The diagrams show (as dotted lines) the system and fan characteristics and opera

Question

The diagrams show (as dotted lines) the system and fan characteristics and operating point for normal design conditions of a typical air flow system. The volume flow rate through the system can be controlled (reduced) by three methods Dampers in the ducting which may be progressively closed Vanes at the fan inlet which can create increasing pre-whirl Speed of driving motor which can progressively reduce fan speed. For each of these three methods show on the diagrams how the system or fan characteristics change to give a new operating point. (a) () Control by duct dampers (ii) Control by inlet vanes. (ii) Control by fan speed. In each case show the new operating point. Retun Page 11 with the examination answer booklet (5 marks) Explain, with reference to the diagrams, why and how the flow is reduced in each case. (b) (0) Effect of duct dampers. (ii) Effect of inlet vanes. (ii) Effect of fan speed. Hint: Sketches of velocity diagrams may be useful in the explanation for (i)

Explanation / Answer

i)In a duct damper presence, reduce the flow volume by restricting area inside damper.In restiction of flow pressure increase inside the duct,in graph shows pressue head incrase and flow rate decrese exponensialy.

ii)Effect of inlet valve:Vanes create pre whirl which create back pressure of actual flow.Due to back pressure flow restriction in duct reduce the flow rate and increse the pressure head.

iii)Driving motor speed will reduce the speed of fan.Results of reducing fan speed , reduce the flow rate inside duct because flow rate in proposonal to speed.In duct, reduction of fan speed flow rate reduce and pressure head in duct increse.

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