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The term duct refers to the air side. While the alternate term in water side is pipe. The AHU ( Air Handling Unit) consists of filter, cooling coil etc. When air passes through the filter and then the cooling coil the air gets treated and it is now ready for supply into the room. When this air has to be delivered into various rooms then ducting is done. The ducting is done over every room. Essential air supply for each room is calculated and the duct is designed as such. This is the SUPPLY air duct. It supplies air to rooms. At the end of the duct grills or registers or diffusers are used. Now when air of certain quantity is supplied into a room, The same air of certain quantity has to be removed in order to maintain the pressure inside the room. So this air is taken back to the AHU for recirculation. The duct that is used to remove air from the room to the AHU is called the RETURN air duct.
There are three things mainly controlled by using plenum box :
1. Duct components vibration control
2. Duct noise control.
3. Duct easily distribution.
First, we should know Why the Plenums are installed and second, Where are they installed ?
1. Why ? : Plenums are installed just before an Air Terminal for ensuring the air is uniformly dispersed. And to reduce the velocity of discharging air in order to meet the FACE VELOCITY of the terminal which should not be more than 2.5 m/s. Also to reduce the hissing sound created by discharging air just by dropping the velocity pressure. Plenums also serve the function to facilitate the Fresh & Return air mixing when the system is designed for Return Air circulation. Plenums are also helpful to provide multiple branches when space is a concern for Ducting to branch out towards multiple locations at a single point.
2. Where ? : Before the Air Terminals, Suction end of the Air Side Equipments viz AHU, FCU for Return and Fresh Air and at the Discharge end or Mouth of the Air side Equipment. And at the suction side of the Exhaust Air fans.
Hope this serves as the answer to your question.
* Usage of plenum box as an example:
- Supply conditioned air to multiple branch ducts.
- For supply, return and exhaust air outlets.
- For supply and return of AHU.
- In case of mixing.
* Usually we use the plenum box for:
- Ensures a continuous flow to the outlets.
- To achieve a stable airflow.
- Sound attenuation.
- Gives a greater flexibility in mounting of outlets.
The main reason of using plenum box is to ease up the installation of grills and/or diffusors. Direct installation is not recommended as per standards in order to eliminate sound, vibration. It also ensures a uniform airflow through the grill and/or diffusors. We install plenum boxes above the ceiling that needs to be flush to ceiling interior surface.
planum box is a junction box used to connect flexible duct to air outlet either linear grill or round or square type.. may be supply type or return type
-----main purpose of using plenum box
. to reduced sound . and for proper air flow through out the air outlet and for rigidly fix the girl into plenum box.
for supply type a air filter may be installed for clean air supply .
---location of installation
it is installed just before end out to air outlet for e.g ( linear grills, round,square diffuser etc
hope this answer will help you
noise and vibration reduction ,easy access,duct distribution ,stability of flow and greater flexibilty of grille,diffuser etc.
For the better air flow and distrbution
