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It's been a long time since I went over heat exchangers and even then it was all theoretical. I'm looking for anyone with real world experience with different types of heat exchangers.

I'm looking at building a external wood burning stove. Most of the designs are pretty much just a firebox surrounded by water which is a step up from just putting a pot of water over a stove. They a bit overpriced/simple for what they are and most are 'dumb' devices. Temp too cold, turn on fan, temp too hot, turn off fan.

Most of my combustion experience is with events that occur multiple times/second rather than in hours. I think there is a lot of room for improvement starting with a proper heat exchanger.

Rough initial estimates:

  • Air Temp: ~1500F | ~800C
  • Water Temp: <212F | <100C (Not making a boiler due to safety).
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  • $\begingroup$ I have an outdoor stove*. I'm not sure how much better you can do than fire box/ water / insulation. One thing to remember is that the inside gets coated with creosote. *My opinion of outdoor wood stoves upon request. $\endgroup$
    – George Herold
    Commented Jan 27, 2015 at 19:25
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    $\begingroup$ If it is building up it means it isn't getting burned the first time. Properly designed there should be no creosote build up because it's burning and contributing to energy into the water. Newer EPA stoves separate the pyrolysis from the actual burn but I think they're using a pretty bad heat exchanger. Your opinion likely comes from a number of shortcomings in engineering design. One of which is a terrible heat exchanger. $\endgroup$
    – JedF
    Commented Jan 27, 2015 at 22:07
  • $\begingroup$ Wouldn't a better heat exchanger simply cool the fire faster, meaning the fan would come on more, meaning it would burn through fuel faster? If my thinking is correct this means you would get more power from it, but it wouldn't necessarily be more efficient in terms of output vs. fuel usage. (though power output vs. physical size would improve). $\endgroup$
    – jhabbott
    Commented Jan 28, 2015 at 0:32
  • $\begingroup$ Designed properly you could "idle" your firewood and have it self relight. Obviously you have to design your system to be properly sized so that you don't waste time between operating states. If you built in weather forecast and properly modeled the heat losses in your house you could even have it predictively react to coming temperatures. $\endgroup$
    – JedF
    Commented Jan 28, 2015 at 2:52

2 Answers 2

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I can't speak from real (=hands on with my own hands) experience, but in industrial applications I see mostly shell and tube type HX. Exhaust is in the tubes, the shell has the water.

Pros:

  • lots of surface
  • when you add flanges opposite the tube openings, cleaning is doable

Cons:

  • Pressure loss in the small tubes

An alternative would be to jacket the (existing) chimney, with water between chimney and jacket. Insulation would be addded on the outside.

Pros:

  • easier to retrofit
  • little change to convection etc. characteristics of the stove

Cons:

  • Less surface, so less heat transfer
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I'm going to have to make this an answer. If someone feels it's inappropriate I can delete it, what follows is the start of my comment.

Grin, I bought my house with an outdoor wood stove, we have a love/ hate relationship.

I love:

  1. It burns all my waste paper and meat products, (Including the treats our cats bring us, and the scraps the dogs drag home during deer season.)
  2. It heats my garage and preheats hot water. (It could heat the house, but... more wood.)
  3. Gathering and processing of wood, yes it's work, but it's also an excuse to be in my woods which I enjoy. I sit on my butt most of the day, so wood = exercise.
  4. That the ashes are all outside.

I hate:

  1. When something breaks down, it's always on the coldest day of the year. (of course that'a not true, but plumbing and outside, windy, below freezing is not fun.)
  2. It belches smoke because of poor burning.
  3. If I leave my house during the winter, (say Xmas) then I either have to drain it or have someone come over and feed it.

Of course some of that is open to engineering.
I'm leary of overly complicated.
and I burn a lot of wood on cold winter nights.

Here's a pic, It's cold here tonight (7 F) and it's burning nicely. enter image description here

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