT and YK centrifugal compressors use Vaned diffusers employ wedge-shaped, airfoil-shaped, stream of the impeller in centrifugal compressors can be of two types; vaned or vaneless.
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or conical tube flow guides that arrest the swirling component of the flow leaving the impeller. The flow is guided into passages that accomplish the desired static pressure rise in a shorter distance than in vaneless diffusers, allowing for a more compact design. There is also potential for slightly higher efficiency at the design point. However, the presence of the vanes near the
noise levels than compressors with vaneless diffusers. Since chillers rarely operate at the design point of full load and design ECWT, the higher peak efficiency of a vaned diffuser is of little value. YORK has chosen vaneless diffuser for better off-design performance and quieter chiller operation.
Compressor Sound Spectra 100 Low Load
90 80 70
High Load
60 50 40 30 20 10 0 63
125
500
250
1000
2000
4000
8000
Figure 2
Octave Band Frequency (Hz) impeller discharge is a significant noise generator analogous to a siren. Since the vaned diffuser has fixed inlet angles, they are less tolerant of flow angle changes that occur as the chiller load changes. Thus, compressors with vaned diffusers may have slightly higher peak efficiency but a more limited capacity range and higher
Performance Map Figure 1 is a typical performance map for a constantspeed compressor with a vaneless diffuser. Each PRV position generates a performance line which moves upward and to the left, from low head (low +T/+P
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between evaporator and condenser) to a higher head, until rotating stall in the diffuser begins. The PRV line continues upward, thru a zone of rotating stall, until the compressor surges.
Since YORK’s single-stage compressors utilize a vaneless diffuser design, Figure 1 is representative of YORK single-stage compressor performance at constant speed.
The points on the PRV lines at which rotating stall be- Surge and Stall gins are connected by a line that is called the “stall envelope”. The points at which the compressor be- A surge is a reverse flow in which refrigerant moves
Compressor Performance Map
100
Temperature Lift %
PRV 100% ced
u ed
R
0%
0
Figure 3
Evaporator Cooling Load %
gins to surge are connected by a “surge envelope”. A considerable distance can separate these envelopes. Vaneless diffuser stall is not an immediate forerunner of surge in the way that impeller stall is.
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backward, from the condenser, thru the compressor, to the evaporator. The condenser pressure goes down when the refrigerant flows back to the evaporator, and the evaporator pressure goes up. This lowers the head against which the compressor is operating, and allows There is no vaneless diffuser stall when the PRVs are the compressor to begin pumping refrigerant in the right wide open. The impeller stall and surge point are at direction again. When the compressor starts pumping the same point with open vanes. again, the condenser pressure goes up and the evaporator pressure goes down. If the +P is still above the limit for the compressor, the unit will surge again. page 5
The flow reversal during a surge unloads the compressor motor. This drops the motor ammeter reading almost to zero. When the compressor starts pumping again, the ammeter swings up again, almost to its fullload reading. The evaporator and condenser pressure gages also swing up and down as the flow swings back and forth through the compressor.
with the compressor operating at high load without stall, the other with the compressor operating at low load with rotating stall in its diffuser.
Noise and Vibration
The noise and vibration associated with rotating stall in a vaneless diffuser can be minimized, if not completely avoided, by keeping the entering condenserwater temperature low when the load is low. Of course this also keeps the compressor power consumption low, and keeps the compressor from surging.
In both cases, the impeller blade frequency appears as a mild peak in the 2000 Hz octave band. An impeller with 17 blades rotating at 7100 rpm would produce discharge pressure fluctuations (noise) at a freIn all forms of rotating stall, only a small amount of the quency of 2012 Hz, almost in the middle of the 2000 total flow recirculates inside the impeller or diffuser. Hz band. Most of the flow is pumped continuously from the evaporator to the condenser. The evaporator and con- At low load, a larger peak occurs in the 125 Hz ocdenser pressure gages are steady during rotating stall. tave band. This is rotating stall noise. The stall noise The motor load is also steady. A centrifugal compres- frequency is about the same as the impeller running sor can operate indefinitely under these conditions. speed (7100 rpm = 118 Hz). Operation in surge, on the other hand, even for a short time, can damage the compressor. Operating Conditions
In surging, flow reversals occur every one or two seconds. Small systems surge at higher frequencies and large systems at lower frequencies. The sound of surging is distinctly different from the sound of rotating stall. A surging unit emits a loud “groan” every few seconds. Surging makes a unit sway rather than vibrate. In Figure 3, two different operating lines are shown Long water pipes attached to a surging unit may swing on the compressor performance map of Figure 1. The back and forth every few seconds. upper line is one of constant entering condenser water temperature (ECWT) at all loads. The lower line is a Stall cells rotate around impellers and diffusers at less typical part-load line with the same ECWT design than the impeller’s rotational speed. But when mul- point. tiple stall cells rotate in a vaneless diffuser, their combined frequency is close to the impeller rotational fre- When the cooling load is reduced below the full-load quency. Three cells, equally spaced around a vaneless (100%) point, the constant ECWT line reaches the diffuser, each rotating at one-third of the impeller run- stall envelope at 71% load. With further load reducning speed, will produce compressor discharge pres- tion, the compressor operates with rotating stall in its sure fluctuations at the frequency of the impeller rota- diffuser until the load reaches 24%, at which point the tion. compressor surges. The discharge pressure fluctuations that are caused by rotating stall in vaneless diffusers are too small to be seen on the condenser pressure gage, but they can be heard as a “roaring” noise, and felt as a condenser shell vibration. Figure 2 is an octave-band analysis of the sound emitted by a typical constant-speed compressor with a vaneless diffuser. Two sound spectra are shown: one
With lower condenser-water temperatures, the reduced ECWT line avoids the stall zone until the load is reduced to 31%. This line never reaches a surge point. Keeping the full-load point some distance away from the PRV 100% OPEN surge point also minimizes stall and surge. If the full-load point in Figure 3 was further
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to the right on the PRV 100% OPEN line, the constant ECWT load line would almost completely avoid surge, and the reduced ECWT load line would almost completely avoid stall.
Recognizing and understanding the difference between rotating stall and surge is the first step in the process of making better chiller selections and helping guide the user on how to best operate installed chillers. When looking at chiller selections, operating points are often referred to as being on the “left or right side” of the compressor map. This is illustrated in Figures 4 and 5. Being on the right side of the map Right Side Selection
Temperature Lift %
PRV 100%
Stall Zone Reduced ECWT Constant ECWT
0%
0
Figure 4
Evaporator Cooling Load %
PRV 100%
Temperature Lift %
Conclusion
Left Side Selection
Stall Zone Reduced ECWT Constant ECWT
0%
0
Figure 5
Evaporator Cooling Load %
gives a design point with the vanes in a full (or near full) open position, further away from both the stall and surge zone. As noted above in the paper, moving to the right gives a wider unloading range outside the surge and stall zone. When one starts with a selection towards the left side of the map, this range is reduced, and the machine may spend more operating hours in the stall region. Operating chillers with lower ECWT(at full and part load) saves energy and minimizes cooling tower fan cycling, reducing system maintenance. This is also the best way to keep machines away from operation in stall and surge conditions. Emphasizing this type of system operation provides a winning combination of efficient and reliable chilled water plant operation.
Draft 1f-Joe Brillhart
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