Download Local Exhasut System - Design and Layout PDF

TitleLocal Exhasut System - Design and Layout
TagsChemistry Procurement Dust Ventilation (Architecture) Particulates
File Size9.0 MB
Total Pages109
Document Text Contents
Page 1

Page 1 of 109

Health and Safety

HSE Books

This is a free-to-download, web-friendly version of HSG258 (Second
edition, published 2011). This version has been adapted for online use from
HSE’s current printed version.

You can buy the book at and most bookshops.

ISBN 978 0 7176 6415 3
Price £20.00

This book provides guidance on the supply of new local exhaust ventilation (LEV)
equipment. It describes the principles and good practice of deciding on, designing,
commissioning and testing cost-effective LEV.

The guidance is written for the suppliers of LEV goods and services, but will also be
helpful for employers and managers in medium-sized businesses, and trade union
and employee safety representatives. All of these groups need to work together to
provide, maintain and use effective LEV and to reduce exposure from inhalation of
hazardous substances.

The book contains information about: the roles and legal responsibilities of
suppliers, and of their clients as employers; competence; principles of good design
practice for effective LEV hoods and their classification; ducts, air movers, air
cleaners; and system documentation – with checking and maintenance schedules,
and the marking of defective equipment.

It also includes guidance on the specification of LEV; the supplier’s quotation;
commissioning; zone marking; the user manual and logbook; testing and hood

This second edition updates the guidance based on feedback from industry.

Controlling airborne
contaminants at work
A guide to local exhaust ventilation (LEV)

Page 2

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Health and Safety

' Crown copyright 2011

First published 2008
Second edition 2011

ISBN 978 0 7176 6415 3

You may reuse this information (not including logos) free of charge in any format
or medium, under the terms of the Open Government Licence. To view the licence
visit, write to the
Information Policy Team, The National Archives, Kew, London TW9 4DU, or email
[email protected]

Some images and illustrations may not be owned by the Crown so cannot be
reproduced without permission of the copyright owner. Enquiries should be sent to
[email protected]

This guidance is issued by the Health and Safety Executive. Following the guidance
is not compulsory and you are free to take other action. But if you do follow the
guidance you will normally be doing enough to comply with the law. Health and
safety inspectors seek to secure compliance with the law and may refer to this
guidance as illustrating good practice.

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Fans and other air movers

186 The fan is the most common air mover. It draws air and contaminant from the
hood, through ductwork to discharge. There are five general categories of fan:

propeller; ■
axial; ■
centrifugal; ■
turbo exhauster; ■
compressed-air-driven air mover. ■

Propeller fans

187 Propeller fans are often used for general or dilution
ventilation. They are light and inexpensive to buy and
run, with a wide range of volume flow rates. However,
they will not produce much pressure and operate best
against low resistance.

188 The fan blades are of sheet material (metal or plastic) mounted in a plate or
cage and on a hub that is attached directly to the shaft of an electric motor, or
belt driven. Generally, they are unsuitable for ducted systems with a moderate
resistance or with particle filters.

Axial fans

189 Axial fans are not suitable for dusts. They are
compact, do not develop high pressures and cannot
overcome the resistance to flow that many industrial
applications require.

190 The impeller fan blades are on a rotating hub mounted in a short cylindrical
casing. The fan is in the duct. Unless the contaminant is flammable or corrosive,
the motor is also in the duct.

Centrifugal fans

191 Centrifugal fans are the most commonly used fans
for LEV systems. They generate large differences in
pressure and can produce airflows against considerable

192 The impeller fan blades are mounted on a back plate, often within a scroll
casing. Air is drawn into the centre of the impeller along the line of the drive shaft.
The air is ejected at a tangent to the impeller.

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Types of centrifugal fan
193 The blade shape characterises the type of centrifugal fan:

Radial blade (most commonly, paddle type). These are robust, easy to maintain, ■
clean and repair. They can convey heavy dust or product loads. Radial blades
are often a solution for dusty contaminant clouds.
Forward curved multivane. These have many relatively small blades. The blade ■
tips incline towards the direction of rotation. Rotational speed is usually lower
than with other types of centrifugal fan. Forward curved multivane blades may
be unsuitable for dusty contaminant clouds.
Backward bladed (curved, flat, laminar, aerofoil). These can overcome high ■
system pressures. With high dust loads, dust can accumulate on the impeller
which can lead to imbalance and vibration.

Turbo exhausters (multi-stage centrifugal)
194 Turbo exhausters can generate the high suction pressures needed to power
low volume high velocity (LVHV) systems: they are not conventional fans. They use
high-precision blades that are susceptible to damage by dust and require a filter to
protect the exhauster.

Compressed-air-driven air movers
195 Compressed-air-driven air movers are appropriate where electrically powered
fans are unsuitable, eg where access is difficult, or where there are flammable
gases. They are small, inexpensive and easily portable. Their main disadvantages
are the high running cost (compressed air is expensive) and high levels of noise for
relatively small amounts of air moved.

Fan location

196 The objective is to have as much of the ductwork as possible under negative
pressure. In particular, indoor ductwork upstream of an air mover should normally
be under negative pressure. Leakage in this ductwork will then be inward and
contaminated air should not escape into the workplace. One solution is to locate
fans and positively pressurised ductwork outside occupied areas.

Volume flow rate

Ventilation hardware










Fan curve


Figure 35 Fan curve showing intersection with system curve

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Term Alternative terms De�nitions; units Comments; conversions

Total pressure The algebraic sum of the static
and velocity pressures.

The pressure exerted by
moving air, were it brought to


Conveying velocity Air velocity to convey particles
and prevent deposition in

Turbulence Non-laminar air movement.


The pressure of a vapour in
equilibrium with its liquid (or
solid) phases.

1 Pa = 9.86 ppm

At 25 °C, mg/m3 = ppm x Mol.
Wt / 24.45

Vector Velocity and

The speed and direction of a
contaminant cloud or draught.

No/low vector, eg vapour from
High vector, eg dust jet from
angle grinder.


Dynamic pressure

Pressure exerted by air due to
its motion.

The difference between the
total pressure and the static


The section within an opening
at which the streamlines first
become parallel after entering
that opening.

Wake Turbulent wake
Recirculation zone

A low-pressure region that
forms downstream of a body in
an airflow.

Complex airflow patterns
can appear in the wake
downstream of a worker.
Contaminant can be drawn into
the breathing zone.

Working zone The volume in the workplace
where an activity is generating a
contaminant cloud.

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Published by HSE 06/11 Page 109 of 109Page 109 of 109109

Health and Safety

Further information

For information about health and safety, or to report inconsistencies or inaccuracies
in this guidance, visit You can view HSE guidance online and
order priced publications from the website. HSE priced publications are also
available from bookshops.

British Standards can be obtained in PDF or hard copy formats from BSI: or by contacting BSI Customer Services for hard copies
only Tel: 020 8996 9001 email: [email protected]

The Stationery Office publications are available from The Stationery Office,
PO Box 29, Norwich NR3 1GN Tel: 0870 600 5522 Fax: 0870 600 5533
email: [email protected] Website: (They are also
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