BTU's for 12 h.p.?

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kentuckyshaker
Posts: 1
Joined: Sat Feb 08, 2003 2:27 pm

BTU's for 12 h.p.?

Post by kentuckyshaker »

Can someone give an estimate for the amount of BTU's needed per hour to run an efficient 12h.p. steam engine putting out 12 h.p.? Also, what size is the smallest 12h.p. steam engine you are aware of? [Email]kentuckyshaker@hotmail.com
McDuffie
Posts: 3
Joined: Fri Jan 31, 2003 12:17 am

Re: BTU's for 12 h.p.?

Post by McDuffie »

I would approach it like this:
You need to have a number value for "efficient". A steam consumption rate of 10 pounds per horsepower hour is really the best you should hope for. 20 pounds per horsepower hour is less optimistic but more realistic in this horsepower range; especially if you want to keep the engine size down. Greater efficiency seems to demand more and more room; more tubing in the boiler, more piston displacement for greater expansion ratio...

So for example, we have 20 pounds per horsepower hour times 12 horsepower ... 240 pounds of steam consumed per hour.

We also need to know the desired steam temperature. For reliability's sake, we need to stay below 800 F or the oil will carbonize. Synthetic oils can do better, some claim 1000 F as the breakdown point. A more important limit is 700F , when your piston rings exceed 700, the life of your piston rings will be shortened. Since there will be some heat loss between the boiler and the throttle, and some heat losses from throttling, and the fact that the rings will be a bit cooler than the incoming steam, I'm going to say the target steam temperature at the boiler is 800 F. Steam tables will show you that there is a very close relationship between the steam temperature and the BTU content. If you look at 800 F steam, the BTU content is very close to 1400 BTU across a wide range of pressures. There are steam tables online here : http://pump.net/otherdata/steam.htm

240 pounds of steam times 1400 BTUs per pound makes 336000 BTUs per hour in the steam.

Since the boiler can't be 100% efficient, the fuel needs to provide the heat in the steam as well as the heat lost up the chimney. Compact forced circulation boilers can be made with efficiency ratings above 90%.

336000 BTUs per hour divided by .9 efficiency makes 373333 BTUs per hour in the fuel.


I'm sorry that I can't recommend an engine ... it's going to be a compromise decision,
and the trade-offs can drive you mad.

You might get the highest efficiency from a poppet valved, high compression uniflow,
but a piston valved, two cylinder compound will be much easier to design.

I suggest that you also post your questions at these two forums.

http://www.steamautomobile.com/ForuM/list.php?f=1

http://steamgazette.com/phorum-3.3/list.php4?f=1
Rolly

Re: BTU's for 12 h.p.?

Post by Rolly »

I am running just about 12 HP in my 35-foot Steamboat Tryall. Im using a 15 HP rated boiler.
I average four gallons of # 2 oil per hour. Say around 570,000 BTU per hour. The burner has a 4.5 Gallon nozzle but cycles on and off with a differential pressure of 25 PSI. 225 off down to 200 on.
The engine is a compound and runs a good 28 inch of vacuum.
http://ourworld.cs.com/revans8770/index.html
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