# what property of steam makes it the preferable motive fluid in jet ejectors? why not air or nitrogen?

I want to know what properties of steam make it the fluid of choice in

1. steam turbines

2. jet ejectors.

I want to understand the mechanism of energy conversion in these equipment and hence understand why steam is the preferred choice. Please leave out economic factors for now and keep your responses rooted only in physics/thermodynamics.

• By leaving out economic factors you may be leaving out the most important reasons. In the absence of those factors it may make no sense to use water in preference to unobtainium. – RedGrittyBrick Aug 5 '14 at 9:15

Turbines are employed to derive work in a power cycle. The choice of working fluid doesn't solely depend on turbine alone, you have to consider whole power cycle. For example in Rankine cycle (power cycle employed in Power Plants), water is widely used due to its thermodynamic properties(like high specific heat), non-toxic, non-reactive, abundance, low cost. And also when water is used work done by pump is very less (around 1-3%) when compared to work produced by turbine. The figure below is the T-S diagram of Rankine cycle using water as workng fluid for a typical coal power plant:

Now carefully observe the plot, we can see water is in steam state(3-4) at a point where turbine is employed and it always be. So, steam turbine is employed.

Turbines obtain power(i.e. work) from steam through two ways: Impulse type and Reaction type. Whatever may be the way, it will be isoentropic process ideally. You can look into the theories reagrding steam turbines in wikipedia: http://en.wikipedia.org/wiki/Steam_turbine

Steam-powered gadgets start with liquid-phase fluid. The amount of pressure (per unit energy applied) gained when a liquid changes to its gaseous phase is fantastically more than that gained when material already in gaseous form is heated.

The choice of steam is based on several facts. Water's plentiful ,nontoxic :-) , and the boiling point is in a reasonable temperature range. If you tried to use, e.g., nitrogen, you'd need a 77K cold dewar to store the liquid-- not to mention the expense of cooling the nitrogen in the first place.

• Hi Carl, thanks. Lts leave out the plentiful/non-toxic angle out for now. From purely a physics point of view is it just the pressure gained per unit energy input that is important here? If so, leaving out availability and other economic/environmental factors, technically there couldbe other liquids that might be able to do what steam can, right? Something that has, say a lower vapor pressure than water, like say acetone. Again, leave out the practicality and consider this just from the phsyics angle. – daraj Jul 1 '14 at 4:36
• also, what about the latent heat requiremrnt? is that also critical? Steam jet ejectors and turbines are for different purposes, so the same property might not be critical for both applications – daraj Jul 1 '14 at 4:38

Why steam?

Because super-heated steam and above almost acts as polytropic, so our equations just work almost as expected. If you know chaos theory you would understand physicists & engineers just got lucky when it comes to all this fluid-mech. + thermodynamics scenarios

I want to understand the mechanism of energy conversion in these equipment

Sorry, but the science isn't that esoteric here, so the energy conversion mech. wouldn't reveal that much

Remember you can (and engineers often have to) change design parameters, like blade design is completely different in a turbine vs. propeller. It is a subject of masters & PhD

Why not air?

Air has so many gases & stuff (read pollens, soot, SPM, etc.). Even if finely filtered; the kind of torture air would have to undergo via primary heaters -> superheater, de-superheater -> pipes -> nozzles -> ... is likely to break down air (seperate it into its components) or cause chemical reaction not good, nightmare.

Also. jetting or more accurately throttling air at high pressures may cause condensation

Remember working fluid is (super)heated to 1. avoid thermal shock & part's erosion by eliminating moisture 2. increase the overall work done

Why not nitrogen?

You are right we can use nitrogen :) it is perfect, since economics isn't an issue.

Nitrogen can absolutely replace steam as choice of working fluid (with appropriate adjustments in design parameters, of course). In fact when we start space voyaging, someday our plans may run on nitrogen on super-cold alien planets :P

Looking at a power cycle you want the largest possible change in volume at a certain pressure - and steam obliges. When water turns to steam its volume increase dramatically. How much depends on the temperature, obviously. The nice thing is that you don't have to do a lot of work to push the volume of water into the boiler because work =$P\Delta V$ and the change in volume is small.

The next step of the cycle is expansion. The hot steam expands and pressure drops as it cools down; but until it gets to around 100C it will remain mostly a gas.

Finally as the water cools down and condenses you can capture the liquid (which is still very hot) and push it back into the boiler. This means less heat is "lost" at the end of the cycle.

This condense - evaporate - condense cycle is thermodynamically relatively efficient; the temperature at which water boils is "reasonable" (easy to reach with conventional fuels) and the emitted vapor harmless and cheap.

All these are good things.