Counter vs. Co-current heat exchanger configuration in boilers

[damon_sisk]

The only installation method I've seen in manuals on gasification boilers has the water moving through the boiler's heat exchanger in the same direction as the combustion gases (up). This forms a co-current heat exchanger configuration. Has anyone ever tried a counter current setup? All one would have to do is to pump the water in the top of the boiler and out the bottom.
It is well known that the counter current configuration is more effective (more heat transfer per unit area of heat exchange surface) than the co-current configuration (http://en.wikipedia.org/wiki/Countercurrent_exchange).
I know that the combustion gas temperature must be maintained above the dew point of the water in the combustion gases to prevent condensation and therefore corrosion on the inside surfaces of the heat exchanger.
In my case however, the outside surface of my single wall stove pipe is about 250°F to 350°F, and my water temperature has a hard time reaching 195°F. The combustion gas temperature must be quite a bit higher, so I think I'm losing heat out my chimney.

The only problem I can imagine is that the little Taco 007 (3 speed pump) that I have might not provide enough flow to force the warmer, bouyant water down.

Anyone ever tried this?

 

[DaveBP]

These guys think it's a good idea:

http://www.koeb-holzfeuerungen.com/kus_tree/powerslave,id,13,nodeid,13,lang,EN.html

Next time I win the lottery, I'm going to look into these.

There have been some rumors about Viessmann investigating a move to N. America with these wood boilers.

 

[Gooserider]

The only installation method I've seen in manuals on gasification boilers has the water moving through the boiler's heat exchanger in the same direction as the combustion gases (up). This forms a co-current heat exchanger configuration. Has anyone ever tried a counter current setup? All one would have to do is to pump the water in the top of the boiler and out the bottom.
It is well known that the counter current configuration is more effective (more heat transfer per unit area of heat exchange surface) than the co-current configuration (http://en.wikipedia.org/wiki/Countercurrent_exchange).
I know that the combustion gas temperature must be maintained above the dew point of the water in the combustion gases to prevent condensation and therefore corrosion on the inside surfaces of the heat exchanger.
In my case however, the outside surface of my single wall stove pipe is about 250°F to 350°F, and my water temperature has a hard time reaching 195°F. The combustion gas temperature must be quite a bit higher, so I think I'm losing heat out my chimney.

The only problem I can imagine is that the little Taco 007 (3 speed pump) that I have might not provide enough flow to force the warmer, bouyant water down.

Anyone ever tried this?

Very common question, and there have been a few folks that have played with it as I recall... While you are quite right about the counterflow theory, the general experience seems to be that the "residence time" of the heat in the firebox is so short that there isn't a lot of benefit to reversing the flow, as the increase in heat transfer to the water is less than the added energy needed to push the water backwards through the boiler against it's natural flow pattern. Keep in mind also that you need to keep SOME heat in the flue, in order to avoid having condensation in the flue, with it's consequent creosote and corrosion issues - given the typical 100°F low reading of a surface thermometer, sounds like you have an actual flue exhaust temp in the 4-500°F range, which is about what you want...

I would look at other possible causes for the problem that you have getting your output water up to 195°F, though again, I am not sure why this is a problem - you get more efficient heat transfer with lower boiler output temps, and if trying to charge storage, it is quite normal and a "good thing" to have trouble getting up those last few degrees until the fire is just about burned out...

Gooserider

 

[benjamin]

Haven't tried it, and probably won't.

Most of the boilers being discussed have firetubes that take the flow of water into account to some degree, I would never mess with that aspect of any advanced boiler. If you're talking about a sectional cast iron boiler, or an outdoor wood boiler then the improved efficiency would be completely lost in the translation, IMHO.

I'm guessing wikipedia is more relevant to heat exchangers that are working with a much smaller temperature difference than even the last pass in a boiler.

 

[pybyr]

I'm a big proponent of countercurrent flow; have pointed it out to several folks regarding transfer of heat to the living space; I've set up my household HXs accordingly.

That said, in a boiler, I have to assume that the MFGs know what they are designing for, and that one should follow their design parameters such that you take the heat off of the top. Perhaps you could do otherwise with a huge-flow pump, but to make sure you don't get steam pockets, you'd have to use a really serious pump, at which point your wire-to-water efficiency may take a dive to the point that any net gains on heat transfer may not be worth it.

I could easily be a founder of Over-Thinkers-Anonymous, but I think, with all due respect for wanting to always hear of open-minded suggestions, that this may not be the best route to pursue.