Desuperheater question

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Desuperheater question

Post by SL Ethel » Fri Apr 05, 2019 6:25 pm

I am putting together a wood-fired ofeldt boiler, using a round firebox with castable refractory lining I already have. The only downside to this firebox in its last incarnation was that the refractory stored a LOT of heat. This was very noticeable even with the 18" diameter firebox under a pressure vessel holding 15 gallons of water. With the ofeldt holding 1/4 of that volume, I think it would probably lead to a lot of steam being made even when the damper is shut, especially after running hard for a while. I'd rather not have a boiler that would blow itself dry just even with the damper shut.

To take more advantage of the radiant heat available, and to reduce thermal lag of the refractory, I was thinking about making a huge superheater in the form of a stainless helical coil just inside of the upper part of the refractory - obviously this would be exposed directly to the hottest part of the fire. It would form something of a water wall (or more properly steam wall) covering up about 1/2 of the refractory area. Then to deal with the way-too-hot steam that would deliver, run it through a desuperheater. In this setup, the desuperheater itself would deliver a large fraction of the overall steam production.

The first objection the occurs immediately is that I might just end up burning out the superheater in short order. The second is that I may be introducing operational challenges that I haven't fully thought through yet. Any thoughts, feedback, or opinions will be gratefully received.

Cheers,
Scott
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Re: Desuperheater question

Post by TriangleTom » Sat Apr 06, 2019 3:02 am

Trying to wrap my head around the concept a little bit.

First, why use a steam coil instead of a water coil? If you're worried about burning out the coil, water would transfer heat away faster, and should prolong service life. It would also make one hell of an economizer, especially if steam production in the main body of the ofeldt decreases as a result of less radiant heating.

Second, why desuperheat the steam at all? It seems like it'd just be a waste of energy when you may as well just feed it to the engine. If you're worried about the steam being too hot, you could run more water through it to lower the temperature, and perhaps just fire the boiler less to reduce overall steam production.
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Re: Desuperheater question

Post by barts » Sat Apr 06, 2019 4:20 am

A radiant superheater is definitely subject to burnout if there's no steam going through it... so a hot (wood - not readily turned down) fire. I think your best solution when stopping the engine for a while is to

1) stop firing a while before if you can predict it.
2) open the firebox door and keep the damper open.... all that heat will keep the air nice and warm that's racing through the boiler.

I use cast Mizzou refractory in my copper tubed boiler; the heat storage is noticeable, but not closing the stack and letting the air rush through seems to take care of the problem.

A desuperheater makes sense to control overall superheat under varying load conditions - but once the need for steam is passed, there's no more control.

-= Bart
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Re: Desuperheater question

Post by steamboatjack » Sat Apr 06, 2019 8:20 am

I agree with TT, an economiser would be better than a superheater.
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Re: Desuperheater question

Post by SL Ethel » Mon Apr 08, 2019 3:09 pm

Thanks for the input. The reason I was thinking about a superheater rather than a feed water heater in the firebox is for efficiency's sake - I wanted to preserve the counter-flow heat exchange of having the hottest heat transfer surface (the superheater element) next to the fire, followed by the generating coils, then the feed water heater. I could also keep steam flowing to keep it from burning out just by bleeding off steam, as opposed to having some secondary powered feed pump to keep water in the feed heater when the engine is stationary.

Putting the feed heater in the firebox would definitely be simpler - the question then is how much efficiency (or more importantly capacity) would I likely be giving up by having the heating surfaces "backward" from a counterflow heat exchanger design?

What about a two element feed heater? Feed water first goes through a coil above the main generating tubes, then (presumably then at the same boiling temperature as the generating coils) goes through a second feed heating coil in the firebox? I would assume this would deliver a lot of steam along with the feed water, or even all steam if the feed was slowed. I'm planning on a 6" dia center drum, so I guess I'd be hoping that it would be big enough to damp out any surging from what would essentially be a poorly controlled monotube boiler feeding into it.

I'm planning on automatic feed control, so there should be at least some water flowing in as feed any time the engine is running. I suppose I would probably want something like a duplex feed pump as well in order to keep some water moving through the coil when the engine is stopped.

This would all be a lot simpler with liquid fuel, but for me, taking away the wood fire would be taking away half the fun.

Cheers,
Scott
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Re: Desuperheater question

Post by fredrosse » Tue Apr 09, 2019 12:57 am

While it is good to make the best thermal arrangement, basically a counterflow heat exchange arrangement, this leads to serious difficulties of potential metal overheating, even with normal automatic controls, and a fire that can be completely controlled in a few seconds time. With wood firing, there are going to be periods of high thermal output and with the presence of water (rather than steam), with its very high heat value required for evaporation, is the only practical way of providing enough thermal inertia to protect boiler metals from overheating.

Within a 150 psi boiler, turning a pound of feedwater to temperatures that are dangerous to ordinary steel boiler tubes requires heating from, say 200F (93C) water to 600F (315C) superheated steam, with the high heat of phase change (boiling), requires 1156 BTU energy.

