Determination and interpretation of total and transversal linear efficiencies in push-pull ventilation systems for...

A real-scale pilot installation simulating an open surface treatment tank with a push-pull ventilation system has been designed. From experiments carried out, typical representations of the total and transversal linear efficiencies show that when total efficiency is related to push flow rate, taking as a parameter the pull flow rate, a parabolic profile is obtained with a maximum point or plateau that increases as the pull flow increases. When the transversal linear efficiency is analysed, three general zones where losses occur to the exterior can be detected: (i) when the push flow rate is low, any distortion in the wall jet, whether external (e.g. in the air flow inside the workshop) or internal (e.g. thermal effects), provokes an escape from contaminant; (ii) in the impact zone, where the push flow impacts on the tank surface, distortion increases as the push flow rate increases; (iii) when the push/pull flow rate ratio increases and preferential currents are produced inside the exhaust hood, these escape and cause substantial losses in efficiency.