Losses in Duct System
a) Surface Frictional Loss.
b) dynamic losses in duct.
c) loss due to enlargement.
d) loss due to sudden construction.
e) pressure losses in elbow and bend.
f) losses at suction and discharge opening.
g) pressure losses in fitting and leakages.
Losses in Duct System
● For carrying smooth operation of any air-conditioning system is depend upon the sufficient circulation of air in the air conditioning space.
● Pressure is lost due to friction between the moving particle of air and internal surface of a duct when the pressure loss occurs in a straight duct i.e. friction loss.
● The pressure is lost dynamically at changes in direction in bend, elbow, and change in cross-sectional area. This losses is called dynamic losses. Therefore the calculation of pressure losses is necessary for the selection of proper duct size.
a) Surface Friction Loss :
The frictional resistance of a duct of any cross-section is given by Darcy's equation,
D = Diameter of circular duct.
V = Velocity of the fluid flowing in m/sec.
f = Friction factor.
L = Length of the duct in meters.
The friction factor f is depend on Reynolds number,
f = 64/Re when Re <2300
b) Dynamic Losses in Ducts:
● Whenever there is change in direction or velocity in the flow through duct, the pressure loss is inevitable. The additional loss called dynamic loss.
● The change in magnitude of velocity occurs when the area of duct changes.
● The change in magnitude or direction which cause accelerating and accelerating force which may be internal or external.
● The pressure loss due to the change of direction of velocity at elbow is known as velocity pressure head.
c) Loss due to Enlargement :
● When the area of duct changes, the velocity of air flowing through the duct changes, when area increase, the velocity decrease with rise in pressure which form eddies at the corner thus sudden or abrupt change is neglected.
d) Loss due to Sudden Contraction:
● When the air is flowing and having a sudden or abrupt contraction, the eddies are formed at the shoulders of the large section and beyond the entry at the smaller section forming a vena-contracta.
● The loss of pressure due to sudden contraction is not due to contraction itself but it is due to sudden enlargement of flow area from vena-contracta to the section of smaller duct.
e) Pressure Losses in Elbow and Bend :
Le = Equivalent length of duct.
Kd = Dynamic loss coefficient.
● The value of (Ld/Kd) is different for different elbow. The value of (Ld/Kd) is mostly affected by the geometry of elbow and surface roughness of duct wall and remains unaffected by the air velocity.
● To minimize the pressure loss in the bend, the splitters are generally used, aspect ratio is small.
◆ Aspect ratio :
● This the ratio of the longer side to the shorter side of the duct. As the aspect ratio increase, more metal surface is required for the duct for sure cross-sectional area. Thus higher aspect ratio increase not only the initial cost but also operating out by way of increased heat gain loss. Therefore this ratio is most important factor for duct design.
◆ Radius ratio :
● The radius ratio is the ratio between the center line of the curve of the elbow and the width (W) of the elbow. An elbow with a radius ratio 1.5 is considered as standard elbow and should be used whenever possible.
● When aspect ratio exceed three, there will be increased turbulence within the elbow resulting in marked increased in pressure loss.
● A full radius elbow has a throat radius of 0.75 D. (where D is depth of the elbow cross-section) and this is considered as optimum.
f) Losses at Suction and Discharged Opening :
● When an abrupt suction opening is provided the air is accelerated at the opening, forming a vena-contracta inside the duct. In this case sudden changes from infinity to duct area and dynamic loss coefficient.
g) Pressure Losses in Fittings and Leakages:
● Whenever air is diverted from the main duct to the branch duct, there is velocity reduction in the main duct.
● If there is no loss, the change in the velocity pressure is completely converted into static pressure. But actual static regain is reduced by the regain factor, Rg for circular duct value of Rg = 0.9 and for rectangular duct of high aspect ratio, the value of Rg is low as 0.5.
