Ductwork Systems and Supply

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Ductwork Systems and Supply

• Arch 432

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What You Need To Know

• Become familiar with the materials sizing
  ductwork
• Understand sizing units

3
Ductwork Considerations

• Shape
• Size
• Aspect ratio
• Velocity
• Static Pressure
• of the system
• Space conflicts

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Terms

• Balanced
• VAV

• Aspect ratio
• Velocity
• Static Pressure
• of the system

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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

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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.

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Duct Sizing

• Duct Size CFM cubic feet of air per minute
• Duct size listed as W x D
• See hand out

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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

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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

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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.

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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

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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

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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

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Thats Huge!
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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.

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Zoning

• There are at least two zones
• Gym Zone A
• Remainder
• Fitness rooms Zone B
• Activity spaces Zone B
• Locker rooms and restrooms Zone C?
• Lobby Zone D
• Offices Zone D?

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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

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Zone for Gym
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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 ..

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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

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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.

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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.

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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.

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Rectangular
W

• Lower aspect ratios
• 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.

D
1 to 1
Aspect ratio W / D
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Prof. Kirks one-of-a-kind, surefire process
guaranteed to result in a mind-numbing law suit.
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When Things Go Wrong

• Mechanical Contractor required to certify that
  duct plenums were adequately protected

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When Things Go Wrong