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    MECHANICAL VENTILATION

                    Compiled by

             Mohd Rodzi Ismail
         School of Housing Building & Planning
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       • Indoor air is withdrawn and replaced by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
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
3

    VENTILATION
    REQUIREMENTS
    Control of ventilation rates - normally
    based on recommendations by
    authorities or code of practice.
    e.g. BS 5720
4




    Table 2.0 - Air changes rates (BS 5720)
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
6

    MECHANICAL
    VENTILATION
    An alternative to the unreliable natural
    systems
    Components involved:
      Fan
      Filters
      Ductwork
      Fire dampers
      Diffusers
7




    Table 1.0 - Fresh air supply rates (BS 5720)
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
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)
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)
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)
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.
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 )
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
15




      Types of fan
          Cross-flow or tangential
     1.
          Propeller
     2.
          Axial flow
     3.
          Centrifugal
     4.
16




     ●   Cross-flow or tangential fan




         Tangential or cross-flow fan
17




     Tangential flow fan
18




     How tangential flow fans work
19




                     Propeller fan




     Free standing propeller fan     Wall mounted propeller fan
20




     Types of propeller fans
21




                Axial flow fan
                                          To protect the fan-cooled
                                          motor in greasy, hot &
                                          corrosive gas situations




     Axial flow fan              Bifurcated axial flow fan
22
23




     Types of axial flow fans
24




                  Counter rotating
     Heavy duty
25




     Bifurcated axial-flow fan
26




     Centrifugal fan




               Centrifugal fan
27




              Air out




     Air in
28




     Centrifugal fan impellers
29
30




                        Wall type
     Centrifugal fans
31




                                                      Tubular
     HVAC duty centrifugal fan   Industrial duty
                                                   centrifugal fan
                                 centrifugal fan
32



     Filters

        Four categories of filters
            Dry
       1.
            Viscous
       2.
            Electrostatic
       3.
            Activated carbon
       4.
33




       Dry filters




     Roll filter     Disposable element filter
34
35
36




     Viscous filters




        Viscous filter
37
38




     Electrostatic filters




                Electrostatic filter
39
40




     Activated carbon filters




            Commercial cooker hood
41
42



     HEPA filters
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
44




     Table 3.0 - Ductwork data
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
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
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
48




     Duct conversion – using conversion chart (simpler
     but less accurate)




         Circular to rectangular ductwork conversion chart
49




       Noise control




     Sound attenuation
50




     Table 4.0 - Recommended maximum ducted air velocities
                 and resistance for accepted levels of noise
51




     Volume &
     direction control




     Air movement control
52



     Fire dampers




      Fire dampers
53



     Diffusers




      Grills &
      diffusers
54




     Diffusers
       airflow
     patterns
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
56




     Suspended ceilings as plenum chambers
57



     SYSTEMS

     Mechanical ventilation systems
       Mechanical extract/natural supply
       Mechanical supply/natural supply
       Combined mechanical extract &
       supply
58

     Mechanical extract/natural
     supply




          Extract ventilation to a commercial kitchen
59




     Extract ventilation to a lecture theatre
60




     Application of shunt ducts to a block of flats
61

     Mechanical supply/natural
     supply




      Plenum ventilation
      system
62

     Combined mechanical
     extract & supply




         Combined mechanical extract and supply
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
64




     Duct design chart
65




           Example 4 (ventilation design calculation)




     Q, air volume flow rate (m3/s) = Room volume x air changes per hour
                                                Time in seconds
66




     Given
       Room volume = 480 m3
       Air changes per hour = 6

     Therefore

         480 x 6
      Q=         = 0 .8 m 3 / s
          3600
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
68




                     (0.4 m3/s)
        (0.8 m3/s)




     0.4 m3/s        0.4 m3/s
69



                                  450



                                  320



     From the design chart:
                              A
     • Duct A = 450 mm Ø
     • Duct B = 320 mm Ø
                              B
70




     From duct design
     chart (equal
     velocity method)
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.
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
73




     From duct design
     chart (Velocity
     reduction method)
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.
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
76




     From duct design
     chart (Equal friction
     method)
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
78




     Duct design chart
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
80



     REFERENCES

     Greeno, R.(1997). Building Services,
     Technology and Design. Essex:
     Longman.
     Hall, F. & Greeno, R. (2005). Building
     Services Handbook. Oxford: Elsevier.
81



     QUIZ

      Name 5 purposes of ventilation
      What is “coanda effect”?

