AIR DISTRIBUTION SYSTEM

FUNDAMENTALS

PRESSURE

• Pressure is the force exerted per unit area

• Pressure is the action of one force against another force. Pressure is force applied to, or distributed over, a surface.

• The pressure P of a force F istributed over an area A is defined as P = F/A

4#

Pressure Measurement Terms• Absolute Pressure

Measured above total vacuum or zero absolute. Zero absolute represents total lack of pressure.• Atmospheric Pressure

The pressure exerted by the earth’s atmosphere. Atmospheric pressure at sea level is 14.696 psia. The value of atmospheric pressure decreases with increasing altitude.

• Barometric PressureSame as atmospheric pressure.

• Gauge Pressure The pressure above atmospheric pressure. Represents positive difference between measured pressure and existing atmospheric pressure. Can be converted to absolute by adding actual atmospheric pressure value.

• Vacuum PressureThe pressure above atmospheric pressure. Is the difference between atmospheric pressure and absolute pressure. Can be converted to absolute by deducting it from actual atmospheric pressure value.

• Differential Pressure The difference in magnitude between some pressure value and some reference pressure. In a

sense, absolute pressure could be considered as a differential pressure with total vacuum or zero absolute as the reference. Likewise, gauge pressure (defined above) could be considered as Differential Pressure with atmospheric pressure as the reference.

ABSOLUTE, GAUGE & VACUUM PRESSURES

Pressure Measurement

VacuumAtmospheric Pressure14.7 psia

Pressure Measurement

14.7 psia =407in. Water

14.7 psia =29.92 in. Mercury (Hg.)

Negative Pressure = Less Than Atmospheric

Positive Pressure = Greater Than Atmospheric

Pressure Relationships

How Do We Make Air Move ?

Flow of air or any other fluid is caused by a pressure differential between two points. Flow will originate from an area of high energy, or pressure, and proceed to area(s) of lower energy or pressure.

FLOWP0P1

P1>P0

Pressure Differential Causes Movement

BURTON 3-6

FLOW LOW HIGH

FAN

Duct air moves according to three fundamental laws of physics

1.Conservation of mass2.Conservation of energy3.Conservation of momentum

LAW OF CONSERVATION OF MASS

Conservation of mass simply states that an air mass is neither created nor destroyed. From this principle it follows that the amount of air mass coming into a junction in a ductwork system is equal to the amount of air mass leaving the junctionIn ductwork, the law of conservation of mass means a duct size can be recalculated for a new air velocity using the simple equation:V2 = (V1 * A1)/A2

Where V is velocity and A is Area

LAW OF CONSERVATION OF ENERGY

The law of energy conservation states that energy cannot disappear; it is only converted from one form to another.

This is the basis of one of the main expression of aerodynamics, the Bernoulli equation. Bernoulli's equation in its simple form shows that, for an elemental flow stream, the difference in total pressures between any two points in a duct is equal to the pressure loss between these points, or:(Pressure loss)1-2 = (Total pressure)1 - (Total pressure)2

LAW OF CONSERVATION OF MOMENTUM

• Conservation of momentum is based on Newton's law that a body will maintain its state of rest or uniform motion unless compelled by another force to change that state. This law is useful to explain flow behavior in a duct system's fitting.

Pressure Terms

• Static Pressure

• Velocity Pressure

• Total Pressure

Airflow through a duct system creates three types of pressures

Static pressure is the measure of the potential energy of a unit of air in the particular cross section of a duct.

Air pressure on the duct wall is considered static.

Imagine a fan blowing into a completely closed duct; it will create only static pressure because there is no air flow through the duct. A balloon blown up with air is a similar case in which there is only static pressure.

STATIC PRESSURE

Static pressure

Duct

Airflow

Probe located on the surface of the

duct

Static pressure (SP) is exerted in all directions.

Static Pressure

What is use of the term “Static Pressure” ?

• Accelerate the air.

• Overcome resistance to friction.

