|Diffusers and Air Distribution|
Air distribution and movement – Diffusers and grilles
The purpose of air distribution in air conditioning systems is to create the proper combination of temperature, humidity and air movement in the conditioned space. The creation of the comfort conditions is a function of the above parameters, which must be within specific limits for each application (e.g. in office premises with high occupancy of people, the recommended air velocity should be smaller than 0.35 m/s).
A feeling of discomfort can be felt if there is a lack of uniformity in the conditioned space, or from time differential. Such feeling can be caused by large local temperature alterations, by air streams of high speed or by poor air distribution in respect to the local loads.
It has been observed that a temperature difference of 0.5 0C results in 0.08 m/s change in air speed. In addition, higher velocities and lower temperatures are better tolerated in the elbow area rather than the neck area. In a uniform space a temperature difference of 1 0C is the maximum value at which there are no complaints. Finally, for a group of rooms in a space (e.g. office floor), a temperature difference of 1.5 0C is accustomed.
In general, temperature differences during the winter period result in a greater feeling of discomfort than equal differences during the summer period.
The effect of air movement in the people occupying the conditioned space is shown in the table below:
In order to achieve the above requisites, air is supplied and extracted to and from the conditioned spaces by means of suitable air diffusers and air grilles through which the air flow, velocity and throw pattern can be regulated for specific demands.
The air comes through the diffusers in much larger velocities and different temperatures than the ones tolerated in the space. Due to diffusion, the velocity and the temperature differences are gradually decreased until they reach the tolerable limits. The phenomenon is described as follows: the air that comes out from a diffuser – the primary airstream, drifts the existing air in the space, creating the secondary airstream. The above masses of air mix together to create the final airstream.
Air diffusers selection
The selection of the suitable diffuser is of high importance, since this selection determines the attainment of the desired conditions. The criteria for the diffusers selection are the following:
1. Air Flow
The air flow required to go through a diffuser is the main selection criterion. The air flow combined with the required throw, the noise level and the allowable pressure drop determine the size of the diffuser.
Throw (T) is defined as the horizontal distance from the diffuser to a point in the mixed airstream, where the maximum sustained velocity has been reduced to a specific value, namely Vt. The terminal velocity Vt is usually determined at 0.2 m/s, while depending on the application can be 0.3 m/s or 0.5 m/s.
The throw for air grilles installed on walls are calculated for 2.5 – 3.0 m height from the floor. When more than one wall grilles are placed opposite each other, they must be selected for equal air flows and a throw equal to half of the distance between them.
For a given throw the air velocity in the room Vr will increase or decrease inversely with the installation height of the grille. Counter to, for a given diffuser size, air flow and Vr, the throw will decrease 10 cm for every meter increase in installation height when this exceeds 3 m.
The throw is a function of air velocity, the reduction of which is proportional to the air quantity of the room that is drifted by the primary airstream. This quantity is proportional to the diffuser’s perimeter; hence, for two diffusers of equal area, the one with the bigger perimeter has the smaller throw.
For grilles installed on walls, the throw is selected to be equal to the ¾ of the distance from the grille to the opposite wall.
Drop is the vertical distance between the grille’s axis and the intersection of the airstream at the end of the throw.
When the air that comes out from a diffuser is colder that the air in the space it has greater density and tends to (being heavier) descent, creating the phenomenon of drop. The drop is proportional to the temperature difference ΔΤ between the cold airstream and the air in the room. Drop is also proportional to the throw and in reverse to the discharge velocity Vk.
If there is no temperature difference, the airstream has a smaller drop due to its vertical expansion as it mixes with the air in the room. The value of the drop is approximately 10 cm for every 2.5 m of throw. The phenomenon of drop is important for the feeling of comfort and its value should be such so that the airstream in an occupied area (at a height of 1.8 m from the floor) does not have a velocity that could cause annoyance.
4. Diffusion and Deflection
The air that comes out of a diffuser, when there are no obstacles in the air stream, takes the form of a symmetrical beam (normally conical). If for a given distance from the diffuser we consider an intersection vertical to the axis of the beam, we call deflection (s) the maximum distance from the beam’s symmetry axis. Diffusion (h) is the maximum distance between the two extremes (Diffusion = 2 x Deflection).
The distance between the centres of two adjacent diffusers should (at least) equal the value of the diffusion of one of the diffusers for the required throw. The distance from the centre of the diffuser to the adjustment wall should (at least) equal the deflection of the diffuser for the required throw.
5. Pressure Drop
Diffusers cause a pressure drop in the airstream that must be taken into account when calculating the pressure drop of the fan. The passage of air through a diffuser causes a pressure drop due to the conversion of the air energy to speed and due to friction phenomena in the diffuser itself.
Special consideration should be given to return/extraction grilles, since the necessary pressure difference should be sufficient to push a mass of air through an opening and accelerate it from a low velocity to the higher duct velocity.
6. Discharge Velocity, Throw Velocity and Room Velocity
Discharge velocity Vk is the velocity of air that comes through a diffuser, measured at a distance of 2.5 cm from the diffuser. Throw velocity Vt is the maximum velocity at the limit of a specific throw distance. Room velocity Vr is the velocity of the air in the space. The room velocity is the value that affects the comfort conditions and is a function of the throw (T), the terminal velocity Vt, the temperature difference, as well as a function of any obstacles the air stream meets (beams, luminaires, etc.). The recommended velocity values for various applications are shown on the table below.
7. Noise Level
As the air comes through a diffuser, noise is generated. The measurement of this noise is necessary in order not to exceed the limits of comfort zone. As the noise level is related to the discharge velocity, the limitation of the velocity holds the noise within normal levels. Indicatory values for the noise level are shown on the table below (TEE-Greek Technical Chamber instructions).
8. Position of diffusers
Diffusers must be positioned in places where unwanted air drafts caused by loads are neutralized. If for example there is a heating source in the conditioned space, its thermal effect can be counterbalanced by directing a cool air stream towards it, or by placing a return grille near it.
As far as supply diffusers are concerned, the best position is near the ceiling in order to avoid annoying drafts. The selection of the required throw should be made conservative and should not exceed the ¾ of the distance from opposite walls. A larger throw will cause annoyance, while a small throw rarely causes any problems. The placement of supply diffusers near windows and glass walls is recommended in order to reduce the effects of radiation and unwanted drafts.
For return grilles it is advised to avoid the use of large grilles in areas occupied by people, as the large masses of return air could create drafts. The use of volume dampers for the flow control should also be taken into account as the turbulence that a closed damper produces results in high noise levels. In general the use of volume dampers in the air ducts and away from the grilles is advised.
Where possible, the return of air through ceiling diffusers and grilles should be avoided; during winter the warm air masses will not reach the areas near the floor, while during summer there is the possibility that the cool air will by-pass before reducing the space loads.
The optimum solution is to use wall grilles for return air, placed near to the floor; in winter the colder air near the floor is extracted first and is replaced by warmer air streams, while in summer the air stream follows its expected flow. In addition, the return of air through floor grilles should be avoided, as the grille will act as a vacuum cleaner resulting in collection of dust in the filters and a reduction in the efficiency of coils in the air conditioning units.
On the side, the intake velocity in return/extraction grilles is noticeable only in areas close to them and does not considerably affect the movement of air in the space. In general, for supply and return ducts of the same area and frontal velocity on the edges, the 10% of this velocity is met at a distance of 30 equivalent diameters for the supply grille, while the same velocity is met at a distance of one equivalent diameter from the return grille.