Myth Busters: Axial Compressors Don't Have Shrouds, So Why Do We Need Them for Centrifugals?

The aerodynamic centerpiece of a centrifugal compressor is the impeller. That's where the mechanical work is transferred to the gas to increase the pressure. There are two types of centrifugal compressor impellers: closed (shrouded) and open (unshrouded). Since the vanes are essentially the place where the energy transfer happens, why is a shroud necessary? There are a number of considerations that aim to increase efficiency, operating range, head-making capability, manufacturability, and reliability.

A high-tip velocity maximizes the head per impeller. But the tip velocity of any impeller is limited by the allowable stresses. The shroud adds to stress levels that must be carried by the blades. Thus, an open impeller usually allows for higher tip velocities while staying within acceptable stress levels for the material. So why select impellers with a shroud?

Looking at the cross section of an impeller, we recognize that the blade height (or aspect ratio) is relatively small compared to blades in axial compressors. Thus, secondary flows from leakage across the blade from the pressure to the suction side are an issue. In open impellers, it’s imperative to maintain tight clearances between the blade and stationary parts in all operating conditions. Shrouded impellers allow control of leakage flows by using labyrinth seals at the impeller eye between the shroud and stationary parts. That greatly obstructs leakage and enables easier control of clearances. Since leakages affect head-making capability and efficiency, the relative advantages depend on the circumstances.

In high-pressure applications, tip clearance losses may be significant and require tight control on the gap between the blade tip and shroud to maintain acceptable stage performance. In applications with significant axial movement and thermal growth, tip clearance must be increased, which can affect performance and even thrust levels. That’s particularly difficult if multiple impellers sit on a single shaft. In that case, both shrouded and open impellers may be used, with the open impellers usually in the higher flow front stages. On the other hand, the stress caused by the centrifugal forces on the closed impeller blade due to the mass of the shroud necessitates thicker blades and limits maximum allowable tip speeds.

Modern impellers are analyzed for vibration-mode shapes to avoid interference with fluctuating forces, such as those from stationary vanes. Keeping the modal frequency high enough—and outside of the critical range—is usually easier with shrouded impellers. Open impellers typically have lower natural frequencies compared to closed impellers. The absence of a shroud removes an additional motion constraint that raises the natural frequency of the blade. The lower natural frequencies of open impellers can have implications for the compressor’s diffuser vane count selection and general robustness.

If impellers are 5-axis milled or welded, open impellers are easier to manufacture. Shrouded impellers can be milled in one piece, but low-flow impellers in particular require that the shroud be brazed or welded to the vanes. If impellers are cast or additive manufacturing is used, there is no significant difference in manufacturing difficulty, because there are no challenges controlling the fillets among the blades, hub, and shroud. In addition, there are also semi-shrouded impellers, where only part of the flow path (usually near the impeller eye) is shrouded, while the exducer part is unshrouded.

The relative advantages of open and shrouded impellers are reflected in the respective applications they are used for. In-line compressors for high-pressure applications—gas pipelines and other upstream, midstream, and downstream applications—typically use shrouded impellers. In these applications, tip velocities are often limited for reasons other than stress limits, such as operating at acceptable, favorable Mach number ranges, or improving efficiency and range.

On the other hand, air compressors and integral gear-type machines for various applications often take advantage of the high-tip velocities achievable with open impellers. In fact, they’re currently part of the discussion about hydrogen compressors, due to hydrogen’s high speed of sound and the fact that very high enthalpy rises are needed for even moderate pressure increases.

The authors want to thank Karl Wygant and Rob Pelton, both at Elliott Turbo, for their contributions to this article.

ABOUT THE AUTHORS

Klaus Brun is the Director of R&D at Elliott Group. He is also the past Chair of the Board of Directors of the ASME International Gas Turbine Institute and the IGTI Oil & Gas applications committee.

Rainer Kurz is the Manager of Gas Compressor Engineering at Solar Turbines Incorpo-rated in San Diego, CA. He is an ASME Fellow since 2003 and the past chair of the IGTI Oil and Gas Applications Committee.

Any views or opinions presented in this article are solely those of the authors and do not necessarily represent those of Solar Turbines Incorporated, Elliott Group, or any of their affiliates.