Recent Question/Assignment

Q1 Gas is compressed in a reciprocating compressor from 1 bar to 6 bar. The FAD (Free Air Delivery) is 13 m3/sec. The clearance ratio is 0.05. The expansion part of the cycle follows the law PV1.2 = C. The crank speed is 360 rev/min.
Calculate the swept volume and volumetric efficiency.
Q2. Provide answers for the following:
A system on site is a pressure sand filter with a service curve. The filter is backwashed using the feed pump, at a higher flow rate, shown as backwash curve.

• Service duty point : 3 m3/h
• Backwash duty point : 9 m3/h
a) What is the speed of the pump needed to achieve the service and backwash flow rates? (4 marks)
b) What would be the best way to control the speed of the pump? Why? (2 marks)
c) The pump is set up as a fixed speed constant pressure system (the pump runs once system pressure reaches 550 kPa, turns off when pressure drops to 450 kPa, plant re-starts) to maintain a system pressure of 500 kPa at the discharge of the pump. (4 marks)
Using pump affinity laws calculate the service and backwash rates for the pump?
A pump station has been designed to lift water out of a 6 metre deep pit (vented to atmosphere) via a centrifugal pump mounted at ground level.
• Liquid conditions 20°C
• Suction pipe work losses 2.0 metres
• NPSH safety factor 5.0 kPa
• Vapor pressure @ 20oC 0.25 metres
a) Calculate the Net Positive Suction Head (NPSH) for the system. (3 marks)
b) In summer the liquid temperature rises to 60 oC, this changes the water vapor pressure to 2.05 metres. What is the revised NPSH for the higher water temperature? (2 marks)
For the NPSH result from part (b) what increase in suction pipe work size is required to have a positive NPSH result as calculated in part (a). (3 marks)