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Mechanical Ventilation

  • 1. 0 MECHANICAL VENTILATION Compiled by Mohd Rodzi Ismail School of Housing Building & Planning
  • 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. 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. 3 VENTILATION REQUIREMENTS Control of ventilation rates - normally based on recommendations by authorities or code of practice. e.g. BS 5720
  • 5. 4 Table 2.0 - Air changes rates (BS 5720)
  • 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. 6 MECHANICAL VENTILATION An alternative to the unreliable natural systems Components involved: Fan Filters Ductwork Fire dampers Diffusers
  • 8. 7 Table 1.0 - Fresh air supply rates (BS 5720)
  • 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. 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. 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. 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. 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. 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. 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. 15 Types of fan Cross-flow or tangential 1. Propeller 2. Axial flow 3. Centrifugal 4.
  • 17. 16 ● Cross-flow or tangential fan Tangential or cross-flow fan
  • 18. 17 Tangential flow fan
  • 19. 18 How tangential flow fans work
  • 20. 19 Propeller fan Free standing propeller fan Wall mounted propeller fan
  • 21. 20 Types of propeller fans
  • 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. 22
  • 24. 23 Types of axial flow fans
  • 25. 24 Counter rotating Heavy duty
  • 26. 25 Bifurcated axial-flow fan
  • 27. 26 Centrifugal fan Centrifugal fan
  • 28. 27 Air out Air in
  • 29. 28 Centrifugal fan impellers
  • 30. 29
  • 31. 30 Wall type Centrifugal fans
  • 32. 31 Tubular HVAC duty centrifugal fan Industrial duty centrifugal fan centrifugal fan
  • 33. 32 Filters Four categories of filters Dry 1. Viscous 2. Electrostatic 3. Activated carbon 4.
  • 34. 33 Dry filters Roll filter Disposable element filter
  • 35. 34
  • 36. 35
  • 37. 36 Viscous filters Viscous filter
  • 38. 37
  • 39. 38 Electrostatic filters Electrostatic filter
  • 40. 39
  • 41. 40 Activated carbon filters Commercial cooker hood
  • 42. 41
  • 43. 42 HEPA filters
  • 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. 44 Table 3.0 - Ductwork data
  • 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. 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. 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. 48 Duct conversion – using conversion chart (simpler but less accurate) Circular to rectangular ductwork conversion chart
  • 50. 49 Noise control Sound attenuation
  • 51. 50 Table 4.0 - Recommended maximum ducted air velocities and resistance for accepted levels of noise
  • 52. 51 Volume & direction control Air movement control
  • 53. 52 Fire dampers Fire dampers
  • 54. 53 Diffusers Grills & diffusers
  • 55. 54 Diffusers airflow patterns
  • 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. 56 Suspended ceilings as plenum chambers
  • 58. 57 SYSTEMS Mechanical ventilation systems Mechanical extract/natural supply Mechanical supply/natural supply Combined mechanical extract & supply
  • 59. 58 Mechanical extract/natural supply Extract ventilation to a commercial kitchen
  • 60. 59 Extract ventilation to a lecture theatre
  • 61. 60 Application of shunt ducts to a block of flats
  • 62. 61 Mechanical supply/natural supply Plenum ventilation system
  • 63. 62 Combined mechanical extract & supply Combined mechanical extract and supply
  • 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. 64 Duct design chart
  • 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. 66 Given Room volume = 480 m3 Air changes per hour = 6 Therefore 480 x 6 Q= = 0 .8 m 3 / s 3600
  • 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. 68 (0.4 m3/s) (0.8 m3/s) 0.4 m3/s 0.4 m3/s
  • 70. 69 450 320 From the design chart: A • Duct A = 450 mm Ø • Duct B = 320 mm Ø B
  • 71. 70 From duct design chart (equal velocity method)
  • 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. 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. 73 From duct design chart (Velocity reduction method)
  • 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. 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. 76 From duct design chart (Equal friction method)
  • 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. 78 Duct design chart
  • 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. 80 REFERENCES Greeno, R.(1997). Building Services, Technology and Design. Essex: Longman. Hall, F. & Greeno, R. (2005). Building Services Handbook. Oxford: Elsevier.
  • 82. 81 QUIZ Name 5 purposes of ventilation What is “coanda effect”?