Subject:- Refrigeration and Air Conditioning

Topic:- Duct Design and Applications

Difficulty:- Low

The pressure is lost due to friction between the moving particles of the fluid and the interior surfaces of a duct. This is termed as friction loss.

$p_f=\frac{(fLρ_a V^2)}{2m} where m=\frac{(cross sectional area (A))}{(wetted perimeter (P))}$

Pressure is also lost dynamically at the changes of direction such as in bends, elbows, etc. and at the cross-section changes of the duct. This is termed as dynamic loss.

3)Pressure loss due to Enlargement in area

The result of enlargement of cross section in ducts is the formation of eddies at the corners which in turn causes pressure loss.

$p_L=C_2 (\frac{V_2}{4.04})^2$ mm of water,where $C_2=(\frac{A_2}{A_1} -1)^2$ known as loss coefficient

4)Pressure loss due to Contraction in area

When air is flowing through a duct having sudden or abrupt contraction eddies are formed at the shoulders of the large section and beyond the entry at a smaller section forming a vena contracta. The equation is same as that of pressure loss due to enlargement.

5)Pressure Loss at Suction and Discharge of Duct

Pressure loss at suction is given by ,$p_L=\frac{〖CV〗^2}{2g} $ m of air,where C is the loss coefficient

Pressure loss at discharge is actually a loss due to the energy of head, which the flowing air has by virtue of its motion.

$p_L=\frac{V^2}{2g}$ m of air

6) Pressure Loss due to Obstruction in duct Pressure losses also occur due to various obstructions in path of air flow from fan to outlet.