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Ventilation

5 Fans

Vladimír Zmrhal (room no. 814)

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

EnvironmentalEnvironmental

EngineeringEngineering

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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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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

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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 Pa

Axial fans

� propeller

� tubeaxial

� vaneaxial

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Types of fans

According to using

� residential fans

� ducted fans

� jet fans

� roof fans

� smoke-ventilating fans

� hot-gas fans

� corrosion-resistant fans

� etc.

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Centrifugal fans

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

frames (6)

� scroll (diffuser) - kinetic energy → pressure energy

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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

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� 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

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a) b) c) d)

Centrifugal fans

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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

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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

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Jet fans

Axial fans

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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

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� 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

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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∆=

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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]

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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∆= =⋅

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∆p = f (V)P = f (V)

ηtot = f (V)

Fan performance curves

n = const.

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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

=

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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 c

c

pt

cη

ρ η∆∆ = −

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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]

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Fan and system pressure relationship

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� dynamic pressure

� total pressure

� total pressure difference of the fan

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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

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Fan and system pressure relationship

2 0lp∆ =

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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

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Working

point

System and fan charakteristics

System

curve

Fan

charakteristics

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Parallel fan operation

System curve

Fan charakteristics

2 fan operation

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Serial fan operation

System curve

Fan charakteristics

2 fan operation

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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

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Fan characteristic change

� economical

� speed control n

Control of the fans

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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

( )1201

fn s

p= −

Control of the fans

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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

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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

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Thank you for your

attention