Turbomachinery ( Chapter 4) 2 Learning Outcomes (Chapter 4) - - PowerPoint PPT Presentation

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Turbomachinery ( Chapter 4) 2 Learning Outcomes (Chapter 4) - - PowerPoint PPT Presentation

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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

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Turbomachinery (Chapter 4)

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Faculty of Engineering and Applied Science

Memorial University of Newfoundland

  • St. John’s, Newfoundland, Canada

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
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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

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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

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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

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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

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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:

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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:

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Faculty of Engineering and Applied Science

Memorial University of Newfoundland

  • St. John’s, Newfoundland, Canada

9

Pumps and pump performance (cont.)

  • Pump selection:
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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, NPSHR, Brake Horse Power, and Efficiency from pump curves.

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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:
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SLIDE 12

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:

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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 NPSHR for the desired characteristics.

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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.

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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:

Kv=6.0

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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”:

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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”:

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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).

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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?

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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).

  • NPSHA is a design parameter, while NPSHR is a characteristic of

the pump.

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Faculty of Engineering and Applied Science

Memorial University of Newfoundland

  • St. John’s, Newfoundland, Canada

21

Cavitation in Pumps (cont.)

  • NPSHA is defined as follow:

– The head losses up to the pump inlet are what are included, nothing else! – This leads to a number

  • f

analysis problems such as: finding vertical placement (Zi), horizontal placement (Li), intake diameter (Di), or minor loss factor (K).

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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 NPSHR=40 (ft)? The vapor pressure of water is 0.26 (psi) and the ρg value is 62.4 (lb/ft3).

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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:

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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:

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Faculty of Engineering and Applied Science

Memorial University of Newfoundland

  • St. John’s, Newfoundland, Canada

25

Pump performance (cont.)

  • Geometrically similar machines:
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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

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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

  • ften not equal.

– Thus, we define:

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Faculty of Engineering and Applied Science

Memorial University of Newfoundland

  • St. John’s, Newfoundland, Canada

28

Fans and Fan Performance

  • Fan Performance
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Faculty of Engineering and Applied Science

Memorial University of Newfoundland

  • St. John’s, Newfoundland, Canada

29

Fans and Fan Performance

  • Fan Performance
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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