Thermal efficiency

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Anne from Little Britan
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Thermal efficiency

Post by csonics » Wed Nov 18, 2009 4:36 pm

Posted on behalf of Ralph B:

Ralph B
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Joined: 17 Oct 2009
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Posted: Sat Oct 17, 2009 5:24 am Post subject: Thermal efficiency
Bart has a reply under the small turbine thread and has some numbers on thermal efficiency. I was quite surpriced to see that a steam engine could obtain numbers in the 30's. (I am assuming this is total vice Carnot) From my understanding, getting 8% out of a compound reciprocating steam engine was high norm.

Not that fuel price is a big concern for hobby steamboaters. But I believe with the price of diesel now-a-days a coal/steam system running at 10% efficiency can compare to diesel. With the exception of space requirements of course.

How many out there are using superheated steam?
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mcandrew1894
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Posted: Sat Oct 17, 2009 6:52 am Post subject:
Hi Ralph,

I am not using superheated steam as that requires consistent and plentiful internal lubrication. I just don't want to deal with it.

There are plenty of areas where thermal efficiency can be improved without superheat....I know I will be scorched probably for this stance, but ....

Just my opinion...worth exactly what you paid for it

Dave
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87gn@tahoe
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Location: South Lake Tahoe, CA
Posted: Sat Oct 17, 2009 9:34 am Post subject:
I am running superheat at about 450-500 degrees F, and of course internal lubrication.. thinking of switching from SCO (Steam Cylinder Oil) to collodial graphite...

I think the matter of running or not running with superheat depends a lot on your engine's design and your own personal goals...

I would think that an engine with a piston valve would be better suited with superheat, as there isn't much of a "fudge" factor if a bit of "wetter" steam comes down the line, as opposed to a conventional slide valve engine. I also think engines with multiple expansions would be better suited with superheat due to the heat lost between stages.

Thats my opinion anyway.
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mcandrew1894
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Posted: Sat Oct 17, 2009 9:49 am Post subject:
Hi Wes,

My concerns are based on keeping oil out of the boiler as I am running condensing.....If your running atm, it's not such an issue, superheat to your hearts content.....

For me 100F worth of superheat is more than enough to make sure I don't have wet steam....but that's pretty tame and it is still wet enough to not dry the cylinders out, forcing lubrication.


Dave
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87gn@tahoe
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Posted: Sat Oct 17, 2009 10:04 am Post subject:
The oil in the boiler is why I am considering using collidal graphite instead.. WAYYYY too much potential for oil getting in the system as it s.

In the end, if we were so worried about effeciency, the steam engine woldn't be the way to go.
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Wesley Harcourt
-S.L. Wayward Belle (Mr. Grosjean was/is a genius.)
-S.L. George H. Sandin (Father's boat. Cut my teeth on that one.)
-'64 Buick Riviera
-'65 Buick Special WAGON
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barts
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Location: Menlo Park, CA
Posted: Sat Oct 17, 2009 10:19 am Post subject:
I'd hoped I was clear; the best measured thermodynamic efficiencies of steam engines is perhaps 20% in full size practice; the theoretical numbers assume a really impractically large engine - or a large turbine.

A small diesel engine in a boat will produce about 10hp/gal/hour; our small steam plants do about a 1/5 th of that, perhaps a little better.

I've run superheat before... it's tricky. Most engines don't like superheat; we've all heard our engines squeaking when closing the throttle partially after a hard run - you're hearing the effects of superheat, as the steam drops pressure but not temperature across the partially closed throttle. Upper cylinder lubrication is essential, and removing that oil from the boiler feed if you're condensing is also very important.

Personally, I'd avoid any more than token superheat - 50->100F to make up for condensation. If you want efficiency, the first place to go feed water heater and
economizer. After that is expanding the steam more: when picking the next engine for your boat, use either a compound (triple!) or a uniflow w/ poppet valves. Failing that, pick a single w/ a riding cut-off on the slide or piston valve, and make it large enough to produce the power you need at 80 psi boiler pressure.

