Steam engine fuel efficiency

[farmerden]

Yes to both questions .I can control steam pressure to the atomizer-from about 3lb to 30 lb.The oil flow is controled with a needle valve. To get the mix right watch the smoke coming out of the stack! All you are doing is adjusting the air-fuel mixture. Somewhere in this site is a video called "the great race" Here you will see what too much fuel and not enough air looks like! Den Look under "rallies" the first McConnel Is entry

[Aheadslow]

Here is a drawing for a type of burner I used to build a shop heater, I believe they call it a babbington burner. The hole for introduction of the air was .010 if I remember correctly and it operated best at about 30psi air.

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[Mjolnir]

Actually the very largest steam plants can reach efficiencies well over 50%, and the most efficient plants are combined cycle plants that include a steam cycle as part of the combination. But this is not very relevant to our small steam plants. One of the local boats, a bit over twenty feet long, was running on used cooking oil and burning a couple of litres an hour cruising around at four to five knots. I suspect that if he had a small diseasel in there he would have burnt much the same quantity of much more expensive fuel. The steam plant would have less thermal efficiency, but the much smaller high speed propellor would have had lower efficiency, so it would all balance out. Except the free fuel was still cheaper than the bought fuel would be, and is also a renewable resource.

What we tend to want to do with these boats tends to mitigate against efficiency anyway. The maximum theoretically attainable efficiency of a heat engine depends on two factors, the temperature at which the heat goes in, and the temperature at which it is rejected. In a condensing boat, the latter is the temperature of the surrounding water. The high end temperature is the temperature at which the steam is entering the engine, so for high efficiency this should be as high as possible....but really highly superheated steam is not nice to handle, and makes severe demands on lubrication. So we will generally not want more superheat than it takes to just dry the steam out, at most. Some prefer to keep the steam wet, which means it effectively is the lubrication. Either way the relatively low temperature puts a severe limit on the efficiency that can be reached. I think we should actually be able to do better than a non condensing locomotive of similar power, although I haven't seen any figures to show that.

(Note that nobody ever obtains anything like the theoretical maximum efficiency, but it is a good guide as to how well you are doing, and what you should change to do better.)

Another thing to consider is that the more efficient the plant is, the bulkier it will tend to be. Although we tend to have more room than a minature locomotive of the same power, the room is not unlimited. So we don't have room for as muich in the way of economisers, preheaters, etc as a big plant does.

regards
John

[87gn@tahoe]

Actually a diesel could run on straight (filtered) used cooking oil. I occasionally run it in my '92 Dodge W250 and my '90 Dodge Ramcharger which I converted to diesel. One can also run straight (filtered) used motor oil. Been doing it for a while now.

On small reciprocating steam "efficiency" one should look into a forced draft (one could use an exhaust steam turbine derived from an automotive turbocharger) monotube steam generator and a condensing poppet valve uniflow with needle and roller bearings throughout, etc. Though then ease of operation without proper controls may become an issue.

[fredrosse]

I have been in the utility power industry for over 40 years, and have never seen a steam electric plant reach 50% efficiency. The best plants can run in the mid-40% range, and that is with 3800 PSIG steam, at temperatures over 1000F.

The 4% locomotive efficiency I quoted was for full size locomotives. 1-1/2 scale locomotives would have much lower efficiency. There have been efficiency competitions on scale steam locomotives, and the efficiency of the winning engine will sometimes just kiss 1%, (coal heat energy to motive power) efficiency.

My domestic heat-power system, using a 4.5 horsepower poppet valve uniflow engine at 1040 RPM, with 135 PSIG steam, slightly superheated, tested at about 7.6% overall efficiency (with 70% boiler efficiency). The high soeed of this engine, coupled with the large admission valve, sharp cutoff, and uniflow configuration is why it managed higher efficiency.

A good launch engine, wity typical lower RPM, should be a little better than the scale loco, but probably not better than a full size loco.

