Question

Briefly discuss the cause of foaming and pri ming in steam drums [3 marks) Discuss the driving force behind natural circulation cooling in a power plant that uses subcritical pressures, indicate a problem associated with relatively high subcritical pressures and a method used when this condition is reached or when safety is paramount at supercritical pressures [3 marks] If natural circulation is reduced due to the effects discussed in (b) highlight potential problems in high heat absorbing areas of the water tubes and some methods typically used to mitigate them [4 marks] A furnace wall riser, 18m long, 76.2mm Outer Diameter (OD) and 6.1mm thick receives saturated water at 80bar and 1.5m/s velocity. Assuming a circulation ratio of 12.5 and a slip ratio of 1.2 and if the quality of the steam at the top of the riser is 8%, determine i) The pressure head developed due to natural circulation if the average density of the liquid and vapour mixture is assumed in the riser 3 Marks] i) The void fraction at the riser exit [3 Marks] ii) The heat transfer rate per unit projected area of the riser tube if the projected area is approximated by the outside diameter and the height of the tube [4 Marks]

Answer 1

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