Effective length of cooling rack

What is the effective length of a cooing rack? I know most would say more the merrier. Has anybody tried temp gauges pre and post rack to see temp difference? I relize that we are trying to achieve close to ambient temp. How short can you go? Guys with removable cooling tubes have, you tried removing and blocking pipes to see the temp differences? I have not seen anybody adding cooling fins to their rack tubing to increase heat exchange, why? Better cooling shorter rack. Sorry about so many questions in one topic.


Hello Bradon

With 500-600f coming into the cooling rack on my trucks by the time I stop and check the temp at the end of the cooling rack the temp will be about 50f over ambient temp . After this the gas has to go through two large metal containers and enough piping from rear of truck to the motor .

If there is plenty of room for cooling tubes they are cheap and easy . If there is very limited space this is where I use the fins ( labor intensive )


BrandonO. there are many variable factors to this.
How much space and weight can you accommodate?

These in-use factors will affect all cooler assemblies.
You WILL have internal liquid (mostly water) collecting inside. So watch your sloping, and allow for collection and drain out points. Do not let accumulated liquid condensate to pool-trap and block gasses flow.
Mobile system this liquid-slug on braking and cornering can slosh and intermittent block fuel gasses flow.
Operating where liquid waters become Ice overnight and this liquid condensate can freeze.
Mike LaRosa used huge diameter cooler piping to always allow hot gasses to flow bypass and thaw out his coolers.

Coolers WILL always have soots building up inside of their passageways. This will insulate for heat transfers. This buildups will slow down and eventually block fuel gasses flows.
So . . . always build to allow for manual rag/brush swabbing out of the cooler tubes; and/or power washing out to clean them.
Wayne Kieth build his systems with oversized capacity; with many parallel flow pathways, to accommodate high amounts of build up between his finally needed cleaning out cycles.

Short advice is go-as-big-as-you-can in the piping diameters and length areas.
Minimum sucks. Will be a maintenance nightmare.

Steve Unruh


What is the effective length of a cooing rack? I know most would say more the merrier.

It’s more about surface area for cooling, calculated using the pipe ID. The sweet spot for surface area varies with the engine size / amount of gas at full throttle, so it’s hard to provide general guidance. The range of “hey it works for me” is wide, say 30 sq ft to 130 sq ft of cooling surface using cooling rails.

I have not seen anybody adding cooling fins to their rack tubing to increase heat exchange

Some folks like @bsoutherland in his charcoal MGB project have very limited room, and take a different approach. In his case, it’s finned tubing from hydronic heating baseboard radiators. Quite efficient with adequate airflow or active cooling - I’ve used them for a stationary generator build.

Echoing a @gasman post I only vaguely remember (will try to find it and reference): a short length of pipe will pull along condensation droplets due to air speed…that will get in the way of condensate collection. Very compact builds have to deal with issues that a cooling rack can sidestep by sheer size. Related: cost of cooling rails using thinwall piping is cheap and simple.


For me, no1 choise for cooling rack is aluminium. Durable, relatively cheap, light and better thermal conductivity.
But as far as condensate goes, l wuld encourage anyone that is space limited for his builds to concider mixing charcoal with wood. Jst a few % is eough. It is some work, yes, but the benefits outweigh the work 10 fold.
Some benefits:
Charcoal will react with water in the gasifier. This will produce dry gas that doesent need as much cooling surface (no water to condence out). Filters can be made smaller as filtering dry gas is easyer thain foggy wet gas. And more effective!
Charcoal will allso boost gasifier temperature but in a good way. Heat is good, heat means fast molecules and reactions, better gas. But it must be on a right plce, and that will be achived with charcoal.
The extra heat will allso expand the range of a gasifier. Steam will not suffocate the gasifier at sudden low demand, and the gasifier will not overdraw.


