Power Recovery Equipment 8241 Turbo Expanders

These units are used instead of letting gas pressure down through control valves or multiple orifice chambers. There are three main applications: fluid catalytic cracker (FCC) regenerator gas at around 750 °C and ~3 bar(g) letting down to near atmospheric pressure; natural gas expanders supplied to a plant at anywhere from 10 to 100 bar(g) and letting down to ~3 bar(g) for fuel gas; and cryogenic expanders linked to compressors in cold box service.

Each service has its own design requirements and constraints. There is one main point to make - if the upstream or downstream pressure has to be controlled, then it is better to make the expander slightly undersized. Turbo expander performance falls off rapidly if the flow and pressure ratio does not match its design. It is therefore better to have some modest by-pass flow through a control valve so that as the process requirement changes, it is this by-pass flow that alters, leaving the

Figure 8.9 Typical inlet guide vanes. 8.2.3.2 Axial Fans

These are generally used in high volume, low head applications. Depending on the service, VFD may be applicable. Variable angle inlet guide vanes also offer good efficiency at part load. For large fans, efficiency can be improved by selecting aerofoil section blades rather then pressed sheet. The possible efficiency improvements detailed in Section 8.2.8 should be checked.

expander at peak performance. In the case of an FCC expander, it is particularly important since catalyst fines will either build up or erode the expander blading.

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