Duct Design - biasew.net

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
SUPPLY
A coil
.25” sp
FAN
COIL
OR
FURN
ACE
R.A.
BOX
RAG CONT
G35W
S.P. 0.045
RETURN AIR
SAG CONT 32W
S.P. 0.04
FILTER
S.P. 0.07
FURNACE
NO SCALE
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
Ratio of
Perimeter Perimeter
to Area
9.03 ft2
40.7 in.
3 ft.
9.0 ft2
Equivalent
Round Duct
Friction
at
15,000 CFM
10.65 ft.
1.18:1
40.7 in.
.07 in. / 100 ft.
12 ft.
1.33:1
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.
3 ft.
9.0 ft2
2 ft.
4-1/2 ft.
1-1/2 ft.
1 ft.
9.0 ft2
6 ft.
9 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
SUPPLY
A coil
.25” sp
FAN
COIL
OR
FURN
ACE
R.A.
BOX
RAG CONT
G35W
S.P. 0.045
RETURN AIR
SAG CONT 32W
S.P. 0.04
FILTER
S.P. 0.07
FURNACE
NO SCALE
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
Duct Calculator
Airflow
(CFM)
Velocity
Pressure
Friction
Loss
Round Duct
Diameter
Equivalent
Rectangular
Duct Sizes
Figure 7
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
Ratio of
Perimeter Perimeter
to Area
9.03 ft2
40.7 in.
3 ft.
9.0 ft2
Equivalent
Round Duct
Friction
at
15,000 CFM
10.65 ft.
1.18:1
40.7 in.
.07 in. / 100 ft.
12 ft.
1.33:1
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.
3 ft.
9.0 ft2
2 ft.
4-1/2 ft.
1-1/2 ft.
1 ft.
9.0 ft2
6 ft.
9 ft.
Figure 8
90 SQ FT
190 SQ FT
50 SF
90 SQ FT
450 SQ FT
120 SQ FT
120 SQ FT
WASHER
(N.I.C.)
50 SQ FT
GAS
DRYER
(N.I.C.)
60 SQ FT
160 SQ FT
120 SQ FT
90 SQ FT
45 CFM
190 SQ FT
95 CFM
50 SF
25CFM
90 SQ FT
45 CFM
450 SQ FT
225 CFM
120 SQ FT
60 CFM
120 SQ FT
60 CFM
WASHER
(N.I.C.)
50 SQ FT
25 CFM
GAS
DRYER
(N.I.C.)
60 SQ FT
30 CFM
160 SQ FT
80 CFM
120 SQ FT
60 CFM
90 SQ FT
45 CFM
190 SQ FT
95 CFM
50 SF
25CFM
90 SQ FT
45 CFM
450 SQ FT
225 CFM
120 SQ FT
60 CFM
120 SQ FT
60 CFM
WASHER
(N.I.C.)
50 SQ FT
25 CFM
GAS
DRYER
(N.I.C.)
60 SQ FT
30 CFM
160 SQ FT
80 CFM
120 SQ FT
60 CFM
90 SQ FT
45 CFM
190 SQ FT
95 CFM
50 SF
25CFM
90 SQ FT
45 CFM
450 SQ FT
225 CFM
120 SQ FT
60 CFM
120 SQ FT
60 CFM
WASHER
(N.I.C.)
50 SQ FT
25 CFM
GAS
DRYER
(N.I.C.)
60 SQ FT
30 CFM
160 SQ FT
80 CFM
120 SQ FT
60 CFM
90 SQ FT
45 CFM
190 SQ FT
95 CFM
50 SF
25CFM
8/8
90 SQ FT
45 CFM
450 SQ FT
225 CFM
120 SQ FT
60 CFM
18/8
120 SQ FT
60 CFM
12/8
WASHER
(N.I.C.)
50 SQ FT
25 CFM
GAS
DRYER
(N.I.C.)
8/8
60 SQ FT
30 CFM
160 SQ FT
80 CFM
120 SQ FT
60 CFM
90 SQ FT
45 CFM
190 SQ FT
95 CFM
8/8
450 SQ FT
225 CFM
50 SF
25CFM
90 SQ FT
45 CFM
120 SQ FT
60 CFM
12/8
120 SQ FT
60 CFM
8/8
WASHER
(N.I.C.)
50 SQ FT
25 CFM
GAS
DRYER
(N.I.C.)
60 SQ FT
30 CFM
160 SQ FT
80 CFM
120 SQ FT
60 CFM
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)