In terms of complexity the uniflow engine itself has much to recommend it - but you need valve gear that will let you cut-off at 5 to 10% - which generally means poppet values, run by cams and not eccentrics. Starting w/o vacuum can be hard, so you'll want either additional clearance that can be added to the cylinder w/ a valve, aux. exhaust valves or a steam ejector on the condenser to generate vacuum at 0 rpm. Note that your BMEP (Brake Mean Effective Pressure) will be lower w/ short cut-offs, so you'll need a larger engine to produce the same amount of power.

- Bart
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mcandrew1894
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Posted: Sat Oct 17, 2009 10:47 am Post subject:
What Bart said!

Those squeeks you hear after throttle down are the cylinder being hotter than the steam entering it. All condensation goes away as the steam superheats, and it's metal on metal..no water lube at all...the squeeking quiets down after the engine cools down and the condensation returns.

This happens in the LP of Sabino when we throttle down and we turn some bypass steam on her LP to quiet her ( and lubricate!) her for a minute or so....the other way is to raise the condensor pressure....but it more problematic and annoying.
Then again I have friends who think I'm nuts for not superheating....

Horses for courses I suppose......It's important to have fun I think....

What ever "floats the boat" works fine by me.

Dave
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Ralph B
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Posted: Sat Oct 17, 2009 11:49 pm Post subject:
Bart...poor wording on my part. I understand you were talking theoretical efficiency, I just didn't think a steam engine with all its moving parts could even dream of achieving that high. A steam turbine, yes, but a steam engine...I had no idea.

Comparing steam to diesel, the only place steam can compete is in fuel cost. If you can get 1/3 the efficiency of a typical diesel fuel costs at the current rate would be even (thats using Pennsylvania anthracite). I only bring this up thinking that if fuel prices skyrocket there could (an unlikely "could" I know) be a resurgence in steam for some commercial craft.

I was thinking superheat just to preserve that latent heat energy as long as possible.

Any diagrams available for feedwater heaters?
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barts
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Joined: 02 May 2009
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Location: Menlo Park, CA
Posted: Sun Oct 18, 2009 12:21 am Post subject:
A traditional feedwater heater is just additional tubing or pipe in the boiler, designed to recover heat that would otherwise go up the stack. The size can be a large a 20 to 30% of the heating surface of the boiler. You'll get the best efficiency here w/ relatively small coper tubing so the velocity of water inside is high, and you should bring outside of the boiler again. A good design has a check value before the tubing, and one afterwards as well. They have been known to fail, so having the means aboard to bypass the heater will make your plant more reliable.

The efficiency will be best if you use a hot well or partial feed pump bypass; feeding the boiler continuously is what you want...

- Bart
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Ralph B
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Posted: Sun Oct 18, 2009 3:37 am Post subject:
Bart, I had thought if it were flue gas heating the feedwater it was an economizer and waste or exhaust steam it was a preheater. I guess names don't really matter as they both heat the feed.

My experience is with extraction steam systems where you pull steam off different stages of the turbine. In most nuke plants 1/3 of the feedwater entering the steam generator (or reactor if its a BWR) is from extraction vice condensate from the hotwell.

I was thinking of a shell and tube prior to the condenser, then from there to the economizer. Hotwell recirc would be prior to the feed preheater as well as ballcheck and then another check after the economizer.
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mcandrew1894
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Posted: Sun Oct 18, 2009 5:49 am Post subject:
Ralph,

My boat is set up as such.

The exhaust feedwater heater is a diesel fuel preheater, about 1 sq foot.

The economizer is 45 feet of 5/16" copper in a horizontally disposed coil on the port side of the boiler.

Dave
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barts
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Location: Menlo Park, CA
Posted: Sun Oct 18, 2009 12:17 pm Post subject:
Feedwater heater vs economizer....

I've never quite sure which is supposed to be which, and some of my old books disagree. In any case, in large scale practice the idea behind a heat exchanger just downstream of the engine is that you can run lower condenser/hotwell temps w/o losing efficiency, and thus pull more vacuum on the engine, gaining power w/o adding more heat in the boiler.

Since many of us run fairly high exhaust temps unlike full-scale practice, the feed water heater is more important since there's more energy to recover.

