Centrifugal compressors: Basics and operational range

Centrifugal compressors are the dynamic type, meaning that compression is accomplished through the conversion of kinetic energy to static energy. The defining characteristic of centrifugal compressors is that head is determined by the volume flow through the unit.

In the case of an axial-inlet centrifugal compressor (typical of integrally geared units), an appropriate length of pipe serves to smooth the flow of gas into the eye of the first impeller.

This article contains excerpts from the paper, “Your Gas Compression Application – Reciprocating, Centrifugal, or Screw?” by Greg Phillippi of Ariel Corporation, Tim Manthey, Aerzen USA, Jonathan Sutter of Elliott Group, Ben Williams of Ariel Corporation and Bruce McCain, an engineering consultant, at the 2016 Turbomachinery Symposium.

For the radial-inlet compressor, an appropriate length of pipe and inlet guide vanes direct the gas towards the center of the unit before a 90° bend redirects the flow (axially) into the eye of the first impeller. In the multistage centrifugal compressor, a vaned radial return channel is used to straighten and smooth gas flow preceding a 90° bend into the eye of the next impeller. The impeller spins on a shaft and is the means by which energy (work) is imparted on the gas. Gas enters the eye of the impeller and once again makes a 90° turn resulting in a radial flow from the center of the impeller to the outer diameter.

Additionally the gas encounters rotating blades in the impeller. These blades push the gas in a circular motion resulting in a static pressure rise (compression) due to the centrifugal force of rotation. For the impeller with the typical backward-leaning blade, approximately two thirds of total static pressure rise of the stage is obtained within the impeller. Upon leaving the impeller, the flow has two velocity components: the first is a component in the radial direction, VR, and the second is a component in the tangential direction, VT (impeller velocity components). The high velocity gas then enters the diffuser.

The diffuser is a stationary component which primarily converts velocity (kinetic energy) to pressure (static energy). The diffuser is a radial passage that is roughly the same width as the impeller blade, but the radial area expands and provides the desired diffusing effect. As the gas exits the diffuser and enters the return bend, the gas has obtained the majority of the static pressure rise for the centrifugal stage (a centrifugal stage being the combination of the impeller and diffuser). The return bend is found in multistage centrifugal compressors and is a stationary component which redirects the gas from a radially outward flow in the diffuser to a radially inward flow into the return channel which contains the next stage’s guide vanes (as previously mentioned, these vanes help to straighten and smooth the gas flow preceding the downstream impeller). The return bend is a 180° passageway that is often partially integral with the upstream stage’s diffuser and partially integral with the downstream stage’s guide vanes. Performance of a centrifugal compressor is often evaluated through the compressor’s head versus flow curve and power versus flow curve. The shape of the head versus flow curve is very much a characteristic of the impeller geometry (Head vs. Flow).

For the typical compressor stage, decreasing the required head increases the volumetric flow. Likewise, increasing the required head reduces the volumetric flow. The ends of the curve are defined by surge at the low-flow end and stonewall at the high-flow end. The power vs. flow curve is also stage dependent. For a given stage, the shape of the power curve is a function of flow, head, and efficiency (Power versus Flow).

With respect to controlling operational range, the order of effectiveness is typically as follows:

• Recycling a portion of the flow from the discharge back to the inlet of the compressor is an effective method of control, but reduces compressor efficiency. An additional downside is the potential increase in after-cooler size if the recycle flow is large.

• Speed variation results in a large operating range. The operational map for the compressor is usually limited by mechanical constraints, such as rotor dynamics or impeller stress.

• Adjustable inlet guide vanes add a component of pre-whirl to the gas before entering the upstream impeller. This is a very efficient means of compressor control, however the effect can be limited when there are many stages of compression

• Throttling via a control valve preceding the compressor is also an effective means of controlling the volumetric inlet flow to a centrifugal compressor but introduces inefficiency into the overall process. Throttling can also be applied at the discharge of the compressor.