Dynamic pressure is the kinetic energy of a unit of air flow in an air stream. Dynamic pressure is a function of both air velocity and density: Dynamic pressure = (Density) * (Velocity)2 / 2

P v = (V/4005)2

V = Velocity through the duct.

DYNAMIC PRESSURE

Velocity pressure

Velocity pressure = Total pressure – Static pressure

Total pressure

Velocity PressureVelocity Pressure (VP) is kinetic (moving pressure) resulting from air flow.

Static pressure

What is use of the term “Velocity Pressure” ?

• Determine the air flow.

• To design the system.

• V = 4005(VP)1/2

Consists of the pressure the air exerts in the direction of flow (Velocity Pressure) plus the pressure air exerts perpendicular to the plenum or container through which the air moves. In other words:PT = PV + PS

PT = Total PressurePV = Velocity PressurePS = Static Pressure

TOTAL PRESSURE

Total pressure

Probe located in the duct, facing into the direction of airflow

Total PressureTotal pressure (TP) is the algebraic sum of the VP and SP.

Static Pressure and Velocity Pressure are Mutually Convertible

When air is accelerated, the static pressure is converted to velocity pressure.

When air is decelerated, the velocity pressure can be transformed back into static pressure.

=

STATIC, TOTAL & VELOCITY PRESSURES

Pressure Upstream and Downstream of the Fan

TP SP VP

Up-stream - - +

Down-stream + + +

Fluids in Motion

All fluids are assumed in this treatment to exhibit streamline flow.

• Streamline flow is the motion of a fluid in which every particle in the fluid follows the same path past a particular point as that followed by previous particles.

Assumptions for Fluid Flow:

Streamline flow Turbulent flow

• All fluids move with streamline flow.• The fluids are incompressible.• There is no internal friction.

Rate of FlowThe rate of flow R is defined as the volume V of a fluid that passes a certain cross-section A per unit of time t.

The volume V of fluid is given by the product of area A and vt: V Avt

AvtR vAt

Rate of flow = velocity x area

vt

Volume = A(vt)

A

Constant Rate of FlowFor an incompressible, frictionless fluid, the velocity increases when the cross-section decreases:

1 1 2 2R v A v A

A1

A2

R = A1v1 = A2v2

v1

v2

v2

2 21 1 2 2v d v d

v1d12 = v2d2

2

The area is proportional to the square of diameter, so:

d2 = 0.894 cm

Example 1: Water flows through a rubber hose 2 cm in diameter at a velocity of 4 m/s. What must be the diameter

of the nozzle in order that the water emerge at 16 m/s?

Example 1 (Cont.): Water flows through a rubber hose 2 cm in diameter at a velocity of 4 m/s. What is the rate of flow in

m3/min?

2 21

1 1(4 m/s) (0.02 m)

4 4dR v

R1 = 0.00126 m3/s

1 1 2 2R v A v A

21

1 1 1; 4dR v A A

3

1m 1 min0.00126min 60 s

R

R1 = 0.0754 m3/min

The Venturi Meter

The higher velocity in the constriction B causes a difference of pressure between points A and B.

PA - PB = rgh

h

AB

C

Work in Moving a Volume of Fluid

P1

A1

P1

A1

P2

A2

A2

P2

h

Volume V

Note differences in pressure DP and area DA

Fluid is raised to a height h.

22 2 2 2

2

; FP F P AA

11 1 1 1

1

; FP F PAA

F1

, F2

Work on a Fluid (Cont.)

Net work done on fluid is sum of work done by input force Fi less the work done by resisting force F2, as shown in figure.

Net Work = P1V - P2V = (P1 - P2) V

F1 = P1A1

F2 = P2A2

v1

v2

A1

A2

h2

h1 s1

s2

Conservation of EnergyKinetic Energy K:

2 22 1½ ½K mv mvD

Potential Energy U:

2 1U mgh mghD

Net Work = DK + DU

2 21 2 2 1 2 2( ) (½ ½ ) ( )P P V mv mv mgh mgh

also Net Work = (P1 - P2)V

F1 = P1A1

F2 = P2A2

v1

v2

A1

A2

h2

h1 s1

s2

Conservation of Energy2 2

1 2 2 1 2 2( ) (½ ½ ) ( )P P V mv mv mgh mgh Divide by V, recall that density r m/V, then simplify:

2 21 1 1 2 2 2½ ½P gh v P gh vr r r r

Bernoulli’s Theorem:

21 1 1½P gh v Constr r

v1

v2

h1

h2

DUCT

A Duct is a passageway or conduit made of noncombustible material for movement of air from one place to the another.