I've used a shell and tube heat exchange on the Otter for years. I made it from a chuck of 2" x24" or so thick wall brass tubing; I silver-soldered in 11/4" nominal or so copper pipe caps, reversed and drilled to form the tube sheets. There are flanges for plates containing the feed water connections silver soldered on each end, and 3/4" copper fittings in the 2" shell at either end on opposite sides for the steam connections. There's a stack of 1/4" copper tubes inside and the whole thing is rather hidden underneath the boiler.

- Bart
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fredrosse
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Posted: Wed Oct 21, 2009 9:51 am Post subject:
In virtually all USA power plant practice, the economizer is a heat exchanger located in the flue gas that exits the boiler, and provides the final step of feedatwer heating.

Feedwater heaters are generally defined as taking their source of heat from steam within the power cycle, (such as exhaust steam from the enfine, or extractions from a turbine) and are located upstream of the economizer.
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fredrosse
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Posted: Wed Oct 21, 2009 9:59 am Post subject:
Steam engine efficiency is often quoted much higher than real experience. Large steam locomotives (which are generally much more efficient than small launch engines) average around 4% efficiency (power output divided by input fuel energy). On controlled tests, even with compound locomotives, a thermal efficiency of 10% was almost never achieved.
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fredrosse
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Posted: Wed Oct 21, 2009 10:11 am Post subject:
On the subject of small plants, an ASME paper describes two "Domestic Heat-Power Systems" operated to heat a home plus supply electricity, at about one horsepower output.

The steam-electric plant, with economizer, superheater, and regenerative feedwater heating, got about 0.6% (yes, less than 1%) efficiency with a small steam turbine generator.

The same plant, with a modern high speed poppet valve uniflow engine could attain 6% overall efficiency. Another poppet valve steam solar project (Whitecliffs) managed steam plant efficiencies into the "teens" , with a much larger machine (25,000 Watts), and 600 psi/600F steam.

Considering these machines, with real and controlled tests, it appears that our typical launch steam plants manage actual efficiencies of only a few percent, at best.

barts
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Joined: 02 May 2009
Posts: 32
Location: Menlo Park, CA
Posted: Wed Oct 21, 2009 3:49 pm Post subject:
fredrosse wrote:
On the subject of small plants, an ASME paper describes two "Domestic Heat-Power Systems" operated to heat a home plus supply electricity, at about one horsepower output.

The steam-electric plant, with economizer, superheater, and regenerative feedwater heating, got about 0.6% (yes, less than 1%) efficiency with a small steam turbine generator.

The same plant, with a modern high speed poppet valve uniflow engine could attain 6% overall efficiency. Another poppet valve steam solar project (Whitecliffs) managed steam plant efficiencies into the "teens" , with a much larger machine (25,000 Watts), and 600 psi/600F steam.

Considering these machines, with real and controlled tests, it appears that our typical launch steam plants manage actual efficiencies of only a few percent, at best.


Yup....

Available btu in 1 gallon diesel fuel: 124000
1 hp is 2545 BTU/hr.

3 hp-hr/gal fuel (good efficiency for a small boat) yields 3 * 2545/124000
is 6% thermal efficiency; most boats don't reach this.

- Bart
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Bart Smaalders
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Ralph B
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Posted: Thu Oct 22, 2009 1:05 am Post subject:
Fred der Grosse (I am reading Castles Of Steel and the SMS Freidrich der Grosse was just mentioned)

Would you have the tetle of that paper? I would like to read it.

I wonder what the cross over point in size is when a turbine becomes more efficient. Also wonder why they didn't use similar sized plants to compare. At 1hp a steam engine should whup a turbine in efficiency, but at 25kW I would hope a turbine could get within spitting distance of 13% (optimal conditions of course).
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fredrosse
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Posted: Fri Oct 23, 2009 5:44 pm Post subject:
The paper is: EXPERIENCE WITH EARLY DISTRIBUTED GENERATION SYSTEMS 2003 IJPGC2003-40192

From the American Society of Mechanical Engineers (ASME) International Joint Power Conference
2003

The cross-over point (where real steam turbines show a better efficiency than reciprocators) is in the vicinity of several hundred horsepower.

It would be possible to bring the "cross-over" power down to less than 100 horsepower or so, but that results in a steam turbine running close to 100,000 RPM, and there is just not much commercial demand for such a machine.
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