generally u shaped cross section the views of fig1 and 3 are of greatly exaggerated dimension in practice the depth of the chamber 28 is about one and one half inches while its surface may be substantially square with sides on the order of 52 inches a pair of ports 32 formed of apertures in the recessed interior portion of the lower plate 30 provide communication between the interior of the chamber 28 and pumping apparatus not shown for drawing avacuum in the chamber a plurality of i beams 24 forms a peripheral frame about the lower unit 12 providing support at the edge of the generally u shaped lower plate 30 in the same manner in which the frame formed in the upper unit 10 by means ofthe i beams 14 provides support or reinforcement at the edge of the upper plate 16 the flanges of the i beams 24 are considerably broader than those of the i beams 14 providing at their inner portions areas for fixing a molded elastomeric rib 36 along the lengths of the beams 24 the rib 36 thereby surrounds the evacuation chamber 28 elevating the height of the membrane 20 allowing the lamination.