The diffuser converts a portion of the remaining kinetic energy in the gas stream (velocity pressure) into static pressure (potential energy), further reducing the volumetric flow. Centrifugal compressor diffusers fall in two broad categories: vaneless and vaned. As indicated by their name, vaneless diffusers contain no vanes.
Conversely, vaned diffusers contain one or more rows of vanes. In general, vaneless diffusers offer the widest flow range because there are no vanes to interfere with the gas as it passes through the diffuser. However, the static pressure recovery in vaneless diffusers is not as high as in their vaned counterparts. Therefore, the peak attainable efficiency for stages with vaneless diffusers is not as high.
This article contains excerpts from the paper, “Centrifugal compressor evolution” by James M Sorokes of Dresser-Rand Business, and Mark J Kuzdzal of DRESSER-RAND Business, Siemens Power & Gas Division at the 2018 Turbomachinery Symposium.
Vaneless diffusers were the dominant style in early centrifugal compressors because of their simplistic design. The parallel or tapered walls were easy to machine via turning. Therefore, it was possible to achieve some very good surface finishes, a necessity for high performance in vaneless diffuser. However, the limited peak static pressure recovery (typically less than 50 percent) restricted the peak efficiency achievable with vaneless diffusers.
Some process compressor OEMs attempted to apply channel diffusers. The name comes from the fact that two adjacent diffuser vanes form a passage or channel. These diffusers do provide superior static pressure recovery, CP, with peak CP levels reaching 75 percent – 80 percent. However, channel diffusers also cause a substantial reduction in flow range, making them undesirable for compressors that must operate over a range of flow conditions. As a result, channel diffusers are rare in process compressors but somewhat popular in air machines, gas turbine gas generator compressors, or turbochargers, which do not require wide flow range.
Vaneless diffusers were the most widely-used style in industrial centrifugals until the late 1980s when some OEMs began applying a style known as the low solidity vaned diffuser. Unlike the channel diffuser, the vanes in a low solidity vaned diffuser (LSD) do not form a channel and also have no true geometric throat.
The most important benefit is that LSDs provide nearly the same operating range as a vaneless diffuser yet provide greater pressure recovery and, therefore, higher stage efficiency. The introduction of the LSD provided a step change in stage efficiency without significantly reducing the flow range. However, the efficiency enhancement seemed to be limited to medium- to low-flow coefficient stages; i.e., f < 0.080 and smaller; with the greatest benefit being in flow coefficients, f, less or equal 0.030.
More recently, select OEMs have re-introduced the rib diffuser, a special class of LSD with vanes that do not cross the entire flow passage. These diffusers were first suggested in the mid-to-late 1970s but did not gain broad acceptance until much later. Again, this style of diffuser provides an efficiency boost in some flow coefficient ranges but is ineffective in others. Those seeking more details on this style are directed to the work of Sorokes and Kopko.
One other class of vaned diffusers deserves mentioning… the so-called tandem or multi-row diffuser. Rather than a single row of vanes, these diffusers have multiple rows (two or more) of vanes with each row contributing to the static pressure recovery process. It is commonly known that if one attempts to achieve too much turning in diffuser vane, the flow will separate from the vane surface, leading to excess losses or possible premature stall. Therefore, if high levels of turning are desired, the turning is better achieved with two or three rows of vanes rather than one.
The multiple rows of vanes also help to control boundary layer growth and reduce losses in the diffuser passage. In recent years, tandem rib-LSD or LSD-LSD diffusers have proven to provide performance benefits when compared to single row cascades.
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