A blog on what's new, notable, and next in turbomachinery

Shrouded or Open Impellers for Compressor Chillers - That is the Question

By Steve Kohr
Aug 2, 2017

  • Facebook
  • Twitter
  • LinkedIn

One of the most important design choices facing a chiller compressor designer is whether to utilize open or shrouded impellers. The impacts of this verdict are wide ranging, which arguably makes this most basic selection the most critical one.

There are three key loadings on any impeller:

  1. Inertial (centrifugal) loading – a direct result of the rotational speed of the machine, along with the impeller material properties

  2. Aerodynamic pressure loading – driven by the fluid flow conditions

  3. Temperature loading - from the fluid the impeller is compressing, along with other boundary conditions on the impeller

In general, for a chiller compressor, the inertia load, which is governed by Equation 1, is dominant. If all the loadings are consistent between an open and shrouded impeller, the response of each to those static loads can be vastly different. In general, the shrouded impeller will have significantly higher stress magnitudes due to the increased mass and radius imposed on the impeller by the shroud. Therefore, the stress margins on the shrouded impeller will be smaller and may drive design changes that otherwise would not be required for the open impeller.

Equation for Centripital Force.jpgEquation 1: Centripetal Force equals Mass of Wheel x Radius x Angular Velocity Squared

Another consideration in impeller design is the modal (vibration) response of the blading and disk.  Most chiller compressors have a wide range of operation, which makes it more difficult to create a design that does not have potential modal interferences. An open impeller will generally have blade modes at lower frequencies and disk modes at much higher frequencies. Shrouded impellers are usually the opposite—disk modes occur at lower frequencies and blade modes at higher frequencies due to the added constraint on the blades from the shroud. Blade modes often present less risk to the overall machine than disk modes. Therefore, a lot more care must be taken when designing a shrouded impeller for dynamic response than for an open impeller, since it is never recommended to operate on a disk mode.

There are some rotordynamic impacts to the design selection of either an open or shrouded impeller as well. The increase in mass with a shrouded impeller will decrease the critical speeds for the entire range of bearing stiffness. There is also potential for decreased stability since the sealing radii for shrouded impellers are generally larger. The larger sealing radii will also increase the seal leakage which can have a negative impact on performance. The negative impact is often offset because the shroud eliminates the tip clearance and associated tip leakage necessary with open impellers. In general, shrouded impellers provide higher efficiency. Axial thrust can also change significantly with a shrouded versus open impeller, depending on where the seal is placed on the shroud. 

Finally, there is the question of manufacturing. Manufacturing open impellers is very straightforward and can be done using point milling or even flank milling for simpler blade geometries. This access to the blading leads to relatively fast cutting times. Shrouded impellers can be manufactured using at least three methods: casting, brazing of the shroud, or by 5-axis milling.

Casting is the fastest and cheapest method and is very common in the automotive industry, but not embraced by the chiller industry. Chiller compressor manufacturers are striving to achieve very high performance, and the roughness of the cast surface as well as tolerance control can have a significant negative impact on performance. Brazing of the shroud is the most common method and it is used across many industries. It is important to consider the structural integrity of the brazed joint in the design process. However, in chiller compressors the tip speed is commonly very low, which can lead to low stress magnitudes that alleviates this concern. Milling, or hollowing, the shrouded impeller from a single piece of stock is the ideal solution with respect to tolerances and stress. It provides a high performing shrouded impeller with little loss from poor surface finish and none of the structural concerns that come from the brazing process. The biggest drawback of this method is the machining time. It can take five to ten times the cutting time to mill a hollowed shrouded impeller compared to the open version of the same blading. It is extremely important to use good Computer-Aided Manufacturing (CAM) software to ensure optimized milling strategies to minimize the machining time.

The seemingly simple choice between whether to use an open or closed impeller in a chiller compressor design is not so simple after all. There are several design impacts that must be studied at an early phase of the design development to ensure that the correct choice is being made. Performance is only one piece of the puzzle. Be sure to get the entire integrated design team involved so that all the trade-offs can be fully understood and the right decision can be made.

So, you will have to decide -

Whether 'tis nobler in the mind to suffer

Tip Leakage of outrageously large radii

Or to take longer to see it machined,

For higher efficiency.

Tags: CAE Software, Compressors, CAM Software, Manufacturing

Subscribe to SpinOffs