CHAN CHUAN CHANG METAL WORKS
The Specialist in Air Diffusion Equipment ...
QUESTIONS & ANSWERS
How to select an appropriate grille?
Grilles are located in the ceiling, sidewall or floor by the designer's choice or by the physical constraints of the building. The type of grille used is also selected by the Architect's aesthetic requirements and the designer's need to overcome stagnant zones and deliver sufficient cooling to occupied spaces.
Two questions have to be asked:
(i) What are the specific room-use characteristics and the structure components of that room and,
(ii) What are the performance requirements required of the actual supply grille required.
Four important aspects evolve from these two questions:
(a) the air pattern requirements
(b) the throw requirements
(c) the air quantity
(d) the desired noise levels
The air delivered into each space is determined by the overall system design. The selection of the number and type of grilles is based on distributing the required amount of air to the space at acceptable pressure drips and noise levels.
The dispersement of air into a space is a function of the shape of the room to be supplied. For example, a large area such as a library, supermarket, open plan office, school room, ballroom or auditorium may be delivered in a series of overlapping space modules with may be square or rectangular to suit:
- Square or rectangular louver face diffusers
- Round ceiling diffusers
- Slot diffusers
- Light air boots
Lighting fixtures, exposed beams, support columns and office partitions may all have a strong bearing of the type of diffuser which best suits that given situation.
What is mean by the air pattern requirement?
Both the positioning of the outlets and the shape of the area to be conditioned effects the dispersement of the air.
For example, a small office may only require a single small ceiling diffuse, located in the center of the room. However, a large area, like a supermarket, library or schoolroom, may require a large number of diffusers, evenly spaced throughout the area, to produce an overlapping movement of air.
Furthermore, lighting fixtures, exposed beams, support columns, office partions and aesthetic consideration may all have a strong bearing on the frame style and core pattern that will meet the specific requirements.
What is throw requirement?
The proper throw condition will be achieved, if the two following extremes of conditioning do not arise:
Inadequate conditioning which fails to adequately cover the total area.
Excessive air quantities relative to the capacity and positioning of the diffuser thus creating drafts.
Basically, the distance from an outlet to the nearest enclosing wall or the distance from a diffuser to the intersection of its airstream with that of a second diffuser, is considered the throw requirement.
For high ceiling applications, the throw is usually measured to extend to the 1500 mm level in the room.
How to determine air quantity?
The total volume of air to be delivered to each area, is determined by the overall system design. Thus the number of outlets per room, determines the volume to be transmitted through each outlets.
What is NR level?
The maximum permissible NR levels from each grille relate directly to the quantity of air transmitted through any given core style.
The following table may be used as a guide to the generally acceptable NR levels for various common use situations:
NR LEVEL TYPICAL APPLICATION
20-25 - Radio, Televisions, Studios, Churches
25-30 - Live Theatres, Opera Halls, Concert Halls, Board Rooms
30-35 - Conference Rooms, Movie Theatres, Lecture Rooms, Private Offices
35-40 - Libraries, General Offices, Laboratories, Restaurants
40-45 - Halls, Corridors, Cafeterias
45-50 - Storerooms, Large Department Stores, Supermarkets
Over 50 - Manufacturing Areas
| OCTAVE BAND CENTER FREQUENCY (HZ)
The computed sound pressure levels were then graphed to obtain spectrum levels in terms of overall NR ratings.
How to avoid 'Dumping'?
In sidewall applications the selection of supply grille needs to be carried out with care to minimize 'dumping' or dropping of the cold air down onto occupied spaces. Drop can be reduced by arching the air upward from a sidewall outlet. For this to most effective the outlet should be located some s\distance below the ceiling. When air is discharged horizontally along a ceiling in a free space (without intervening walls), the air stream drops near the end of this trajectory due to the buoyancy effect of the cool air. In heating applications the opposite occurs.
The drop of the air steam is primarily dependent on the quantity of the air and the temperature differential. Since the drop is greater with a large quantity of air from a given outlet, multiple outlets with smaller quantities of air can be used to reduce the drop. Drop from linear diffusers can be reduced by dividing the diffusers into active and inactive sections, with the active sections handling an effectively smaller quantity of air. In order to provide a positive breakout of the air stream, the active portions of the continuous grille should be separated by inactive lengths of between 300 mm and 1000 mm dependent upon length of active section.
