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Mild Steel boiler - can it cope with the heat from a batch box?  RSS feed

 
John Harrison
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Thanks to all the information and help on this forum I'm finally at the stage where I need to decide on which boiler to fit at the top of my 5" batch box heat riser.

I recently bought a used Clearview 750 wrap around mild steel stove boiler (45,000 btu/hr) which I plan to connect via a 'Neutraliser' to our Heating and DHW circuit. The system is open vented (unpressurised) and is fitted with a heat leak radiator. I'm making provision to fit a circulating pump on the rocket boiler side of things in case the boiler 'kettles' when exposed to the fierce heat of the batch box unit. The whole batch box firebox and boiler is to be encased in a brick 'bell' with the final exhaust routed via an existing masonry chimney.

I'm concerned that after talking to different plumbers (who are all giving me different advice...!) that the boiler may spall when in contact with the 700 degrees C plus output from the heat riser. Some say the cooling action of the water in the boiler will save the steel, others say the boiler will fail after a few weeks use. In addition some have mentioned that the steel boiler may rob too much heat from the exhaust and, combined with the cooling effect of the masonry bell and chimney, stall the draught of the system. I've reduced the planned size of the brick bell somewhat to hopefully compensate for the cooling effect of the boiler.

What does the forum think?
 
Satamax Antone
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Well, i'm not absolutely sure the outside of the boiler would spall. But it's so easy to fit a stainless shield to the boiler.  I would weld the stainless shield on the part facing the heat riser's output. With 312 rod.

As for the stalling, take the outside of the boiler as ISA

For a 150mm. You need 53sqft or 5m2. Including boiler's outside surface.

http://donkey32.proboards.com/thread/1822/sizes-single-bell-systems
 
Peter van den Berg
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As long as the boiler is filled with water that 700º C doesn't mean anything. The boiler is open vented so the water won't be able to get above boiling point. Whether or not the water will take too much heat I don't know, sorry for that.
 
Glenn Herbert
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My opinion on the relative effect of the water-filled boiler is that it will be more efficient at absorbing heat, since its surface may be much cooler (212F at most) than the interior masonry surfaces of the bell (no real limit on maximum temperature). Therefore, I would weight it higher, by an unknown factor, than the masonry surfaces in ISA calculations. If an ISA of 5m2 is indicated, I would make your total combined ISA slightly smaller.

To be safe, I would build in the ability to expand or shrink the ISA of the bell as your experience shows to be warranted. There could be many ways to do this.
 
Satamax Antone
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Glen, Walls of the bell, when cold, are at the same temp as the boiler if cold. I seem to recall the heat absorption was dependent on the delta of temperature. Due to convection in the tank, and circulation, it can't reach more than 90C°. But But up to the point the tank is at 90, the walls should heat up at the same rate, pretty much, or faster may be. But this shouldn't effect the bell's ISA too much, if at all.

Anyway, your idea of adjustable bell's size is good. I would implement this with an adjustable plunger tube.

John, a plunger tube is the chimney descending in form of a tube, inside the bell from the top. If you do this with two tube one sliding inside the other, you can adjust the height of the chimney entrance, and hence adjust the size of the active bell. Anything bellow the bottom of the plunger tube being some kind of lost heat exchange surface. So your effective ISA is less.

 
Glenn Herbert
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Yes, during initial heating, there will be little or no difference between the masonry and boiler surfaces of the bell. If heated for long periods, the difference could grow to be significant, with the boiler staying effective at absorbing heat while the masonry absorbs less due to the smaller delta T. I suppose this would not be any kind of detriment, as the system needs to work well while coming up to temperature anyway, and would only become relatively more efficient compared to an all-masonry system at higher heat states.
 
Satamax Antone
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Glenn, as well, heat radiated from the walls of the bell will heat up the boiler until everything is at equal temps.
 
John Harrison
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Satamax, Glenn and Peter - thanks very much for your quick and helpful reply's (as always). Sorry for the delay in thanking you all, our internet signal in the wilds of Wales is very poor at the moment...

The idea of the adjustable bell is pure genius!

I'm planning on installing an inspection door in the bell where the boiler will be located to allow me to monitor how the exhaust from the heat riser is affecting the boiler surface. I think I'll try a thin stainless steel 'protector baffle' between the heat riser exit and the boiler and see how the system performs.

Thanks again for your help Gentlemen - I'll keep you posted as the build progresses.
 
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