Tom Lawrence
University of Georgia

Duct systems II - # 1

Air Distribution Flow Analysis

• Design velocities
• Fan inlets / outlets
• Leakage
• Duct sizing / design methods

– Equal friction
– Static regain
– T-method

• Duct systems design

– Total pressure
– Static pressure

REFERENCE:
20001 ASHRAE Fundamentals Handbook,

Chapter 34

Tom Lawrence
University of Georgia

Duct systems II - # 2

Design Velocities

• Air velocity rates

– Low velocity  Larger unit size for higher

cost, too big

– High velocity  Smaller unit size, but larger

pressure loss

• Typical design velocities for P, noise:

Ducts:

Components:

Air Flow
(cfm)

Air Velocity
(ft/min)

> 20,000
2,000 –
20,000
500 – 2,000
<500

Air Flow (cfm) Air

Velocity
(ft/min)

Intake Louvers
Exhaust
Louvers
Filters
Heating Coils
Cooling Coils

Tom Lawrence
University of Georgia

Duct systems II - # 3

Airflow 

S

ta

ti

c

P

re

ss

u

re



Design system
pressure drop

Fan Inlets, Outlets

• Poor fan performance can be caused by:

– Error in estimated system resistance
– Poor fan inlet and/or outlet connections

• Common fan inlet or outlet connection design

problems

– Non-uniform or swirl in flow at fan inlet
– Outlet duct flow directions changes or splits too

close to fan

• Non-optimum fan inlet and outlet connections

add another net flow loss

Actual system
pressure drop

Flow Deficiency

Tom Lawrence
University of Georgia

Duct systems II - # 4

Fan Outlet Design

• Effective duct length after fan discharge to give

flow stabilization (recovery)

• Sometimes system design dictates elbow, etc.

before complete recovery
 system loss coefficients from ASHRAE

Fundamentals Handbook for these fittings

Tom Lawrence
University of Georgia

Duct systems II - # 5

Fan Outlet Effective Length

Centrifugal

Axial

Inlet
collar

Blast area

100% Effective
Length

Velocity Profiles

Tom Lawrence
University of Georgia

Duct systems II - # 6

Fan Inlet Design

• Non-uniform flow at inlet most common

reason for fan performance not being that
which was expected

– Example: Elbow near inlet which

introduces turbulence, spin and/or
uneven flow into fan inlet

• Ideal condition:

–

Tom Lawrence
University of Georgia

Duct systems II - # 7

Duct Leakage

• All duct systems will leak!

• Duct leakage classifications in ASHRAE

handbook (or SMACNA Air Duct Leakage
Test Manual)

• Example unsealed seam leakage rates

Duct P < P ambient

Leakage Into Duct

Duct P > P ambient

Leakage Out of Duct

Type of
Duct

Average Leakage
(cfm/ft)

Rectangular
Round

Tom Lawrence
University of Georgia

Duct systems II - # 8

Duct Design/Sizing Methods

Equal Friction Method
• Ducts are sized to provide ~ the same pressure

loss per unit length of duct

• Simple method:

– Perform initial sizing and P calculations
– Resize ducts as needed to provide ‘uniform’

pressure loss characteristics

Tom Lawrence
University of Georgia

Duct systems II - # 9

Duct Design/Sizing Methods

Static Regain Method
• Objective is to provide the same static pressure

at a diverging flow junction (tee) by adjusting
the downstream duct sizes

• For the same flow rate

Increase duct size:

 Lower velocity Higher

Decrease duct size:

 Higher velocity Lower

WHY DOES THIS HAPPEN?

Tom Lawrence
University of Georgia

Duct systems II - # 10

Duct Design/Sizing Methods (Cont’d)

Static Regain Method (Cont’d)
Example:

Objective: Find duct size such that P

1

= P

2







Tom Lawrence
University of Georgia

Duct systems II - # 11

Duct System Design Procedure

1. Based on system flow requirements, building or

structure layout, etc.; arrange duct system inlets and
outlets to provide proper airflow and distribution

2. Set outlet sizes from manufacturer’s data or exhaust

design considerations

3. Sketch the duct system, connections, supply outlets,

return or suction inlets, etc. with the fan or central air
handling station.

4. Divide into sections and number each
5. Size each section based on equal friction or static

regain methods; size fan

6. Layout detailed duct system. Resize fan if necessary

based on final system design

7. Resize duct sections to balance pressures or address

other design issues (i.e. noise levels)

Tom Lawrence
University of Georgia

Duct systems II - # 12

Branched Duct Systems

Simple example:

Equal pressure loss from point ‘2’ to points 3

through 6

• If flow is ‘known’ by requirements, then

determine duct sizes to match

• If duct sizes are known, find resulting flow rates

for each individual branch











