All You Need to Know about DUCT DESIGN. This is the best chapter of duct design from ASHRAE publication.
COMMERCIAL, industrial, and residential air duct system design must consider (1) space availability, (2) space air diffusion,
(3) noise levels, (4) duct leakage, (5) duct heat gains and
losses, (6) balancing, (7) fire and smoke control, (8) initial investment
cost, and (9) system operating cost.
Deficiencies in duct design can result in systems that operate
incorrectly or are expensive to own and operate. So, poor air distribution
can cause discomfort, loss of productivity and even adverse
health effects. Moreover, lack of sound attenuators may permit objectionable
noise levels.
Poorly designed ductwork can result in unbalanced
systems. Faulty duct construction or lack of duct sealing produces
inadequate airflow rates at the terminals. Then, proper duct insulation
eliminates excessive heat gain or loss.
The terms head and pressure are often used interchangeably;
however, head is the height of a fluid column supported by fluid
flow, while pressure is the normal force per unit area.
For liquids, it is convenient to measure the head in terms of the flowing fluid. With
a gas or air, however, it is customary to measure pressure on a column
of liquid.
Type of Pressure
The term p/ρg is static head; p is static pressure.
On the other hand, the term V square/2g refers to velocity head, and the term ρV2/2 refers
to velocity pressure.
Pressure-Space Relationship
Space pressure depends on fan location and duct system
arrangement. For example, a supply fan that pumps air into a space
increases space pressure; an exhaust fan reduces space pressure. If we use
both supply and exhaust fans , space pressure depends on
the relative capacity of the fans.
Space pressure is positive if supply
exceeds exhaust and negative if exhaust exceeds supply. System pressure variations due to wind can be minimized or eliminated by careful selection of intake air and exhaust vent locations.
All You Need to Know about DUCT DESIGN
