Book - School of Science and Technology

Chapter 16Ductwork designHaving decided on the type of air system to be employed, made the calculations of airquantity and temperature, and considered the type and location of the air terminal devicesand central plant, it is now necessary to consider in more detail the characteristics of theductwork system which will convey the conditioned air about the building. Most of thediscussion which follows applies equally to ventilating or air-conditioning systems.One of the fundamental relationships is that between air speed and pressure, given by:p v ˆ 0:5v 2wherep v ˆ velocity pressure (Pa ) ˆ specific mass of fluid (kg/m 3 )v ˆ velocity (m/s)for standard airthus ˆ 1:2kg/m 3pv ˆ 0:6v 2Figure 16.1 gives the relationship graphically for air velocities encountered in ventilationwork. For other temperatures and pressures, correction is necessary:P v2 ˆ p v (P/101:325)[293/(273 ‡ t)]whereP ˆ alternative pressure (kPa )t ˆ alternative temperature ( C)Other properties of air used in deriving the standard data for air flow in ducts are:temperature ˆ 20 Catmospheric pressure ˆ 101:325 kParelative humidity ˆ 43%