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Chemical Engineering

Chemical Engineering
Chemical Engineering 374 Fluid Mechanics Pumps 12 Spiritual Thought Moses 1:33 33 And worlds without number have I created; and I also created them for mine own purpose; and by the Son I created them, which is mine Only Begotten.3 Potential SeminarFluids Roadmap5 Key Points • Pumps/Turbines – Chp 14.114.2 (today), 14.214.3 (Wed.) 14.4145 (Friday) • Pumps – Add energy to fluid (increase pressure, not speed) • Liquids  pumps • Gases – Fans: Low DP, High Flow, 1 psi – Blowers: Med DP, Med Flow, 40 psi – Compressors: High DP, Low Flow, 40 psi • Pumps – Positive displacement – Dynamic6 Positive Displacement Pumps • Displace fluid by moving parts with low clearance – Piston/cylinder – Turning gears – Screws • Lower flow rates – 1000 gpm • Self priming • High pressures ( 500 psi) – Need safety devices • High viscosity fluids – Oil, foods • Pulsating flow, hard to control flow rate7 Centrifugal Pumps • Centripetal forces accelerate fluid and increase pressure. • Flow enters axially and is accelerated to the outside where pressure rises. • High flow rates ( 300,000 gpm) • Large gaps • Lower pressures (relative) 100 psi • Not self priming • The industry standard for moving gases and liquids. – If it’s a pump, its probably a centrifugal pump8 Centrifugal Pumps9 Performance Parameters • Brake Horsepower – Shaft work – Work supplied to the pump – Some is lost  inefficiency • Water Horsepower – mgH is the work imparted to fluid across the pump • Efficiency • Inefficiency – Leakage of fluid between spaces ሶ 𝜌𝑉 – Fluid friction in pump 𝑏ℎ𝑝 – Mechanical friction in pump – Does not include the motor 𝑔𝐻10 Pump Performance C B • Key parameters are V and H A • Most pumps are on or off • Consider pump to three elevations A, B, C V • Pump head lifts fluid • Ignore any pipe losses • A: Pump just “throws” fluid, • Note, head increases over the but H=0 pump, then drops over the ሶ – W = rgH𝑽 load. • B: Start elevating, flow rate • Load can be KE or elevation, drops and head increases or loss or pressure. • C: At some point flow stops • Could think of this as and head is maximum11 Pump Curve Schematic12 Pump Operation Curves • Piping system requires a given V and a given H. – H is friction and minor losses, etc. loss • Pump has a corresponding V and H. • These must match, forming the operating point. – This may not be the best efficiency. • Select a pump so that the best efficiency point (BEP) occurs at the operating point. • Generally oversize the pump a bit – higher flow for given H req – or Higher H for given flow avail – Add a valve after pump  raises H to match req H for given flow avail – Somewhat wasteful, but offers control. – Also may increase efficieny. (But higher efficieny may not compensate for extra work wasted in the valve (see example 14.2)13 Example Find the flow rate and required pumping power for the system to the 2 2 right. The pumping curve is defined by H = 200 m – v s /m, with p P = Z a pump efficiency of 90. 2 2 6 atm ε = .045 mm P = 1 1 atm Pump curve: L = 200 m 2 H =200v p Z = 0m 1 D = 0.1 m 250 232.223 Operating Point is at intersection of two lines: H = 149.149 m op 200 v = 7.131 m/s op H (v) 3 .req 𝑣ሶ = 0.055 m /s 150 H (v)ሶ 𝜌𝑉 .P 𝑏ℎ𝑝= 𝜂 100 ℎ𝑏𝑝= 89.35 61.662 50 0 2 4 6 8 10 0 v 10 𝑘𝑊 𝑔𝐻 𝑜𝑝14 Pump Performance Curves15 Cavitation • Pressures inside pumps can decrease locally in some spots (like the low pressure side of a blade) • Recall flow separation and wakes • Cavitation causes local boiling, bubble collapse. – Think of th pinging you hear when water bubble start to form on the stove. – Causes erosion and pitting of blades.16 Net Positive Suction Head (NPSH) • Think of NPSH as the pressure drop inside the pump. – If pump NPSH is 10, then you need at the pump inlet to be more than 10. • NPSH is specified for a given pump. Operate ABOVE it. req • NPSH increases with flow rate (higher flow, more cavitation tendency. req • NPSH of the operating system decreases with increasing flow. – Higher flow means more pressure drop means lower pressure at the pump inlet, means lower NPSH. • Locate pumps down low (below tanks and columns. (To maximize P) H pump • Lower temperature is better (lower P ) v • As increase T, and/or Flow rate, watch NPSH hav e out for cavitation NPSH req (cavitates) 17 NPSH
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