Helium engines

Gallery opened: 29 April 2015

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Making an engine that uses helium as its working fluid is going to require some strong justification. Helium is expensive, and it is effectively the ultimate non-renewable substance. You can always make oil out of coal if you feel the need; the technology existed in 1932. However, helium is an element, and the only way to 'make' it is to sit around and wait for a radioactive substance to emit alpha particles, which are helium nucleii.

The only sensible reason to make a helium engine is to liquefy helium. As for liquid air, using an expansion engine rather than a Joule-Thomson throttle valve to drop the gas pressure, as in the Linde cycle, gives more efficient cooling because work is being taken out of the system. A further complication fully explained in the link is that a gas must be below its inversion temperature to be liquefied in this way, and so simple Linde cycle liquefiers cannot normally be used to liquefy neon, (BP -246.1 degC) hydrogen, (BP -252.9 degC) or helium (BP -269 degC) and the use of an expansion engine is then essential for reasonable efficiency.

Left: Helium piston engine for wet expansion

Perhaps the most remarkable thing about this engine is how conventional it looks. It is designed to work as a wet expander- in other words liquid helium forms actually inside the engine. Claude had dreadful troubles with lubrication when was liquefying air inside an engine; the fact that it can be done routinely with helium presumably demonstrates what progress has been made in dry lubrication since 1900.

Its operation is pretty much self-explanatory, but note that the valves are at the bottom, presumably so the liquefied helium is easily cleared through the exhaust valve.

Piston expanders like this one are commonly used in the temperature range 20 - 40 K. (Kelvin or degrees Absolute) Wet expanders are preferred as they are more efficient than dry expanders, in which the gas cools but does not liquefy. Typically wet piston expanders are 80% efficient compared with dry piston expanders at 70 - 75%.

It is well known that in steam power generation turbines are more efficient and more reliable than reciprocating engines. In the helium business things do not work the same way. Turbines can be and are used as expanders, but piston expanders are in general more efficient, and maintain their efficiency over a wide range of flow rates, making plant operation easier.

The power output is tiny compared with that required to compress the helium in the first place, and is usually not worth trying to exploit it. It is usually dissipated in a small fan.

From Cryogenic Engineering Ed B A Hands. Academic Press 1986

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