In the 2013 ASHRAE Fundamentals chapter 21, the friction loss (pressure loss) hydraulic formulas are indicated, and based on those equations the duct sizing chart has been created (page 21.8). Also there are two methods for duct sizing or duct design, equal friction or static regain methods. but the most common way for calculating or estimating a pressure loss in a duct work is equal friction method. In this method a constant static pressure should be considered, which for low pressure system (air velocity lower than 2,000 fpm) the 0.08” wg/ 100 ft is a practical number for the noise control and the smooth air stream. By using the duct sizing chart or a tool (ductlator) at a desired air flow (CFM) and equal static pressure loss 0.08”/100 can size the ductworks, but at the same time the air velocity need to be checked to be at the range of 1200- 1500 fpm for the noise control.
For calculating the total pressure loss in the longest run in a ductwork system, the developed length should be calculated:
L = LD + LE
L= Developed length of the longest duct run
LD= total length of the longest duct run
LE= the equivalent length of the all trims (elbows, tees, transitions, etc.)
The equivalent length of the trims can be obtaining from the related charts in ASHRAE fundamentals, but by rule of thumb considering a 20% of the duct’s length for the trims’ equivalent length would work. Therefore:
L= LD + 25 LD/100 = 1.25 LD
By considering the 0.8”/100 ft pressure loss, the total pressure loss (friction loss) in the longest duct run would be:
PL = 1.25 LD x 0.8/100
The pressure loss of the last diffuser must be added to the PL as well to obtain the total pressure loss in the longest duct run for selecting the blower (fan).