Classification of duct systemsDucts are classified based on the load on duct due to air pressure and turbulence.

The classification varies from application to application, such as for residences, commercial systems, industrial systems etc. For example, one such classification is given below:

– Low pressure systems: Velocity ≤ 10 m/s, static pressure ≤ 5 cm H2O (g)

– Medium pressure systems: Velocity ≤ 10 m/s, static pressure ≤ 15 cm H2O (g)

– High pressure systems: Velocity > 10 m/s, static pressure 15<ps ≤ 25 cm H2O (g)

Classification of duct systems

Pressure Losses or Resistance to Flow

Pressure loss is the loss of total pressure in a duct or fitting. There are three important observations that describe the benefits of using total pressure for duct calculation and testing rather than using only static pressure.

1. Only total pressure in ductwork always drops in the direction of flow. Static or dynamic pressures alone do not follow this rule.

2. The measurement of the energy level in an air stream is uniquely represented by total pressure only. The pressure losses in a duct are represented by the combined potential and kinetic energy transformation, i.e., the loss of total pressure.

3. The fan energy increases both static and dynamic pressure. Fan ratings based only on static pressure are partial, but commonly used.

PRESSURE LOSS COMPONETS

Pressure loss in ductwork has three components

1. Frictional losses along duct walls 2. Dynamic losses in fittings and 3. Component losses in duct-mounted equipment.

FRICTION LOSS IN DUCTs

Frictional losses in duct sections are result from air viscosity and momentum exchange among particles moving with different velocities.

Factors affecting friction loss are 1. Air Velocity2. Duct size & shape3. Duct material roughness factor4. Duct Length

DUCT VELOCITY

High velocities in the ducts results in:

1. Smaller ducts and hence, lower initial cost and lower space requirement2. Higher pressure drop and hence larger fan power consumption3. Increased noise and hence a need for noise attenuation

Recommended air velocities depend mainly on the application and the noisecriteria.

Typical recommended velocities are:

1. Residences: 3 m/s to 5 m/s2. Theatres: 4 to 6.5 m/s3. Restaurants: 7.5 m/s to 10 m/s

If nothing is specified, then a velocity of 5 to 8 m/s is used for main ductsand a velocity of 4 to 6 m/s is used for the branches. The allowable air velocitiescan be as high as 30 m/s in ships and aircrafts to reduce the space requirement.

ASPECT RATIO

RATIO OF LONGEST DIMENSION TO THE SHORTEST DIMENSION

Recommended Max Aspect ratio is 4:1

Aspect ration of an elbow is the dimension of the side where the radius of the curve lies to that of the other side.

SURFACE ROUGHNESS OF DUCTs

RECOMMENDED FRICTION RATES

FRICTION LOSS DETERMINATION

Frictional loss per unit length can be determined using

1. Friction Chart (ASHRAE, 1997) 2. The Darcy-Weisbach Equation

FRICTION CHART

Darcy-Weisbach Equation

• where f is the dimensionless friction factor, L is the length of the pipe/duct and D is the diameter in case of a circular duct and hydraulic diameter in case of a noncircular duct.

hl

D VL

gV

DfLhl 2

2

DYNAMIC PRESSURE LOSSESDynamic losses are the result of changes in direction and velocity of air flow. Dynamic losses occur whenever an air stream makes turns, diverges, converges, narrows, widens, enters, exits, or passes dampers, gates, orifices, coils, filters, or sound attenuators. Velocity profiles are reorganized at these places by the development of vortexes that cause the transformation of mechanical energy into heat. The disturbance of the velocity profile starts at some distance before the air reaches a fitting. The straightening of a flow stream ends some distance after the air passes the fitting. This distance is usually assumed to be no shorter then six duct diameters for a straight duct.

