Turbomachinery ( Chapter 4)
2 Learning Outcomes (Chapter 4) • Classification of turbomachines – Pumps – Fans – Compressors • Sizing, selection, and performance of turbomachines • Series and parallel components • Cavitation in turbomachines • Similarity laws of turbomachinery Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
3 Introduction • Turbomachines add or extract energy from a fluid stream. • In this course, we are mainly concerned with performance and system modeling. • We will examine those which add energy to the fluid stream. These includes: – Pumps – Fans – Compressors Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
4 Pumps and pump performance • Positive Displacement Pumps – Gear pumps – Vane pumps Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
5 Pumps and pump performance (cont.) • Kinetic Pumps – Centrifugal Pumps Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
6 Pumps and pump performance (cont.) • We can also categorize pumps based on the flow direction. Axial flow pumps Radial flow pumps Mixed flow pumps Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
7 Pumps and pump performance (cont.) • For a simple centrifugal design, one can show that the theoretical pump head is: • In reality, pump performance is more readily modeled as: Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
8 Pumps and pump performance (cont.) • Pump performance: – Actual head: – Actual fluid power: – Brake (impeller) power: – Efficiency: Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
9 Pumps and pump performance (cont.) • Pump selection: Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
10 Pumps and pump performance (cont.) • Pump curves: – We can read H vs. Q, NPSH R , Brake Horse Power, and Efficiency from pump curves. Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
11 Pumps and pump performance (cont.) • Matching system and pump curves: Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
12 Pumps and pump performance (cont.) • If the system and pump curves are given by simple expressions as follows: The operating point is found at the intersection of the two curves: Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
13 Example 4-1 (Problem 4-1) • A piping system requires a pump to be selected to deliver at least 75 (GPM) of flow at 400 (ft) of head. The pump is to operate on a 60 (Hz) fixed nominal speed of 3500 (RPM). Select a pump using Fig. 4-2 and determine the nominal impeller size, operating efficiency, and NPSH R for the desired characteristics. Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
14 Example 4-2 (Problem 4-2) • Given a pump curve of the form: and system curve of the following form: find the system operating point. Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
15 Example 4-3 (Problem 4-5) • Consider the closed loop pumping system sketched below. If the total length of the piping is 60 (m), with the diameter of 5 (cm), and a roughness of 0.0001 (m), what is the resultant flow in the system if the pump has the following characteristic: and the filter has the following pressure loss: We also know: K v =6.0 Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
16 Pumps performance (Series) • Pumps are combined in series to increase pumping head when discharge is satisfactory. – We add head “H” at constant flow rate “Q” : Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
17 Pumps performance (Parallel) • Pumps are combined in parallel when pumping head is adequate but discharge is not. – We add flow rate “Q” at constant head “H” : Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
18 Example 4-4 • Consider a pump with the following performance characteristics: Find the equivalent pump curve for two pumps in series (2PS) and two pumps in parallel (2PP). Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
19 Example 4-5 (Example 4-3 cont.) • A continuation of problem 4-5. If the desired discharge were m=25 (kg/s) and the pump was normally run at 1750 (RPM), can the desired discharge be achieved with two pumps in series or two pumps in parallel? Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
20 Cavitation in Pumps • Cavitation is the formation and collapse of bubbles in the impeller housing of a pump. • It can lead to erosion pitting of the impeller leading to a loss of pump performance. • We ensure cavitation does not occur by insuring that Net Positive Suction Head (NPSH) available (A) exceeds that required by the pump (R). • NPSH A is a design parameter, while NPSH R is a characteristic of the pump. Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
21 Cavitation in Pumps (cont.) • NPSH A is defined as follow: – The head losses up to the pump inlet are what are included, nothing else! – This leads to a number of analysis problems such as: finding vertical placement (Z i ), horizontal placement (L i ), intake diameter (D i ), or minor loss factor (K). Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
22 Example 4-6 • A particular pump is required to pump 24000 (GPM) of water whose free surface is at atmospheric pressure. If the losses leading up to the inlet at this flow rate are 6 (ft) of head, where should the pump be placed with respect to the free surface to avoid cavitation if the NPSH R =40 (ft)? The vapor pressure of water is 0.26 (psi) and the ρ g value is 62.4 (lb/ft 3 ). Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
23 Pump performance • Scaling – In general pump performance varies according to: – For geometrically similar machines we only consider: Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
24 Pump performance (cont.) • Pump performance – Power: – Flow: – Pressure: – Efficiency: Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
25 Pump performance (cont.) • Geometrically similar machines: Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
26 Pump performance (cont.) • Partially similar machines: – “ i ” : impeller – “h” : housing Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
27 Fans and Fan Performance • Fan Performance – Fan performance and scaling is much the same as is for pumps – Major difference is that for many fans, total pressure is used in the performance curve as the inlet and outlet areas are often not equal. – Thus, we define: Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
28 Fans and Fan Performance • Fan Performance Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
29 Fans and Fan Performance • Fan Performance Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
30 Fans and Fan Performance • Flow Control – a) Flow control device (system controlled), b) pump/fan controlled, c) both Faculty of Engineering and Applied Science Memorial University of Newfoundland St. John’s, Newfoundland, Canada
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