The Steamboating Forum

Now I've been told the figure 1:1600 or 1:1700 as the ratio in volume of parts water to steam when allowed to fully expand under ideal conditions.

I know the concept of Steam Tables and have looked at them.

But what about at least a ball park eatiofor compressed air expanding from a compressed air at the same pressure, say 200PSI.

I have never heard a ballpark figure for this - but always wondered...

-CB

"I've been told the figure 1:1600 or 1:1700 as the ratio in volume of parts water to steam when allowed to fully expand under ideal conditions."

That is the change in volume, when EVAPORATING liquid water into saturated steam, not expanding steam like in a cylinder. At atmospheric pressure, 14.7 PSIA, 1 cubic foot of 212F water will turn into 1603 cubic feet of saturated steam at 212F. THis ratio is different for different pressures, for example, evaporating one cubic foot of water to saturated steam at 147 PSIA will result in about 160 cubic feet of saturated steam.

"But what about at least a ball park eatiofor compressed air expanding from a compressed air at the same pressure, say 200PSI?"

I think you are asking what is the change in volume when air at 214.7 PSIA is expanded (in an ideal cylinder or in free discharge to atmosphere) down to 14.7 PSIA?

This is an entirely different process, as the air is never a liquid, and is always a gas. Two processes are identified in basic thermodynamics, adiabatic expansion, where no heat transfer to the gas during expansion occurs, and isothermal expansion, where the temperature of the air is kept constant, by feeding heat to the air while it is expanding. A pound of air at 215 PSIA, 600F has a volume of 1.83 cubic ft, and in isothermal expansion, the end result at 14.7 PSIA is a volume of 26.7 cubic feet, 600F. For adiabatic expansion to 14.7 PSIA, the expanded air volume is 12.41 cubic feet, 32F. In real cylinders, the actual final temperature is somewhere in between, as heat from the cylinder walls is transferred to the gas during expansion.

fredrosse wrote:That is the change in volume, when EVAPORATING liquid water into saturated steam, not expanding steam like in a cylinder.
This is an entirely different process, as the air is never a liquid, and is always a gas.
.

Correct.

But even though though it is an entirely different process you can generally run a reciprocating steam engine on compressed air.

fredrosse wrote:A pound of air at 215 PSIA, 600F has a volume of 1.83 cubic ft, and in isothermal expansion, the end result at 14.7 PSIA is a volume of 26.7 cubic feet, 600F. For adiabatic expansion to 14.7 PSIA, the expanded air volume is 12.41 cubic feet, 32F. In real cylinders, the actual final temperature is somewhere in between, as heat from the cylinder walls is transferred to the gas during expansion.

12.41/1.83=6.7814207650
or
26.7/1.83=14.59

So if we split the difference we can just say 1:10.

If this is correct, even though it's evaporation vs expansion it gives a nice comparison to give the general public as a ballpark figure to give an idea of the unique properties of steam that make it so extraordinary...

1:10 vs 1:1600 - Two orders of magnitude difference!

-CB

"But even though though it is an entirely different process you can generally run a reciprocating steam engine on compressed air."

Yes you can run a steam engine on compressed air, however the EVAPORATION of the steam occurs in the boiler, not anything like the expansion of a gas in a cylinder at all.

"If this is correct, even though it's evaporation vs expansion it gives a nice comparison to give the general public as a ballpark figure to give an idea of the unique properties of steam that make it so extraordinary...

1:10 vs 1:1600 - Two orders of magnitude difference!"

You are comparing two processes that are entirely different and not remotely connected to anything in common, except they occur in a steamboat. The air expander is on the order of 1:10 on expansion ratio, and a steam engine will have very nearly the same ratio when you are talking about a gas, either steam or air, expanding. The fact that the liquid water changes to a very large volume of steam is due to the PHASE CHANGE (LIQUID TO GAS), and virtually all substances exhibit the same phenomenon, weather you boil liquid Nitrogen, Propane, Water, or Mercury.

Dry air is a pretty much ideal gas. Ignoring any heat change, which makes minimal difference, Volume / Pressure = Volume / Pressure.

That is a 10 Litre tank at 100psi if expanded to 100 Litres will give you 10psi.

Hence why air pressure is a useless energy transfer medium. The real power of steam is the temperature differential in the water. Water holds a LOT of energy - 4185 Joules per 1kg per 1C change. Over 1 second that gives you 4kW. Your boiler is 600 Pounds an hour. Your engine changes Steam from 180C to 120C (hence why an air pump makes such a difference) is equal:

60C change, 272kg of water = 68299200 Joules = 18,972 Watts = 26.6 Horse Power.

If you imagine the size of air compressor required to even compress 26.6 HP worth of air, you realise how inefficient air must be.

fredrosse wrote:You are comparing two processes that are entirely different and not remotely connected to anything in common, except they occur in a steamboat. The air expander is on the order of 1:10 on expansion ratio, and a steam engine will have very nearly the same ratio when you are talking about a gas, either steam or air, expanding. The fact that the liquid water changes to a very large volume of steam is due to the PHASE CHANGE (LIQUID TO GAS), and virtually all substances exhibit the same phenomenon, weather you boil liquid Nitrogen, Propane, Water, or Mercury.

Ok, school me please - How can I use the volume ratio of 1:1600 of parts water to parts steam to make a comparison to compressed air at a similar PSI. - That's ultimately what I am asking ...

-CB

I don't think one really can.. Apples and bananas

Why can't I say the compressed air in a aircompresser tank expands 10 times in volume to ambient while the volume of water in my boiler evaporates to 1600 times the volume in steam?



-CB

cyberbadger wrote:Why can't I say the compressed air in a aircompresser tank expands 10 times in volume to ambient while the volume of water in my boiler evaporates to 1600 times the volume in steam?



-CB

You probably could but it wouldn't have any real meaning apart from the fact that compressing air is a really poor way of transferring energy.

Ken

marinesteam wrote:
You probably could but it wouldn't have any real meaning apart from the fact that compressing air is a really poor way of transferring energy.

Ken

I was going for the converse that steam is a really good way of transferring energy.

The reason I ask is to try and relate this figure for non steam folks and draw a comparison that for the average modern man(person) is more familiar with now days - compressed air.

-CB