Dynamic losses are proportional to dynamic pressure and can be calculated using the equation:Dynamic loss = (Local loss coefficient) * (Dynamic pressure)

where the Local loss coefficient, known as a C-coefficient, represents flow disturbances for particular fittings or for duct-mounted equipment as a function of their type and ratio of dimensions. Coefficients can be found in the ASHRAE Fittings diagrams.

DYNAMIC LOSSES AND EQUIVAENT LENGTHS

COMPONENT PRESSURE LOSSES

Due to physical items with known pressure drops, such as hoods, filters, louvers or dampers.

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System Resistance

Sum of Static, Dynamic & Component pressure losses in system• Configuration of ducts, pickups, elbows• Pressure drop across equipment

Increases with square of air volume• Long narrow ducts, many bends: more

resistance• Large ducts, few bends: less resistance

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System ResistanceSystem resistance curve for various

flows

calculated

Actual withsystemresistance

PRESSURE COMPONENTS IN DUCTS

PRESSURE COMPONENTS IN DUCTS CONSIDERING FRICTION

DUCT DESIGN CRITERIA

1. Space Availability2. Installation Cost3. Air Friction Loss4. Noise Level5. Duct heat transfer and airflow leakage6. Codes and standards requirements

SPACE AVAILABILITY

INSTALLATION COST

AIR FRICTION LOSS

• DUCT SIZE• DUCT SHAPE• DUCT ROUTE• MATERIAL OF CONSTRUCTION• FITTINGS USED

NOICE LEVEL

HEAT TRANSFER AND LEAKAGE

CODES AND STANDARDS

• SMACNA• DW 144

DUCT DESIGNING METHODS

1. Velocity method2. Equal Friction Method3. Static Regain method

VELOCITY METHODThe various steps involved in this method are:

1. Select suitable velocities in the main and branch ducts. 2. Find the diameters of main and branch ducts from airflow rates and velocities

for circular ducts. For rectangular ducts, find the cross-sectional area from flow rate and velocity, and then by fixing the aspect ratio, find the two sides of the rectangular duct

3. From the velocities and duct dimensions obtained in the previous step, find the frictional pressure drop for main and branch ducts using friction chart or equation.

4. From the duct layout, dimensions and airflow rates, find the dynamic pressure losses for all the bends and fittings

5. Select a fan that can provide sufficient FTP for the index run6. Balancing dampers have to be installed in each run. The damper in the index

run is left completely open, while the other dampers are throttled to reduce the flow rate to the required design values.

VELOCITY METHODMerits and Demerits

• The velocity method is one of the simplest ways of designing the duct system for both supply and return air.

• However, the application of this method requires selection of suitable velocities in different duct runs, which requires experience.

• Wrong selection of velocities can lead to very large ducts, which, occupy large building space and increases the cost, or very small ducts which lead to large pressure drop and hence necessitates the selection of a large fan leading to higher fan cost and running cost.

• In addition, the method is not very efficient as it requires partial closing of all the dampers except the one in the index run, so that the total pressure drop in each run will be same.

EQUAL FRICTION METHODIn this method the frictional pressure drop per unit length in the main and branch ducts (Δpf/L) are kept same.

Then the stepwise procedure for designing the duct system is as follows:1. Select a suitable frictional pressure drop per unit length (Δpf/L) so that

the combined initial and running costs are minimized.2. Then the equivalent diameter of the main duct (A) is obtained from

the selected value of (Δpf/L) and the airflow rate. 3. From the airflow rate and (Δpf/L) the equivalent diameter of the main

duct can be obtained either from the friction chart or using the frictional pressure drop equation.

4. Since the frictional pressure drop per unit length is same for all the duct runs, the equivalent diameters of the other duct runs, can be obtained.

