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05 VEN Fans - cvut.czzmrhavla/VENT/05_VEN_Fans.pdfTypes of fans According to using residential fans...

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1 1 Ventilation 5 Fans Vladimír Zmrhal (room no. 814) http://users.fs.cvut.cz/~zmrhavla/index.htm Dpt Dpt. . Of Of Environmental Environmental Engineering Engineering 2 Introduction Fans air pump that creates a pressure difference and causes airflow generally for gas transport rotary blade machine not only for HVAC applications Fan parameters total pressure difference p volume airflow rate V power input P
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  • 1

    1

    Ventilation

    5 Fans

    Vladimír Zmrhal (room no. 814)

    http://users.fs.cvut.cz/~zmrhavla/index.htm DptDpt. . OfOfEnvironmentalEnvironmental

    EngineeringEngineering

    2

    Introduction

    Fans

    � air pump that creates a pressure difference and causes airflow

    � generally for gas transport

    � rotary blade machine not only for HVAC applications

    Fan parameters

    � total pressure difference ∆p� volume airflow rate V

    � power input P

  • 2

    3

    Types of fans

    According to the direction of airflow

    � centrifugal fan

    � axial fan

    � mixed-flow fan

    � cross-flow fan

    Drive arrangements

    � direct drive

    � coupling drive

    � belt drive

    4

    Types of fans

    According to pressure rise across the fan

    Centrifugal fans

    � low pressure ∆p < 1000 Pa� medium pressure ∆p = 1000 to 3000 Pa� high pressure ∆p > 3000 PaAxial fans

    � propeller

    � tubeaxial

    � vaneaxial

  • 3

    5

    Types of fans

    According to using

    � residential fans

    � ducted fans

    � jet fans

    � roof fans

    � smoke-ventilating fans

    � hot-gas fans

    � corrosion-resistant fans

    � etc.

    6

    Centrifugal fans

    � impeller wheel (1), inlet (2), discharge (3), scroll (4), motor (5),

    frames (6)

    � scroll (diffuser) - kinetic energy → pressure energy

    2

    4 65

    3

    1

    14

    Centrifugal fans blow air at

    right angles to the intake of

    the fan; the impeller

    rotates, causing air to enter

    the fan near the shaft and

    move perpendicularly from

    the shaft to the opening in

    the scroll-shaped fan

    casing

  • 4

    7

    � impeller design

    a) forward-curved blades

    b) backward-curved blades

    c) radial blades

    d) aerofoil blades

    � low pressure fans with forward-curved blades – most widely used

    in ventilation and air-conditioning – lots of blades (40 to 50),

    ηm = 0,55 to 0,65� medium pressure and high pressure fans with backward-curved

    blades – small number of blades (6 to 15), ηm= 0,8 to 0,85

    Centrifugal fans

    8

    a) b) c) d)

    Centrifugal fans

  • 5

    9

    Axial fans

    � the axial-flow fans have blades that force air to move parallel to

    the shaft about which the blades rotate

    � because the discharge opening is inline with its entrance, and

    axial flow fan offers the advantage of simplified duct

    arrangement

    � wide variety of applications, ranging from small cooling fans for

    electronics to the giant fans used in wind tunnels

    � standard axial flow fans have diameters from 300–400 mm or

    1800 to 2000 mm and work under pressures up to 800 Pa

    10

    Propeller fans

    � low pressure fans

    Tubeaxial fans

    � generally considered to be heavy-duty

    propeller fans

    Vaneaxial fans

    � basically tube axial fans plus vanes.

    � behind the fan blades are vanes which

    straighten the spiral flow of air, thus

    increasing the static efficiency

    Axial fans

  • 6

    11

    Jet fans

    Axial fans

    12

    Mixed-flow fans

    � in a mixed flow fan the air flows in both axial and radial direction

    relative to the shaft.

    � mixed flow fans develops higher pressures than axial fans

    � the angle of leaving air < 90°

    � example: roof fans

  • 7

    13

    � also tangential fan or tubular fan

    � main flow moves transversely across the impeller, passing the

    blading twice

    � forward-curved blades, the fan is usually long in relation to the

    diameter L = (1 to 5)D

    � example: fan-coil units, ηm = 0,45 to 0,55

    Cross-flow fans

    14

    Volume airflow rate V [m3/s]

    � volume of air, which is transferred by fan

    � performance data are based on dry air at standard conditions

    101,325 kPa and 20 °C → ρ = 1,2 kg/m3

    Total pressure difference ∆∆∆∆p [Pa]� the fan have to pass the system pressure losses (static

    pressure)

    Electrical power P [W]

    Fan parameters

    tot

    V pP

    η∆=

  • 8

    15

    Specific fan power SFP [W/(m3/s)]

    tot

    P pSFP

    V η∆= =

    Fan parameters

    Energyco

    nsu

    mption[M

    Wh/a]

    Volume airflow rate V [m3/h]

    16

    Energy consumption

    [kWh/year]

    τ … working time of the fan [hours]

    00

    n

    totE Pd Pτ

    τ= =∑∫

    1 1 2 21 2

    1,1 ,2 ,

    1...

