# Pot boiler run time question



## scoobster28 (Sep 15, 2008)

Please let me first state that I am going to make a couple of broad generalizations, and second let me point out that I only know what I know based on reading online forums, other websites, etc. I have very little personal knowledge on the subject. That being said...

It seems to me that a Meths fired pot-boilered engine will run for a lot longer duration then a gas fired one. In reading websites reviewing different live steam engines (like Marc Horovitz's excellent site, as well as others) the impression I get is that with an alcohol-fired engine you frequently must top up the fuel, but rarely if ever the boiler water, and the run time is around 40 minutes. When I read reviews of similar small engines that burn gas/butane, it seems that they run for only about 20-25 minutes.

Is this a bad comparision of apples and oranges? Is there something mechanical that I am missing (like the size of the boilers), or does the fuel actually improve steam efficiency and thus allow for longer runs?

Thanks.


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## Jack - Freshwater Models (Feb 17, 2008)

Your query is way more complex than you realize! There are a lot of variables involved. For two given boilers of the same physical size the one that holds the most water can produce the most steam. The amount of fuel you carry on board determines the length of run. Add a boiler feed pump and you can add to water in boiler. Large drivers will eat less steam over a given distance than small drivers if the cylinders are the same. You can keep some meths locos in steam indefnitely by stopping for water and meths periodically. 

What exactly are you trying to do??????????? 

Jack


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## David BaileyK27 (Jan 2, 2008)

You will find that most Meths fired boilers have an axle pump which keeps the water topped up in the boiler, whereas most gas fired engines do not. 
David Bailey www.djbengineering.co.uk


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## Shay Gear Head (Jan 3, 2008)

I've had runs, as has Dave Hottman, of over one hour with my Accucraft 2 cylinder Shay.

That's on one gas filling but much water added. The boiler pressure is somewhere around only 5 pounds.

That's running light - no cars. And the fire is absolutely adjusted to it's lowest point just before going out.

After that long you sort of get bored.

John Garrett got a 2 hour and 57 minute run on his 4 truck modified Shay. Larger gas tank (one filling) and an axle pump to keep the boiler topped off.

At some point you become a track HOG and other people want to run their loco too!


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## Jack - Freshwater Models (Feb 17, 2008)

Bruce, 

I have to agree about getting bored and beng a track HOG. I enjoy watching others trains run as much as I do running my own. In a steam up I would think 30 minutes would be max I would want to run. At home one might want to run longer but with meths and a hand pump one can always stop to add water and meths (and maybe some oil). 

Jack


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## seadawg (Jan 2, 2008)

When I converted my C-16 to straight propane I could run for probably 4 hours (estimated) without a fuel stop on a half a tank of gas, topping up the water as required. I would probably attempt to add some cylinder oil somewhere along the way. Talk about a track hog!!


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## steveciambrone (Jan 2, 2008)

Posted By scoobster28 on 17 Feb 2010 06:39 AM 
Please let me first state that I am going to make a couple of broad generalizations, and second let me point out that I only know what I know based on reading online forums, other websites, etc. I have very little personal knowledge on the subject. That being said...

It seems to me that a Meths fired pot-boilered engine will run for a lot longer duration then a gas fired one. In reading websites reviewing different live steam engines (like Marc Horovitz's excellent site, as well as others) the impression I get is that with an alcohol-fired engine you frequently must top up the fuel, but rarely if ever the boiler water, and the run time is around 40 minutes. When I read reviews of similar small engines that burn gas/butane, it seems that they run for only about 20-25 minutes.

Is this a bad comparision of apples and oranges? Is there something mechanical that I am missing (like the size of the boilers), or does the fuel actually improve steam efficiency and thus allow for longer runs?

Thanks.
It has nothing to do with efficency. My example is my vintage pot boiler meth fired Roundhouse Colonial. It will run a good 35 minutes and does not have anyway to refill water, meths is topped off about every 7-10 minutes. It can run long without the need to add water because of the space normally used for the center flue in a gas burner contains water in the pot boiler which gives it a greater capacity for water compared to a gas fired center flue boiler. It is hard for it to oversteam since the meths gives a constant level of heat, compared to a gas burner which normally needs to be turned down after it all heats up. On some of these forums it is almost forgotten that Live steam started with many pot fired meths burnering designs.

