Encyclopedia of Environmental Science and Engineering, Volume I and II

520 INDUSTRIAL HYGIENE ENGINEERING

by changing the diameter of the drive and driven pulleys and

with gearboxes by changing the spur wheels. If increases in

fan speed are contemplated they should be done in consul-

tation with the manufacturer as motor and shaft loadings,

impeller materials and bearing sizes are designed for a par-

ticular maximum duty and power in mind and any increase

over and above the manufacturers’ maximum can lead to

failure of one of the above components and could cause a

serious accident.

Air Cleaning and Filtration This topic can be divided

into two fields of speciality depending upon the nature of the

dust and its concentration in the airstream. Where air is sup-

plied to a workplace from outside then its particulate con-

centration is low but of a wide range of sizes and materials.

Filtration is required to clean it to the standard required of

that workplace, which in some cases can be very stringent,

for example, in clean room atmospheres.

In the case of captured air from a dust emission source

the loadings can be very high but often of a specified material

and of a limited size range often much larger in aerodynamic

diameter than with external air pollution. This requires the

use of different techniques to provide the necessary air qual-

ity standard.

In the former case filtration is by inertial separation

using the particles’ changes of direction as it zig-zags its

way through dry fabric filter media. The particle is caught

on the surface of the material which is sometimes coated

with a viscous medium to assist capture and to improve load-

ing capacity. The size of the particle captured will depend

upon the matrix size of the medium. Filter materials consist

of cotton, wool, nylon, viscose fibers, paper, glass fiber,

asbestos, cellulose and many others and are either packed

Radial

curved

Forward

curved

Backward

curved

P

P

P

Q

Pressure

Casing or volute

Air out

Cut off

Shroud Backplate

Air in

Blades

Impeller

FIGURE 6 Centrifugal fan and graph showing its characteristic curves.

FIGURE 7 Matching of fan and system.

Volume flow rate (m^3 s–1)

Total pressure

Fan system

Pressure (N m

–2

) Fan power

watts

Duty power

Fan duty point

Fan total press

ure

Fan^ p

ower^ c

urve

System resistance curve p = o^2

Fine blade tip

clearance

Downstream

guide vanes

Aerofoil blades

Air flow

Blade

angle

Rotation Pressure

Stall point

Power

Power Pressure

Q

FIGURE 5 Axial flow fan and graph showing its characteristic curves.

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