EQUAL FRICTION METHOD PROCEDURE

1. If the ducts are rectangular, then the two sides of the rectangular duct of each run are obtained from the equivalent diameter of that run and by fixing aspect ratio as explained earlier. Thus the dimensions of the all the duct runs can be obtained.

2. The velocity of air through each duct is obtained from the volumetric flow rate and the cross-sectional area.

3. Next from the dimensions of the ducts in each run, the total frictional pressure drop of that run is obtained by multiplying the frictional pressure drop per unit length and the length.

4. Next the dynamic pressure losses in each duct run are obtained based on the type of bends or fittings used in that run.

5. Next the total pressure drop in each duct run is obtained by summing up the frictional and dynamic losses of that run

6. Next the fan is selected to suit the index run with the highest pressure loss.7. Dampers are installed in all the duct runs to balance the total pressure loss.

EQUAL FRICTION METHODMERITS AND DEMERITS

• Equal friction method is simple and is most widely used conventional• method. This method usually yields a better design than the velocity method as• most of the available pressure drop is dissipated as friction in the duct runs,• rather than in the balancing dampers. This method is generally suitable when

the• ducts are not too long, and it can be used for both supply and return ducts.• However, similar to velocity method, the equal friction method also requires• partial closure of dampers in all but the index run, which may generate noise. If• the ducts are too long then the total pressure drop will be high and due to• dampening, ducts near the fan get over-pressurized.

STATIC REGAIN METHOD

This method is commonly used for high velocity systems with long duct runs, especially in large systems. In this method the static pressure is maintained same before each terminal or branch.

STATIC REGAIN METHOD

The procedure followed is as given below:

1. Velocity in the main duct leaving the fan is selected first.

2. Velocities in each successive runs are reduced such that the gain in static pressure due to reduction in velocity pressure equals the frictional pressure drop in the next duct section. Thus the static pressure before each terminal or branch is maintained constant.

• Static Regain method yields a more balanced system

• Does not call for unnecessary dampening. • As velocity reduces in the direction of airflow, the

duct size may increase in the airflow direction. • Velocity at the exit of the longer duct runs may

become too small for proper air distribution in the conditioned space.

STATIC REGAIN METHODMERITS AND DEMERITS

General rules for duct design

1. Air should be conveyed as directly as possible to save space, power and material

2. Sudden changes in directions should be avoided. When not possible to avoid sudden changes, turning vanes should be used to reduce pressure loss

3. Diverging sections should be gradual. Angle of divergence ≤ 20o

4. Aspect ratio should be as close to 1.0 as possible. Normally, it should not exceed 4

5. Air velocities should be within permissible limits to reduce noise and vibration

6. Duct material should be as smooth as possible to reduce frictional losses

DUCT DESIGN PROCESS STEPS

DETERMINE NO OF ZONES

DESIGN STEP-1

PERFORM COOLING AND HEATING LOAD ESTIMATES

DESIGN STEP-2

• DETERMINE SPACE, ZONE & BLOCK AIR FLOWS

DESIGN STEP-3

• SELECT DUCT MATERIAL, SHAPE & INSULATION

DESIGN STEP-4

LAY OUT DUCTWORK FROM AHU TO AIR DISTRIBUTION DEVICES

DESIGN STEP-5

FIT MAIN DUCT TO BUILDING

CREATE A SYSTEM SIZING

SCHEMATIC

SUMMARISE DUCT CFM AND LABEL DUCT SCHEMATIC

DESIGN STEP-6

SIZE DUCTWORK FROM FAN, OUT TO EXTREMITIES

USE STANDARD METHODS

DESIGN STEP-7

DUCT SIZING USING FRICTION CHART

1. Select desired velocity in first duct section2. Enter friction loss chart, read round duct diameter a

intersection of CFM and Velocity lines3. Read resulting friction loss value at bottom of friction

chart; verify that it is acceptable.4. If sizing round duct, we have completed sizing the first

section. Proceed to the next duct section using desired friction rate.

