Pumps, Compressors, Fans, Ejectors and Expanders

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Pumps, Compressors, Fans, Ejectors and Expanders

• Chapter 20
• ChEN 4253 Design I
• Terry 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)
  power from 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 occur separately
  or simultaneously when the pump is operated 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 pressure energy

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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 the pump 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 the pump 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 in metric units can be
  expressed in metric units as
• head (p2 - p1)/(?g) (v22- v12)/(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 acceleratiosn of gravity (9.81)  m/s2
• v2 velocity at the outlet (m/s)

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Pump Efficiency

• Centrifugal Pump

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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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Pump Performance Curves
Resistance
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Net Positive Suction Head-NPSH

• Pumps can not pump vapors!
• The satisfactory operation of a pump requires
  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 the liquid. 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
HX
Head Designed into Installation
Cool a few Degrees To overcome suction head
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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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Piston Pumps
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Gear Pumps
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Lobe Pumps

• food applications, because they handle solids
  without damaging the pump.
• Particle size pumped can be much larger in these
  pumps than in other PD types

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Screw Pump
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Pump Costs

• Cost based upon Size Factor
• Centrifugal Pump
• SQH1/2
• Gear Pump
• SQ
• Piston Pump
• S Power (brake)
• Must cost Electric Motor
• SPcPB/?M

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Compressors

• 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 ?sPowerisentropic/Powerbrake
  (h2,isentropic-h1)/(h2-h1)
• Cost of Compressors
• Size Factor is Compressor Power

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Positive Displacement Compressor
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Positive Displacement Compressor
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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Compressor Heuristic 36

• Estimate the number of gas compression stages, N,
  assuming a maximum compression ratio of 4 for
  each stage. Assumes a specific heat ratio of 1.4
• Optimal interstage pressures correspond to equal
  HP for each compressor. Use a pressure drop of 2
  psi for each intercooler.

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Compressor System
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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 ?sPowerbrake/Powerisentropic (h2-h1)
  /(h2,isentropic-h1)
• Cost
• Size factor Power

http//www.city-compressors.co.uk/
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Fans and Blowers

• Types
• Centrifugal (103-105 acfm, P1-40 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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Fan/Blower Heuristic 34

• Use a fan to raise the pressure from atm to as
  high as 40 in water (g). Use a blower or
  compressor to raise the gas pressure to as high
  as 30 psig. Use a compressor for higher
  pressures.

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Producing VacuumSteam 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
  low pressure) 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 for Distillation Columns
• Fluid (P Psat)
• Steam
• for suction pressure below 100 mbar absolute,
  more than one ejector will be used, with
  condensors between the ejector stages
• Air
• Water
• Collects Particles in Gas Stream
• Venturi Scrubber