[Mjolnir]

Well, I thought I had seen figures for power plants that were higher than 50%, maybe they were for combined cycle plants, which these days reach 60%. Note that this is the overall efficiency from thermal to electrical, so includes any loss in the electrical side of the plant...for boats and vehicles we are only concerned with thermal to mechanical efficiency. Since electrical equipment like generators and transformers does require cooling, we can be pretty sure it is not 100% efficient in itself

The French (Chapelon for instance) acheived thermal efficiencies of 7% for compound steam locomotives, the best results from IMLEC have been around 2%. Since the latter are limited to 100 psi, and since locomotives can't condense very effectively, there is no way they are going to acheive very high figures anyway, even in terms of what is possible for small reciprocating plant. (yes, the South Africans did condense, but only to get the water back, they didn't gain any power or efficiency from it.)

Incidently don't fall into the trap of thinking that because the boiler efficiency quoted is a nice high percentage, that all the problem must be in the engine part. Boiler efficiency is only quoting how much of the thermal energy from the fuel has been transferred into the water. If the temperature of the resulting steam is low, then the thermal energy is no longer available to be converted into work. The heat engine part of the system can only work on the temperature difference between the steam and the exhaust, and the lower that is, the lower the efficiency is going to be.

regards
John

[Bob Cleek]

This may be a stupid question, but humor me... I'm still learning.

I have an "energy star" natural gas fired hot water heater that claims to be something like 95% efficient. In any case, the exhaust is so cool that the stack is nothing but a piece of PVC pipe that barely gets warm to the touch. That suggests to me that the rest of the heat generated by the burner is going into the water being heated. Now, my dumb question is, "How do you do that with a steam boiler on a small steam boat?" Or is it impossible?

[fredrosse]

The short answer to your question: Yes, this can be done.

I teach power plant technologies to engineers at my company, so here is a rather "long winded" answer:

A boiler or other combustion/energy device would theoretically have 100% efficiency if the combustion process is complete (all the fuel burns, ie. reacts with oxygen, releasing the energy), and all of the products of combustion are brought back to the initial temperature of the ambient air and fuel before entering the boiler.

The very high efficiency heating units must bring the temperature of the combustion products (exhaust gas) down to a low temperature, and in the case of Natural Gas Fuel, much of the H2O (Steam/Water) in the products of combustion must be cooled to the point of actually condensing some of the steam into liquid water in the exhaust gas.

When the "stuff" being heated is at a temperature low enough, then the exhaust gasses may be brought to an exit temperature approaching this low temperature, it is just a matter of providing more heat transfer surface area and thus allowing the exhaust gas temperature to "approach" the temperature of the "stuff". This is the method typically used in high efficiency water heaters, warm air furnaces and the like.

If the "stuff" is at a high temperature, for example a boiler operating at 100 PSIG (6.9 BARg) steam pressure, corresponding to a saturation temperature of 338F (170C), some extra tricks are necessary. Conventional heat transfer technology limits the products of combustion exit temperature to something over 338F, which is not low enough to give efficiency in the 95% range.

In order to get the higher efficiency, a combustion air heater attached to the back end of the process is necessary. This device takes in the combustion air (and sometimes fuel gas in a separate stream), and picks up heat from the exhaust gas thru a heat exchanger. This apparatus can, in theory, bring the exhaust gasses down to an exit temperature nearly approaching the temperature of the ambient air that is being used for combustion. As the heated combustion air enters the burner at a higher temperature, the flame temperature experiences a similar temperature increase.

In typical large utility boiler practice, the steam is generated at about 662F (350C) boiling temperature (2400 PSIG -n 165 Bar, Steam), and superheated to 1000F (538C) or more. Feedwater enters the boiler's economizer at about 482F (250C), but the air heater brings the exit combustion products down to about 300F (149C), well below the temperatures of any steam or feedwater within the boiler.

The utilities typically will not bring the exit combustion gas to a lower temperature, although this could easily be accomplished. The reason for this is simple economics (paying for a bigger air heater), plus the fact that utility fuels typically will produce liquid acids if the exhaust gas is brought to a low enough temperature such that condensation is present in the exhaust gas. This would require exotic materials in the air heater, and thus would be way too expensive.

[Bob Cleek]

Oh, well... never mind! LOL

Thanks for a great explanation. I don't see me pursuing that level of efficiency in a small marine steam plant, but I'm glad a question that's been bugging me for a while is now answered.

[87gn@tahoe]

Not quite as efficient, but the monotube in my boat has about 400*F exhaust temp taken about 3" above the tube nest with about 450*F steam temp.