There are various meanings associated with Expanders in compressed air and gas systems.
- A turbo-expander, also referred to as an expansion turbine, is a centrifugal or axial flow turbine in which a high-pressure gas expands to produce useful work, generally to drive equipment or machinery. The device often provides an attractive option for recovering energy when the pressure of a gas stream needs reducing — and so finds use in a wide variety of plants. Because the work comes from the expanding high-pressure gas, the expansion is approximated by an isentropic (nearly constant entropy) process; the reduced pressure exhaust gas from the turbo-expander is at a lower temperature than that of the inlet gas.
In a simple, single-stage turbo-expander, the high-pressure gas flows through variable inlet nozzles (or inlet guide vanes) and then through the wheel, exhausting at a lower pressure and substantially colder temperature. In many applications, the outlet gas goes to a downstream process; therefore, turbo-expander nozzles are used to control the gas flowrate and conditions to maintain the operating conditions (flowrate, pressure, etc.) required downstream.
Such Expanders are installed in gas processing facilities around the world.
- A screw expander is based on the inversion of a twin-screw compressor and under certain conditions may be a suitable alternative to a turbine as a thermal cycle expansion device for the transformation of thermal energy into mechanical energy. These conditions are basically defined by the parameters of the heat source. A screw expander is particularly used when the heat source is limited, such as Solar or Geothermal energy or Waste heat. The classical steam Rankine cycle is not suitable for the use of low-potential heat, and therefore working substances with a lower boiling point than water are usually used. If such a working substance is used, a screw expander is usually an integral part of the Organic Rankine Cycle (ORC), the binary cycle or the Kalina cycle. In the case of low-potential heat applications, a liquid phase may appear during the expansion of the low-boiling working fluid, causing erosion of the turbine blades. In such a case, a screw expander capable of working without damage should be used, even with a relatively large proportion of liquid phase.
Below schematic diagram shows using ORC for Low Pressure heat to drive Screw elements to operate as Expander to generate electricity.
Below schematic diagram shows using High Pressure gas to drive Mono Screw element to operate as Expander to generate electricity and use the Low-Pressure gas for other applications.
- Demand Expander term is sometimes used for ‘Flow Controller’ also.
(See also, ‘Flow Controller’ as Demand Expander)