The same simple, robust geometry, positive displacement, and efficient use of the working volume that make StarRotor compressors so attractive, confer similar benefits when applied to expanders. Like our compressors, StarRotor expanders are compact, efficient, simple, and quiet. In some cases, the significance of these advantages may be different because of differences in application.
Isentropic efficiencies of expanders are typically slightly higher than for compressors of the same scale and technology. In simple terms, a compressor pushes a gas against a pressure gradient, which is inherently inefficient. In contrast, an expander allows a gas to flow with the pressure gradient, which is inherently efficient.
Except as primary components of power generation plants and engines, the use of expanders to convert enthalpy into shaft power is usually optional. The price of energy, the efficiency of the expander, and the cost of the equipment drive the decision whether or not to recover waste energy. Conventional turbo-expanders and other dynamic machines scale down poorly, and often are not economic below the megawatt scale.
StarRotor expanders can be efficient at scales that are too small for other technologies to justify their costs. Because they are positive-displacement machines, they can recover more energy from low-quality heat streams than dynamic machines. This combination of strengths makes it practical to recover waste heat at scales, and at temperatures, that may not otherwise be economic.
Phase changes are not usually a problem for compressors because the heat of compression keeps the working fluid in a gaseous state. In contrast, cooling in expanders makes liquid dropout more of a problem. The high surface speeds in dynamic machines make them vulnerable to droplet erosion, as do the valves of reciprocating machines. StarRotor expanders have low surface speeds and robust shapes that are less prone to damage. Furthermore, StarRotor expanders require no valves.
The liquid tolerance of the StarRotor design can be particularly valuable in processing steam or other vapors, for cryogenic service, and where isentropic expansion is desired. StarRotor has developed intellectual property for LNG process improvements that are enabled by our expander technology.
Depending on size, pressure regime, and service requirements, StarRotor expanders (and compressors) can be built with either greased or oil lubricated bearings. They can be operated with a variety of fluids, with mixtures of fluids, and at varying speeds. Cycling all of the chambers with every rotation smooths out pressure pulses for better multi-stage balance and quieter operation.
In principle, an expander is simply a compressor running in reverse. In practice, most technologies find this promise hard to deliver. StarRotor machines can be designed to operate efficiently in both directions, and with saturated vapors that may partially liquefy during expansion.