Ductwork Systems and Supply
Arch 432
What You Need To Know
Become familiar with the materials
sizing ductwork
Understand sizing units
Ductwork Considerations
Shape
Size
Aspect ratio
Velocity
Static Pressure
of the system
Space conflicts
Terms
Balanced
VAV
Aspect ratio
Velocity
Static Pressure
of the system
Formulas
Btu H / 1.08 (TD) = CFM
What is the TD in this case?
•
Temperature leaving the heating or cooling coils
and the indoor design temperature
What is the 1.08?

It is the number of Btu H required to increase the
temperature of 1 CFM o f air by 10 F
Four Steps for Air Distribution
1) Multiply total tons by 400 to estimate total
CFM.
2) Distribute CFM in proportion to floor area.
3) Adjust CFM to reflect anticipated variations in
Btu H due to specific building conditions.
4) Round off air supply for each room to nearest
25 CFM.
Duct Sizing
Duct Size CFM
= cubic feet of air per minute
Duct size listed as W x D
See hand out
Example
A room has a peak winter sensible heat loss of
20,000 Btu H. How much 1300F air must be
supplied to maintain room temperature at
700F?
Btu H /[(1.08)(TD)] = CFM
20,000/[1.08 (130-70)] = CFM
20,000 / 64.8 = 308.64 = 325 CFM
Duct size - 350 CFM 6 x13 8x9 10x7 12x6 14x6
Example
The same room has a peak summer sensible
heat gain of 10,000 Btu H. How much 550F
air must be supplied to maintain room
temperature at 720F?
Btu H /[(1.08)(TD)] = CFM
10,000/[1.08 (72-55)] = CFM
10,000 / 18.36 = 544.66 = 550 CFM
Duct size – 600 CFM 6x20 8x14 10x12 12x10
14x8 & 16x7
Which Duct Size Do I use?
The bigger one!
Design around 600 CFM
If ceiling space is not a problem choose
the 10” x 12” W X D
If space is a problem choose another
BUT be careful.
Activity Center
Btu H Heat Loss 2,324,056 Btu H
Btu H Heat Gain (Cooling)
Sensible 714,997 Btu H
Latent 534,810 Btu H
Total 1,249,807 Btu H
Activity Center
Btu H Heat Loss 2,324,056 Btu H
Btu H /[(1.08)(TD)] = CFM
2,324,056 /[1.08 (130-70)] = CFM
2,324,056 / 64.8 = 35,865 CFM
Need a duct size of 60 x 52
W
D
Activity Center
Btu H Heat Gain (Cooling)
1,249,807 Btu H
Btu H/[(1.08)(TD)] = CFM
1,249,807 /[1.08 (72-55)] = CFM
1,249,807 /18.36= 68,072 CFM
Need a duct size of 60” x 100”
W
D
That’s Huge!
Zoning
Activity Center
Because in this case cooling duct size
govern, we need to zone accordingly.
2,324,056 Btu H cooling load
775,700 cubic feet


Gym 553,681 c.f.
Remainder 222,019 c.f.
Zoning
There are at least two zones


Gym – Zone A
Remainder
 Fitness rooms
 Activity spaces
 Locker rooms and restrooms
 Lobby
 Offices
Zone
Zone
Zone
Zone
Zone
B
B
C?
D
D?
Activity Center
Zoning A
Example Gym:
553,681 CF volume =71% of the space
1,249,807 Cooling load
71% of 1,249,807 = 887,363 Btu H
Btu H Heat Gain (Cooling)
887,363
887,363/18.36= 48,331 CFM
Duct size of 60 x 100 leaving the system
Zone for Gym
Zoning Gym
12 units in gym



48,331 CFM Needed / 12 = 4,028 CFM
4,028 CFM per unit
Duct size for each unit
20x24, 24 x20, 28 x16 …..
Activity Center
From slide number 6
Air handlers typically circulate about 400 CFM for each
ton of capacity.
On ton of air conditioning capacity is defined as a heat
removal rate of 12,000 Btu H
48,331/12,000 = 4 tons
Duct Sizing
Duct size of 20 x 24
- or Duct size of 24 x 26
“Velocity pressure is the directional push of an air
stream due to its speed” Trost
So we trade static pressure and velocity pressure
by changing duct size.
Duct sizing
Duct size 20 x 24 or Duct size 24 x 26
Static plus Velocity = Total Pressure
A decease in duct size forces air speed and
velocity pressure to increase as static
pressure decreases.
Enlarging a duct will cause air speed and velocity
pressure to decrease as static pressure
increases.
Facts
• Insulation ½” 1” or 2”
• Add two times the thickness of the duct
•
•
•
insulation specified.
Size return air ducts 2” larger than supply
ducts.
Reserve 4% of the total building floor area to
accommodate air handlers.
Reserve 2% of the total building floor area for
boilers, chillers, pumps, and electrical
switchgear.
Rectangular
W
Lower aspect ratios

D

1 to 1
Aspect ratio = W / D
More energy efficient
Use less ductwork
Velocity and Area have
an inverse relationship
to the CFM (CFM = V x
A)
Velocity and Pressure
Drop have a direct
relationship
Duct dimensions are
inside of duct.
Prof. Kirk’s one-of-a-kind,
surefire process guaranteed to
result in a mind-numbing law
suit.
When Things Go Wrong
Mechanical
Contractor required
to certify that duct
plenums were
adequately
protected
When Things Go Wrong