6 Understanding Fans

AMC - The Business of Mining

BASIC MINE VENTILATION

6.0 Understanding Fans


How do they work?

What are the different types?

What is the difference between primary, booster, auxiliary, secondary, development, etc. fans?

Where are fans best installed?


“Any device that produces a current of air by the movement of broad surfaces can be called a fan

………Fans fall under the general classification of of “turbomachinery” and have a rotating impeller at least partially encased in a stationary housing.

………Fans are similar in many respects to pumps. Both are turbomachines that transfer energy to a flowing fluid. It is easy to distinguish between fans and pumps: pumps handle liquids; fans handle gasses.

………Broadly speaking, the function of a fan is to propel, displace,or move air or gas, .”

(Howden Buffalo Inc.“Fan Engineering” (1999))

WHAT IS A FAN?


Impeller rotates and transfers energy to the air. (wheel, rotor, runner, propeller)

Blade the working surface. (vane, paddle)

Shroud supports the blades ( cover disc, inlet plate,backplate, rim, flange)

Hub attaches to the fan/motor shaft but may support the blades directly (axial) or indirectly through a shroud (centrifugal). (boss, disc)Housing encloses the impeller and guides the air to and from the impeller. (casing, scroll, panel, ring, volute)

Centrifugal housing include side plate and scroll sheets.

Axial housing includes the outer and inner cylinder, belt tube

Inlet the opening to the impeller. (eye, suction, suction eye, inlet cone, inlet bell, inlet nozzle)

Outlet the opening leaving the fan. (discharge, discharge cone, evase, diffuser)Guide Vanes when installed before the impeller are called pre-rotation vanes or inlet guide vanes. If they are adjustable they are called variable inlet vanes or simply VIV’s. When installed after the impeller they are termed straightening vanes or discharge guide vanes.

Fan parts


Fan types

Four distinctive fan types are classified according to the direction of flow through the impeller.

Axial-flow. - Air flows through the impeller parallel to, and at, a constant distance from the axis. The pressure rise is provided by the direct action of the blades.

Centrifugal or Radial-flow. - Air enters parallel to the axis of the fan turns through 90º and is discharged radially through the blades. The blade force is tangential causing the air to spin with the blades and the main pressure rise is attributed to this centrifugal force.

Mixed flow. - Air enters parallel to the axis of the fan turns through an angle which may range from 30º to 90º The pressure rise is partially by direct blade action and partially by centrifugal action.

Cross flow. - Air enters the impeller at one part of the outer periphery flows inward and exits at another part of the outer periphery.Mixed and cross flow fans have very limited application for underground mines and will not be discussed further in this course.


Impeller typesAxial Centrifugal


Performance characteristic (1)The theoretical pressure-quantity curve of an ideal fan (no losses) is a straight line between zero volume and zero pressure.

Quantity

Pres

sure

Theoretical pressure – quantity (P-Q) characteristic

Friction losses

Shock lossesUseful P-Q curve


Performance characteristic (2)

Normally referred to as the “fan curve”.

Three parameters are desirable on a fan performance characteristic.

FLOW RATE - although mass flow can be used it is most common to use volume expressed in cubic metres per second (m3/s). (air volume, inlet volume, quantity, Q). Should mass flow rate be given the volume is calculated by dividing this rate by the air density.

PRESSURE - May be given as total pressure or static pressure and expressed in Pascals (Pa). [see next slide]

INPUT POWER – The power (electrical or otherwise) required to drive the fan and expressed in kilowatts (kW). (power, shaft power, absorbed power)


Fan pressure

Fan Total Pressure (FTP)The difference between the average total pressure at the fan inlet and the average total pressure at the fan outlet.

Fan Velocity Pressure (FVP)The average velocity pressure at the fan outlet.

Fan Static Pressure (FSP)The difference between the fan total pressure (FTP) and the fan velocity pressure (FVP). FSP is therefore the difference between the average static pressure at the fan outlet and the average total pressure at the fan inlet.


Fan efficiencyEfficiencyExpressed in percent (%) and describes the ratio of the fan output power (kW) to fan input power (kW).

Fan (motor) input power Is the power to the motor that is required to drive the fan and may include elements of any train considered to be part of the fan. (e.g. belt drive, gear box).

Fan output power Is the product of the fan pressure and the fan volume (AP = PQ) (air horsepower, air power)

Where AP = air power (watts), P = pressure (Pa = N/m2) and Q = quantity (m3/s). Substituting

PQ = N/m2 x m3/s= Nm/s

Since 1Nm = 1 Joule and, 1 Joule/second = 1 watt, then

AP = watts.

