12-l1-l2-pumps etc.ppt

7/28/2019 12-L1-L2-Pumps etc.ppt

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Pumps, Compressors, Fans,

Ejectors and Expanders

Chapter 20

ChEN 4253 Design ITerry A. Ring


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Pumps

Moves Liquid, Creates Pressure Vapor bubbles

Causes Cavitations

Erodes Impeller

Solids Erode Impeller Pump Types

Centrifugal

Positive Displacement

Piston

diaphragm

Pump Power = Q*P = brake (delivered) (horse) powerfrom motor


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Centrifugal Pumps

Two Basic Requirements for Trouble-

Free Operation of Centrifugal Pumps

no cavitation of the pump occurs throughout

the broad operating range

a certain minimum continuous flow is always

maintained during operation

Pump around loops


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Reduced Flows

Unfavorable conditions which may occurseparately or simultaneously when the pump isoperated at reduced flows

Cases of heavy leakages from the casing, seal, and stuffing

box Deflection and shearing of shafts

Seizure of pump internals

Close tolerances erosion

Separation cavitation

Product quality degradation

Excessive hydraulic thrust

Premature bearing failures


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Centrifugal Pump

Electric Motor


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Centrifugal Pump

Electric

Motor


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Centrifugal Pump

Converts

kinetic

energy to

pressureenergy


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Impellers


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Converts Kinetic Energy to

Pressure Energy


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Different Types of Pump Head

Total Static Head - Total head when the pump is not running

Total Dynamic Head (Total System Head) - Total head when thepump is running

Static Suction Head - Head on the suction side, with pump off, if the

head is higher than the pump impeller Static Suction Lift - Head on the suction side, with pump off, if the

head is lower than the pump impeller

Static Discharge Head - Head on discharge side of pump with thepump off

Dynamic Suction Head/Lift - Head on suction side of pump with

pump on Dynamic Discharge Head - Head on discharge side of pump with

pump on


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Pump Head

The head of a pump can be expressed in metricunits as:

head = (p2- p1)/(g) + (v22- v1

2)/(2g) + (z2-z1)

where

h = total head developed (m)

p2= pressure at outlet (N/m2)

p1 = pressure at inlet (N/m2)

= density of liquid (kg/m3) g = acceleration of gravity (9.81) m/s2

v2= velocity at the outlet (m/s)
http://www.engineeringtoolbox.com/pump-energy-equation-d_631.htmlhttp://www.engineeringtoolbox.com/pump-energy-equation-d_631.html


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Pump Efficiency

Centrifugal Pump


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Pump Performance Curves

Resistance


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Pump Design Scaling

Pump Flow rate Q2 = Q1 x [(D2xN2)/(D1xN1)]

Pump Head H2 = H1 x [(D2xN2)/(D1xN1)]2

Pump Brake Horse Power BHP2 = BHP1 x [(D2xN2)/(D1xN1)]3

D = Impeller Diameter

N = specific speed


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Net Positive Suction Head-NPSH

Pumps can not pump vapors!

The satisfactory operation of a pumprequires that vaporization of the liquid

being pumped does not occur at any

condition of operation.


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Net Positive Suction Head

Required, NPSHR

As the liquid passes from the pump suction to the eye of the impeller, the velocity

increases and the pressure decreases. There are also pressure losses due to

shock and turbulence as the liquid strikes the impeller. The centrifugal force of the

impeller vanes further increases the velocity and decreases the pressure of theliquid. The NPSH required is the positive head (absolute pressure) required at the

pump suction to overcome these pressure drops in the pump and maintain the

liquid above its vapor pressure.


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Net Positive Suction Head

Available, NPSHA

Net Positive Suction Head Available is a function of the system in which the

pump operates. It is the excess pressure of the liquid in feet absolute over its vapor

pressure as it arrives at the pump suction, to be sure that the pump selected does

not cavitate.

Head to Feed Pump Subcooling before Pump

To overcome suction head

Head

Designed

into

Installation

HX

Cool a few DegreesTo overcome suction head


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Piston Pumps


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Gear Pumps


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Lobe Pumps

food applications,

because they

handle solids

without damagingthe pump.

