Top tips for choosing a centrifugal compressor

Match your compressed air system to your plant needs.

By Amin Almasi, WorleyParsons Services

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In brief:

  • Compressor selection should be based on an evaluation of technical and commercial factors and this should be completed before placing an order. Integrally-geared compressors are relatively new to process applications and involve more complex dynamic behavior, more vibration and dynamic factors, and usually narrower operating margins. But, the integrally geared configuration can be one of most efficient and compact designs for centrifugal compressors.
  • One of the main applications for integrally geared machines is small- and medium-size constant-speed motor-driven centrifugal compressors where a variable speed drive isn’t feasible. Integrally geared machines driven by synchronous motors require special considerations. Integrally geared centrifugal compressors are usually around 15% to 30% less expensive, around a 10% to 15% shorter fabrication period. In spite of lower prices, they introduce more risks, less reliability and more maintenance
  • Integrally-geared machines can reach an equivalent pressure using fewer impellers and higher maximum tip speeds. Both efficiency and pressure coefficient are higher for semi-open overhung three-dimensional impellers, usually found in integrally geared machines. One disadvantage of an integrally geared compressor is that each impeller needs to be sealed, an arrangement for which labyrinth seals are acceptable.

Centrifugal compressors are dynamic machines in which rotating impellers accelerate the gas. Main flow is radial. The velocity head is converted into pressure, partially in the rotating elements and partially in stationary diffusers. There are many configurations of centrifugal compressor that can do this. The integrally geared configuration is an overhung style machine installed on a gear system that uses the gear pinion shaft extensions to mount impellers. The individual compressor casing is attached to the gear system main casing.

The most common form is a packaged plant air compressor (covered by American Petroleum Institute (API) 672) but in the most recent two decades, integrally geared centrifugal compressors were introduced for process applications and recognized by API 617. Evaluation and purchasing of centrifugal compressors is an involved optimization process with respect to the full array of compressor data. Otherwise, you might end up with equipment that isn’t suitable for the full operating range, reliability, maintainability, availability, commercial terms or power requirement you expect.

Figure 1. The pinions in an integrally geared multi-stage centrifugal compressor don’t need to rotate at the same speed.
Figure 1. The pinions in an integrally geared multi-stage centrifugal compressor don’t need to rotate at the same speed.

A concern in the bidding stage is the correct choice between classic, reliable, low risk conventional type (driver, gear unit and multi-casing machine) and new, compact and relatively cheaper integrally geared machines. Some people evaluate reliability and risk issues first, with the initial cost and other factors as secondary. This reliability-versus-other-factors trade-off usually helps to bring more centrifugal compressors into various process applications. However, this approach might lead to selecting conventional-type centrifugal compressors for every application without considering advantages of integrally geared machines for specific applications.

Compressor selection should be based on a thorough evaluation of technical and commercial factors to ensure the best machine is purchased. But you’ll need to know the different design and operation aspects of both the conventional type and the integrally geared type.

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