5. If sizing rectangular duct, we must convert round sizes to equivalent rectangular sizes using Table

Find the size of round duct to carry 1800 cfm by keeping the velocity below 1500 fpm and max friction rate of 0.10” wg

16” DUCTVelocity – 1300 fpm

Friction rate – 0.14” wg

18” DUCTVelocity – 1000 - 1200 fpm

Friction rate – 0.08” wg

CIRCULAR EQUIVALENT DIAMETERS OF RECTANGULAR DUCTS

CALCULATE AIR SYSTEM PRESSURE LOSSES

DESIGN STEP-8

SELECT FAN AND ADJUST SYSTEM AIR FLOWS

DESIGN STEP-9

FANS

Primary air moving devices used in industrial applications

96

Fan Curve

Performance curve of fan under specific conditions• Fan volume• System static pressure• Fan speed• Brake horsepower

97

Operating PointFan curve and system curve intersect

Flow Q1 at pressure P1 and

fan speed N1

Move to flow Q2 by reducing fan

speed

Move to flow Q2 by closing damper

(increase system resistance)

98

Fan Laws

TYPES OF FANS

Basic groups of fans are:

Axial fansCentrifugal fansSpecial type fans

100

Basic Fan Types

• Centrifugal– Backward Inclined Airfoil-blade – Backward Inclined Flat-blade– Forward Curved Blade– Radial Blade– Radial Tip

• Axial– Propeller / Panel Fan– Tubeaxial– Vaneaxial

• Special Designs– Power Roof Ventilators– Tubular Inline Centrifugal– Mixed Flow– Plenum/ Plug

101

Centrifugal: Backward Inclined Airfoil-Blade

• Name is derived from the “airfoil” shape of blades• Developed to provide high efficiency• Used on large HVAC and clean air industrial systems where

energy savings are of prime importance

102

Centrifugal: Backward Inclined or Curved Flat-Blade

• Backward inclined or curved blades are single thickness or “flat” • Efficiency is only slightly less than airfoil blade• Similar characteristics as airfoil blade• Same HVAC applications as airfoil blade• Also for industrial applications where airfoil blade is not acceptable

because of corrosive or erosive environment

103

Backward Inclined or Curved Flat & Airfoil-Blade

• High volume at moderate pressure

• Non-overloading power characteristic

• Stable performance characteristic

• Low noise

104

Centrifugal: Forward Curved Blade

• Blades are curved forward in the direction of rotation

• Must be properly applied to avoid unstable operation

• Less efficient than Airfoil and Backward Inclined

• Requires the lowest speed of any centrifugal to move a given amount of air

• Used for low pressure HVAC systems• Clean air and high temperature

applications• Typically smallest size selection• Rising power overloading characteristic

105

Centrifugal: Radial Blade

• The blades are ‘radial’ to the fan shaft • Generally the least efficient of the centrifugal

fans• For material handling and moderate to high

pressure industrial applications, rugged construction

• Low volume at high pressure• Large wheel diameter for a given volume-

higher cost• Material handling, self cleaning• Easy to maintain• Rising Power overloading characteristic• Suitable for dirty airstream, high pressure,

high temperature and corrosive applications

106

Centrifugal: Radial Tip

• The blades are radial to the fan shaft at the outer extremity of the impeller, but gradually slope towards the direction of wheel rotation

• More efficient than the radial blade but less than backward inclined

• Offers wear resistance in mildly erosive air streams

107

Axial: Propeller or Panel Fan

• One of the most basic fan designs• For low pressure, high volume applications • Often used for ventilation through a wall• Available in square panel or round ring fan• Maximum efficiency is reached near free

delivery• Reversible blade for reversible flow

applications like jet tunnel fans• Many axial fans can overload at shutoff

108

Tube axial Fan

• More efficient than the panel fan • Cylindrical housing fits closely to outside diameter

of blade tips• For low to medium pressure ducted HVAC systems• Also used in some low pressure industrial

applications• Performance curve sometimes includes a dip to

the left of peak pressure which should be avoided

109

Vane axial Fan

• Highest efficiency axial fan• Cylindrical housing fits closely to outside

diameter of blade tips• The straightening vanes allow for greater

efficiency and pressure capabilities• For medium to high pressure HVAC systems.