Hi Kristijan, you might remember we talked about adding charcoal to the wood mix in one of the discussions at Argos 2019. I run my truck the whole time there on a mix of 10 to 1 wood to charcoal. I noticed more power, and my Muffler Preheater was not inservice at the time. Just had cold air entering the Drop Box Preheater. It helped getting the temperature up to running temperature quicker, one the first morning start up of the gasifier. I am going to use a the 10 to 1 mix.


I remember, ofcorse. Even had the priviledge of driveing it :wink:

A WK or any monorator hopper does a similar job actualy. The hopper condensation helps draw water out of the wood, “charing it” some, for better efficiancy. Its all in the balance of C to H2O ratio. Idealy its around 1:1. Raw wood has way to big of a ratio, say 1:1.3, that 0.3 parts need to go somewhere and need energy to boil off.
Pure Charcoal has a way to low ratio, say 1:0.1. althugh it that is useable it is wasteing potential, it culd crack more water to produce that valuable H2.

Now, we can drive off those 0.3 parts water from raw wood via hopper condensate harvest, or kiln dry the wood but on a small sysyem that can be chalangeing. But, with adding a bit of charcoal the ideal ratio can be restored.


Hi KristijanL.
I like to consider this as the Carbons to Volatilizes ratio in the fuel wood base.
My soft wood Douglas Fir always had too low of carbons. A fast energy release, yes. But always carbons(char) shy. Nearly impossible to make handleable char chunks from my DF. except with old growth knots and old growth tight grained limbs.
True hard woods of oak, ash, maple, hickory, apple/cherry, etc have much higher carbons(char) base. Easy to make dense char chunks out of nearly any tree portion.

Some raw wood volatilizes are much different from just cured, then re-wetted wood moisture.
I can woodstove and gasify well raw Pacific Red Alder, Holly and Madrone. The live-sap volatilizes have combustible sugars energy. Contributing O’s, H’s and more Carbons.
Dried concentrated liquid pitch and crystallized pitch introduced another hard to handle variable. Far too much long chained carbons. Missing lost O’s and H’s for balance. Sooty as hell.

I know many will dispute my observations.
These have been hard-learned. Hard won.

Yep. Yep.
“Cheat (with) the Fuel”
Manipulated the fuels inputs to get better results.
You’ve learned and passed on a valuable wisdom there.
Steve Unruh


Ofcorse fuel sorce is the main setpoint with any gasification. I bet it is possible to make charcoal even with your fir thugh, just not the conventional way of charing big peaces of wood then breaking them down to engine grade. Making char out of woodchips or chunks will resault in litle to no dust in the final product. But then agen, why bother if there is no need. Most woodgasers have raw wood gasification perfected well enough to fit their needs, but there comes situations where different actions must be taken.
Well the water vs carbon ratio was just a simplification. Ofcorse the chemistry is much more complex. But wood (cellulose) being a carbohydrate can be crudely displayed as a molecule containing 6 atoms of carbon and 5 molecules of H20 + 1 molecule H20 represented by inevitable wood moisture of at least 10%.

But you are right, carbon to volitiles is a better term.


I think what Steve observes is correct. Conifer wood is almost entirely cellulose, whereas hardwoods (including poplars), contain lignin, which must have a greater carbon content, I’m guessing.

As for mixing charcoal and wood, great innovation. But in practice won’t they tend to stratify while bumping down the road? Having charcoal of similar size to the wood might help, but reduce the reactivity? Or am I overthinking how much of a problem this would be?

Garry, you’re right. Sometimes they do. Char glues to the funnel tar too.
On those rare occations when I use char, I put a pile of on top of the wood to try make it sprinkle down as evenly as possible. Sometimes I get a tight charbed after a while anyway and then back to coarse, towards the end of a hopper.


I am with JO. On this one.
I can put 2 gal of char on top of full hopper of wood and run about a half hopper of wood and shut it down and when it cools down there will be no sign of char in the wood. The char bed will be be starting plug.