Encyclopedia of Environmental Science and Engineering, Volume I and II

INDUSTRIAL HYGIENE ENGINEERING 519

In order to establish the type and size of fan to provide

the motive power to move the air it is necessary to establish

the total pressure loss throughout the system by adding the

losses for each item from one end to the other including any

filters and discharge louvres. The chosen volume flow rate

plus the total pressure required will identify the fan which

can be chosen from catalogues. If a multi-branched system

having several extract or supply points is designed then the

total volume flow of the fan is the sum of the flows in each

of the terminals but the total pressure loss is that from the

furthest end to the other and must not include pressures lost

in intermediate branches.

Multi-branch duct systems need to be properly balanced

to ensure that the correct flow rate is passing through each

branch. This can be done either by inserting dampers in each

branch and adjusting the dampers to provide the correct flow

or by inherently balancing the system by careful selection of

duct sizes in each branch, also constant volume flow devices

can be inserted to automatically set the branch to the required

flow. Where dust is to be carried then any obstruction inside

the duct such as a damper becomes a place on which dust

can be trapped leading to possible deposition and alteration

of flow characteristics, therefore inherent balancing is to be

preferred.

Also damper balanced systems have the disadvantage

that individual control dampers can be tampered with and

adjusted to the detriment of all the other branches in the

system. Once a system is balanced then the dampers should

be locked in their set positions.

Fans Fan types vary widely and are named: propeller,

axial flow, cross flow, mixed flow and centrifugal depending

upon their geometric shape. Some have their electric motors

in the airstream and some have them outside. If dirty, hot or

corrosive air is handled then it is important to ensure that the

motor is out of the airstream. Also many atmospheres con-

tain inflammable gases or vapours then flameproof motors

and switchgear are required to minimize explosion risks. In

some countries it is a legal requirement that where handling

flammable gas mixtures, fan motors must also be out of the

airstream.

Fan impellers and casings are made of various materials

to suit the type of air and contaminant that is being han-

dled. Corrosive contaminants will quickly destroy mild steel

fans; thus plastic or stainless steel is required, this would

also apply to the materials of the ductwork and fittings. Also

impellers accidentally striking the casing as they rotate can

give rise to sparking thus where there is an explosion or fire

risk materials that produce high temperature or high energy

sparks such as aluminium or magnesium or any alloys of

those elements must not be used.

A fan performs according to a characteristic curve of

pressure against volume flow rate which depends upon

its geometric shape and speed of rotation of the impeller

(Figures 4–6). The position of the duty point on this curve

is governed by the resistance to airflow of the ventilation

system against which it has to work. The resistance of the

system can be represented by a curve whose shape is a

parabola passing through the origin of the form: resistance

pressure proportional to volume flow rate squared ( p ∝ Q^2 );

see Figure 7. The fan efficiency also varies over its range of

duties; thus, it is important to choose one whose duty point

on the system lies on the most efficient part of the charac-

teristic curve. The manufacturer will advise on this and most

fan catalogues indicate efficiencies.

The majority of fans are driven by electric motors

although other sources of motive power can be used. Fan

drives can either be direct, where the impeller is keyed on to

the shaft of the motor or through some drive arrangements

such as V-belts and pulleys, gearboxes or variable speed cou-

plings. It is unusual for direct driven fans to be variable in

speed unless the electric motor itself has the facility of speech

change. Changes of speed can be arranged with V-belt drives

Sheet steel blades

Direct coupled

motor

Large blade tip

clearance

Support struts

Pressure

Pressure

Power

Power

Q

FIGURE 4 Propeller fan and graph showing its characteristic curves.

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