If the FTP is used then the efficiency is expressed as the fan total efficiency and if FSP is used the the efficiency is expressed as the fan static efficiency.


Summary of Fan Power

Air powerIs product of the inlet volume flow and the fan pressure.

Fan shaft powerthe mechanical power supplied to the fan shaft

Motor input powerIs the power to the motor that is required to drive the fan and may include elements of any train considered to be part of the fan. (e.g. belt drive, gear box).

Fan manufacturers talk of efficiency in terms of the Fan shaft power

and users of fans generally think of efficiency in terms of motor input power.

powershaft FanAirpower efficiencyinput shaft Fan =

powerinput MotorAirpower efficiencyinput Motor =


Centrifugal FansImpeller blades are manufactured either laminar (flat, constant thickness) or aerofoil shape and generally hollow. Aerofoil blades have generally been regarded as having greater efficiencies (up to 90%) that those achievable with constant thickness blades, with the advantages of efficiency spread over the characteristic and lower noise generation. However with careful attention to design of blade curvature, inlet eye detail and impeller shrouding, comparable efficiencies can be achieved with constant thickness blades. Not withstanding this aerofoil blades are freely used particularly when blade stresses are high and extra stiffening is required.

Laminar (flat) blades Aerofoil blades


Centrifugal Fans


Centrifugal fan - Basic description

As the impeller rotates it draws air in through the eye and throws it out in a radial direction through the blades.

Can be direct drive (impeller attached to the motor shaft) or indirectly driven by belts, gear box or friction clutch

Blades can be either flat or aerofoil, straight or curved, forward or backward

Performance control is achieved by altering the speed, adjusting VIV’s, or adding blade extensions

When rotated in the wrong direction air will continue to flow into the eye and out through the blades

Can be either single or a double inlet


Forward inclined bladesProvide large quantities (Q) of air at low running speeds.

Pressure rise limited by blade stresses

Generally considered to be the most compact, quietest with the most competitive first cost.

Efficiency limited to 60% to 70% at most.

Steeply rising power characteristic

Straight blades Curved blades Quantity

Pres

sure

Shaf

t pow

erDuty pointPressure

Power


Backward inclined bladesHigher tip speeds than forward inclined blades

High total efficiency

Bulkier casing

Fewer blades with greater depth

Non overloading power characteristic. (i.e. power input does not peak at either free flow or no flow)

Curved bladesStraight blades Quantity

Pres

sure

Shaf

t pow

er

Pressure

Power

Duty point


Radial bladesAir performance sits between backward and forward inclined blades

Steeply rising power characteristic

Relatively low efficiency

Good self cleaning properties make them exceptional for handling air with high concentrations of dusts or other larger airborne particles (backward and forward incline blades tend to clog up more quickly)

Radial blades Quantity

Pres

sure

Shaf

t pow

er

Pressure

Power

Duty point


Performance characteristics of centrifugal fans

Quantity

Pres

sure

Shaf

t pow

er

Forward curved

Forward curved

Radial

Radial

Backward curved

Backward curved

Pressure

Power

Duty point


6.0 (a) Centrifugal fans

FAN SLIDE SHOW


Axial flow fans


Axial flow fan – Basic description

Can be direct drive (impeller attached to the motor shaft) or indirect drive (belts, gearbox)

Tip clearance (distance between the tip of the impeller blades and the fan casing) is typically 0.25% of the impeller diameter.

Blades can be either flat (generally steel plate) or aerofoil (generally cast alloy). Aerofoil sections can apply greater force to the air, increasing pressure and maintaining better efficiency over a wider range. Increasing the thickness and curvature, increases stiffness allowing operation at higher speeds.

When rotated in the backward direction they will reverse the direction of flow through the fan and deliver 60% to 70% of the forward quantity of air. (A true reversible fan will have alternate blades rotated through 1800 and deliver 85% of the normal setting in either direction)

Performance control is achieved by altering speed, adjusting impeller blade pitch angle or adjusting VIV’s.

Performance is enhanced by installation of inlet cone, inlet or outlet guide vanes, tail fairings, and diffusers (evase).


Drive arrangements for axial fans

Typical arrangement for secondary fans in

metal mines


AccessoriesInlet cone (inlet bell) Reduces inlet shock losses

Nose dome - Streamlines flow over the impeller hub

Guide vanes – Downstream reduce swirl and enhance pressure. Particularly important if exhausting into a long high velocity duct as the swirl will exist for long distances significantly increasing pressure drop.

Guide vanes – Upstream induce a swirl in the opposite direction to the rotation of the impeller. Not often used as they increase noise and are less efficient than downstream guide vanes. However will increase peak pressure of the fan.

