The Home Machinist!

I've read numerous suggestion here that Chloe boilers should have 1/2" not 3/8" I.D. tubes and 6 inch boilers for better steaming. Is "6 inch" referencing 6 inch tube (O.D.) or 6 inch pipe (nominal I.D.)?
Thanks much.

6 inch pipe, which has 1/4" wall and is 6 1/2" diameter. yes, also use fewer tubes and make them from 1/2" type K copper, which has an OD of 0.625". my chloe is built this way and it always made more steam than it could use, even with the cylinders bored out to 1.5" bore.

extremely good steamer, I would definitely build it this way, without a doubt. especially if you intend to burn coal, the larger tubes are a must unless you want to be brushing them out frequently.

Use 7/8inch K copper flues. Makes it easier to clean and will steam just as a good as more of the smaller flues. You won't be disappointed.

Bigger the better for firebox and boiler. Might be able to make the firebox 2 inches longer and an inch or so wider. More grate area=easier to fire.

I think someone put a stainless steel arch in the firebox, but don't remember.

A pair of eccentric engineer small or intermediate injectors should be perfect for it. Not sure I would even bother with the crosshead pump.

i like 7/16 od flues in boilers of that size....

Thank you for the input, that's quite a range of tube sizes.

I think most people used to be of the mindset that more smaller flues = better. So more 5/8 flues are better than a few less 3/4. But I think what people have started to realize is that an engine with 7/8 will actually steam freer since less likely to have cinder obstructing air flow and most of the heat transfer is on the firebox sheets anyway.

EDIT - propane vs coal. Above info is for Coal. If you want to fire using propane you can use 1/2 inch even. I think that's what Bill Conner used on his engines, but keep in mind 1/2 will not work with coal, so you would not be able to convert it to coal down the road if you wanted to.

What fuel do you plan to use? The smaller fire tube sizes are OK for propane, but not for coal. Conversely, really large fire tubes might not provide enough heat transfer with propane fuel, unless you introduce some turbulence within them via "turbulators" (twisted or corrugated metal strips inside the fire tubes).

interesting that so many folks say 1/2” won’t work for coal. I have a 2 - 1/2” gauge (17/32” scale) Coventry 4-6-2 and it has seven 1/2” tubes...and it is a coal burner. Never been a problem yet! Just screen the coal to get rid of the dust first.

Keith Taylor

Every 7.5 gauge engine I've seen with tiny flues have had issues with getting clogged after multiple hours running. You can run all weekend using 7/8 flues. It's more just a matter of why bother if the larger flues are better suited for coal.

1/2" won't work for coal? er, yes, it most definitely WILL work for coal. The determining factors for all boilers are L/D ratios, slope (vertical firetube is a different kettle to horizontal...), draft, fuel, tube area to grate area.

L/D ratios of around 70 is "ideal". I don't have a Chiloe drawing, but if the boiler is less than 17" between tube sheets, then .250 ID tubes fit into the ratio. 7/8" tubes give a L/D of 22 at that- your smokebox is going to glow red if you work the boiler.

I would tend towards 1/2" OD K wall copper tubing, giving an ID of about 7/16", for a boiler of 16-20" between tubesheets. (L:D of 38, at 17") If the boiler is much shorter than 16", then the next step down is probably 7/16" OD/ 3/8 ID tubing (L:D of 45) . I wouldn't go much smaller- the 3/8" OD/1/4" ID (L:D of 68) is getting small for cinder cutting issues & clogging in a horizontal firetube boiler. Our experience would suggest not going that small, but sticking to the 7/16" OD tubing. But I would not be using 5/8" as tubes (L:D 34) on less than 20" between tubesheet boilers, and not even considering 7/8" (L:D 22) for tubes.

If the Naysayers Say Nay, I've melted Aluminum in the smokebox on the Traction Engine, with 5/8-3/4" tubes, and about 24" between sheets...so the smokebox temp was >600F. Dad had at the drawing for it, and it has more, larger, tubes than as designed. That makes it a forgiving boiler in some ways, but not really suited to heavy work. We knew it when we bought it- that if you flog the engine (like, say, climbing 600' in a mile, on a 40 MPH highway, with plates on it & all...) that the smokebox was going to get hot. It does. It also means you can do that at 6 pm at night, after the show is over and you want to get home, and it will still produce enough steam to do so without having to brush the tubes first.

What James said, mostly.
The formula is D² = L/55~75, where D is the flue ID and L is the effective flue length.

Plugging in a factor of between 55 & 75 will give a flue ID (D) which will be optimized (so far we're able to optimize such things) for the best combination of gas flow against rate of heat transfer. The ID value produced will rarely hit spot on a standard size so after varying the factor value the rule of thumb is, if it's in between two sizes, use the nearest one, but if it's a tossup fudge to the next size larger for coal and fudge next size smaller for liquid or gaseous fuels.

The O.P. said this is a Chloe boiler. Most the responses assume coal firing, but the type of fuel was not stated. Based on my experience with boilers that size, fired on coal, any fire tubes smaller than 5/8" O.D. will have trouble with clogging with cinders. YMMV.