Grilles and Diffusers
A grille is a device for supplying or extracting air vertically without
any deflection.
A diffuser normally has profiled blades to direct the air at an angle
as it leaves the unit into the space, as shown below.
Grilles and Diffusers can be manufactured in:
Aluminium
Mild steel
Stainless steel
Plastic
Finishes can be white, any colour, brushed or anodised
aluminium, stainless steel or chrome.
Grilles and diffusers may be mounted in ceilings, floors, walls,
doors and in ducts. Some are suitable for horizontal or vertical
mounting while floors grilles tend to be especially strong to
withstand foot traffic.
Types of Grille and Diffuser
There are several types of grille and diffuser to choose from, as
follows;
Egg grate grille
Bar grille
Transfer grille
Louvre bladed diffuser
Straight bladed diffuser
Linear slot diffuser
Less commonly used diffusers are as follows;
o
o
o
o
o
o
o
o
Swirl diffuser
Floor outlet diffuser
Jet diffuser
Punkah diffuser
Barrel diffuser
Perforated diffuser
Valves
Plain face diffuser
Egg Crate Grille
The Egg Crate Grille is probably one of the simplest and cheapest
grilles.
In some cases the plenum box above the grille is visible from the
room below.
If air is to be removed by an extract ventilation system then a
diffuser with profiled blades to direct the air is not necessary and
an egg grate grille can be used.
Bar Grille
The blades of this type of grille are shaped as a bar compared to a
narrow blade.
The bar profile may be T shaped to reduce see through
visibility.
Some linear bar grilles have adjustable blades or angled blades to
reduce see through.
Transfer Grille
These are often used in doors and walls to provide ventilation but
stop the spread of smoke and fire should it occur. An intumescent
fire damper is incorporated in some Transfer grilles as a means of
isolation in the event of a fire.
Louvre Bladed Diffuser
These are used to supply air at ceiling level.
The curved blades deflect air in one, two, three or four directions
depending on where the diffuser is situated.
Diffusers may also be circular as shown below.
Straight Bladed Diffuser
These are cheaper than Louvre Bladed Diffusers.
Some types have adjustable bladed as shown below.
Linear Slot Diffuser
These are used for an alternative air distribution pattern and for
aesthetic reasons.
Air can be delivered around the perimeter of a room as opposed
to point sources interposed in a ceiling space.
Linear slots can be used for return air as well as supply.
In many cases blanking plates are used to create dummy slots
so that a continuous linear effect is created.
One to four slot widths are common depending on how much air
is to be delivered.
Some proprietary plenum boxes are quite tall are require a large
ceiling void, to overcome this a special box can be manufactured
to fit the available space.
Some Slot Diffusers have an adjustable air pattern so that the air
distribution can match the room shape.
If sound insulation is required in an air conditioning system then a
lined plenum box may provide sufficient sound attenuation.
There are some specialised grilles and diffusers as described
below.
Swirl Diffuser
High turbulence occurs immediately within the proximity of the
diffuser, which allows high air change rates to be successfully
supplied into the room.
With careful selection swirl diffusers can handle up to 30 air
changes per hour while still satisfying relevant comfort criteria.
Floor Outlet Diffuser
There is an advantage in supplying cool air conditioned air at low
level.
This can be achieved by floor outlets.
Floor diffusers are also useful in areas with raised floors so that
ductwork can be accommodated under the floor.
Another area for floor outlets is under large areas of glass so that
condensation and cold down draughts are eliminated.
Jet Diffuser
Used in areas where the ceiling is high and large air velocities are
needed to reach the occupied space.
The high volume and long throw of these diffusers makes them
suitable for large halls
Some jet diffusers can be reversible and rotatable which allows
the air jet to be adjusted for both pattern and deflection.
Punkah Diffuser
Used to give flexible air direction, sometimes in the back of seats
so that occupants have control over flow and direction of air.
Perforated Diffuser
Perforated diffusers are suitable for installation in 600mm ceiling
tiles.
The face plate may be pivoted down to gain access.
The diffusers are constructed from aluminium or stainless steel
the flat face plates are perforated, this means that they are easily
cleaned and are used in some kitchens.
Valves
Valves are used where the air flow needs to be regulated or
throttled and are suitable for supply and exhaust applications.
Diffuser Terminilogy
There are several terms used in diffuser and grille systems as
follows;
o Isovel
A contour of equal velocity.
o Throw
The distance from the terminal to the
position where the velocity has decayed to 0.5 m/s.
The lower velocities of 0.5 m/s and 0.25 m/s
are important because this is near the
comfort zone.
o Spread
This is the width of the 05 m/s isovel.
o Drop
This is the vertical distance from the
centre-line of the terminal to the bottom edge of the 025
m/s isovel.
Sizing Diffusers
Design Criteria for Diffuser Design
There are three criteria that determine diffuser size and number.
These are:
o Throw
o Pressure loss
o Noise level
The throw is discussed on the previous page.
Pressure loss is the pressure resistance to air flow and is required
in duct and fan sizing calculations.