Tail piece (fairing) –Reduce turbulence (shock losses) caused by the motor.

Diffuser – minimisesoutlet velocity pressure and maximises fan static pressure


Solidity


6.0 (b) Axial fans

FAN SLIDE SHOW


Performance control

Dampers

Can be either inlet or outlet

Change the resistance of the system


Performance control

Variable inlet vanes

Installed at the inlet close to the impeller

Spin the air in direction of rotation of the impeller

Provide a resistance as well as flow modification

Alter the performance characteristic of the fan


Performance control

Variable pitch blades

Axial fans only

Each setting has a different performance characteristic

Can be altered while in motion but it is usual to withdraw the fan from service to make an adjustment


On-load pitch adjustment

is possible but extremely expensive and high maintenance

Off-load blade pitch adjustment

requires the fan to be withdrawn from service for an extended period of time

blade attachments are often extremely difficult to release after quite short periods of use

an expensive and time consuming activity

In some cases, a new set of pitch plates will be needed for each duty change

many axial flow fan installations are allowed to operate at less than optimal flow rates for long periods of time simply because it is so difficult to change the blade pitch, resulting in a considerable waste of expensive power.


Adjustment using pitch plates

Adjustment using pitch markings

Blade pitch (angle) setting


Adjustment using a protractor

Impellers without markings require the use of a protractor to set the appropriate angle.

Some manufacturers specify the the blade pitch angle in terms of the “tip chord”. Those who refurbish fans often incorrectly set these angles as the at the blade root rather than the tip of the blade. Depending upon the twist of the blade this could be as much as 30 resulting with the fan performance less than expectations.

Blade pitch (angle) setting (2)

Rotation Direction

Airflow Direction

Underside or blade root

angle

Tip chord angle

Impeller Hub

Blade tip

Blade root

Boss


Performance control

Variable speed

Each change of speed develops a new fan performance characteristic(see fan laws)


Performance control

Fans in series

Increase pressure (P)

Slight increase in quantity (Q)

New curves can be drawn simply by adding the pressure (P) at a constant quantity (Q)


Performance control

Fans in parallel

Increase quantity (Q)

New curves can be drawn simply by adding the quantity (Q) at a constant pressure (P)

Note:

the quantity is NOT doubledsimply by adding an extra fan.


Start-up

Single fan - variable speed

On start up a fan goes from standstill to full speed

Will follow path 1,2,3,4 if equilibrium is reached instantaneously

Will follow path 1’, 2’, 2’, 4 if equilibrium is not established instantaneously (i.e. if the system has a significant volume or it is necessary to blow the bag up)

In any case all points are on the negative part of the curve and therefore stable.


Start-up

Single fan – Damper control

Dampers set to open when a predetermined pressure is reached (i.e no flow until operating pressure is reached)

Fan must follow the parabola over the hump and the fan may become unstable during this stage

Miners have found by experience that jog starting fans with long lengths of lay-flat duct reduces excessive power draw and prevents the fan from shaking violently.


Start-up

Single fan with dip in the curve

In this case the curve has a dip as well as a hump

In this case all intersections are on the negative (stable) part of the curve


Start-upTwo fan is series

If started simultaneously they will act in the same manner as a single fan

If one fan is started the operating fan sees a higher resistance caused by the non-operating fan. At the start-up of the second fan the system resistance is lowered and the first fan comes down the curve whilst the second fan moves from a free flow (air from the first fan) situation until both fans are at the same speed and contributing to the combined fan curve


Start-upTwo fans in parallel

If started simultaneously they will act in the same manner as a single fan

When one fan is started it will run up and settle on on the system. The second fan (no flow) will start and when acceleration is sufficient it will move to the right at the same time that the first fan is moves up its curve until both fans are at the same speed and contributing to the combined fan curve

Note that the second fan must move over the hump and could cause serious instability if the curve has a dip as well as a hump


Parallel fans with a dip (1)

Parallel Fans

Quantity (m3/s)

Stat

ic p

ress

utre

(Pa)

1 Fan 2 Fans 3 Fans System

Apparent operating points


Parallel fans with a dip (2)P arallel Fans

Qua ntity (m 3/s)

Stat

ic p

ress

utre

(Pa)

1 Fan 2 Fans 3 Fans Sy s tem

Apparent operating points

Eck line

In this case it is highly likely that the third fan would become unstable on start-up, to the point that it would not recover. To start fans with this particular type of characteristic requires the use of variable speed drives, VIV’s or adjustable pitch in motion blades.


Fan operating duty pointThe theoretical operating (duty) point of any fan is determined as the intersection of the Actual P-Q curve and the resistance curve of the system in which it will operate.

