In a boat with plenty of room for experimentation a negative pressure furnace could be emplyed where a fan sucks the PofC out of the heat exchanger, into a duct filled with a secondary heat exchanger that has feed water in it. If a an exhaust steam feedwater heater was also employed, as well as a condenser (steam, not flue gasses) then a fairly efficient plant could be made, and technically shouldn't be too much larger than a conventional highly innefficient plant as the heat input could be so much smaller [ie the 1st heat exchanger 'the boiler'].
Just a thought...up for debate...
Greg
Steam engine fuel efficiency
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gondolier88
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fredrosse
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Re: Steam engine fuel efficiency
The stack causes a slight negative pressure in the furnace, and the stack draft can often be increased by use of a steam jet exhausting into the stack. "Induced Draft" fans are used in large plants to suck flue gas out of the boilers, but these are not a realistic option for a small launch steam plant.
A small (1 HP) non-condensing plant (atmospheric pressure exhaust) with 100 PSIG saturated steam (338F) will use about 60 Pounds Per Hour steam flow. Feedwater taken from the lake, at 70F, for a steam cycle efficiency of about 3.5%. Including typical small boiler efficiency of 70%, we have an overall plant efficiency of 2.5%.
The same machine operated condensing, (exhausting to perhaps 22 inches Mercury Vacuum) will need only about 80% of the steam generation, so overall plant efficiency goes up to about 3.1%.
Exhaust steam feedwater heating can be applied to either plant, however the non-condensing plant can heat the feedwater to about 200F, and the condensing plant can only heat the feedwater to about 140F, because it's exhaust steam is at a lower temperature.
Using exhaust steam to heat feedwater, the non-condensing machine can reduce boiler heat duty about 11%, resulting in an overall plant efficiency increase from 2.5% up to 2.8%. For the condensing machine, the boiler heat duty can be reduced by only about 6%, so the condensing plant efficiency increases from 3.1% to 3.3%.
The feedwater preheat coil located in the flue gas downstream of the boiler section is called an "economizer", and is used on almost all larger plants. By supplying extra heat transfer surface area, overall heat transfer efficiency goes up and less fuel is required.
Assuming the economizer heats the feedwater up to 300F, the boiling section heat duty goes down:
For the non-condensing plant with no exhaust steam feedwater heater, about 20% fuel reduction is achievable with the economizer, overall efficiency goes from 2.5% up to 3.1% This economizer has to have the largest surface area in this comparison.
For the non-condensing plant with an exhaust steam feedwater heater, about 10% fuel reduction is achievable with the economizer, overall efficiency goes from 2.8% up to 3.1%. This economizer has the smallest surface area in this comparison.
For the condensing plant with an exhaust steam feedwater heater, about 15% fuel reduction is achievable with the economizer, overall efficiency goes from 3.3% up to 3.9%.
These numbers are only approximate, and one can study the extra cost of various economizers and feedwater heaters compared to fuel savings for a large number of options to attain the true optimized design.
There are other "tricks" that can be used to increase plant efficiency, such as superheated steam, compound expansion engines, uniflow engines, special valve gear, etc. Feedwater heating with exhaust steam and economizers are two components that can be relatively easy to retrofit on a small launch plant. As always "every stick has two ends", and there are extra costs, extra maintenance work, and operating restrictions that come with these modifications.
A small (1 HP) non-condensing plant (atmospheric pressure exhaust) with 100 PSIG saturated steam (338F) will use about 60 Pounds Per Hour steam flow. Feedwater taken from the lake, at 70F, for a steam cycle efficiency of about 3.5%. Including typical small boiler efficiency of 70%, we have an overall plant efficiency of 2.5%.
The same machine operated condensing, (exhausting to perhaps 22 inches Mercury Vacuum) will need only about 80% of the steam generation, so overall plant efficiency goes up to about 3.1%.
Exhaust steam feedwater heating can be applied to either plant, however the non-condensing plant can heat the feedwater to about 200F, and the condensing plant can only heat the feedwater to about 140F, because it's exhaust steam is at a lower temperature.
Using exhaust steam to heat feedwater, the non-condensing machine can reduce boiler heat duty about 11%, resulting in an overall plant efficiency increase from 2.5% up to 2.8%. For the condensing machine, the boiler heat duty can be reduced by only about 6%, so the condensing plant efficiency increases from 3.1% to 3.3%.
The feedwater preheat coil located in the flue gas downstream of the boiler section is called an "economizer", and is used on almost all larger plants. By supplying extra heat transfer surface area, overall heat transfer efficiency goes up and less fuel is required.
Assuming the economizer heats the feedwater up to 300F, the boiling section heat duty goes down:
For the non-condensing plant with no exhaust steam feedwater heater, about 20% fuel reduction is achievable with the economizer, overall efficiency goes from 2.5% up to 3.1% This economizer has to have the largest surface area in this comparison.
