rutman, it is a pleasure to see you noticing the detail logic of how heat exchange is done in the gek system. you are correct that we have a single system used everywhere except the air intake.
it is certainly possible to use the "tube in tube" method for the air in / gas out too. it will work fine, but will present a different sum of pros and cons than the ss tube method. i considered the solution you are suggesting and went with the tubes as i find they result in a much higher total transfer, as well as particulate separation due to the turbulence in the rising section, while also eliminating a bunch of otherwise close tolerance fits and plumbs. i actually find the tubes to be a fab "get out of jail free" solution. flex makes things easy, much easier than a ton of welding. unfortuantly, they are also the single most expensive to source part on the gek.
if you are going to do the "tube in tube" heat exchange method, remember that you want your flows as perpendicular to heat exchange surfaces as possible. all surfaces build up a laminar layer, which slows heat transfer. simple parallel flow to the walls won't do much to break up this flow. this is why you put turbulators in water heater flue pipes.
i solve this in all our tube in tube exchangers with a series of baffles that make the gas in the gas space pass back and forth across the center tube. think baffles that look like horseshoes, wrapping about 2/3 of the way around the center tube. this makes somewhat of a standard multipass tube in shell heat exchanger, though with only one tube. here's what the inside of the drying bucket looks like. you can do the reactor the same way.
this is the solution we use on the drying bucket, pyrocoil and bek biochar retort. it is the same solution used again and again. it works well and is easy to make. i could also do it for the air intake preheat too, but i find the particulate separation of the tubes, and the ability to start cooling the gas right at the lip of the reduction bell, to be worth the effort. also, we get the air hotter and the outgoing gas colder for the effort. we're equally concerned with both
if you want to do the tubes, you can also do them from rolled round tube. you don't need to use the water heater lines. or, you can do straight tubes in a vertical orientation, then put baffles in the gap to get the gas to pass back and forth over them. this is the one that has tempted me recently. you do the vertical tubes like this: http://www.gekgasifi...lery/gek-v07-2/