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
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