# compressed air vs steam



## hammers-n-nails (Jul 6, 2009)

i was reding a post that basically claimed that additional work couldnt be gained by alowing compressed air to expand in a cylinder like steam. this doesnt make sense to me because all gases behave in basicaly the same way as far as expansion. the post claimed that as soon as cutoff occured all the work that could be gained from the air had been expended so in a compound engine the hp cylinder would be the only one that makes power. can anyone prove or disprove this?


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## gbritnell (Jul 6, 2009)

It's pretty simple actually. Compressed air is just that, a compressed gas. After the compression is released there is no more pressure. Steam on the other hand is an expansive gas which is created by heat. As long as there is heat there is expansion. When the high pressure steam is admitted to the high pressure cylinder it does X amount of work. When it is exhuasted to the next cylinder it has lost some of its effeciency (heat) and therefore the cylinder is made larger (surface area) to maintain the same amount of pressure on the crankshaft thereby balancing the crank output. Without looking it up I don't know what the amount of thermal loss is in relation to the difference in cylinder sizes. By that I mean, how much larger should the second and third stages of a compound engine be in relation to the intial pressure of the steam to get equal work from each cylinder?
gbritnell


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## Kermit (Jul 6, 2009)

hammers-n-nails  said:
			
		

> i was reding a post that basically claimed that additional work couldnt be gained by alowing compressed air to expand in a cylinder like steam. this doesnt make sense to me because all gases behave in basicaly the same way as far as expansion. the post claimed that as soon as cutoff occured all the work that could be gained from the air had been expended so in a compound engine the hp cylinder would be the only one that makes power. can anyone prove or disprove this?



If you added heat to the gas before allowing it admission to the engine, then it would behave more like the gas you describe.

All these engines are heat engines. The added heat allows the gas to continue expansion. Same principal on which hot air ballons work. A decided advantage with water vapor as the heated 'gas' is the creation of a vacuum when the water turn from gas to liquid.

lots of old books available through www.books.google.com. Search using the "full view only" check box found in the advanced search options. Prevents you from getting booksearch returns which you can not read.

Kermit


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## hammers-n-nails (Jul 6, 2009)

so what your saying is that the difference lies in the fact that steam pressure is created by heat instead of physical compression at ambient temp. but you know airs temp, when its decompressed will fall below ambient air temp. so the pressure to temperatue relationship is still in effect even if to a lesser extent. i still dont undersand.


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## Kermit (Jul 6, 2009)

You would get some expansion if the compressed air "retained" its heat gained from the compression. Adding more heat would then make the gas truly expansive against atmospheric pressure.  These substance remain gases though. Water vapor returns to a liquid form, so that creates some differences. The attributes of superheated steam(water vapor that doesn't condense into water when worked) approach those of the ideal gas model used by scientists...

All I can say is to read up on how heat engines work. heat engine theory might be a good search term.


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## PTsideshow (Jul 6, 2009)

In addition to the thermal dynamics of steam that have been lightly, touched on here. The other major difference is the expansion of steam. Depending on which source you care to site from 1200 times to 1600 times the same volume of of water.

Air has no such properties. And it requires more energy to compress than steam requires to generate.

If this has something to do with the fact that most indoor shows and some outdoor shows ban the use of steam to run the models or full size engines.
Most locals have strict fire and liability laws, when the public is involved.
The insurance carriers balk at covering such a wide group of builders with no inspections or paper work.

Google the word steam and then use the links, for steam engine, steam tables they are surprisingly good. Through the years there even has been applications for other liquids being turned into steam. You may recognize the pictures from the Live Steam magazines. As the info is from some of the current article writers. 
glen


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## agr (Jul 6, 2009)

I recently read a post/book/article on just this very topic, unfortunately at the moment I am unable to locate it.

However, my understanding is that Steam varies from compressed air in that in addition to the energy stored in the form of pressure, steam also contains 'latent heat', that is, the energy required to initially convert the water to steam. Unfortunately the book I have at hand (Model STationary and Marine Steam Engines by K.N.Harris) only gives British units, but I hope it will illustrate the principal.

To raise the temperature of 1lb of water from, say, 62deg.F to 212deg.F requires 150 B.T.U (British Thermal Units) of energy, but then a further 970 B.T.U of "Latent Heat" to convert it to steam at atmospheric pressure. So at atmospheric pressure, 1lb of steam contains 1151 B.T.U. of energy/heat, before you even begin to 'compress' it above atmospheric pressure.

However, much of this is irrelevant in small scale, as Harris states in his book that to realise the advantages (increased efficiency) of a compound they need to run at 110 PSI or higher and be at least 10HP or larger.


Tony.


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## steamer (Jul 6, 2009)

ahhh Tony beat me to it. ;D

Yes latent heat is the key......

Dave


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## Davyboy (Jul 6, 2009)

Ya but, ( all my girls have a big but  ),my model engines run on compressed air with NO LOAD. The air is under pressure up to the valve, which opens, and the regulator needle will dip briefly. The air is expanding to ambient pressure, including air in the line. If I were to have a load, as a steam locomotive or tractor, then wouldn't there be some pressure in the cylinder, available to a secondary expansion stage??? Listen to a steam tractor at a sawmill sometime, they run nearly silent with no load, but when that saw blade hits wood, sweet music!  There is no pressure in the cylinder until there is a restriction or load. Yes I know that steam is more efficient and much more expansive than compressed air, but in my simple world of model engines, running friction is about all the work I can overcome. I'm asking as much as telling, you fellows seem to have a much better grasp on this than I do, please help me to understand.

Davyboy


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## steamer (Jul 6, 2009)

Usually, when the tractor of saw mill is under load, the throttle is also further opened either by a governer or by the operator.  When this happens more steam is being let into the cylinder, raising it's Mean effective pressure.  When the exhause valve opens, the terminal pressure, as a result, is higher and you get the "sweet music" ...and boy it is.

Combine this with the latent heat of the steam being released and you get the bark that you hear. You don't get the same bark from an engine running on air.....it's just not the same. It's all the energy stored in the steam that is so much greater that that stored in air.

Dave


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## hammers-n-nails (Jul 7, 2009)

alright well thats starting to make some sense so i guess my next question is does anyone have a design/plans for a boiler. i cant find my formulas for matching an engine to a boiler and they may not evn be correct for model scale but i need to produce about 2cubic feet of steam per minute at 100-125psi (this is for 100% load). if anyone knows where to locate the formulas to determine sq.ft. heating surface, btu per hour needed,evaporation rate, etc i could possibly dsign my own boiler? i thought i used to have them on google books but i cant seem to find them.


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## tel (Jul 7, 2009)

You'll get some info here


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## Ripcrow (Jun 21, 2013)

I would have thought that you steam engine running on air would have been running only by the shot of air at inlet only and continued to run by the force of the initial blast.no work expansion at all is required to run because you have already overcome friction .Seen a commercial air motor advertised to run at 5 psi can't remember what is was deigned to drive but I wouldn't think anything much.


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