Guidance on the Control of Odour and Noise from ... - Defra

Table 4.11 Maximum velocity for supply and return air openings (grilles and
terminals) [12]
Permitted air velocity (m/s)
Critical Normal Uncritical
Supply 1.5 2.5 3
Return 2 3 4
Notes
A critical situation may require a design criterion of less than NR35
A normal situation may require a design criteria of about NR40
An uncritical situation may permit a design criterion of more than NR40
• Silencers may be required where additional attenuation is necessary. A range
of silencers is available and it may be necessary to insert in-duct silencers both
upstream and downstream to prevent radiation of fan noise through ductwork.
Figure 4.6 shows examples of dissipative duct attenuators. These should be
fitted as close to the fan as possible (but not so close as to lead to a nonuniform
air–flow velocity across the face of the silencer). Where this is not
possible, the intervening ductwork should be acoustically lagged. It may also
be necessary to enclose or lag the fan. Where fans are used to push gases up
a stack, silencers containing absorbent material can sometimes be mounted
directly on top of the stack. However, where gases are hot, wet or dirty, the
infill may need to be protected.
• Acoustic louvres on exhausts and inlets can greatly reduce environmental
noise. However, their performance can sometimes increase back-pressure or
the velocity of the air flow leading to increased noise.
4.9 Fire Suppression
The significant presence of flammable grease and related particles contained within
kitchen extract systems, combined with the possibility of fire ignition caused by the
cooking equipment, creates a hazard level above that which is normally encountered
in other ventilation systems.
Where ventilation systems exhaust grease laden air and vapour mixtures from a
kitchen, the design should provide an acceptable level of protection to ensure the
well being of occupants and fire fighting personnel, and limit the damage to the
property and cooking equipment.
The correct maintenance and use of cooking appliances and ventilation systems
should prevent potential fire. It is however important that in the event of a fire it is
immediately detected and extinguished and prevented from spreading to other areas
of the building.
Fire suppression is the most commonly used form of protection from fire. These
systems use agents, which provide protection to the kitchen canopy.
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