The maximum Noise level is determined for various room types.
Example 1
Design the diffuser system for the supply and return air
ventilation for the room shown below.
DATA:
Total dehumidified air flow required to the room = 500 l/s
The Noise Rating level from Table 1.17 CIBSE guide for Television
(Audience Studio) is 25dB.
The throw required is Ceiling height 3.0 minus 0.8m= 2.2
meters.
Keep the pressure drop below 20Pa.
Use 4-way diffusers.
2.
Look at Waterloo catalogue and choose suitable sizes of
diffusers to provide air into the room and meet the design
criteria.
The design criteria are;
NR 25 and 20Pa.
Throw 2.2m,
If two diffusers are used each flow rate is; 500 /
2
=
250 l/s
From the sizing chart below for a flow rate of 270 l/s, an
M450/M500 gives 2.5m throw, 33NR, 33 Pa.
This is too noisy with too much pressure drop.
-----------------------------------------------------------------------------------------------If three diffusers are used each flow rate is; 500 /
3
=
167 l/s
From the sizing chart below for a flow rate of 180 l/s, an
M450/M500 gives 2.0m throw, 23NR, 15 Pa.
This is good, the throw is just slightly less than 2.2m.
-------------------------------------------------------------------------------------------------
If four diffusers are used each flow rate is; 500 /
4
=
125 l/s
From the sizing chart below for a flow rate of 135 l/s, an
M450/M500 gives 1.5m throw, - NR, 9 Pa.
The throw is not enough but the other criteria are met.
On the basis of the above data three
M450/M500 diffusers are chosen.
3.
Show diffuser layout on a drawing.
The final diffuser layout should consider ceiling layout and lighting
layout.
If ceiling tiles are used the diffusers position will be determined by
the tile grid pattern.
Careful co-ordination is required to install all the equipment into
modern ceilings including warning devices and other services.
Note! Noise Rating - NR - is common used in Europe. The Noise
Criterion - NC - is more common in USA.
Recommended Noise Rating - NR - Levels
The Noise Rating level for different uses should not exceed the Noise Ratings indicated in the table
below:
Noise rating
curve
Application
NR 25
Concert halls, broadcasting and recording studios, churches
NR 30
Private dwellings, hospitals, theatres, cinemas, conference rooms
NR 35
Libraries, museums, court rooms, schools, hospitals operating theaters and wards, flats,
hotels, executive offices
NR 40
Halls, corridors, cloakrooms, restaurants, night clubs, offices, shops
NR 45
Department stores, supermarkets, canteens, general offices
NR 50
Typing pools, offices with business machines
NR 60
Light engineering works
NR 70
Foundries, heavy engineering works
NR - Noise Rating Diagram
NR - Noise Rating Example
Recommended Noise Criterion - NC
The noise in different types of rooms should not exceed the Noise Criterion limits listed below:
Recommended NC Level
NC Curve
Equivalent Sound Level
dBA
Apartment Houses
25-35
35-45
Assembly Halls
25-30
35-40
Churches
30-35
40-45
Courtrooms
30-40
40-50
Factories
40-65
50-75
Private Homes, rural and suburban
20-30
30-38
Private Homes, urban
25-30
34-42
- Individual rooms or suites
25-35
35-45
- Meeting or banquet rooms
25-35
35-45
- Service and Support Areas
40-45
45-50
- Halls, corridors, lobbies
35-40
50-55
25-30
35-40
Type of Room - Space Type
Residences
Hotels/Motels
Offices
- Conference rooms
Type of Room - Space Type
Recommended NC Level
NC Curve
Equivalent Sound Level
dBA
- Private
30-35
40-45
- Open-plan areas
35-40
45-50
- Business machines/computers
40-45
50-55
- Private rooms
25-30
35-40
- Operating rooms
25-30
35-40
- Wards
30-35
40-45
- Laboratories
35-40
45-50
- Corridors
30-35
40-45
- Public areas
35-40
45-50
- Lecture and classrooms
25-30
35-40
- Open-plan classrooms
35-40
45-50
Movie motion picture theaters
30-35
40-45
Libraries
35-40
40-50
Legitimate theaters
20-25
30-65
Private Residences
25-35
35-45
Hospitals and Clinics
Schools
Type of Room - Space Type
Recommended NC Level
NC Curve
Equivalent Sound Level
dBA
Restaurants
40-45
50-55
TV Broadcast studies
15-25
25-35
Recording Studios
15-20
25-30
Concert and recital halls
15-20
25-30
Sport Coliseums
45-55
55-65
Sound broadcasting
15-20
25-30
Example - Estimating NC - Noise Criterion
The Noise Criterion - NC - of a noise spectrum like
62.5 Hz : 40 dB
125 Hz : 50 dB
250 Hz : 55 dB
500 Hz : 60 dB
1000 Hz : 50 dB
2000 Hz : 55 dB
4000 Hz : 45 dB
8000 Hz : 45 dB
can be estimated to
NC = 57 dB
