Trane TRG-TRC014-EN Duct Design, Components of a VAV System, period two, Equal friction method

period two

Components of a VAV System

notes

Duct Design

▲Equal friction method

◆Equal static pressure drop per unit length of duct

◆Design can be performed by hand

▲Static regain method

◆Relatively constant static pressure throughout system

◆Desirable for VAV system duct design

◆Design often requires a computer program

Figure 53

Designers commonly use two methods to engineer the supply duct system:

nEqual friction

nStatic regain

Equal friction duct systems are designed for an equal static-pressure drop per foot of duct. This results in a static pressure that is very high near the fan, and which steadily decreases until it is very low near the far terminal units. Equal friction duct systems can be easily designed by hand.

Static regain duct systems strive to maintain a fairly consistent static pressure throughout the entire duct. Therefore, for VAV systems, the static regain method is recommended for sizing the supply ducts upstream of the terminal units. This relatively constant static pressure at the terminal unit inlet allows for improved selection and control. The design of a static regain duct system often requires the use of a computer program.

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