Expansion engine is an apparatus for extracting work from a gas by allowing the gas to expand between two pressure levels. As the energy is extracted, the gas temperature is lowered. It is a vertical single acting reciprocating type engine.
The Electric Motor is used initially to start the machine. Thereafter the engine is moved by the air pressure itself and during this time the engine motor retains the speed by acting as a brake. Work is done by air in rotating the Flywheel and thus there is loss in the heat content. This process is called enthalpy. Due to this the air gets cooled. This cooling is more than that of an expansion in an expansion valve.
A high expansion engine is more efficient than a low expansion engine, but in a high expansion engine the gas cools considerably as it expands.
Most gas liquefiers use expansion engines to pre-cool the gas close to its liquefaction. He and H2 liquefiers usually contain two or more expansion engines at successive temperatures, with multiple heat exchangers. In gas expansion engines, the gas engine functions as a compressor in reverse. There is no combustion in gas expansion engines.
The Expansion Engines can be considered as three major units:
- The drive unit.
- The Cylinder Unit for air expansion.
- The Hydraulic Systems for operating the Valves.
Piston Type Engine - In order to run an expansion engine with liquified gas there must be a heat source to vaporize the liquid which then expands over the piston to produce work. Piston engines are used for low-speed flight. They have the advantage of being relatively inexpensive to produce. An airplane piston engine works on the same principle as an automobile engine. It releases the energy stored in fuel by burning the fuel in a "four-stroke" cycle.
Turbo (Turbine) Expander Type Engine - This converts pressure drop in the inlet gas flowing through the turbine vanes into mechanical energy through a rotatable shaft attached to the turbine blades. Turbo expander engines are well known in the art.