Ductwork Systems and Supply

Arch 432

What You Need To Know

Become familiar with the materials sizing ductworkUnderstand sizing units

Ductwork Considerations

ShapeSizeAspect ratioVelocityStatic Pressure

of the systemSpace conflicts

Terms

BalancedVAV

Aspect ratioVelocityStatic Pressure

of the system

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

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)] = CFM20,000/[1.08 (130-70)] = CFM20,000 / 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)] = CFM10,000/[1.08 (72-55)] = CFM10,000 / 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 HLatent 534,810 Btu HTotal 1,249,807 Btu H

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

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

That’s Huge!

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.

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?

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

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 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

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 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.

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

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

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