Re-Rating a Centrifugal Compressor from 8.5 to 5 bar Cuts Specific Power 14.55%

Many plants run compressors that are “overpowered for a light load”. A manufacturer in Jiangsu Province operated a SIND TurboPure series centrifugal compressor designed for 8.5 bar, but its production processes only needed 5 bar. Generating 3.5 bar more pressure than required wasted a significant amount of energy every operating hour.

Replacing a large, otherwise sound centrifugal compressor would have been expensive. The plant wanted to recover the efficiency without buying a new machine and without disrupting production.

Instead of replacing the compressor, it was re-rated to the plant’s actual operating point:

• Data first. Eight days of operating data were collected on site to define the real flow and pressure profile.
• Aerodynamic redesign. Core components, including the impeller and volute, were optimised for the new duty.
• Matched compression ratio. The compression ratio was adjusted to the 5 bar requirement, so the machine now runs at an efficient operating point instead of throttling down from its 8.5 bar design.

The work was planned so that it did not affect production.

• Specific power reduced by 14.55%.
• About 3,475 kWh of electricity saved per day, based on the unit’s maximum flow.
• About CN¥800,000 (≈US$112,700 at CN¥7.1/US$) saved in electricity costs per year.
• No new compressor purchase and no production disruption.

The same approach can be applied to existing centrifugal compressors in several situations:

• lowering or raising the discharge pressure;
• reducing or increasing air delivery;
• recovering heat from the cooling system with SIND heat recovery;
• optimising the whole station, including the match between compressor and post-treatment;
• load-sharing group control;
• automatic start-stop;
• performance evaluation of existing compressors;
• pipeline leak detection and repair.

An existing compressor can often regain high efficiency through re-rating, without the cost of a new machine.