Particle size

pumped can be

much larger inthese pumps than

in other PD types


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Centrifugal

Pump


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Positive Displacement Pumps

Piston Pumps

Gear Pumps

Lobe Pumps

Diaphragm Pumps The lower the speed of a PD

pump, the lower the NPSHR.


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Pump Costs

Cost based upon Size Factor

Centrifugal Pump

S=QH1/2

Gear Pump

S=Q

Piston Pump

S= Power (brake)

Must cost Electric Motor also S=Pc=PB/M


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Compressors

Types Centrifugal

Others Piston

Lobed

Screw

Methods of Calculation in Simulators Polytropic, PVk-1/k= constant,

Polytropic - This model takes into account both a rise in temperature in the gas as well assome loss of energy (heat) to the compressor's components. This assumes that heat mayenter or leave the system, and that input shaft work can appear as both increasedpressure (usually useful work) and increased temperature above adiabatic (usually lossesdue to cycle efficiency). Compression efficiency is then the ratio of temperature rise attheoretical 100 percent (adiabatic) vs. actual (polytropic). (k-1)/k = polytropic coefficient

Isentropic, s(T1,P1)=s(T2,isentropic,P2)

Theoretical Power Powerisentropic= FlowRate*(h2,isentropic-h1) Efficiency s =Powerisentropic/Powerbrake s = (h2,isentropic-h1)/(h2-h1)

Cost of Compressors Size Factor is Compressor Power

s

k

k

P

PT

TT

1

1

1

2

1

12
http://en.wikipedia.org/wiki/Polytropic_processhttp://en.wikipedia.org/wiki/Polytropic_process


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Positive Displacement Compressor


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Positive Displacement Compressor

http://www.city-compressors.co.uk/
http://www.city-compressors.co.uk/http://www.city-compressors.co.uk/http://www.city-compressors.co.uk/http://www.city-compressors.co.uk/


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Centrifugal Compressors

Rotors

Stators

Jet

Engine

Design


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Piston Compressor


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Expander

Reverse of Compressor Let flow produce shaft work

Types Centrifugal

Positive Displacement

Piston Lobed

Screw

Methods of Calculation in Simulators Polytropic, PVk-1/k= constant,

Isentropic, s(T1,P1)=s(T2,isentropic,P2)

Theoretical Power

Powerisentropic= f*(h2,isentropic-h1) Efficiency s=Powerbrake/Powerisentropic= (h2-h1) /(h2,isentropic-h1)

Cost Size factor = Power

http://www.city-compressors.co.uk/
http://www.city-compressors.co.uk/http://www.city-compressors.co.uk/http://www.city-compressors.co.uk/http://www.city-compressors.co.uk/


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Fans and Blowers

Types

Centrifugal (103-105 acfm, P=1-40 in H2O) Backward Curved

Straight radial Vane Axial

Tube Axial

Cost of Fans and Blowers

Size factor = Volumetric Flow Rate

Motor


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Choice to Increase Pressure

Heuristic 34 Use a Fan

Atm to 1.47 psig

Use a Blower < 30 psig

Compressor (or staged system) > 30 psig

Heuristic 34 - Number of Stages

Up to a Compression ratio 4 for each stage With intercooler between stages (P=2 psi)

Equal Hp for each stage (equal compression ratio)

P d i V


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Producing Vacuum

Steam Ejector


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Producing Vacuum

Types Ejector - advantage = large volumetric flow rate

Multi-Stage with interstage condensers

Liquid (Oil) Ring Vacuum Pump

Dry Vacuum Pump (rotary screw, lobe) (advantage =lowpressure) Designs similar to Expanders

Design for Flow Rate at suction plus

Air Leakage Rate

Function of pressure and Volume of vessel Cost

Size factor = Flow Rate at suction

Motor for pumps


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Ejector

Produces Vacuum

Provides Low Pressures forDistillation Columns

Fluid (P Psat)

Steam for suction pressure below 100 mbar

absolute, more than one ejector will beused, with condensors between theejector stages

Air

Water

Collects Particles in Gas Stream

Venturi Scrubber

12-l1-l2-pumps etc.ppt

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