gas water heater for exhaust heat exchanger?

[d34]

I saw this posted under the SOMRAD group:

Crumpite said,
"I've got another 50 gal. tank, but gas heated, that I'm going to pipe the exhaust gasses through to act as a heat exchanger.
Preliminary numbers say that it ought to work at least 80% as well as a custom designed exchanger, with the bonus of extra heat storage and ease of cleaning."

I thought about this a little over a month ago but have never heard of anyone doing it.  Is there anyone with experience doing this?  If so can you post up info about it?  Also any links to other people doing it? 

[vdubnut62]

A gas water heater just has a hole through the middle of it where the gas flame burns. If one pipes the engine exhaust through there, it's pretty much the same principle.
Ron

[Crofter]

The exhaust tube would have approximately the same area as the balance of the tank bottom so it is significant for heat transfer. I have seen some units that had a set of linked baffles that promote turbulence for better heat transfer. They can be pulled out with limited headroom for cleaning but a well adjusted gas flame is a fair bit cleaner than diesel. I think the baffles would help but no way as thorough as a cleaning brush to take the insulating black velvet off the surfaces.

[Crumpite]

Folks,

In the water heaters I have the central baffle is removable, but more importantly, it can rotate too.
It's possible to hook up a little crank to the baffle and run it out of the top of the exhaust piping inside an air tight bushing sticking out of the top.
Just rotate the crank and the edges of the baffle will scrape the carbon off, where it will fall into a catch cup located on the bottom of the tank.

I went through the nasty number crunching, and it looks like it will work pretty well.

I'm quite impressed with my own cleverness, now if it will only work as planned. 
Jeez, I hate it when my reputation is on the line.

Crumpite

[mobile_bob]

look into adding a stack to the that is about equal to the length of the firetube in the tank

at the intersection of the two pipes insert another pipe perpendicular that will be the input from
the engine exhaust,

it will  need to enter the junction so as to intersect the side of the tube and establish a swirling of the exhaust
into the tube,

now block the bottom of the firetube

what you have created is a vortex tube, the exhaust flow going in will drive the heat down into firetube
and separate a cooler mass of gas out the top tube.

i have built vortex tubes before for constant airflow, but haven't for intermittent flow exhaust, but i am betting it will work

the upside is the heat will be intensified in the firetube, and with the bottom blocked there will be no draft to suck away the heat
when the engine is shut down.

otherwise, just blowing hot exhaust up the firetube is not going to be very efficient, the carbon will collect like mad, and it will form
a dimpled surface and much like a golf ball it will produce eddy's and a boundary layer which will further insulate the heat flow.

that boundary layer is a killer and works very well to support laminar flow which keeps the heat suspended away from the metal tube.

that twisted baffle that is used in gas hot water tanks is pretty good and getting a swirl established, but the amount of heat available,
along with the relatively cold surface will collect the carbon very effectively.

cleaning is something that would need to be done very frequently

ymmv

bob g

[Crumpite]

Interesting ideas !

As for the carbon buildup, you could just hook, say, a 1 RPM motor to the top shaft and have the baffle/tube scraper running full time.
I like that idea too.

The vortex tube concept applied this way is another interesting one.
You'd likely have to do a fair bit of experimentation to find the best dimensions and tube lengths, but with a greater delta-T it sure would increase heat transfer.
Also with the increased velocity of the vortex, it would help greatly in heat transfer.

Alas, I've only got a 8 foot ceiling to work with, I don't think I'd have enough room to build one.
I'll still do a little research to see what I can find out.

We used to use vortex tubes for localized cooling of sensitive electronics gear mounted in the field.

Crumpite.

[Crofter]

It doesn't take much of a dust or slime film either side of a heat transfer surface to cause a high percentage loss of conductivity. An efficient transfer of heat out of a gas is upflow through a falling liquid. It is what is used in cooling towers, column stills etc. It is easier after to get the heat back from the liquid.

With diesel exhaust the resulting acidification of the coolant and filtering off of solids stickied with oil complicates matters. Maybe you would need two units so you could alternate and hit the idling one with a high temperature burn off. Never heard of a runaway exhaust burnout on a diesel but if it was a contraption you had installed in a building it might be something to consider.

Exhaust heat energy from a diesel certainly is one of the more difficult losses to recover for a small cogen system. I was operator for several years at a small coal fired lime kiln and the exhaust scrubber was a constant maintenance headache. Abrasion from cyclone coal ash and limestone dust and the collection of it from a settling pond. Acidification was not a problem as the lime dust was enough to cancel the acid produced by the burning coal but the solids had to be continuously dredged and hauled to land disposal. Even if you scale it down to homeowner size the same  basic issues exist. At that time in the early 70's we weren't even trying to recover the heat.

[vdubnut62]

Bob, only you would remember a vortex tube AND come up with a way to use engine exhaust to power it!
Why must you keep reinforcing my inferiority complex?
Ron

[M61hops]

Hi Jens.  You're right, a water heater will lose substantial heat through convection flow when the flame is off.  An energy efficent model will have a damper that closes when the flame is off.  In the past I saw add on dampers that were opened/closed by a bi-metallic spring for upgrading home water heaters but they didn't seem to catch on.  I wondered if the spring could react fast enough on the home add on units  .  The commerical energy effecient units usually have electrically operated dampers.               Leland

[Crumpite]

With the listeroid exhaust piped directly into the center draft tube and the exhaust at the other end piped outside, there isn't any way for convection to carry away the heat.
Or perhaps I misunderstand the question...

Another thought I had was to use a spiral wound steel brush mounted in place of the steel spiral.
The brush would do a better job of carbon removal, and the spiral nature of the steel wires might tend to induce the same spiral, turbulent flow that the solid spiral does.

I wonder if McMaster-Car has 4" X 5' long SS brushes in stock. (and at a price I can afford !)

Crumpite

[Crumpite]

You're correct, the all of the older one's I've seen have no draft prevention when the flame is off.
What were they thinking, I don't know. 

Crumpite

[rosebud]

This is a good idea...

I just happen to have a an old "oil fired" hot water tank, minus the burner, leftover from an another solar project. Likely not as efficient as gas fired, but it is priced right for a DIY's . Cleaning may be a different challenge?

thanks for the tip,

rosebud

[buickanddeere]

  It maynot be practical but............ For last last 1-2% of efficiency. The ideal model would introduce the exhaust gasses into the top of the old gas water heater whee the hottest water is. And discharge the exhaust gasses at the base of the old water heater where the water is the coolest.
   Keeps the highest average delta T across the heat exchanger. 

[LowGear]

Was Crumpite suggesting that the heat from the radiator be channeled up the water heater as well as the exhaust using the exhaust as a motivator and establishing a convection stream for both?

Casey