hot air plenum 80 constituting part of the main housing the hot air plenum is defined by an upper wall 81 connected to the upper edge of the rearward wall 74 and extending rearwardly therefrom along the under side of the main support beams 16 the hot air plenum has a rearward wall 82 extending downwardly from the upper wall and communicating at its lower edge with a lower wall 83 the hot air plenum has a forward wall 84 extending from the lower wall to a position roughly in alignment with the lower edge of the trough portion 71 of the forward wall 70 as best shown in fig2 the main housing 60 has a cylinder chamber wall 90 composed of a main arcuate portion 91 constituting the lower boundary of the cylinder chamber 64 the cylinder chamber wall includes a forward arcuate portion 92 which is connected to the forward wall 84 extending in concentric relation to the discharge duct portion 72 of the forward wall 70 to a rearwardly facing duct mounting flange 93 it will be understood that the main housing has opposite lateral walls disposed in spaced substantially parallel relation to each other and interconnected by the walls heretofore described extending in substantially normal relation therebetween the walls of the main housing thus form a substantially airtight housing the cylinder chamber wall 90 has an adjustable portion 94 extending upwardly from the main arcuate portion 91 for adjustment as will hereinafter be described from.