A scroll compressor comprising a scroll pumping mechanism having two co-operating scrolls mounted within a pump housing so that on relative orbital movement of the scrolls fluid is pumped from a pump inlet in the pump housing to a pump outlet in the pump housing. An air cooling arrangement 16 located in the housing between the inlet and an inlet to a duct 18 extending to an air outlet 20. A variable orifice assembly is mounted in the orifice channel and includes a fixed orifice passage extending axially through a variable orifice body having a sealing flange that can seal against a valve seat via an orifice spring. A display shelf might have a projector and a computer. The first, underlying layer includes a dielectric enhancing additive and a thickening agent to enhance the impermeability of the wall.