3.1 Ventilation

Question 1

General ventilation or ‘dilution’ ventilation is a term used to define the flow of air into
and out of a working area, so that any contaminants are diluted by adding some
fresh air. It can be provided by:

Answer 1

 Natural ventilation which relies on wind pressure and temperature differences to
move fresh air through a building and is usually not fully controllable.
 ‘Forced’ or mechanical ventilation which uses mechanical supply and/or
extraction to provide fresh air and is controllable.

Question 2

LEV might not be the right control solution when:

Answer 2

 There are a large number of widely-spaced sources
 The source is large and LEV is impossible to apply over the entire source
 The source position is not fixed
 The source emits relatively small amounts of contaminant (which is not toxic)
 The contaminant is offensive but not harmful.

Question 3

For LEV to work effectively the hood has to be carefully matched to the
contaminated air source that needs to be controlled. Consideration needs to be
given to the following:

Answer 3

Particle size of contaminant clouds
Visibility of particle clouds
Movement of particles in air
Abrasive or corrosive particles
Sticky dust, mist and condensate
Flammable or combustible substances

Question 4

The basic components of an LEV system are:

Answer 4

 An inlet, for example: a hood or enclosure, to collect and contain the
contaminant close to its source.
 Ductwork, to convey the contaminant away from the source.
 A filter / air-cleaner to remove the contaminant from the extracted air-stream
(Note: the filter should normally be located before the fan).
 A fan or other air-moving device to provide the necessary airflow.
 Further ductwork to and outlet or exhaust which discharges the cleaned air to
the outside atmosphere at a suitable point.

Question 5

Types of hood

Answer 5

Enclosing hoods
Receiving hoods
Capturing hoods

Question 6

LEV systems work effectively when the airborne contaminant cloud is contained,
received or captured by the hood. The effectiveness of LEV can be judged by:

Answer 6

 How much the hood constrains the contaminant cloud.
 How well the LEV induced airflow carries the contaminant cloud into the system.
 How little of the contaminant cloud enters the process operator’s breathing zone.

Question 7

Types of LEV Filter / air cleaner

Answer 7

Particle collectors [Fabric filters, Cyclones, Electrostatic precipitators]

Scrubbers [Venturi scrubbers, Self-induced spray collectors, Wet cyclone scrubbers, Packed tower scrubbers]

Question 8

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:

Answer 8

 Propeller e.g. general or dilution ventilation
 Axial not suitable for dusts
 Centrifugal are the most commonly used fans for LEV systems
 Turbo exhauster
 Compressed-air-driven air mover.

Question 9

The following factors can reduce the effectiveness of LEV:

Answer 9

 Poor design: inappropriate inlet for type and size of contaminant cloud, or
underpowered fan unable to capture contaminated air.
 Poor use: system not switched on when needed, or inappropriate positioning of
moveable hood.
 Unauthorised modification: can imbalance a system and adversely affect air
flows.
 Inadequate maintenance: damaged ducting, congested filters and damaged fan
blades will compromise the effectiveness of the LEV.
 Changes of work activity: generating more contaminant than the LEV was
designed to cope with.

Question 10

How often the LEV system should be checked depends on how complicated the
system is, how likely it is to fail, and the consequences if it does.

Regular maintenance inspections should be made of the following areas:

Answer 10

 moving parts that may wear, such as fan bearings or filter shakers
 non-moving parts, such as hoods, ductwork and seals (which can suffer physical
or chemical damage and wear)
 parts that deteriorate with use, such as filters or flexible ducting
 items that need regular attention, such as filters that need replacing, or removing
sludge from a wet scrubber.

This may involve daily operator checks and periodic checks by managers.

Question 11

A variety of qualitative and quantitative methods can be used to assess the
performance of LEV.

Qualitative assessment techniques

Answer 11

The ‘dust lamp’ or ‘Tyndall illumination’ makes fine particles visible. The ‘Tyndall
effect’ is the forward scattering of light. This is commonly seen when a shaft of
sunlight entering a building shines through mist, dust or fume in the air. The ‘dust
lamp’ reproduces this effect by producing a powerful parallel beam of light (see
Figure 3.7). It shows the density and movement of particle clouds in its path. The
lamp should be moved to illuminate different parts of the cloud and indicate the full
cloud size and behaviour.

Other forms of observational assessment can be made using smoke from pellets,
smoke tubes or smoke generators.
These can:
 show the size, velocity and behaviour of airborne contaminant clouds
 identify capture zones and boundaries
 confirm containment within a hood
 identify draughts, their direction and size
 show the general movement of air.

Question 12

A variety of qualitative and quantitative methods can be used to assess the
performance of LEV.

Quantitative assessment techniques

Answer 12

Inlet
Face or capture velocity. See Figure 3.10.

Ducts
Transport (duct) velocity is measured in a
straight section of duct, well downstream
of bends and other turbulence sources.
The duct velocity must be sufficient to
keep the particles airborne (otherwise)
particles may settle out in the ductwork
and affect the overall efficiency of the
system. The actual dust velocity required
is determined by considering the nature of
the particles that the system is handling
such as size and density. An anemometer e.g. thermal anenometer or hot wire, velometer 
or a pitot-static tube could be used. The choice of instrumentation is governed by the air velocity in
the duct (a thermal anemometer is more suitable if this is less than 3ms-1).
An anemometer responds to the cooling effect of airflow and the pitot-static tube
measuring, via a pressure gauge, velocity pressure which can then be converted to
velocity.

Fan/air mover
Measures can be taken of: fan direction and speed; the pressure difference across
the fan; and the volume flow rate. A big difference in pressure across the fan (i.e.
resistance) indicates that the system will not be as efficient.

Air cleaner
Filters: pressure differential across the filter.

Wet scrubber: pressure difference at the inlet and outlet, and the water pH if
relevant to the scrubbing performance.

Air sampling

Additionally, air sampling may be carried out as proof of the efficiency of a LEV
system and effective control of the contaminant where the hazard and potential risk
is great. (See element B4).

Air sampling may also include testing emission levels from the exhaust e.g. volatile 
organic compounds (VOCs). 

When carrying out testing all instruments should be calibrated; and where there may
be flammable atmospheres intrinsically safe instruments should be used.

Question 13

A thorough examination and testing of LEV can be considered to comprise three
stages:

Answer 13

Stage 1: a thorough visual examination to verify the LEV is in efficient working order,
in good repair and in a clean condition.

Stage 2: measure and examine the technical performance to check conformity with
commissioning or other sources of relevant information.

Stage 3: assess whether the control of worker exposure is adequate.