More compact than centrifugal fans of same duty

• Aerodynamic stall causes the performance curve to dip to the left of peak pressure which should be avoided. However anti-stall options available for both unidirectional and reversible axials

110

Power Roof Ventilators• A variety of backward inclined centrifugal wheels or axial impeller designs• Also available in up blast damper design to discharge air away from the

building• For low pressure exhaust systems of all building types (roof mounted)

111

Inline Centrifugal Fan• Cylindrical housing is similar to a vane axial fan• Wheel is generally an airfoil or backward inclined type• Housing does not fit close to outer diameter of wheel• For low and medium pressure HVAC systems or industrial

applications when an inline housing is geometrically more convenient than a centrifugal configuration

112

Mixed Flow Fan• Specific Speed between a centrifugal and axial fan• Cylindrical housing is similar to a vane axial fan • High volume advantages of axial fans• Low sound, high efficiency advantages of tubular

centrifugal fans

113

PLENUM / PLUG FAN

Housed vs plenum fan

This is basically a centrifugal wheel and inlet in a frame without a scroll or housing. The ‘housing’ is the AHU box.Offers tremendous flexibility for inlet and discharge in a AHU applicationMore efficient than a scroll centrifugal for high flows and low SP. All SP rise occurs in the blade passage Wall clearance rules must be followed to avoid significant system effect losses

TYPES OF FAN DRIVES

• Belt-drive blowers have two bearings on the fan shaft and two bearing on the motor

• Motor pulleys and fan motor pulleys can be adjusted to change fan speeds

• Direct-drive motors use no pulleys or belts• Direct-drive motors can be multi-speed motors • Speeds can be changed by changing motor wire

leads

BLOWER

MOTOR

BOTH THE DRIVE AND DRIVEN PULLEYS MUST BE PERFECTLY

ALIGNED

Belt-driven Assembly

DIRECT DRIVE MOTOR ASSEMBLY

THE MOTOR AND THE BLOWER TURN AT THE SAME SPEED

117

SO YOU HAVE ALL THESE CHOICES OF FANS TYPES AVAILABLE…WHAT SHOULD YOU DO TO PICK THE RIGHT FAN FOR YOUR APPLICATION?

118

119

120

Type Dia (in) Spd (rpm) BHP SE % (Static Efficiency)

LwiA (Inlet Sound Power ‘A’)

1 Forward Curved- SW (Centrifugal)

30 476 5.09 61.7 89

2 Backward Airfoil – SW (Centrifugal)

36.5 650 3.82 80.0 77

3 Plenum 33 800 4.25 74.0 80

4 Tubular Mixed Flow

27 1074 4.48 70.2 81

5 Tubular Vane Axial

28 1438 4.77 65.9 86

6 Propeller (Axial)

30 1998 4.92 54.4 103

All fans selected at peak SE (Static Efficiency) for Airflow=10,000 cfm, Static Pressure (SP)~2 I wc

121

Fan Selection based on Specific Speed

Dimensional Specific Speed, is the fan speed required to raise the SP by 1 iwc with 1 cfm airflow.

Ns = N * (Q)^0.5/(SP)^0.75Where, N = Speed (rpm)

Q = Airflow (cfm) SP = Static pressure (iwc) Density = 0.075 lbm/cu ft

122

123

Type Specific Speed, Ns

Max Static Efficiency (SE%)

1 Forward Curved-SW (Centrifugal)

26,300 61

2 Backward Airfoil-SW (Centrifugal)

40,000 80

3 Plenum 50,000 75

4 Tubular Mixed Flow 65,800 70

5 Tubular Vane Axial 90,000 65

6 Propeller (Axial) 126,000 59

All fans selected at peak SE (Static Efficiency) for Specific Speed, Ns

Peak efficiency or Best Efficiency Point (BEP)

Airfoil

Tubular

Forward

Effic

ienc

y

Flow rate

Backward

Radial

Airfoil

Tubular

Forward

Effic

ienc

y

Flow rate

Backward

Radial

Type of FanPeak

Efficiency Range

Centrifugal fans:

