Fig. National Library Board Singapore. Building systems diagram. Blown up detail of the acid-etched precast concrete arcade units. Units would be precast on-site and assembled after which reinforcement is positioned in them and concrete is poured into them. Air is supplied to the rectangular library block through the floors. The sub-floor acts as a plenum through which air is distributed and supplied via small openings in what is termed as 'a displacement system of air-conditioning'. The low-velocity air enters the space at the floor, rising upward, and is returned through slots in the ceiling via the sub-floor of the floor above. The air is introduced at the feet-level at a slighly higher temperature which avoids cold-air drafts of typical ceiling supplied, higher velocity systems. The returning air is ducted through the sub-floor to the four corners of the building where shafts are provided to return the air to the mechanical room at the basement level.