A large number of different duct sizing methods use arbitrary initial parameters based on engineering experience. These parameters are either initial velocity or pressure loss per unit of length. Two of the most widely used duct sizing methods, presented in the ASHRAE 2001 Fundamentals Handbook [ASHRAE, 2001], are the Equal Friction and the Static Regain methods.
The static regain ductwork design method has been the choice of engineers for many years even though it is more difficult and time-consuming than the equal friction technique. Static regain designs have been attributed to yielding more balanced systems that have better flow characteristics than equal friction systems. However, the equal friction method can provide equally efficient designs when experienced engineers use careful initial design assumptions.
A correctly sized duct system appropriately distributes design airflows throughout the facility. Sizing the duct system requires selecting all duct cross sections to result in a pressure-balanced system for the facility. Because there are practically an unlimited number of duct sizes and arrangements that will satisfy a facility's design air-flow requirements, sizing duct is still an art. The mathematical technique of "numerical analysis" incorporates iteration procedures that can provide a solution to duct-sizing problems. However, the solution is "constrained" by the "initial guess" which is provided by design engineers based on their individual experience. Also, it is important to remember that manual duct design methods, such as Equal Friction or Static Regain, do not minimize system life-cycle cost.
