The unit casing shall consist of high rigidity Pentapost framework with double skinned infill panels. Panels shall contain inert high density infill. Panel material shall be heavy gauge Aluzinc corrosion resistant steel. Fans shall be of high efficiency backward curved centrifugal design, manufactured in galvanised steel. Fans shall be direct drive with high efficiency IE2 motors to IEC60034-30:2008 where applicable (e.g. single speed motors, rated at 0.75kW and above). The efficiency rating of other motors will vary by size and type.