How to design air ducts (Duct Design) according to international standards

How to design air ducts (Duct Design) 
For designing air ducts (Duct Design) according to international standards (such as or ) commonly used by engineers. There will be 3 main methods. Each method will have different strengths and limitations. Suitable for each type of work site. This is a summary of both 3 methods for Teacher Thep to use in teaching or choose to use.

1. Equal Friction Method (method of equal friction)
This is the most popular method. in our home and around the world for designing general air conditioning systems (Comfort Air Conditioning)
 * Principle: Set "Friction Loss per unit length" to have the same constant value throughout the system.
   * Standard values commonly used: 0.08 - 0.1 inches of water per 100 feet (in.wg/100 ft) or approximately 0.8 - 1.0 Pa/m
 * Advantages:
   * Easy and fast design (can use Duct Slide Rule or Friction Chart)
   * Save time than other methods
   * Suitable for systems that are symmetrical (Symmetrical Layout)
 * Disadvantages:
   * There may be problems with balancing the air in small branch pipes. It is necessary to install a Volume Damper (VD) to adjust the wind on site.
 * Suitable for: general building work, offices, hotels, cold air duct systems and general ventilation pipes

2. Static Regain Method (Static Regain Method)
It is the most detailed and energy-saving method. But the calculation method is much more complicated (usually requires software to help).
 * Principle: Designed in such a way that the "increased Static Pressure" (Regain) from reducing the air speed in the next section of pipe compensates for the "Friction Loss" lost in that section of pipe (P_{gain} = P_{loss}) so that the Static Pressure in front of every air distribution head has a similar value.
 * Advantages:
   *Self-balancing system (Self-balancing) very well
   * Energy saving fan Because the total pressure drop of the system is usually lower than other methods.
 * Disadvantages:
   * Hand calculation is very difficult and takes a long time
   * The size of the air pipe at the end may not be much smaller. (wasting galvanized sheets at the end)
 * Suitable for: Large systems, VAV (Variable Air Volume) systems, High Velocity systems or buildings that focus on advanced energy saving ()

3. Velocity Reduction Method (method for determining speed / reducing speed)
It's a traditional way. Relying mainly on the experience of the designer
 * Principle: Set the initial air speed at the fan, then gradually "speed down" in each section of duct as appropriate. (or according to the recommended table)
 * Advantages:
   * Easy, not complicated
   * Control noise (Noise) well because we set the speed so that it does not exceed the threshold from the beginning.
 * Disadvantages:
   * Not engineeringly accurate (Based on criteria)
   * Finding the size of the fan (Static Pressure) is difficult because the friction force is not the same in each section.
   * Balancing is the hardest.
 * Suitable for: Small scale work, short pipes, or rooms that are serious about sound.

Summary comparison table (for teaching use)
| Comparison topics | Equal Friction (most popular) | Static Regain (the theoretical best) | Velocity Reduction (easiest)
| Calculation difficulty | Medium | Hard (requires computer) | Easy |
| Air balancing (Balancing) | Fair (requires damper) | Very good (self-balancing) | Poor |
| Air pipe size | Reduce according to the air volume | End pipe may still be large | Reduce according to the specified speed |
Popularity | Very high | Medium (high spec work) | Low (small work) | Plus: Another 1 method for industrial work.

4. Constant Velocity Method (constant velocity method)
 * Used for material conveying systems (Material Handling) or industrial dust suction pipes (Dust Collector).
 * Must maintain a constant high wind speed (such as 3,500 - 4,500 FPM) in order to prevent dust or materials from settling in the pipe. Will not be used for general air conditioning work.