Quantity

Pres

sure

Theoretical pressure – quantity (P-Q) characteristic

Friction losses

Shock lossesUseful P-Q curve


Fan and System Resistance curves

Quantity (m3/s)

Pres

sure

(Pa)

P-Q curve

00

5 10 15 20

500

1000

System resistance curveOperating point12.0 m3/s 700 Pa


MULGA HILL FAN COMPANYQWER 1250-2300-12 990 rpm 1.2kg/m3

0

500

1000

1500

2000

2500

3000

3500

0 20 40 60 80 100 120 140 160 180 200

Fan

Sta

tic P

ress

ure

(Pa)

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120 140 160 180 200

Volume (m3/s)

Fan

Shaf

t Pow

er (k

W)

1020

30 40

50

60

70

10 20 3040

50

60

70

Reference P'nW 2345

Impeller blade pitch setting

Pressure sometimes given as Fan Total Pressure

Manufacturers reference number

Fan performance sets

Manufacturers Code for the fan type usually provides, fan type, hub diameter, impeller diameter, number of blades, fan Speed & air density

Fan operating point

AP = P x Q

Fan Shaft power at the operating point


325 kW

Airpower = 200 x 1500 = 3000 W = 300 kWFan shaft efficiency = 300 / 325 = 92%


System ResistanceQ (m3/s) P (Pa)

0 020 4040 16060 36080 640

100 1000120 1440140 1960160 2560180 3240200 4000220 4840240 5760260 6760

Assume duty100 m3/s 1000 Pa

180 kW

Efficiency 77%??


Example80 m3/s 1.0kPa

Blades set at 400

350 kW motor

What is the maximum flow possible with this fan?


ExampleMaximum blade angle 520

Due to wear on blades would be prudent to set at 500

80 1000System ResistanceQ (m3/s) P (Pa)

0 020 6340 25060 56380 1000

100 1563120 2250140 3063160 4000180 5063200 6250


Relative merits of axial and centrifugal fans

Axial fans offer better efficiency over a wider range of duties whereas the centrifugal fans can have a higher efficiency, albeit over a smaller range, on a single performance curve.

The performance of a single speed axial fan can be altered simply by adjustment to the impeller blade pitch angle

The performance of a single speed centrifugal fan requires the installation of variable inlet vanes.

Axial fans are generally considered to be more easily accessible for maintenance

Axial fans generally run faster than centrifugal as a consequence are much noisier.

Axial fan impellers are generally manufactured from aluminium in an effort to keep weight to a minimum. As a consequence the potential for erosion is greater, particularly if there is water in the shaft


Relative merits of axial and centrifugal fans (2)

The light material used in the blades along with the high rotational speed of axial fans make them prone to erosion, and even in good (dry) conditions it is reasonably expected that this erosion will have significantly reduced the fan performance within five years.

Centrifugal fan impellers are fabricated from plate and are generally hollow. As a consequence when there is water in the shaft the nose of the blade is prone to pitting allowing water to enter the hollow section. Sufficient water in this section will cause the impeller to become unbalanced, and if allowed to continue it will result in high vibration and eventual failure of the impeller shaft.

Centrifugal fans traditionally require the construction of large concrete foundations for the motor and ductwork. The cost of these foundations significantly increases the capital cost of the fan


Relative merits of single and multiple fan installations

Single fan installations are generally less expensive than multiple fan installations.

Multiple fan installations have the advantage of airflow redundancy, i.e. a percentage of airflow will always be available whilst a fan is off line for maintenance or component change out.

Single fan options do not provide any capacity for redundancy airflow. The purchase of spares (motor, impeller, shafts, bearings, blades etc) is good management and should be included as upfront capital expenditure.



6.4 Fan Laws



6.5 Fan Performance Testing



6.6 Operating Multiple Fans


Acknowledgements

DALY, B.B., 1978 “Woods Practical Guide to Fan Engineering”(Published by Woods of Colchester 1978)

Le ROUX, W., “Le Roux’s Notes on Mine Environmental Control” Fourth Edition. (The Mine Ventilation Society of South Africa).

JORGENSEN, R. 1983 “Fan Engineering Eighth Edition” (Buffalo Forge Company. Buffalo, New York.)

BURROWS, J., 1989 “Environmental Engineering in South African Mines” (The Mine Ventilation Society of South Africa)

DERRINGTON, A.S., 2002 “Control of Water Discharge from Mine Ventilation Shafts” (Proceedings of Underground Operators Conference 2002, pp317-326 (The Australian Institute of Mining and Metallurgy: Melbourne)

Documents

6 Understanding Fans