and positioned beneath the upper support plate 211 the upper support assemblies define facing surfaces 224 that are directed toward one another and that partially define therebetween the plenum chamber 214 the upper support assemblies also define bottom surfaces 226 that are directed toward the vacuum box 204 in the illustrated embodiment each bottom surface 226 defines an elongated recess 228 in which an upper pneumatic loading tube 230 is fixedly mounted the upper pneumatic loading tubes 230 are suitably centered the cross machine direction and desirably extend over the full width of the wet web the stationary components 220 of the air plenum 202 also include a pair of lower support assemblies 240 that are spaced apart from one another and vertically spaced from the upper support assemblies 222 the lower support assemblies define top surfaces 242 and facing surfaces 244 the top surfaces 242 are directed toward the bottom surfaces 226 of the upper support assemblies 222 and as illustrated define elongated recesses 246 in which lower pneumatic loading tubes 248 are fixedly mounted the lower pneumatic loading tubes 248 are suitably centered in the cross machine direction and suitably extend over about 50 to 100 percent of the width of the wet web in the illustrated embodiment lateral support plates 250 are fixedly attached to the facing surfaces 244 of the lower support assemblies and function to stabilize vertical movement of.