directly through the duct wall

Heat conduction in the wall in the flow direction

For a good recuperator, the heat resistance perpendicular to the wall
(in the direction of the desired heat flow, see fig. 1) has to be much
lower than the heat resistance in the medium. Contrary the heat
resistance longitudinal in the wall has to be high to prevent direct heat
conduction from the warm to the cold side of the recuperator, causing a
decrease of the apparent temperature difference between the supply
temperatures.

 


Figure 1 direction of the heat flows in the separation wall


Though normal intuition leads to the idea that a good recuperator
needs a wall material with good heat conductivity, the opposite is true;
insulating material is the best wall material! Fig. 2 shows the
effectiveness of a recuperator with an average wall thickness of 80 μm
and a length of 300 mm for a range of materials with increasing heat
conductivity.

 

 

Figure 2 effectiveness as function of the heat conductivity of the wall
material (thickness 80 μm, length 300 mm)


From insulation material (l = 0,03 Wm-1K-1) to the better conducting
plastics the optimum is rather flat at the design value of 93%. For metals
the effectiveness starts to drop (for non existing super conductors to
50%). For non-existing super insulators the effectiveness goes to zero,
but still for the best vacuum insulation (l = 0.001 Wm-1K-1) the
effectiveness is higher than for an aluminum wall. Plastics like polystyrene
are nearly optimal material.


Using aluminum, the effectiveness drops with more than 10%.