    1000 1000 1000

    ni i

    tot i

    tot tot tot i

    V p V p V pE τ τ τ

    η η η∆ ∆ ∆= + + =⋅ ⋅ ∑

    . .1000

    tot

    tot

    V pE SFPVτ τ

    η∆= =⋅

  • 9

    17

    ∆p = f (V)P = f (V)

    ηtot = f (V)

    Fan performance curves

    n = const.

    18

    Fan laws

    n = var.; ρρρρ = const. ρρρρ = var.; n = const

    22 1

    1

    nV V

    n= 2 1V V=

    2

    22 1

    1

    np p

    n

    ∆ = ∆

    3

    22 1

    1

    nP P

    n

    =

    22 1

    1

    p pρρ

    ∆ = ∆

    22 1

    1

    P Pρρ

    =

  • 10

    19

    Temperature rise across fans

    tot

    c

    p SFPt

    c cρ η ρ∆∆ = =

    P M Q= +

    tot tot

    P pVQ M pV V c tρ

    η η∆= − = − ∆ = ∆

    (1 )fan cc

    pt

    ρ η∆∆ = −

    20

    Temperature rise across fans

    Tem

    parature

    rise

    across

    thefan

    ∆∆ ∆∆t fa

    n

    Totaltem

    parature

    rise

    ∆∆ ∆∆t to

    t

    Total pressure difference ∆∆∆∆p [Pa] Total pressure difference ∆∆∆∆p [Pa]

  • 11

    21

    Fan and system pressure relationship

    22

    � dynamic pressure

    � total pressure

    � total pressure difference of the fan

    2

    2d

    wp ρ=

    2 1 1 2 1 2 2t t t t l l dp p p p p p p p∆ = − = ∆ + ∆ = ∆ + ∆ +

    t s dp p p= +

    Fan and system pressure relationship

  • 12

    23

    Fan and system pressure relationship

    2 0lp∆ =

    24

    Duct system charakteristics

    Pressure losses of ductwork

    � friction

    � local losses

    2 2 22

    2 2 2loss

    local lossesfriction

    l w w l wp KV

    d dλ ρ ζ ρ λ ζ ρ ∆ = + = + =

    ∑ ∑1424314243

    λ ρζπ

    ⋅ ⋅ = + ⋅ ∑ 2 4

    8lK

    d d

  • 13

    25

    Working

    point

    System and fan charakteristics

    System

    curve

    Fan

    charakteristics

    26

    Parallel fan operation

    System curve

    Fan charakteristics

    2 fan operation

  • 14

    27

    Serial fan operation

    System curve

    Fan charakteristics

    2 fan operation

    28

    System characteristic

    change

    � dampers

    � reducing of airflow by

    increasing the system

    pressure required

    � incresing power

    consumption

    2 2 2 '( )P V p p∆ = ∆ − ∆

    Control of the fans

  • 15

    29

    Fan characteristic change

    � economical

    � speed control n

    Control of the fans

    30

    a) variable speed motors

    � variable speed motors (twin or three speed motors)

    � number of motor poles p (s…slide of the motor)

    b) frequency control

    � optimal control

    � control in wide range od 0 do 100 %

    � for bigger systems

    c) voltage control

    ( )120 1fn sp

    = −

    Control of the fans

  • 16

    31

    d) EC motors

    � electronically commutated

    (EC) motors

    � brushless DC motors where

    the direction of the electric

    current is switched using

    electronic controllers

    � variable speed control,

    without the drawbacks of

    brushes

    � DCV (demand control

    ventilation systems)

    Control of the fans

    32

    Adjustable pitch-blades

    a) Controllable-pitch in suction of centrifugal fan

    � for backward-curve impellers only

    b) Controllable-pitch of impeller wheel

    � for axial fans

    � high operation efficiency

    � complicated mechanism a expensive

    Control of the fans

  • 17

    33

    Thank you for your

    attention


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