Cooling rail sizing

“Wet shunt tube with flap” could you please explain this? 15- 20 square inches per liter of displacement. Engine displacement? that seems awful small? I am sure velocity must play an enormous roll in reheating as well as cooling?


Hi, Jim!
Yes, ~2 days back the parallel tube for unheated, damp gas with a regulation flap was described.

If the reheater makes the cool, damp gas unnecessarily hot, it can be modified (cooled) with some cool & wet gas which is parallelling (bypassing) the reheater and then mixed in with the “too hot” gas to achieve the wanted temperature.
The flap has to be well centered on its axis in order to stay neutral against “flowpower”. Otherwise a setting can go out of hand…

Yes, the reheating area is determined by the motor displacement.
In fact, the air preheating around the firetube and in the heatexchanger
will “take back” about half of the heat energy leaving the grate-area.
Because of this, it is a good precausion to increase the reheating area for the wet & cool gas to:

31"2 per liter displacement
31"2 X 4,7 = 145,7"2 or ~150"2
150"2 = 1,041667’2

The reheater mantel around part of the heatexchanger cylinder would be ~1" thick.

This seems to be enough; the increase in temperature needs not to be very big, even if the reheated gas has to take the “burden” of the cold secondary air at the mixer. Otherwise cold and heavily humid secondary air can lead to condensation again.

All this counts at a deep frost startup without gasoline.

And the delivery tube/metal hose from the reheater to the mixer has to be insulated.


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I’m sorry Max but you lost me again. We were at 15-20squre inches per liter of displacement. 20 x7.4=148squre inches

Hi, Jim!
I just adjusted the reheating area a bit upward, as you were not satisfied, by all means you were right! Was the rest consumable?

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I think Max originally said 15-20 square inches per liter, based on a hotter gas temperature, needing less preheat. As you rightly point out, it’s a bit small… there’s no downside to increasing the surface area.

So, 31 square inches per liter will provide double the heat exchange, and perform a bit better at cold startups.

Another thing for folks reading to keep in mind, European decimals are marked by commas, and American usage is periods. When Max writes 4,7 that means the same as we would write 4.7 and so on.

Hopefully this is also “consumable” ie understandable.


Hi, Jim and Chris!

I am Sorry!

Holy cow! At last my eyes opened; 7.4 liter had become 4.7 liter,
which is a “natural size” in the subconscious mind…

So the conscious numbers are:

31"2 X 7.4 liter = 229.4"2

The calculator batteries are OK!



Yes 7.4 liter Max. I still find it amazing That it takes 12 squre meters to cool the gas and only 1 1/2 square feet to reheat it. Much less temp change on the reheat though. I have got a pretty good idea in my head now on how it will all fit togather. At least 1 more trip to jhe junk yard for a few RV bumpers for manifolds on my drive shaft cooler modules.


Hi, Jim!
For the same gasflow (motor consumption) the heat transfere is dependent on the
area X temperature differens.

The initial cooling aims at shrinking the volume and condensing out as much as possible of the different steams.
Active area limited only in
sub zero Celsius (freezing) conditions.

Theoretically, and practically you want to start up a motor with a “summer-warm” gasmix, or at least a mix which is not immediately making icecrystals on the sparkplug insulators…

Sub-zero C air contains very little humidity in absolute terms = very dry in relative terms.

But the cooled down gas, which cannot be cooled beyond the freezing point
(without a snowfilter) will have 100% humidity = satisfied = like thick fog = mist.
So, to get the gas’ relative humidity down beyond a renewed condensation (100%) as it mixes with the sub-zero C air, the reheat has to theoretically go

equally high above zero as the cool secondary air is below zero C to “balance” on the “condensation edge”.

To be on the dry (safe) side, one has to reheat the gas at least to the double value above zero C as the ambient air is below zero C.

Measured at the entrance of the mixer.

Figure that,


I have a question, is the reheat needed if the outside temperatures are around freezing and above.

Hi, Jim Goes!

Above the freezing point the mixture will not develope icecrystals anymore and the motor will not “condense” anything to ice either without vacuum.
The ambient air can “carry” more humidity with increasing temperature.

But the cooled gas remains 100% wet if not reheated.

The benefit diminishes with ambient temperatures rising above freezing as long as the relative humidity in ambient air is low or modest.
No sharp limits recognized. Quality and taste…

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Thanks Max
All on DOW that I know of hasn’t used reheat. I do seem to remember Wayne and I think Carl having a little water in the breather housing but didn’t effect the operation any.

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The only time reheating is necessary is to make the gas go through a paper filter. 100% humid or “foggy” gas will get the paper wet. If you’re not paper filtering, it’s not really an issue.

Moisture condensing in the air breather is a symptom of hot gas ie loose char bed, taking longer to cool down and condense. Summer time this is more likely as well. Be sure there are drain screws at all the system low points.


Chris, Do you drain the lower lines daily if it is freezing temps or do you just wait until the temps warm up to drain the water off.

Every two or three months I will drain condensate from the gas lines under the truck. Usually no more than a few spoon fulls .


Freezing temps make the cooling rails more efficient, and so I hardly ever need to drain these points in winter. More of a summer thing.

Like Wayne said, only a few tablespoons come out.


Thanks Wayne and Chris, I am really looking forward to picking up the 92 Dakota truck in May and driving it up to the Argos Event. Getting to meet and learn from all of you is going to be a real treat. Years and years of people who are in the know on how to Gasifiy and make truck or car run on it. It doesn’t get any better than this until you are doing it on your own. Thank You All for making this possible.

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I get very little water passed the hay filter but I get quit abet in the hay filter.
some times about as much as in the back tank.


there has been a few posts about poor running that ended up with low pipes full of water so keep in mind if something is not acting right.


Has anyone ever looked into using the 1/6 (sixlet) beer kegs as cooling racks? Occasionally I can see them on craigslist for $30. Sorry if this is off topic, I wasn’t sure where to ask this.

Hello Aaron .

I have never used any beer kegs for cooling . Seems like there would be little surface area on the keg compared to lengths of pipe or tubing.

I think there would be several places on a gasifier where the kegs could be of some use .