What is the Aspect Ratio of the air duct?

What is the Aspect Ratio of the air duct?
Aspect Ratio of a duct is the ratio between the long side and the short side of a rectangular duct.
Simply written is
Aspect Ratio = long side ÷ short side
Example:
Air duct size Aspect Ratio
400 × 400 mm 1:1
600 × 300 mm 2:1
800 × 200 mm 4:1
1000 × 200 mm 5:1
The higher the Aspect Ratio value, the flatter and longer the air pipe.
How necessary is it?
It is very necessary because Aspect Ratio affects
1. Pressure Drop
2. Noise
3. Air velocity in the pipe.
4. Fan/ESP size
5. Installation area above the ceiling.
6. Price of production and installation work.
7. Strength of the air duct.

Important principles
A good air duct should be close to square.such as
* 400 × 400 mm
* 500 × 400 mm
* 600 × 500 mm
This type of pipe will have
* Less surface area exposed to air
* Lower Friction
* Less noise
* Uses less fan power
* Easier to produce

Pipes that are very flat will have problems.
For example:
1000 × 200 mm
Even though the cross-sectional area is equal to
500 × 400 mm
is 200,000 mm² equal.
But the 1000 × 200 mm pipe will have
* The circumference is greater than
* Greater friction surface area
* Pressure Drop is higher
* Sound over
* Must use a stronger fan.
* The air pipe frame is easier to vibrate.
* Need to strengthen more

Simple comparison example
Assume the same cross-sectional area, approximately 0.20 m².
Pipe size, cross-sectional area, perimeter, Aspect Ratio
500 × 400 mm 0.20 m² 1.80 m 1.25:1
800 × 250 mm 0.20 m² 2.10 m 3.2:1
1000 × 200 mm 0.20 m² 2.40 m 5:1
It can be seen that even though the area is the same But the pipe is flatter. Has more circumference
This means that more air comes into contact with the pipe surface. Therefore causing more friction.

Recommended Aspect Ratio values for HVAC work
In general, in the design of air ducts
Aspect Ratio suitability
1:1 to 2:1 Very good.
2:1 to 3:1 works fine.
3:1 to 4:1 can be used, but be careful of Pressure Drop.
More than 4:1 Should be avoided if not necessary.
More than 5:1 is not recommended except in areas where it is really required.

Guidelines for actual use:
Try not to exceed the design estimate. 4:1
If possible, it should be in the range 1:1 to 3:1.

And if above the shallow ceiling Do I have to use a flat pipe?
You can use it, but you have to be careful.
For example, there is limited space above the ceiling, requiring the use of pipes.
* 1000 × 200 mm
* 1200 × 250 mm
* 1400 × 250 mm

In this case, you should check further:
* How much does Friction loss increase?
* Velocity still not too high?
* Is the noise from the air nozzle too much?
* Is the air duct trembling or swollen?
* Do I need to increase the thickness of the sheet or add support?
* Is the ESP fan enough?
* Is there enough space to do Transition or Elbow work?
Impact of high Aspect Ratio

1) Pressure Drop increased.
The pipe is very flat, causing more air to rub against the surface of the pipe.
Makes Friction Loss higher The result is
* Fan requires more pressure.
* ESP value increased
* Consumes more electricity
* The destination wind may not arrive.

2) The sound became louder.
When the pipe is too narrow on one side The wind will not disperse well.
especially the area
* Elbow
* Take-off
* Transition
* Damper
* Diffuser Neck
May cause wind noise, turbulence, and vibration more easily.

3) Air ducts are more difficult and expensive to produce.
A more flat pipe will have more surface area.
Therefore, more steel plates are used. and may need to reinforce the frame moreThe result is
* Higher material prices
* Higher production wages
* Use Support more
* More difficult to install

4) Risk of bending or shaking of the air pipe.
Pipes with very wide sides, such as 1200 × 200 mm
The wide side has a chance

* bend
* Shake
* There is a moaning sound.
* Reinforcement required

5) Balancing the wind is more difficult.
Pipes that are very flat, especially before entering the dispenser or branch
This may result in poor velocity profile distribution. The result is

* The air flows unevenly.
* Difficult to adjust Balancing
* Some nozzles deliver strong wind, some nozzles have weak air.

Real example
Case 1: Well designed
Requires a pipe area of approximately 0.24 m².
Choose:
600 × 400 mm
Aspect Ratio = 600 ÷ 400 = 1.5:1
Considered very good, easy to use, Pressure Drop is not too high.

Case 2: Above the ceiling limit
Similar cross-sectional areas
Choose:
1200 × 200 mm
Aspect Ratio = 1200 ÷ 200 = 6:1
Can be used only when necessary, but must check for Pressure Drop, noise and pipe structure well.

Easy to remember summary
Low Aspect Ratio = fat pipe / close to square / good for the system
High Aspect Ratio = Flat pipe / High friction / High Pressure Drop / High noise / More expensive

Instructions for design work
For general HVAC work, it is recommended that:
Best: 1:1 to 2:1
Still good: no more than 3:1
Fair: not more than 4:1
Should be avoided: more than 4:1
Not recommended: more than 5:1 If there are really no space limitations.

Brief summary:
Design the air duct for “Not too flat” will help ensure good wind flow, low noise, low ESP and a more energy efficient system.