HVAC Air Distribution
Fundamentals
1. Duct is simply a path to distribute and manage the air flow to the required dessined zone/Areas
2. Duct shape may be circular,square,Rectangular or spiral
3. Galvanised iron, Aluminium,stainless steel,Black iron etc....are some of the commonly used duct materials
4. The thickness of the duct sheet is mentioned by its gauge
5. Aspect ratio is the ratio of duct width to hight
6. Aspect Ratio 1 to 4 is generally advised for duct sizing
7. The size and shape of the duct will depend upon the CFM, Available space for Ducting application etc......
8. The Duct is connected to the equipment through flexible Connector
9. All the brancehs are fitted with VCD
11. Thermal insulation is done for supply air duct
12. Accaustic insulation is done for supply duct
Some Fundamentals
1. Air out lets are selected based on the manufactures Design data
2. Type and Location of the air terminals will depend up on the room design and AESTHAICS
3. Louvers are fitted at the fresh air duct entry
4. Fittings and transitions are designed according to SMACNA And ASHRAE
5. Flexible Canvas connection should be used to connect the equipment and ducting system
6. Access should be provided for Dampers
7. AHU should be provided with drain pan and U trap
8. Gaskets to be used between duct joints to prevent leakage
Type of Ducts
Duct Designing Methods
1. Velocity reduction method
2. Equal friction method
3. Static regain method
Velocity (FPM) - Velocity (CFM) - / area (SQFT)
Main duct velocity - 1200 to 1500 fpm
Branch Duct velocity - 750 to 900 fpm
COLLAR/DROPPER Velocity - 300 to 500 fpm
PLENUM Boxes - 500 Fpm
Return air duct velocity shall be less than supply air duct velocity
Considers the total ducting will bear the same frictional loss
0.08 water/100 ft to 1 water/100 feet
The range will differ depending up on the application and use of space
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