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Mechanical Ventilation

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Mechanical Ventilation

  1. 1. 0 MECHANICAL VENTILATION Compiled by Mohd Rodzi Ismail School of Housing Building & Planning
  2. 2. 1 INTRODUCTION Definition “the process of changing air in an enclosed space” • Indoor air is withdrawn and replaced by fresh air continuously • From clean external source
  3. 3. 2 The importance of ventilation – to maintain air purity, i.e.: preservation of O2 content – this should be maintained at approximately 21% of air volume removal of CO2 control of humidity – between 30 & 70% RH is acceptable for human comfort prevention of heat concentrations from machinery, lighting and people prevention of condensation dispersal of concentrations of bacteria dilution and disposal of contaminants such as smoke, dust gases and body odours provisions of freshness – an optimum air velocity lies between 0.15 and 0.5 ms-1
  4. 4. 3 VENTILATION REQUIREMENTS Control of ventilation rates - normally based on recommendations by authorities or code of practice. e.g. BS 5720
  5. 5. 4 Table 2.0 - Air changes rates (BS 5720)
  6. 6. 5 Conversion from “m3/hour per person” to “air changes per hour” Air supply rate x nos. occupants Room volume Example 1 A private office of 30 m3 volume designed for 2 people 43 x 2 = 2.86 air changes per hour 30
  7. 7. 6 MECHANICAL VENTILATION An alternative to the unreliable natural systems Components involved: Fan Filters Ductwork Fire dampers Diffusers
  8. 8. 7 Table 1.0 - Fresh air supply rates (BS 5720)
  9. 9. 8 Fans Provide the motive for air movement (imparting static energy or pressure and kinetic energy or velocity) It’s capacity for air movement depends on Type Size Shape Number of blades Speed
  10. 10. 9 Basic law of fan capabilities (at a constant air density): Volume of air varies in direct proportion to 1. the fan speed, i.e. Q2 N 2 = Q1 N1 where, • Q = volume of air (m3/s) • N = fan impeller (rpm)
  11. 11. 10 Pressure of, or resistance to, air 2. movement is proportional to fan speed squared, i.e. P2 ( N 2 ) 2 = P ( N1 ) 2 1 where, • P = pressure (Pa)
  12. 12. 11 Air and impeller power is proportional to 3. fan speed cubed, i.e. W2 ( N 2 )3 = W1 ( N1 ) 3 where, • W = power (W or kW)
  13. 13. 12 Example 2 A fan of 2kW power discharges 4 m3/s with impellers rotating at 1000 rpm to produce a pressure of 250 Pa. If the fan impeller speed increases to 1250 rpm, calculate Q, P and W.
  14. 14. 13 Q 2 1250 Q2 N 2 = = 1. therefore, Q2 = 5 m3/s Q1 N1 4 1000 (1250 ) 2 P2 ( N 2 ) 2 P2 = 2. = 2 therefore, P2 = 390 Pa P1 ( N 1 ) 2 250 (1000 ) W 2 (1250 ) 3 3 3. W2 (N 2 ) = = therefore, W2 = 3.9 kW 3 W1 ( N 1 ) 3 2 (1000 )
  15. 15. 14 As fans are not totally efficient, the following formula may be applied to determine the percentage Total fan pressure x air volume 100 Efficiency = x Absorbed power (W) 1 So, for the previous example, 390 x 5 100 Efficiency = = 50% x 3900 1
  16. 16. 15 Types of fan Cross-flow or tangential 1. Propeller 2. Axial flow 3. Centrifugal 4.
