SWEGON AB Indoor Climate Systems 2004 - Air distribution products - Rev. 5 June, 2007
www.swegon.comMixing ventilation
General
Mixing ventilation can generally be used in comfort ventilation, i.e., regardless of whether the ventilation air is used for cooling or heating.
In the chapter "Ventilation principles", the flow forms for the different supply air alternatives were discussed. The design must take into consideration the following:
- degree of activity/room type
- room dimensions
- air volumes etc.
- possible cooling requirement
- resulting air velocity in the room
- resulting sound level
- It is important to determine the degree of activity in order to decide what limits to apply for comfort.
- The room dimensions influence the flow pattern and thus the comfort of the room. It is essential during the planning stage to correct throw data to ensure compliance with current desig regulations.
- The least possible air volume is based on the hygiene requirements. For general ventilation in offices, 12 to 15 l/s, person can be considered to be the minimum outside air volume.
- A calculation in which the internal and external loads as energy accumulation in the building are taken into consideration, must form the basis for calculating the required cooling. Together with the comfort requirements, this forms the foundation for the selection of the system solution and a suitable supply air flow.
- The diffusers are described with a throw with a final velocity of 0.20 m/s. In different operating situations, this final velocity can be corrected so that the correct flow can be attained without draught problems. A description of this procedure is given in this section.
- A calculation should always be made of the resulting sound from air terminals and duct systems in relation to the actual sound absorption in the room. The procedure for this calculation is given in the "Acoustics" section.
Other aspects which must be considered include vertical spread pattern:
When cooled or heated air is supplied vertically to a room or with a certain vertical injection angle, the heated air shortens and the cool air lengthens the throw depending on the density of the supply air. These conditions can be calculated and Swegon have developed a special computer program for this type of operation. The air flow, temperature variations between supply air and room air and the injection angle are given in this program.Alternative mounting
The stated throw for slot diffusers, cone diffusers and perforated diffusers relates to ceiling-mounted units. If the supply air terminal is mounted in a freely suspended position and the jet is directed so that it adheres to the ceiling, the throw is reduced as a result of the induction at both sides of the supply air jet. The following conditions exist:
l0.2 free suspended = ky x l0.2 |
|
where ky = correction factor depending on the distance, y, between the diffuser and the ceiling. | |
Figure 32.Correction factor ky as a function of the distance, y, between the device and ceiling.
The information applies to wall jets, i.e. with adherence to the ceiling. If the grille is mounted more than 0,2 m from the ceiling, the throw decreases in accordance with the formula:
l0.2 up to ceiling = ky x l0.2 |
|
where ky = correction factor depending on the distance, y, between the diffuser and the ceiling. | |
Figure 33.For wall-mounted grilles, where the throw is measured with the diffuser mounted 0.2 m from the ceiling, the above graph illustrates ( l0.2 ) for other distances between the grille and the ceiling.
Combining supply air jets
When two or more supply air terminals are positioned so closely to each other that their jets combine, the throw is lengthened. To calculate the extended throw, please refer to our calculation program ProAir, which can be accessed on our website or at your nearest sales office.
Figure 34.Combining of supply air jets.
Throw
General
The throw should be stated with a terminal velocity of 0.2 m/s in accordance with VVS-AMA. For calculations using other terminal velocities, please refer to the ProAir calculation program.
Conversion of throw
For different reasons, a higher air velocity can be accepted when a supply air jet reaches the occupied zone or meets an obstruction, such as a wall. In a limited area the air velocity can be calculated according to the figure below.
Figure 35.Calculations of air velocity at the distance x from diffuser.
x = | distance in m from the air terminal to the point in the jet where the air velocity is vx m/s. |
Vx = | air velocity at distance x from the air terminal device. |
Example:
An air terminal has a throw of l0.2 = 3 m. The throw l0.3 then becomes:
The minimum distance between supply air terminals
The minimum distance between two supply air terminals, which have their jets directed at each other, can be reduced. This is because the velocity of the core jets can be higher at the mixing point, without the combined velocity of the jets in the occupied zone exceeding 0.2 m/s. This is a result of the strong mixing effect of the two jets, which retards their velocities. The following relationship is applicable:
Lm = kv (l0.2 unit 1 + l0.2 unit 2)
Lm = The minimum distance between the supply air terminals.
kv = Correction factor, see Figure 37.Example:
Two supply air terminals, each with a throw of l0.2 = 5.0 m have a minimum distance at Dt = 6°C of Lm = 0.72 (5.0 + 5.0) = 7.2 m.
Figure 36.The minimum distance Lm between supply air terminals.
B = Occupied zone
The minimum distance between the supply air terminals and a wall
A jet which strikes a wall is permitted to have a higher terminal velocity than 0.2 m/s due to the retarding effect and the deflection which occurs.
The following relationship applies:
Lv = kv· l0.2
Kv is obtained from figure 37. Note that the formula above does not generally apply to outer walls, where convection flow or a cooling effect can occur.
Example:
A supply air terminal with a throw of l0.2 = 5.0 m and Dt = 4°C can be placed Lv = 0.67 · 5 = 3.35 m from the wall.
Figure 37.The relationship between correction factor kv and the temperature difference Dt°C (tsupply - texhaust)
The minimum distance between the supply air terminals with increased ceiling height
The stated throw applies to a normal ceiling height of 2.7 m. With higher ceilings, the distance between the ceiling and the occupied zone is considered to be a retardation distance for the air jet. The figure illustrates the relationship between the distance between two supply air terminals and the distance to the occupied zone.
LmA = Lm - A
Example:
Two supply air terminals, each with a throw of l0.2 = 5.0 m and Dt = -6°C and mounted in a ceiling with a height of 4.5 m, can have a distance of 5.4 m. Lm = (5.0 + 5..0)· 0.72 = 7.2 m. Calculation of the distance LmA = 7.2 - (4.5 - 2.7) = 5.4 m, i.e., the devices can be placed with 5.4 m between the inner edges.
The figure shows the relationship for the distance between the supply air terminal and the wall, which can also be corrected because the air jet has a longer retardation distance.
LVA = LV - A
Figure 38.Minimum distance between supply air terminals with increased distance A + 2700.
B = Occopied zone
Minimum distance between the supply air terminals
Hanging obstructions, such as light fittings, should not be placed near the supply air terminal.
Various alternatives for this situation:
Figure 39.Different alternatives for obstructions in ceilings.
1 = Ceiling terminal
2 = Side wall terminal
3 = Wall grille
4 = Front, via window sill
For all the alternatives, the minimum distance, Lmin, depends on the spread pattern