For the non-condensing plant with an exhaust steam feedwater heater, about 10% fuel reduction is achievable with the economizer, overall efficiency goes from 2.8% up to 3.1%. This economizer has the smallest surface area in this comparison.
For the condensing plant with an exhaust steam feedwater heater, about 15% fuel reduction is achievable with the economizer, overall efficiency goes from 3.3% up to 3.9%.
These numbers are only approximate, and one can study the extra cost of various economizers and feedwater heaters compared to fuel savings for a large number of options to attain the true optimized design.
There are other "tricks" that can be used to increase plant efficiency, such as superheated steam, compound expansion engines, uniflow engines, special valve gear, etc. Feedwater heating with exhaust steam and economizers are two components that can be relatively easy to retrofit on a small launch plant. As always "every stick has two ends", and there are extra costs, extra maintenance work, and operating restrictions that come with these modifications.
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87gn@tahoe
Re: Steam engine fuel efficiency
One could use an exhaust turbine to induce draft. Using "free" power. My father has the stock turbo off of my '87 Buick Grand National on his boat inducing draft.
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fredrosse
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Re: Steam engine fuel efficiency
How is the turbo configured? I would imagine all the steam engine exhaust going thru the turbocharger turbine, with the turbocharger compressor either blowing into the firebox, or into the stack, to make an injector type draft inducer?
How is oil fed to the turbo bearings, and how have they held up in the steam environment?
How is oil fed to the turbo bearings, and how have they held up in the steam environment?
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87gn@tahoe
Re: Steam engine fuel efficiency
The turbine side runs off of the engine exhaust before it enters the condenser. The compressor side was originally configured to blow into the ash pit creating a pressurized firebox, but the combustion gasses were leaking out of the firebox door and were quite unpleasant (oil fired). My father then ran the compressor side into the stack, centering it, pointing upward, with a venturi as a nozzle. It works quite well and induces quite a draft.
As far as the bearings go, the turbo was pretty much shot when I took it out of my GN. There was a lot of end play, so much so that the compressor blades had hit the housing and one had broken off. Not good for an automotive application, but good for a steamboat experiment. My father fitted the turbine cartridge with a zerk fitting for grease (trailer bearing grease, bearings had lots of play anyway) on the oil feed side and a valve for draining any water on the return side. Once the grease warms up a little it spins quite freely.
I have given my father another GN turbo that is in MUCH better shape with little to no end-play which indicates good bearings and potentially good seals. He will probably experiment with drip feeding steam turbine oil, or colloidal graphite for lubrication with a drain to the engine bilge. Temperatures and turbo rpm are nowhere near an automotive app (1200*F+ on exhaust side, 100,000+ rpm in an automobile).
Overall it's dead reliable, using free power, and pushes air in the correct direction whether the engine is in ahead or astern (unlike a belt-driven fan). It would be even more effective if it were sized properly to the engine displacement and RPM.
The only issue I see with the use of an exhaust turbine is the noise. The compressor sucking in air is quite noisy and needs some type of ducting or a silencer of some sort.
As far as the bearings go, the turbo was pretty much shot when I took it out of my GN. There was a lot of end play, so much so that the compressor blades had hit the housing and one had broken off. Not good for an automotive application, but good for a steamboat experiment. My father fitted the turbine cartridge with a zerk fitting for grease (trailer bearing grease, bearings had lots of play anyway) on the oil feed side and a valve for draining any water on the return side. Once the grease warms up a little it spins quite freely.
I have given my father another GN turbo that is in MUCH better shape with little to no end-play which indicates good bearings and potentially good seals. He will probably experiment with drip feeding steam turbine oil, or colloidal graphite for lubrication with a drain to the engine bilge. Temperatures and turbo rpm are nowhere near an automotive app (1200*F+ on exhaust side, 100,000+ rpm in an automobile).
Overall it's dead reliable, using free power, and pushes air in the correct direction whether the engine is in ahead or astern (unlike a belt-driven fan). It would be even more effective if it were sized properly to the engine displacement and RPM.
The only issue I see with the use of an exhaust turbine is the noise. The compressor sucking in air is quite noisy and needs some type of ducting or a silencer of some sort.
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gondolier88
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Re: Steam engine fuel efficiency
Simple and effective, well done on a great idea!87gn@tahoe wrote:Overall it's dead reliable, using free power, and pushes air in the correct direction whether the engine is in ahead or astern (unlike a belt-driven fan). It would be even more effective if it were sized properly to the engine displacement and RPM.
The only issue I see with the use of an exhaust turbine is the noise. The compressor sucking in air is quite noisy and needs some type of ducting or a silencer of some sort.
It's a shame having positive pressure fireboxes on traditional launches is so difficult to do as your turbo would lend itself so well to being mounted in a unit that comprised a heat exchanger and the turbo together that ducted the air from the compressor into the ashpan- preheated high volume combustion air....
Perhaps a source of better seals for the firedoor and a simple lever mechanism that swapped the duct from pressurising the firebox when the door was opened could be the way to go?
Greg