In VAV applications, selections should be made on the basis of a satisfactory air pattern at the lowest flow condition. This should then be re-checked at the maximum flow condition to ensure that both the noise criterion and the air pattern are acceptable. This is often difficult with marginally aerodynamic grilles and diffusers and often some degree of comprise is required. In some applications adjustable pattern diffusers may be necessary to achieve satisfactory performance.
The 'rule of thumb' to use is always select the most aerodynamically efficient grille or diffuser - it will generally perform better under all circumstances.
How to select Floor Grille?
The Floor grille throw data is the sum of the vertical and horizontal projections based on the outlet being mounted close to the wall and with wall and ceiling effect. The air stream is attracted to the wall even if the projection is deflected 15 degrees towards the room and then travels along the ceiling until its terminal velocity is met. This system is particularly suitable for northern hemisphere winter applications but has been found to be energy intensive for southern hemisphere or tropical climates.
If used with window drapes it is preferable to use 0 degrees deflection pattern and project the air stream directly up the drapes. When the air hits the ceiling, it fans out in all directions. Normally a space will be effectively cooled below the vertical throw of the air stream, thus with a 2700 mm ceiling the maximum throw from the outlet should be at least 2700 mm when the outlet is located on the floor.
How to select Linear Grille?
The linear grille was originally conceived to handle large quantities of air at low noise levels and provide an air pattern of exceptional flexibility. This has been achieved and their use has been widespread particularly in open plan offices and large commercial spaces such as lobbies, foyers, airport terminals etc. their design philosophy was to keep the mass flow/surface area ratio of the supply air as small as possible thereby providing good control of throw and drop.
To select a linear grille:-
a) Determine the flow/meter necessary to meet the ventilation design requirement.
b) Select the number of slots necessary to meet these volume requirements.
c) Check the pressure drop at the selected flow rate.
d) Check the noise level generated at this flow rate.
e) If either (c) or (d) are outside the specification limits, reselect to lower noise level and pressure drop.
f) Recheck throw to ensure adequate.
g) Design the plenum to suit these linear diffusers by reference to the "Plenum Duct Design for Linear Grilles".
What is the plenum duct design for Linear Grilles?
Vipac's Air Distribution Laboratory has conducted extensive tests on various duct and diffuser lengths and sizes, and inlet connections to determine plenum duct design requirements for good air distribution from continuous length diffusers.
The controlling air flow conditions for the plenum duct design criteria considered during testing were primarily:
Equal discharge velocities along a continuous diffuser length.
A discharge air stream perpendicular to the axis of the diffuser. If the discharge angle is much less than 90 degrees from the axis of the diffuser the air distribution will be poor. Some parts of the space will get practically no air whilst others are over supplied.
Two types of plenum duct in inlets were tested:
Center branch duct connection to the plenum duct.
End supply inlet to a plenum duct.
Depending upon the application - sidewall, sill, floor or ceiling - the center supply duct approaches the plenum duct either from the bottom, the top or the side. For an end supply inlet, two basic designs were tested.
Ducts having constant cross-section
Due to the complexity of design and marginal performance, end supply inlets are not recommended unless there are no other alternatives.
What are the procedures in selecting plenum duct?
Knowing the diffuser model and length required for the flow, throw and sound limits specified, refer to the recommended Plenum Duct Size Table and select a size that will suit the available space. If none of the recommended plenum sizes will fit the available space, determine the cross-sectional area.
Determine the number of branch duct connections required for the diffuser length being used, based on one connection per 4 m of diffuser.
Determine the size and location of the perforated distribution baffles if required.
Why there is ceiling smudge?
Investigations in the VIPAC Engineers and experience in the field both indicate that ceiling smudge is usually caused by the dirt in the secondary air (the air in the space being conditioned). To equalize temperature and to reduce discharge velocities, air diffusers mix the secondary room air with the primary air being delivered to the space being conditioned. In the process of this mixing, the dirt in the secondary air may impinge on the ceiling can cause smudging.
To minimize smudging the design and adjustment of the diffuser should be such that the dirt does not impinge on the ceiling. CCC ceiling diffusers are designed to reduce impingement. They can be adjusted so that the air stream will lop away from the ceiling, and therefore, greatly reduce the impingement.
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