Fire bed

Is there a hard and fast dictate or rule of thumb, to calculate how thick the fire bed should be in relation to lets say the bottom row of fire tubes or the bottom of the fire hole

Help, information and advice always gratefully accepted. Colin

No, it depends on the locomotive's firebox, the fuel, the type of grate etc.. You just have to arrive at a method for your loco and available fuel. I mostly fill my Simplex until coal is falling back out the fire door, especially when we had the perfect fuel, Auschar, sadly no longer available.

One of the problems is finding coal that is big enough as so much of it is supplied as gravel for screw feeders. You want large enough lumps to keep gaps for the airflow wo typically the best size is teh largest that will fit through the firehole door down to half that size. If it is all small stuff a thick layer just beds close blocking the air so goes out or gets hole blown in the layer so it might as well be out.

I think I'd modify lump size according to whether the fire is wide but shallow, or narrower but deep, because a miniature loco's fire is thin compared to the coal nuggets even with a deep box. So though you don't want the fuel clogging the air-flow, neither do you want holes in the fire. That half-door size seems a good general guide though.

The fire-bed should not reach the tubes. In most designs I've looked at the lowest tubes are no lower than the door sill.

How much the coal coalesces is a matter of its quality, but I have seen some bituminous coals cake into masses needing frequent breaking up.

I reckon with miniature engines that the Fireman is more skilled than the Driver!

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Incidentally, though the fire does not scale very well, the BR handbooks recommended coal about the size of a man's fist, for full-size locos, and not excessively thick fires; apparently irrespective of firebox proportions;. It seemed it was left to the firemen to control the individual boiler by its performance and his experience, based on that guide. (A little booklet just about firing, as well the full Handbook). Their big headache though was clinker, not coal lumps, blocking the air-flow. The graded embankment of a former railway line near my home is covered in the stuff!