MER 439 Design of Thermal Fluid Systems

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• MER 439 - Design of Thermal Fluid Systems
• Pumps and Fans
• Professor Bruno
• Fall Term 2006

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Introduction

• A pump is a machine that expends energy to
  increase the pressure of a fluid (liquid or gas)
  and move it from one point to another.
• Pumps gt Liquids
• Fans gt Air
• In this lecture we will learn about pump types,
  pump performance curves and some general
  guidelines for selecting a pump.

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Pump Types

• Two General Categories
• (1) Dynamic Pumps These pumps use a rotating
  component to impart energy to the fluid in the
  form of high velocity, high pressure or high
  temperature.
• (2) Positive Displacement Pumps Have a fixed
  volume chamber that takes in and discharges the
  pumped fluid.

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Dynamic Pumps

• Dynamic Pumps can be classified according to the
  direction of the flow with respect to the
  rotation axis.
• (1) Radial Flow
• (2) Axial Flow
• (3) Mixed Flow

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Radial Flow (Centrifugal) Pumps

• Two main components
• The impeller consists of a number of blades
  (vanes) arranged in a regular pattern about the
  shaft.
• The casing (housing) encloses the impeller

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Radial Flow (Centrifugal) Pumps

• Energy is added to the fluid by the rotating
  blades
• Both pressure and absolute velocity are increased
• The casing is designed to slow the fluid and
  convert the KE of the fluid to an increase in
  pressure.

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Axial Flow Pumps

• Used in low pressure
• drop applications.
• The motor rotates on a shaft onto which the
  impeller is attached.
• The rotating shaft is enclosed in a housing.
• Flow passage is bounded by this housing.

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Positive Displacement Pumps

• Reciprocating Pump a reciprocating piston draws
  in fluid on an intake stroke and moves it out on
  the discharge stroke.
• Rotary Gear Pump Two meshed gears rotate within
  a housing. Fluid enters between the gears and is
  drawn into the volumes between adjacent teeth.

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Positive Displacement Pumps
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Performance Maps

• Pump performance is typically characterized by a
  pressure head versus flow capacity curve.
• Manufacturers tests the pumps and provide the
  curves.

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Performance Maps

• Usually show different rotational speeds
• DP decreases as flow rate increases
• Iso-efficiency curves are shown

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Performance Maps

• Manufacturers supply summary plots of all types
  of pumps.

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Example

• A pipeline that conveys water to an elevated
  tank at a campsite is shown. The elevated tank
  supplies water to people taking showers.

The 40 ft long pipeline contains 3 elbows and
one ball check valve and is made of 6-nominal
schedule 40 PVC pipe. The pump must deliver 250
gpm. Use the figure on the previous slide to
select a pump for this system and calculate the
pumping power.
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Solution

• Water Properties r 62.4 lbm/ft3,
• m 1.9 x 10 -5 lbf-s/ft2
• Pipe Properties ID 0.5054 ft
• A 0.2006 ft2
• The steady flow energy equation is

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Solution

• P1 P2 Patm 0 and V1 V2
• Z1 0 Z2 30 ft
• Q 250 gal/min 0.555 ft3/s
• V Q/A 2.76 ft/s
• Re 62.4(2.76)(0.5054)/1.9x10-5(32.2)
• 1.43x105
• e/D smooth gt f 0.0165 (Moody Diagram)

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Solution

• Minor Losses
• SK 3Kelbow Kinlet K valve Kexit
• From a fluids book
• SK 3(0.31) 1 70 1 72.9

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Solution

• Solving for hp we get
• A pump corresponding to region 01 will be
  suitable for this application. The power is

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System Characteristics and Pump Selection
The energy equation between points 1 and 2 in the
system shown is hp is the actual head gained
by the fluid from the pump and ShL represents all
friction and minor losses.
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The System Equation
From our study of pipe flow we know that
typically hL varies approximately as the flow
rate squared Where K depends on the pipe
sizes, lengths and types and on the minor loss
coefficients. This system equation shows how
the head gained by the fluid from the pump is
related to the system losses.
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The System Equation
We can combine the system equation with the pump
performance curve to determine the operating
point. The intersection of the pump performance
curve and the system curve is the operating
point.
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Combining Pumps

• Pumps can be arranged in series or parallel to
  provide additional head or flow capacity.
• (a) In Series - add heads at same flow rate
• (b) In Parallel - add flow rates at same head.

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Example Problem
Water is to be pumped from one large open tank
to another as shown. The pipe diameter is 6 in
and the total length of the pipe between the pipe
entrance and exit is 200 ft. Minor loss
coefficients for the entrance, exit and the elbow
are shown on the figure and the friction factor
can be assumed to be 0.02.
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Example Problem
A certain centrifugal pump having the
performance characteristics shown is suggested as
a good pump for this flow system. With this pump
what would be the flow rate between the tanks? Do
you think this pump would be a good choice?
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Example Problem

• The steady flow energy equation between points
  (1) and (2) is
• With P1 P2 0 and V1 V2 0 and z2 -z1 10
  ft, f 0.02, D 0.5 ft and L 200 ft

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Example Problem

• The velocity is related to the flow rate
• The system curve can be expressed as

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Example Problem

• We can plot this against the pump curve to locate
  the operating point

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Example Problem

• The curves intersect at Q 1600 gal/min with the
  corresponding head again of 66.5 ft.
• Another concern to consider is the pump
  efficiency. Although the pump is not operating at
  peak efficiency it is close 84. Thus this pump
  is a satisfactory choice. The power needed to
  drive the pump is

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Net Positive Suction Head (NPSH)

• Low pressures are commonly encountered on the
  suction side of a pump
• Cavitation can occur
• Cavitation occurs when a liquid pressure at a
  given location is reduced to the vapor pressure
  of the liquid. Vapor bubbles form and the liquid
  starts to boil.
• Causes loss in efficiency and structural damage
  to the pump.

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Net Positive Suction Head (NPSH)

• To characterize the potential for cavitation the
  difference in the total head on the suction side
  near the pump impeller inlet and the liquid vapor
  pressure head is used

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Net Positive Suction Head (NPSH)

• The required NPSHR is the value that must be
  maintained to avoid cavitation.
• The available NPSHA represents the head loss that
  actually occurs for the system.

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NPSH Example

• Example
• Q 0.5 ft3/sec
• NPSHR 15 ft
• T 80 oF
• P 14.7 psi
• kL 20, D 4 in
• Determine the maximum z1 for no cavitation

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NPSH Example

• The available NPSHA is
• The maximum z1 will occur when NPSHA NPSHR

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NPSH Example

• The velocity is
• The head loss is
• At T 80o F the vapor pressure of water is
  0.5069 psia, and r 62.22 lb/ft3

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NPSH Example
To prevent cavitation the pump should not be
located higher than 7.65 ft above the water
surface.