  17. 17. 16 ● Cross-flow or tangential fan Tangential or cross-flow fan
  18. 18. 17 Tangential flow fan
  19. 19. 18 How tangential flow fans work
  20. 20. 19 Propeller fan Free standing propeller fan Wall mounted propeller fan
  21. 21. 20 Types of propeller fans
  22. 22. 21 Axial flow fan To protect the fan-cooled motor in greasy, hot & corrosive gas situations Axial flow fan Bifurcated axial flow fan
  23. 23. 22
  24. 24. 23 Types of axial flow fans
  25. 25. 24 Counter rotating Heavy duty
  26. 26. 25 Bifurcated axial-flow fan
  27. 27. 26 Centrifugal fan Centrifugal fan
  28. 28. 27 Air out Air in
  29. 29. 28 Centrifugal fan impellers
  30. 30. 29
  31. 31. 30 Wall type Centrifugal fans
  32. 32. 31 Tubular HVAC duty centrifugal fan Industrial duty centrifugal fan centrifugal fan
  33. 33. 32 Filters Four categories of filters Dry 1. Viscous 2. Electrostatic 3. Activated carbon 4.
  34. 34. 33 Dry filters Roll filter Disposable element filter
  35. 35. 34
  36. 36. 35
  37. 37. 36 Viscous filters Viscous filter
  38. 38. 37
  39. 39. 38 Electrostatic filters Electrostatic filter
  40. 40. 39
  41. 41. 40 Activated carbon filters Commercial cooker hood
  42. 42. 41
  43. 43. 42 HEPA filters
  44. 44. 43 Ductwork Circular, square or rectangular cross-sections More efficient, less frictional resistance to airflow Convenience, more easily fitted into building fabric Circular & rectangular ductwork
  45. 45. 44 Table 3.0 - Ductwork data
  46. 46. 45 Duct conversion For equal velocity of flow 2ab d= a+b For equal volume of flow 0.2 ⎡ (ab) ⎤ 3 d = 1.265 x ⎢ ⎥ a+b⎦ ⎣ where • d = diameter of circular duct (mm) • a = longest side of rectangular duct (mm) • b = shortest side of rectangular duct (mm) • 0.2 = fifth root
  47. 47. 46 Example 3 (duct conversion) A 450 mm diameter duct converted to rectangular profile of aspect ratio 2 : 1 (a = 2b). For equal velocity of flow: 2 x 2b x b 4b 2 4b 2ab d= 450 = = = a+b 2b + b 3b 3 3 x 450 b= Therefore, b = 337.5 mm and a = 2b = 675 mm 4
  48. 48. 47 0.2 ⎡ (ab) ⎤ 3 For equal volume of flow: d = 1.265 x ⎢ ⎥ a+b⎦ ⎣ 3 0.2 ⎡ (2b x b) ⎤ 450 = 1.265 x ⎢ ⎥ ⎣ 2b + b ⎦ 0.2 ⎡ (2b ) ⎤ 23 450 = 1.265 x ⎢ ⎥ ⎣ 3b ⎦ From this, b = 292 mm and a = 2b = 584 mm
  49. 49. 48 Duct conversion – using conversion chart (simpler but less accurate) Circular to rectangular ductwork conversion chart
  50. 50. 49 Noise control Sound attenuation
  51. 51. 50 Table 4.0 - Recommended maximum ducted air velocities and resistance for accepted levels of noise
  52. 52. 51 Volume & direction control Air movement control
  53. 53. 52 Fire dampers Fire dampers
  54. 54. 53 Diffusers Grills & diffusers
  55. 55. 54 Diffusers airflow patterns
  56. 56. 55 “Coanda effect” – created by restricted air and pressure at the adjacent surface due to limited access for air to replace the entrained air above the plume
  57. 57. 56 Suspended ceilings as plenum chambers
  58. 58. 57 SYSTEMS Mechanical ventilation systems Mechanical extract/natural supply Mechanical supply/natural supply Combined mechanical extract & supply
  59. 59. 58 Mechanical extract/natural supply Extract ventilation to a commercial kitchen
  60. 60. 59 Extract ventilation to a lecture theatre
  61. 61. 60 Application of shunt ducts to a block of flats
  62. 62. 61 Mechanical supply/natural supply Plenum ventilation system
  63. 63. 62 Combined mechanical extract & supply Combined mechanical extract and supply
