Duct Design

Presented by

Dave Janquart

Overview of Topics

• Duct design

• Duct insulation

• SMACNA Standards

Factors Influencing Duct Design

• Equipment Losses

• Air Velocity

• Duct Material

• Duct Size & Shape

• Length of Duct

Friction Losses

FURNACE

NO SCALE

SUPPLY

FAN

COIL

OR

FURN

ACE

R.A.

BOX

FILTER

S.P. 0.07

RE

TU

RN

AIR

RAG CONT

G35W

S.P. 0.045

SAG CONT 32W

S.P. 0.04A coil

.25” sp

Air Distribution System

Components

• Supply fan

• Supply Ductwork

• Transition fittings

• Discharge grills

• Return grills

• Return ductwork

Calculate Air System Pressure

Losses

• The air pressure losses from both the supply and return duct systems plus the loss through external a/c coil is often referred to as the external static pressure.

• Manufacturers generally publish fan ratings based on external static pressure (internal losses due to coils, filters, etc. are usually accounted for in the cataloged fan ratings).

Air Velocity in Duct Design

• As Air Velocity Increases Noise Increases

• Increasing Air Velocity Causes More

Turbulence

• Recommended Velocity 700-900 FPM

Duct Material Roughness

– Duct material roughness refers to the inside surface of

the duct material.

– The rougher the surface, the higher the friction loss.

– Most duct sizing tables use the roughness factor for

smooth, galvanized sheet metal as the reference value.

– The back side of the Duct Calculator has a conversion

table for converting the material roughness from

smooth sheet metal to other materials.

Select Duct Material (sheet metal,

fiberglass, flex)• Decide which duct construction material to

use.

• Common choices: fiberglass duct board, galvanized sheet metal, “flex” vinyl coated with helical wire core.

• Each material has advantages.

– First cost (price & ease of installation)

– Performance (friction loss, fan energy)

– Acoustic properties

– Thermal properties (heat gain & loss; internal/external insulation)

Round vs. Rectangular Duct

of Equal Friction Rate

40.7 in.

3 ft.

3 ft.

2 ft.

4-1/2 ft.

1-1/2 ft.6 ft.

1 ft. 9 ft.

9.0 ft2

9.0 ft2

9.0 ft2

9.03 ft2 10.65 ft.

Ratio ofPerimeter

to Area

EquivalentRound Duct

Frictionat

15,000 CFMPerimeter

1.18:1 40.7 in. .07 in. / 100 ft.

1.33:112 ft. 39.4 in. .086 in. / 100 ft.

13 ft. 1.45:1 38.7 in. .095 in. / 100 ft.

15 ft. 1.67:1 37.2 in. .113 in. / 100 ft.

20 ft. 2.22:1 34.5 in. .156 in. / 100 ft.

Figure 8

Fitting Influences

• Selection of the proper fittings is critical

Design Friction Rate

• When sizing a duct system the designer will usually choose a design friction rate.

• This is the desired friction loss in inches w.g. per 100 ft. of equivalent length (in. w.g./100 ft. E.L.) of duct.

• The design friction rate is generally determined based on the velocity of the air in the first section of ductwork.

• Most designers use a design friction rate somewhere between 0.08 and 0.10 in. w.g. per 100 ft. E.L.

Determining Friction Rate/100 ft.

• Equipment pressure drop

• Duct pressure drop

• Terminal device pressure drop

Terminal Device Pressure Drop

• Pressure loss for supply air grill

• Pressure loss for return air grill

Friction Losses

FURNACE

NO SCALE

SUPPLY

FAN

COIL

OR

FURN

ACE

R.A.

BOX

FILTER

S.P. 0.07

RE

TU

RN

AIR

RAG CONT

G35W

S.P. 0.045

SAG CONT 32W

S.P. 0.04A coil

.25” sp

Equal Friction Method

• Same friction loss per foot of ductwork

• Most commonly used method

• Accomplished by use of a Ductulator

• Requires less balancing

– Important to have balance dampers at each

outlet

• Results in reasonable duct sizes

Velocity Scale

Velocity

Pressure

Airflow

(CFM)

Friction

Loss

Round Duct

Diameter

Equivalent

Rectangular

Duct SizesFigure 7

Duct Calculator

Duct Calculator

Duct Calculator (Reverse)

Recommended & Maximum

Duct Velocity Ranges

Design toward smaller end of range for quieter systems

and toward larger end of range to minimize duct sizes.

Things to Consider

• Aspect ratio no more than 3:1

• Residential 2 times # of 6” RD take offs

plus 2” for ducts that are 8” high

• .5 cfm per Sq Ft

• No more than100 cfm per 6” RD

Round vs. Rectangular Duct

of Equal Friction Rate

40.7 in.

