Book - School of Science and Technology

Chapter 15Air distributionEffective distribution of air within an occupied space is the key to successful operation ofa ventilation or air-conditioning system. It is of little use to provide plant and ductworkdistribution arrangements ideally suited to meet load demands if the methods used tointroduce the air supply do not provide for human comfort or process needs.Successful air distribution requires that an even supply of air over the whole area beprovided without direct impingement on the occupants and without stagnant pockets, atthe same time creating sufficient air movement to cause a feeling of freshness.This definition indicates what is probably the key to the problem of successful distribution:that unduly low velocities of inlet are to be avoided just as much as excessively highones and that distribution above head level not directly discharging towards the occupantswill give the necessary air movement to ensure proper distribution over the wholearea without draughts. Low level floor supplies, however, introduce air directly into theoccupied zone and so need special attention.To produce satisfactory conditions in the comfort zone of a space to be held at normaltemperature, the distribution system should produce an air velocity, at a measurementpoint 1.8 m above the floor and not less than 0.15 m from a wall, of between 0.1 and0.25 m/s and never less than 0.05 m/s. Where activity is high and spot cooling needed, as ina factory, a velocity of up to 1 m/s might be acceptable. Laminar flow, used in cleanrooms, and other special distribution methods are outside the scope of this book.General principlesThere are five general methods of air distribution.. upward. downward. mixed upward and downward. mixed upward and lateral. lateral.The choice of system will depend on:. whether simple ventilation or complete air-conditioning is employed. the size, height and type of building or room. the position of occupants and/or heat sources. the location of the central plant, and economy of duct design. constraints imposed by the building structure and internal layout.