How Does a Turbo Expander Work?

Written by richard rowe
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How Does a Turbo Expander Work?
Turbos use expanders to harness an exhaust's energy potential. (turbo image by Elijahu from Fotolia.com)

Turbo expanders are one of the lesser known but most important components in gas-handling and pressurisation systems. Turbo expanders work on one of the simplest of all natural laws to convert fluid (or gas) pressure into mechanical movement. These devices can be found in many applications such as turbochargers, jet engines, power generation plants, commercial refrigerators and at least one interesting form of power regeneration.

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Conservation of Energy

Otherwise known as the "First Law of Thermodynamics," this law states that energy can never be created or destroyed; it can only change forms. For instance, when an engine burns fuel to move your car, it's essentially just converting energy stored in the form of a liquid (gasoline) into kinetic energy (movement). A turbo expander works by harnessing the pressure and heat of pressurised gas to spin a shaft.

Parts and Function

A basic turbo expander has two parts: an outer housing contains the gas, and a turbine-like expander wheel "catches" the gas as it passes by. The gas exerts pressure on the expander wheel, causing it to turn. After pushing on the expander wheel, the gas exits through a hole in the housing. Because it has expended a certain amount of its stored energy to turn the shaft, the exiting gas leaves the turbo expander at a much lower temperature than it went in.

Expander Wheel Shape

If you've ever seen a child's pinwheel, then you're somewhat familiar with how an expander wheel works. The find on the expander wheel's fins catch pressurised gas coming in from the side. If those fins were completely vertical (like a paddlewheel), most of the pressure would just pass over the top. To increase flow, engineers "twist" the whole assembly into a helical shape. This allows gasses to pass through quickly while expending as much force as possible on the expander wheel.

Compressor-Drive Applications

Turbo expanders form exactly half the body of a turbocharger and a big chunk of any jet engine. Although it might not exert much torque (force) per revolution, pressurised gas can make significant power once the expander wheel gets up to speed. The expander wheel in compressor-drive applications connects to a matching centrifugal compressor (which works just like the turbo expander, but backward) via a shaft. The shaft spins the compressor wheel, which shoves air into the engine's combustion chamber.

Cooling

Gas exiting the expander is colder and lower-pressure than it was when it went in. The drop in temperature makes turbo expanders useful in refrigeration applications; pressurised gas goes in at a certain temperature, comes out cold and transfers its newfound coldness to the air with a heat exchanger (radiator).

Power Regeneration

When natural gas leaves the processing plant, it has to do so at enormous pressure to travel the hundreds or thousands of miles to wherever it's going to be used. Before it reaches the end user, the gas must go through a series of valves and restrictors to reduce the pressure so it doesn't blow anything up at the point of use. These pressure-drop requirements present an opportunity for energy-recapture using turbo expanders; the turbo expander drives a generator that sends electricity back into the electrical grid.

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