3 ft.

3 ft.

2 ft.

4-1/2 ft.

1-1/2 ft.6 ft.

1 ft. 9 ft.

9.0 ft2

9.0 ft2

9.0 ft2

9.03 ft2 10.65 ft.

Ratio ofPerimeter

to Area

EquivalentRound Duct

Frictionat

15,000 CFMPerimeter

1.18:1 40.7 in. .07 in. / 100 ft.

1.33:112 ft. 39.4 in. .086 in. / 100 ft.

13 ft. 1.45:1 38.7 in. .095 in. / 100 ft.

15 ft. 1.67:1 37.2 in. .113 in. / 100 ft.

20 ft. 2.22:1 34.5 in. .156 in. / 100 ft.

Figure 8

GAS

DRYER

(N.I.C.)

WASHER

(N.I.C.)

120 SQ FT

160 SQ FT

60 SQ FT

120 SQ FT

450 SQ FT

90 SQ FT

120 SQ FT

50 SQ FT

190 SQ FT

50 SF

90 SQ FT

GAS

DRYER

(N.I.C.)

WASHER

(N.I.C.)

120 SQ FT

160 SQ FT

60 SQ FT

120 SQ FT

450 SQ FT

90 SQ FT

120 SQ FT

50 SQ FT

190 SQ FT

50 SF

90 SQ FT

95 CFM

45 CFM

25CFM

45 CFM

60 CFM225 CFM

25 CFM

60 CFM

80 CFM

30 CFM

60 CFM

GAS

DRYER

(N.I.C.)

WASHER

(N.I.C.)

120 SQ FT

160 SQ FT

60 SQ FT

120 SQ FT

450 SQ FT

90 SQ FT

120 SQ FT

50 SQ FT

190 SQ FT

50 SF

90 SQ FT

95 CFM

45 CFM

25CFM

45 CFM

60 CFM225 CFM

25 CFM

60 CFM

80 CFM

30 CFM

60 CFM

GAS

DRYER

(N.I.C.)

WASHER

(N.I.C.)

120 SQ FT

160 SQ FT

60 SQ FT

120 SQ FT

450 SQ FT

90 SQ FT

120 SQ FT

50 SQ FT

190 SQ FT

50 SF

90 SQ FT

95 CFM

45 CFM

25CFM

45 CFM

60 CFM225 CFM

25 CFM

60 CFM

80 CFM

30 CFM

60 CFM

GAS

DRYER

(N.I.C.)

WASHER

(N.I.C.)

120 SQ FT

160 SQ FT

60 SQ FT

120 SQ FT

450 SQ FT

90 SQ FT

120 SQ FT

50 SQ FT

190 SQ FT

50 SF

90 SQ FT

95 CFM

45 CFM

25CFM

45 CFM

60 CFM

225 CFM

25 CFM

60 CFM

80 CFM

30 CFM

60 CFM12/8

18/8

8/8

8/8

GASDRYER

(N.I.C.)

WASHER(N.I.C.)

120 SQ FT

160 SQ FT

60 SQ FT

120 SQ FT

450 SQ FT

90 SQ FT

120 SQ FT

50 SQ FT

190 SQ FT

50 SF

90 SQ FT

95 CFM

45 CFM

25CFM

45 CFM

60 CFM225 CFM

25 CFM

60 CFM

80 CFM

30 CFM

60 CFM8/8

12/8

8/8

Select and Adjust Fan

• The total static pressure is the pressure that the fan

must overcome to deliver the required amount of air.

This total amount is also used to select the required fan

motor size or fan speed.

• Perform air balance next. Per Comm 23.18

• Measured airflow values should be compared against

the required values.

• Adjust fan and balancing dampers as necessary to

deliver the correct amount of air to each zone.

Duct Insulation

• 22.17 UDC Duct system insulation. (1)

except as provided in sub. (4), all heating

and cooling duct systems, or portions

thereof, that are located in unheated or

uncooled spaces respectively, shall be

provided with insulation with a thermal

resistance of at least R-5.

Duct Insulation cont.

• R-4.2 and R-6 are common industry

standards for insulated flex

• R-5 and R-8 are common industry standards

for duct wrap

Installation of Underground

Ducts SMACNA Standards

Installation of Underground

Ducts cont.

Installation of Underground

Ducts cont.

• 23.08 UDC supply air ducts shall be

insulated with a moistureproof material

having a resistance value of a least R-5.

Underground Duct cont.

Underground Ducts cont.

• Uninsulated duct will require 152 btuh per linear foot of duct for heating

• 1” insulated duct will require 91 btuh per linear foot of duct for heating (40% less energy)

• 2” insulated duct will require 74 btuh per linear foot of duct for heating (51% less energy)