Heating saturated steam to the same "danger zone" of overheated tubing requires only 127 BTU energy, about 10% of the previous heating value. And that is with the assurance of constant flow through the circuit. Any interruptions of flow will allow the superheater tube temperature to soar very quickly. With liquid water inside the tube, the boiling character gives at least 10x the margin in time to get things under control.

If you want a wound coil around your firebox, send water through it, not steam. I would recommend a wound coil surrounding your firebox, either directly fed with feedwater, or, better yet, a small economizer coil near the stack outlet (preheats feedwater and gets good boiler efficiency), followed with the furnace coil, what is called a "steaming economizer" that then enters the boiler evaporating section. That is precisely the arrangement I used on my Domestic Heat-Power Module, and the carbon steel furnace coil has lasted for decades. That was a stoker fired boiler, with feedwater pump and stoker on the same circuit.
Last edited by fredrosse on Tue Apr 09, 2019 3:07 pm, edited 1 time in total.
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Re: Desuperheater question

Post by SL Ethel » Tue Apr 09, 2019 2:45 pm

Thanks for the input, Fred.

I think the conclusion, then, is that I'm likely just going to burn up an oversized superheater. This boat is supposed to be a relaxed cruising boat, so I don't want to set it up such that it needs constant fiddling just to not self-destruct.

I think it makes sense just to build it without any feed or superheater in the fire space and see how it does. It would be easy to come back and fit an additional length of feed heater in that space at a later time.

Thanks all for the help!

Cheers,
Scott
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Re: Desuperheater question

Post by DetroiTug » Fri Apr 12, 2019 12:34 pm

I run an Ofeldt in my carriage with economizer and superheater, the economizer or any feedwater heater is much more important, some of the navy ships had as many as five feedwater heaters. A Feedwater heater (Mike are you reading this :) ) is far more critical in it's design and necessary.

I have two feedwater heaters, a coil in the muffler that uses exhaust steam which is very effective as well. I can take on feedwater and actually experience higher steam production while doing so, however only for a period of time - say one to two minutes, after that, the feedwater heaters begin to chill down and become less effective. This can also be tweaked by sizing the pump accordingly for lower feedwater velocity, ideally a feed pump should run all the time and just keep up or slightly surpass the feedwater needs. Adjust the bypass to do that and you'll see what I mean.

The superheater is all after the fact and great for a slight performance boost and excellent for dealing with a bit of errant carry over, but it's useless on a boiler that can't keep up because it's being chilled with cold feedwater. In that case, It's lipstick on a pig..

The superheater while it can offer performance boost can also be very detrimental to the engine and definitely the throttle if it is after the superheater, always situate the throttle before the superheater. On a new system especially in a boat, I would forgo the superheater at first and see if it is even needed. I'm liquid firing and can shut the fire off instantly if I have to stand idle, a woodfire, that can't be done and the superheater can overheat which in turn sends very high temperature steam to the engine once the throttle is opened again. If needed, On a boat I would start off with a very small bit of superheat, with only a foot or two of tubing. On the other hand, too much economizer is a good thing, making steam in the economizer? Good. Some of the Stanley boilers use a "waterwall' a feedwater heater which is a coil of tube around the boiler, some have as much as 300 feet of tube. Although I'm not a fan of that, all economizer should be in the exhaust flue gas space only.

-Ron
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Re: Desuperheater question

Post by barts » Sat Apr 13, 2019 10:38 pm

So the right order to plumb the feed side after the engine driven feed pump is:

1. Tee to float valve in hotwell so we bypass water from the hotwell back into the hotwell w/o heating it.
2. Lift check valve to keep steam (if any) formed in economizer out of the hotwell.
3. Pressure relief valve in case some idiot (Captain) closes boiler feed valve.
4. Feed water heater... if the feed water heater is big enough, the feed water will come up to the temperature of the exhaust steam of the boiler, since there are a lot of BTU in the leftover steam - far more than it takes to heat the feedwater back up to the same temperature. Any heat salvaged here doesn't end up in the lake/ocean.
5. Economizer - as much as you can fit in. Relatively small diameter tubing works well here, since this transfers heat better due to higher flow. Even relatively efficient steam boat boilers see flue gas temps of 400F or more - all BTU that can be put to use.
6. Boiler lift check valve
7. boiler.

Since either the feed water and economizers can fail, having bypasses made up for repair purposes is a good diea.

- Bart
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Re: Desuperheater question

Post by fredrosse » Sun Apr 14, 2019 3:21 am

Adding a couple of points to Bart's list, needed to comply with the ASME Boiler and Pressure Vessel Code:

No. 6 Boiler check valve, followed by an Isolation Valve

If the economizer can be isolated and bypassed, then the economizer itself requires pressure relief protection because it then becomes a "fired pressure vessel" independent of the boiler proper.
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