Short-circuiting of ventilation air - a factor to be considered in the design of ventilation systems.

Short-circuiting of ventilation air reduces significantly the efficiency of ventilation in diluting and removing contaminants and excess heat. Therefore the prevention of short-circuiting is very significant to consider at the design stage of air distribution in ventilated rooms. The paper reports on results obtained experimentally from both full-scale and small-scale laboratory tests. Air distribution was quantified by measuring the local mean age of air and the air-exchange efficiency using a trager gas technique. At isothermal conditions short-circuiting may occur due to that the extract air point is located in the primary air stream (jet), close to the supply air point and thereby directly 'stealing' the ventilation air. A large amount of internal fittings may cause short-circuiting because the jet cannot penetrate (blockage effects) into the occupied zone. Fornon-isothermal conditions, the combination of having both supply and extract located at the ceiling level and supplying heated air (negatively buoyant air) may give rise to e very strong short circuiting and the air is impaired to penetrate into the occupied zone. Supply of heated air at floor level (positively buoyant air) together with an improper location of the extract air terminal may also cause the air to bypass the occupied zone.