Expansion Engine

It is not uncommon to utilize both the isentropic and isenthalpic expansions in a cycle. This is done to avoid the technical difficumes associated with the formation of liquid in the expander. The Claude or expansion engine cycle is an example of a combination of these meth-... 

Reciprocating expansion engines have been used since the early twentieth century and are still used to some extent, especially for volumetric flows below 10 ft /min. Reciprocating machines often suffer from high maintenance, excessive size, valve problems, and tlie fact that liquid will damage the valves. For these reasons they have largely been replaced by turboexpanders, even down to sizes around 1 hp. 

Generally, reciprocating expanders are selected when the inlet pressure and pressure ratio are high and when the volume of gas handled is low. The inlet pressure to expansion engines used in air-separation plants varies between 4 and 20 MPa, while capacities range from 0.1 to 3 m3/sec. Isentropic efficiencies achieved are from 70 to 80%. 

The flow diagram for the Claude process, shown by Fig. 9.10, is the same as for the Linde process, except that an expansion engine or turbine replaces the... 

Figure 6.35 Pressurized synchronous generator powered by a four-cylinder reciprocating natural gas expansion engine.

In Fig. 6.35, a pressurized synchronous generator is coupled with a four-cylinder reciprocating expansion engine. 

If refrigeration is to be accomplished at a temperature range where no suitable liquid exists to absorb heat by evaporation, then a cold gas must be available to absorb the heat. This is generally accomplished by using a work-producing expansion engine. 

A significant improvement to the process outlined above was the use of an expansion engine. Typically, in ASUs turbo expanders are used. An ideal turbo expander is isentropic and reversible. Illustrated in Figure 3.9, air at -150°F (172 K) and 90 psia (620 kPa) is expanded to 20 psia (138 kPa). In an isentropic expansion A-B, the expansion follows the isentrope with a net change in enthalpy. In reality the expansion will not be reversible and will follow a curve similar to A-C. The actual enthalpy change divided by the isentropic enthalpy change is a measure of the expander efficiency. 

The Claude s cycle is a nonideal isentropic plus an isenthalpic expansion. A preheated gas is firstly isothermally compressed from p to p2 followed by a fast isobaric cooling through 3 from T 300 K. There, an expansion engine with another precooled gas helps the hydrogen gas to step to point 4 and then to T2 80 K. An expansion valve helps the cooled gas to go isenthalpically to point 5 at an even lower temperature T3 30 K and p3 being lower than p2. An isentropic expansion finally produces the liquefied gas at the initial T. ... 

Replacement of valves with expansion engines to produce work. 

Turbine, expansion engine, compressor, fan, blower, or pump. 

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