Duct Sizing Using Equal Friction method
The Equal Friction method creates an initial guess for duct sizing by establishing a constant pressure loss per unit of duct length. Several sources recommend using 0.1 in. wg (~25 Pa) pressure loss per 100 ft (30 m) total length. This length is taken for the critical path, considering the longest branch in an air distribution system, and possibly have the highest sum of total pressure loss on the run. However, shorter runs may have more elbows, fittings, and other flow restrictions casing greatest friction loss than the longest straight runs.
- Get the target critical path having maximum pressure loss.
This is the longest branch between fan and terminal outlets.
- Total pressure losses to each section of the critical path are assigned as the recommended pressure loss per unit of length multiplied by the actual section length.
- Calculate cross sections for the critical path using previously assigned total pressure losses, and correct these if necessary in order to satisfy velocity and geometrical constraints.
Pressure loss in junctions cannot be calculated until branched cross sections are assigned. At this time, the pressure loss in junctions can be ignored; a constant pressure loss can be assumed for any junction, or the same cross sections can be used in branches as in trunk ducts.
You should achieve pressure balancing by selecting proper duct cross-sections rather than by using dampers (Duct Sizing utility provides alternate duct sizes pull down list and allow locking one or more values in computing). - Sum the pressure losses in the critical path and select a fan so that fan total pressure is close to the sum of total pressure losses in the critical path.
This pressure is called the root pressure, ideally same as the fan pressure. If the selected fan does not satisfy the pressure requirement, change the assigned pressure loss per unit length and recalculate by assigning revised pressure losses in sections.
- Assign a total pressure at each node of the critical path.
To achieve pressure balancing, node pressures must be dissipated in corresponding branch sections.
- Exclude sections that belong to the critical path and recalculate to select the longest branch from the remaining sections.
This will be the new critical path and the node pressure acts as root pressure.
- Compute the total pressure loss per unit length of the new branch as its root pressure divided by its length.
This pressure loss should be larger than the initial pressure loss per unit length assumed for the initial critical path.
- Repeat the calculation for the new critical path.
Continue this process until cross sections are calculated for all sections.
The challenge in this process is to satisfy the noise and geometry requirements. For example, a short section located close to the fan must be balanced with the long critical path. Often, overcome only by dampening flow. However, this creates noise caused by high velocities in the damper. If the layout of the system permits fan pressure can lowered to prevent noise.
The Duct Sizing utility is designed to follow the equal friction method workflow in computing losses. The critical paths having maximum pressure loss are addressed by refining flow parameters and fittings in the duct work. The summary of Loss Coefficient run in tandem with Duct Sizing summary. It allows resetting coefficient and correction factor or overriding the loss for a selective component of fitting. After resizing the system is recalculated to arrive at acceptable loss paths.