Airfoil, Backward curved/inclined

79-83

Modified radial 72-79

Radial 69-75

Pressure blower 58-68

Forward curved 60-65

Axial fans:

Vane axial 78-85

Tube axial 67-72

Propeller 45-50

Fan Selection at a Density Other Than Standard

Fan performance is affected by changes in gas density. Corrections must be employed if density varies by more than 5% from the standard

0.075lbm/ft3

Corrected Pressure is given by: Pe = Pa (0.075/ρa) Pe = Equivalent or corrected pressure Pa = Actual pressure ρa = Actual density, lbm/ft3

Actual power requirement is given by PWRa = PWRt (ρa/0.075)

PWRa = Actual power requirementPWRt = Power requirement in rating table.ρa = Actual density, lbm/ft3

Fan selection at non-standard density requires knowledge of actual volumetric flowrate, actual pressure requirement and the density of gas at the fan inlet .

Fan Selection

Considerations for fan selection are :1. Capacity:

Flow rate based on system requirements. Expressed as actual cubic feet per minute (acfm).

Pressure requirement based on system pressure requirements. Expressed as FSP or FTP in inches of water gauge.

2. Air stream: Material handled through fan.

Small amount of smoke or dust - backward inclined centrifugal or axial fan is selected.

Light dust fume or moisture - backward inclined or radial fan is preferred. Heavy particulate loading - radial fan is selected.

Explosive or flammable material. Spark resistant construction is used. Explosion proof motor is used.

3. Physical Limitations: Fan size is determined by

Performance requirements Inlet size and location Fan weight

The most efficient fan size may not fit the physical space available.

4. Drive arrangements: Electric motor is the power source of fans.

Unlike packaged fans, for larger units the motor is coupled directly to the fan or indirectly by a belt drive.

Fan Selection

Standard drive arrangements are: Direct drive:

Offers more compact assembly and assures constant fan speed. Fan speeds are limited to available motor speeds.

Belt drive: Offers flexibility in changing the fan speed. Important in applications where changes in system capacity or pressure

requirements are needed.

5. Noise: Generated by turbulence within he fan housing. “White” noise which is a mixture of all frequencies is mostly produced. Radial blade fans produce a pure tone at a frequency BPF. BPF = rpm * n * CF.

Where: BPF - blade passage frequency.

RPM - rotational rate.N - number of blades.CF - conversion factor, 1/60.

Fan Selection

6. Safety and accessories: Safety guards are required at inlet, outlet, shaft, drive and cleanout

doors. Accessories help in future maintenance requirements. Flow control can be done using dampers.Outlet dampers: Mounted on the fan outlet. Adds resistance to the system when partially closed.Inlet dampers: Mounted on the fan inlet. Pre-spins air into the impeller. Lowers operating horsepower.

Fan Selection

6. Safety and accessories: Safety guards are required at inlet, outlet, shaft, drive and cleanout

doors. Accessories help in future maintenance requirements. Flow control can be done using dampers.Outlet dampers: Mounted on the fan outlet. Adds resistance to the system when partially closed.Inlet dampers: Mounted on the fan inlet. Pre-spins air into the impeller. Lowers operating horsepower.

Fan Selection

Various factors effecting fan selection are: Volume required (cfm) Fan static pressure Type of material handled Explosive or inflammable material Direct driven vs belt driven Space limitations Noise Operating temperature Efficiency Corrosive applications

Fan Selection

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Summary

• Fan selection is not a trivial process for a given application.• Example shown applies to one design operating point. The selections

will change for other operating points. • There is no magic fan that will result in least cost, best efficiency and

low noise for a wide range of operating points.• Compromises should be well understood upfront.• Direct Drive (DD) selection speeds may further limit selections.

Varying width options can optimize DD selections.• Mechanical design requirements like balancing and vibration levels,

spark and high temp resistance, corrosion resistance, arrangements, motors, bearings, drives can further challenge the selection process.