LECTURE-11 FAN AND DUCT SYSTEM-2

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1 L ecturer: -D D r. E sam M ejbil A bid Subject: A ir Conditioning and R efrigeration Year: Y Fourth B.Sc. D E P A R TM E N O F M E CH A N ICA L E N G IN E E R IN B abylon U niversity College of E ngineering D epartm ent of M echanical Engineering LECTURE-11 FAN AND DUCT SYSTEM-2 Friction Factor for Ducts: The value of friction factor, for smooth ducts may be obtained by using the following expressions: 1. For laminar flow, the friction factor For turbulent flow, the friction factor.. 16 In case of rough pipes or ducts the friction factor depends upon the roughness factor (e/d), where (e) is the absolute roughness of the surface and (D) is the diameter of duct. The friction factor for rough pipes or ducts is:

2 D E PA R TM E N O F M E CH A N ICA L E N G IN E E R IN. 17 Friction Chart for Circular Ducts: From equations 11 and 13 the frictional pressure loss for circular ducts (N/m 2 )...18 The frictional pressure loss for circular ducts (mm of water) for various velocities (m/s) and duct diameter (m) may be obtained directly from the friction chart. In these charts, the vertical ordinates represent volume flow rate of air and the.horizontal ordinates represent frictional pressure loss in mm of water per unit length of the circular duct (P f /L). Dynamic Losses in Ducts: The dynamic losses are caused due to the change in direction or magnitude of velocity of the fluid in the duct. The change in direction of velocity occurs at bends and elbows. The dynamic pressure loss,. (19) Where (C) is the dynamic loss coefficient. The dynamic pressure loss expressed in terms of an additional equivalent length (Le) of the duct is given by:. in mm of water (20)

3 D E PA R TM E N O F M E CH A N ICA L E N G IN E E R IN From equations 19 and 20, the relationship between the dynamic loss coefficient (C) and equivalent additional length is.. (21) Duct Design: The object of duct design is to determine the dimensions of all ducts in the given system. The ducts should carry the necessary volume of conditioned air from the fan outlet to the conditioned space with minimum frictional and dynamic losses. The area changes must be gradual where possible and limited to not more than 20 o for diverging area and 60 o for convergent area. For rectangular ducts the aspect ratio of 4 and less is desirable but it should not be greater than 8 in any case. The velocities in the ducts must be high enough to reduce the size of the ducts but it should be low enough to reduce the noise and pressure losses to economize power requirement as table (1) Recommended Velocities in m/s Designation School, theater and Residences public building Industrial building Outdoor air intakes Filters Heating coil Air washers Fan Outlet Main Duct Branch Duct Branch Risers

4 D E PA R TM E N O F M E CH A N ICA L E N G IN E E R IN Methods for Determination of Duct Size: The following three methods for determination of duct size are important as: 1. Velocity Reduction Method The various steps involved in this method are: i. Select suitable velocities in the main and branch ducts ii. Find the diameters of main and branch ducts from airflow rates and velocities for circular ducts. For rectangular ducts, find the cross-sectional area from flow rate and velocity, and then by fixing the aspect ratio, find the two sides of the rectangular duct. iii. From the velocities and duct dimensions obtained in the previous step, find the frictional pressure drop for main and branch ducts using friction chart or equation. iv. From the duct layout, dimensions and airflow rates, find the dynamic pressure losses for all the bends and fittings. v. Select a fan that can provide sufficient Flow for the index run vi. Balancing dampers have to be installed in each run. The damper in the index run is left completely open, while the other dampers are throttled to reduce the flow rate to the required design values. The Velocity Method Proper air flow velocities for the application considering the environment are selected. Sizes of ducts are then given by the continuity equation like: A = q / v..(22) A = duct area (m 2 ) q = air flow rate (m 3 /s) v= air speed (m/s)

5 F A N A N D D U CT SYSTE M D E PA RTM E N OOFF M E CH A N ICA L E N G IN E E RRIN IN A proper velocity will depend on the application and the environment. The table below indicate commonly used velocity limits: 2. Equal friction method In this method the frictional pressure drop per unit length in the main and branch ducts ( pf/l) are kept same, (23) Then the stepwise procedure for designing the duct system is as follows: i. Select a suitable frictional pressure drop per unit length ( pf/l) so that the combined initial and running costs are minimized. ii. Then the equivalent diameter of the main duct (A) is obtained from the selected value of ( pf/l) and the airflow rate. As show wn in Figure (1), airflow rate in the main duct is equal to the sum total of airflow rates to all the conditioned zones.

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