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Ductwork Systems and Supply

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. Balanced VAV. Aspect ratio Velocity

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Ductwork Systems and Supply

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  1. Ductwork Systems and Supply Arch 432

  2. 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

  4. Balanced VAV Aspect ratio Velocity Static Pressure of the system Terms

  5. 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 Formulas

  6. Multiply total tons by 400 to estimate total CFM. Distribute CFM in proportion to floor area. Adjust CFM to reflect anticipated variations in Btu H due to specific building conditions. Round off air supply for each room to nearest 25 CFM. Four Steps for Air Distribution

  7. Duct Size CFM = cubic feet of air per minute Duct size listed as W x D See hand out Duct Sizing

  8. 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

  9. 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 = 550CFM Duct size – 600 CFM 6x20 8x1410x12 12x10 14x8 & 16x7 Example

  10. 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.

  11. 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

  12. 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

  13. 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 Activity Center

  14. That’s Huge!

  15. 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.

  16. 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?

  17. 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 Activity Center

  18. Zone for Gym

  19. 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 …..

  20. 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 Activity Center

  21. 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

  22. 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. Duct sizing

  23. 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. Facts

  24. 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

  25. Prof. Kirk’s one-of-a-kind, surefire process guaranteed to result in a mind-numbing law suit.

  26. When Things Go Wrong • Mechanical Contractor required to certify that duct plenums were adequately protected

  27. When Things Go Wrong

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