  64. 64. 63 VENTILATION DESIGN Three methods of designing ductwork and fan: Equal velocity method • the designer selects the same air velocity for use through out the system Velocity reduction method • the designer selects variable velocities appropriate to each section or branch of ductwork Equal friction method • the air velocity in the main duct is selected and the size and friction determined from a design chart. The same frictional resistance is used for all other sections of ductwork
  65. 65. 64 Duct design chart
  66. 66. 65 Example 4 (ventilation design calculation) Q, air volume flow rate (m3/s) = Room volume x air changes per hour Time in seconds
  67. 67. 66 Given Room volume = 480 m3 Air changes per hour = 6 Therefore 480 x 6 Q= = 0 .8 m 3 / s 3600
  68. 68. 67 Equal velocity method Air velocity throughout the system (duct A & duct B) = 5 m/s (selected based on Table 4.0) Q, the quantity of air = 0.4 m3/s is equally extracted through grille Duct A will convey 0.8 m3/s; duct B will convey 0.4 m3/s
  69. 69. 68 (0.4 m3/s) (0.8 m3/s) 0.4 m3/s 0.4 m3/s
  70. 70. 69 450 320 From the design chart: A • Duct A = 450 mm Ø • Duct B = 320 mm Ø B
  71. 71. 70 From duct design chart (equal velocity method)
  72. 72. 71 The fan rating relates to the frictional resistance obtained in N/m2 or Pa per unit length of ductwork From the design chart Duct A = 0.65 Pa x 5 m effective duct length = 3.25 Pa Duct B = 1.00 Pa x 10 m effective duct length = 10.00 Pa Total = 13.25 Pa Therefore, the fan rating or specification is 0.8 m3/s at 13.25 Pa Effective duct length – the actual length plus additional allowances for bends, offsets, dampers, etc.
  73. 73. 72 Velocity reduction method Selected air velocity in duct A = 6 m/s Selected air velocity in duct B = 3 m/s Q, the quantity of air = 0.4 m3/s is equally extracted through grille Duct A will convey 0.8 m3/s; duct B will convey 0.4 m3/s From the design chart Duct A and B are both coincidentally 420 mm Ø
  74. 74. 73 From duct design chart (Velocity reduction method)
  75. 75. 74 Friction in duct A = 1.00 Pa x 5 m = 5.0 Pa Friction in duct B = 0.26 Pa x 10 m = 2.6 Pa Total = 7.6 Pa Therefore, the fan rating or specification is 0.8 m3/s at 7.6 Pa Effective duct length – the actual length plus additional allowances for bends, offsets, dampers, etc.
  76. 76. 75 Equal friction method Selected air velocity through duct A = 5 m/s Calculated airflow through duct A = 0.8 m3/s Calculated airflow through duct B = 0.4 m3/s From the chart: Duct A at 0.8 m3/s = 450 Ø with a frictional resistance of 0.65 Pa/m Duct B (using the same friction) at 0.4 m3/s = 350 Ø with an air velocity of approximately 4.2 m/s The fan rating is 0.8 m3/s at 0.65 Pa/m x 15 m = 9.75 Pa
  77. 77. 76 From duct design chart (Equal friction method)
  78. 78. 77 Determination of sufficient air changes e.g.: Library (max. velocity of 2.5 m/s with a max. resistance of 0.4 Pa/m length) – from Table 4.0 From the chart: Maximum air discharged, Q = 0.1 m3/s Duct size = 225 mm Ø
  79. 79. 78 Duct design chart
  80. 80. 79 From Q = Room volume x air changes per hour Time in seconds and, Air changes per hour = Q x time seconds Room volume = 0.1 x 3600 180 Thus, 2 changes per hour would be provided
  81. 81. 80 REFERENCES Greeno, R.(1997). Building Services, Technology and Design. Essex: Longman. Hall, F. & Greeno, R. (2005). Building Services Handbook. Oxford: Elsevier.
  82. 82. 81 QUIZ Name 5 purposes of ventilation What is “coanda effect”?

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