baffle 50 and because of their differing masses such relative movements take place as the structure changes its temperature by reason of the rigid mounting of the support tubes 52 to the t bars 55 and the end t bars 55a to the mounting plates 63 the support tubes 52 on the hot side also function to push the side channels 27 outwardly as the temperature of the support tubes 52 and the baffles 50 rises so as to provide additional space between the side channels 27 to accomodate the damper blades 46 which also expand longitudinally of their shafts 47 as their temperature increases conversely if the temperature of these structures drops all of them contract at varying degrees by reason of their differing coefficients of expansion and the mounting so far described provides for this movement as well however because the hot side of the damper 20 i e the lower portion in fig9 is subjected to more extreme variations in temperature than is the cold side i e the upper baffles 50 as shown in fig9 there results a different relative expansion and contraction provision for differential expansion and contraction between the hot sides of the baffles 50 and their cold sides is illustrated in fig1 instead of mounting the end t bars 55a rigidly on studs 64 as is the case with the baffles 50 on the hot side of the damper 20 the end t bars 55a on the cold side are mounted for movement relative to the mounting plates 63 in.