Demand controlled air ductwork.

A demand controlled air ductwork should be so dimensioned that the flow controllers have good flow and acoustical operation conditions. From the air flows in a room and its highest permissible sound level, the highest differential pressures allowable to the air flow controllers (duct air flow controllers and terminal devices) are selected. The required minimum differential pressure is 50-100 Pa or a higher differential pressure determined according to the outdoor temperature (-20°C), the height of the building and the air conditioning system required to control the thermal disturbing forces. From these differential pressures and the preadjustability of the air flows are determined the average pressure level of the duckwork and the highest permissible pressure losses of the air ducts, which at most may be about half the average pressure level. Also determined on the basis of the pressure level of the ductwork are the air tightness requirements in order to have control over the leakage noise. Suitable duct sizes have been ready-calculated. The pressure losses of the ducts are simply estimated from the pressure losses from local losses (bends and branches). Friction losses in the ducts are disregarded. The dimensioning of the ducts should be adapted to the standards of the equipment technology (flow, acoustics, air tightness) that is available. The recommended total pressure loss (ducts and air flow controllers) of a demand controlled ductwork should be a maximum of around 200-300 Pa excluding the pressure loss of the central unit.