Subject:- Refrigeration and Air Conditioning

Topic:- Duct Design and Applications

Difficulty:- Low

Duct design methods:

1. Velocity reduction method

• In this method, the velocities in the ducts are assumed such that they progressively decrease as flow proceeds.

• Pressure drops are calculated for these velocities for respective branches and main duct and the fan is designed to overcome the pressure losses.

• Duct sizes are determined based on assumed velocities and known quantities of air to be supplied.

• This method is usually adopted for very simple systems and is easiest. Also the velocities can be adjusted to avoid noise.

• The major disadvantage is that considerable judgement and experience is required in selecting velocities.

2. Equal pressure drop or friction loss method

• In this method, the size of duct is designed to give equal pressure drop (or friction loss) per metre length in all ducts.

• The velocities are automatically reduced in the branch ducts as the flow is decreased.

• This method however does not balance pressures at the outlets if branches are of different lengths and hence dampers are required for balancing pressure drops in these branches.

• The high velocities in shorter runs to reduce high pressures may create objectionable noise and hence noise absorbing outlets must be provided.

3) Static regain method

In this method, size of duct is decided to give equal pressure at all outlets, for perfect balancing of the air duct layout system.

This can be done by equalizing the friction loss in each branch to the gain in pressure due to reduction in velocity.

The static pressure regain is given by,

SPR=$R.ρ_a \frac{[(〖v_1〗^2-〖v_2〗^2)}{[2]}=R(p_v1-p_v2 )$ Where R is the static regain factor.

The method allows for balancing but reducing velocity increases duct size. Very long branches may not be possible to design economically. Hence it is sufficient to design the main duct for complete regain.

Friction chart:

The frictional pressure loss for circular ducts (in mm of water) for various velocities (in m/s) and duct diameters (in mm) may be directly obtained from the friction chart as shown in figure.

The vertical ordinates represent volume flow rate of air (Q) in m3/s and the horizontal ordinates represent frictional pressure loss in mm of water per unit length of the circular duct.

These charts are valid for 20°C and 1.013 bar and clean galvanized iron ducts.

Correction factors must be used if duct material and air temperature is other than mentioned above. The charts may also be used for rectangular ducts if equivalent circular duct diameters are obtained first.