by johnemack » Mon Aug 11, 2008 12:00 pm
Hello John,
You ask >how much heat will eventually migrate through<. Calculating that is rather tedious – and what is the use of knowing it is x Watts? (I find it best to design for minimum conduction in every detail.)
So, go for a long cylinder of the thinnest low-conductivity material. For LTD that will be of plastic (babies’ bottles can withstand over 100C). Or, since it does not have to fit the displacer closely, it can be of wrapped film with a lapped joint. (On miniature LTD’s I’m using 12u Mylar, half-thou thick, but it buckles a bit under negative pressure.) For MTD’s it will probably be of glass, carefully cut from labware. For HTD’s it will be of stainless foil for low conductivity or of (machinable?) ceramic. Keep to the thinnest wall.
To reduce heat loss from the wall and so preserve the temperature gradient for the regeneration, the cylinder can usefully be lagged, but not so thickly as to cause longitudinal conduction. That also protects the thin wall from handling damage.
Clamp bolts should be as thin and long as you can make them, and of stainless for low conductivity. On small engines use snap stainless wire clips, instead, because they are thin and make poor thermal (ie, point) contact with the hot and cold plates. (Recently, on miniature LTD’s, I eliminate them by bonding the cylinder as a sealed unit. There’s nothing to service in there, anyway). Alternatively, use clamp rings with short bolts at each end.
Your >despite the insulator< might refer to the cylinder gaskets. Keep these as thick and as narrow (radially) as possible. (I’m now using silicone rubber tubes, slit along their length and snapped round the ends of the cylinder wall. They are clamped by the end plates and require surprisingly small end load to seal.)
You refer to > reroute the heat back to the original power
source?< Heat will flow only from hot to cold! However, some waste heat can be recovered by using it to pre-heat the burner air before recuperating from the flue.
There is much scope for ingenuity in maximizing efficiency (but I still haven’t done my sums).
Another John.