Book - School of Science and Technology

428 Air distributionair temperature has been established,* but is not in common use in Britishpractice.High-velocity supply fittingsThe use of high velocity air distribution in ducts has been referred to in connection withthe induction, dual-duct and variable air volume systems. High velocity air distributionmay also be the most practical and economical method both in cost and space for use withany, otherwise normal, ventilation or air-conditioning system where extensive ductwork isinvolved. This is particularly the case in multi-storey buildings where duct sizes are muchreduced by use of higher velocities. Duct velocities up to 30 m/s may be used, although it ismore usual to limit these to the range of 15±20 m/s.At the terminal end, it is necessary to break down the high velocity to low velocity forintroduction into the room, and a silencing or attenuating box in some form is required:this subject has been referred to previously and illustrated in Chapter 14.Combined lighting and air distributionMention has previously been made of the use of special fittings which enable some part ofthe heat generated by lighting to be dealt with at source before it enters the room. Earlytypes of such fittings used `boots' mounted to conventional lighting enclosures, as Figure15.19, but air handling functions have been developed in an integrated design, as previouslyreferred to, Figure 15.26. The performance of the lighting apparatus can beimproved by such arrangements, where the air extracted is drawn over the tubes thusproducing temperature stability, and the heat entering the room may be reduced veryconsiderably, as described in Chapter 18. In these cases it is important to refer to thelighting manufacturers' requirements, if the return air flow rate is too high over the tubesthey can overcool and loose efficiency.(a) Non-air handling(b) Air handlingExtract void(c) Diffuser fittings* ASHRAE Handbook, Fundamentals, 1993.