Steve


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## Jack - Freshwater Models (Feb 17, 2008)

Pot boilers have a lot to offer. I'm just starting a 10mm scale British 0-6-0t and I am hoping that I can use a pot boiler. The original design calls for a Smithies boiler but it doesn't have much water capacity. My first choice is a pot boiler to keep it simple and increase water capacity. The pot boiler will be much easier to make and plumb. The down side to a pot boiler on this loco is that it has a single inside cylinder and fitting in a burner my be a challenge. If I can't design a burner to fit the alternative will be a type B boiler with 4 fire tubes. In either case I plan on having the ability to add water. 

Scoobster mentioned Marc Horivitz's site. There is a wealth of info to be gleaned from that site and/or Marc's book. 

Jack


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## xo18thfa (Jan 2, 2008)

As Jack said, there are a zillion variables. Things such as the total swept volume of the cylinders, how fast the engine runs, what pressure, the heating surface of the boiler, the type and design of the boiler, how much water it holds. all kinds of friction, the load of the train.

The only real conclusion that can be drawn here is that gas has much more calories than alcohol. 21,000 BTU for butane and 11,000 for meths


A very interesting book is "Model Boilers and Boilermaking" by K.N. Harris. Mr Harris covers all the factors in understandable terms.


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## steamboatmodel (Jan 2, 2008)

I have a model Steam Launch that had a pot Boiler, I have fired it with both Gas ( Straight Butane Butane/propane mix) and Meths. Same boiler and engine just different burners, The Meths usually took longer to bring up steam, and did not produce steam as fast as the Gas did. I think that every situation is going to be different so you just have to experiment (Play around) and see what works best for you. 
Regards, 
Gerald.


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## Jack - Freshwater Models (Feb 17, 2008)

Bob is wise to bring up Harris's book. It really is understandable and does explain what a hobbyist needs to know. I still feel in the dark since I don't know what Scoobster is trying to do. 

Jack


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## weaverc (Jan 2, 2008)

The answer to your question is a matter of variables. I have a vertical pot boiler flamed by a butane fed ceramic burner underneath. It is a 2-1/2" copper pipe with lots of flues. It will run at least an hour and a half, limited by the size of the fuel tank. Water is continuously fed to the boiler from a bunker through a check valve via a Regner pump. Since the motor is a dual oscillating Osmotor (Colin Binnie Steam Motor), lubrication via the displacement lubricator is not an issue because while it's running, I occasionally squirt oil on the oscillator faces to assist the lubricator, which still has oil in it after a long run.


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## xo18thfa (Jan 2, 2008)

Hey Carl: Could you post a photo of your Colin Binnie motor. I am a fan.

Thanks


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## weaverc (Jan 2, 2008)

Bob,

Here are the gears that are part of the Osnotor









Collin Binnie Steam Motor









Here it is running:


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## scoobster28 (Sep 15, 2008)

Thank you for the wealth of responses. To help clear up my earlier post, this is where I was coming from: 

First, I was thinking of small, two-axle engines (Roundhouse, Accucraft) that had large horizontal boilers (no "Crab" style engines, no vertical boiler trams, etc.) 

Second, I was assuming that such luxuries like axle pumps for water, as well as hand pumps, would not be factored. in. Topping off of the fuel WOULD be factored in, but not the water. 

With those considerations in mind, the answer that I think explains my question best is that in a gas-butane engine, the tube that runs through the boiler is effectively removing extra water capacity space. Additionally, because gas fires need to be adjusted occasionally to take into account the load, grades, fuel storage temperature, etc, it can lead to reduced efficiency. 

Finally, while I have seen people keep their engines in steam for hours (at my live steam club, some of the 1.5" scale engines will run for 8-10 hours straight!) I too think that a run time of around 30-45 minutes is the perfect duration.


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## Shay Gear Head (Jan 3, 2008)

Check out the Sidestreet Banner Works loco reviews and look for the Sgt. Murphy. Pot boiler with a run time of over and hour for water normally and several top-offs of fuel.


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## steamboatmodel (Jan 2, 2008)

If it has a tube that runs through the boiler, it is not a true pot boiler, but a fire tube boiler. If you compare two boilers of the same volume, I think the the gas fire tube one will generate steam faster. 
Regards, 
Gerald.


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