Energy Savings for Centrifugal Compressors

Current design improvements of both the rotating and stationary aerodynamic components of centrifugal compressors can greatly increase the efficiency of vintage machines. A centrifugal compressor built in the 1970’s or 1980’s might have an external polytropic efficiency in the high 70’s, whereas a newer compressor might have an efficiency over 85%.

Centrifugal compressors are designed to operate at a best efficiency point (BEP) shown on its performance map. Many compressor operators, in an effort to increase production will operate their machine at a flow higher than the BEP, sometimes even exceeding the predicted choke point. Not only is operating past the choke point very inefficient, but it could also cause damage to the compressor. Conversely, over time, changing process conditions may be causing the compressor to operate in a recycle mode. This is also an inefficient operating point.

Whether a centrifugal compressor is operating at the BEP, near choke, near the surge point, or in recycle, the operator may reduce the total energy consumption by rerating the installed machine with new, higher efficiency aero components. For example, a steam turbine driven compressor that was operating at a very inefficient point had a reduction in power of nearly 11% by installing higher efficiency aero components.

By rerating a compressor instead of replacing it, the user can save a great deal of money by reusing the existing compressor casing, foundation and process piping and minimizing the time it takes to implement the change.

Whether the compressor is driven by an electric motor, steam turbine or gas turbine, costs for operating these drivers is ever increasing and how much it may go up in the future is uncertain. With a greater worldwide focus on protecting the environment by reducing greenhouse gases, improving the efficiency of an installed centrifugal compressor by rerating can go a long way toward that goal.

This presentation will explain the reasons for rerating a compressor and provide details of the rerate process.