Why use steam water and not other fuild in steam engines?

Question

Given this answer, I understand the importance of phase change (liquid --> gas) for the fluid used in a steam engine. Thus I understand it is important to use a fluid that is liquid at pressure and temperature this engine may work in (between 600 and 1000hPa, between -20°C and 50°C (I may take large margin for extreme climate condition including high altitude mountains)). Thus, we may use any fluid that is liquid in this condition and easy to boil (I'm thinking of ethanol that has a lower boiling point but other fluids may be suited).

My question is: why water and not other fluids such as ethanol?

Edit: I cite ethanol but I don't want the answer to be focused on this fluid in particular. It can be any fluid that is liquid in normal temperature and pressure condition and easy to boil. I have no idea what fluid can meet this requirement.

Answer 1

There's some interesting information on a number of alternative working fluids here, including ammonia, alcohol, mercury, sulpur dioxide, freon, and many others.

Refrigerants are used today in organic rankine cycle applications which operate at lower temperatures than steam systems and are used to generate electricity from waste heat sources. These are small systems which are not practical for large power generation applications.

Some of these materials were considered in the past because high temperatures could be achieved at lower pressures than with water, but modern metallurgy has given us metals that easily contain the necessary pressures in water boilers.

Water has great benefits over these other materials because is has very low cost, is non-toxic, is ready available, and is non-combustible. As a result, there's no compelling reason to seek other fluids.

Answer 2

I can imagine using other liquids than water in a steam engine, but it would have to be for a very specific reason and purpose. In general, water is in so many ways superior.

Steam engines designs are very inefficient in terms of preserving the boiled liquid. In most cases, the steam is just released into the air, resulting in a very simple and inexpensive design. Capturing, cooling and condensing, pumping, etc. would be a far more complicated process that is not only much more expensive, but far more likely to experience mechanical breakdowns.

Consider a steam engine (as in what pulls a train). Its water continuously boils away and it has to stop to refill from a trackside storage tank. Early trains needed to do this every 10 km or so, and later, onboard storage tanks extended the range to 200 km. Those trackside tanks can be automatically kept full using a windmill powered well pump.

Now imagine the same process using ethanol:

• Who is going to keep the trackside tanks full of ethanol, and how?
• Who is going to keep people from stealing and drinking the alcohol?
• What is going to happen when the ethanol "steam" released from the engine gets near the sparks pouring out of the chimney?

So yes, in theory any liquid with a reasonable boiling point could be used, but in practice there really isn't even the slightest justification for it.

Answer 3

Water has a very high specific heat and heat of vaporization, so it takes more energy to raise it's temperature and to boil it than almost any other substance.

In the same way that it takes more energy to melt aluminum than copper, even though aluminum has a lower melting point than copper because aluminum has a higher specific heat and heat of fusion than copper.

Also, other substances could have a better vapor pressure curve than water, which means there could be a greater increase in pressure for the same increase in temperature.

If there was an advantage to using another substance than water it would have to be enough to still be advantageous even after the expenditure of the energy required to recover and recycle the expansion medium, also the engine would have to be stationary, as in an electrical power generation plant where an increase in efficiency of a few percent would be enough to justify implementing the modifications.

So, preferably we are looking for a slightly polar molecule with a low molecular weight and preferably nonflammable.

Also, it might be possible to use lower boiling liquids under higher pressure because there could be a very good vapor pressure curve with a large increase in pressure for the same increase in temperature.