Ductwork Systems and Supply
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Transcript of 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 ductworkUnderstand sizing units
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Ductwork Considerations
ShapeSizeAspect ratioVelocityStatic Pressure
of the systemSpace conflicts
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Terms
BalancedVAV
Aspect ratioVelocityStatic Pressure
of the system
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Formulas
Btu H / 1.08 (TD) = CFMWhat 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)] = CFM20,000/[1.08 (130-70)] = CFM20,000 / 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)] = CFM10,000/[1.08 (72-55)] = CFM10,000 / 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 HLatent 534,810 Btu HTotal 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)] = CFM2,324,056 /[1.08 (130-70)] = CFM2,324,056 / 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)] = CFM1,249,807 /[1.08 (72-55)] = CFM1,249,807 /18.36= 68,072 CFM
Need a duct size of 60” x 100” W D
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That’s Huge!
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Zoning
Activity CenterBecause in this case cooling duct size govern, we need to zone accordingly.
2,324,056 Btu H cooling load775,700 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 AExample Gym:
553,681 CF volume =71% of the space 1,249,807 Cooling load
71% of 1,249,807 = 887,363 Btu HBtu H Heat Gain (Cooling)
887,363887,363/18.36= 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 6Air 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 PressureA 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
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 relationshipDuct dimensions are inside of duct.
W
D
Aspect ratio = W / D
1 to 1
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Prof. Kirk’s 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