Rotating Equipment:
Pumps, Compressors, &
Turbines/Expanders
Chapters 2 & 9
Topics
Fundamentals
▪ Starting relationships
Equipment
▪ Pumps
• Thermodynamic relationships
• Centrifugal pumps
• Bernoulli’s equation
• Reciprocating pumps
▪ Simplifications
• Pumps – constant density
compression
• Compressors – reversible ideal gas
compression
▪ Use of PH & TS diagrams
▪ Multistaging
Efficiencies
▪ Adiabatic/isentropic vs. mechanical
▪ Polytropic
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
• Gear pumps
▪ Compressors
• Centrifugal compressors
• Reciprocating compressors
• Screw compressors
• Axial compressors
▪ Turbines & expanders
• Expanders for NGL recovery
• Gas turbines for power production
o
What is “heat rate”?
2
Fundamentals
Updated: January 4, 2019
Copyright © 2017 John Jechura (jjechura@mines.edu)
Review of Thermodynamic Principals
1st Law of Thermodynamics – Energy is conserved
▪ (Change in system’s energy) = (Rate of heat added) – (Rate of work performed)
Ê = Q − W
▪ Major energy contributions
• Kinetic energy – related to velocity of system
• Potential energy – related to positon in a “field” (e.g., gravity)
• Internal energy – related to system’s temperature
o Internal energy, U, convenient for systems at constant volume & batch systems
2
u
g
Eˆ = Uˆ +
+ h
2gc gc
o
Enthalpy, H = U+PV, convenient for systems at constant pressure & flowing systems
2
u
g
Eˆ = Hˆ +
+ h
2gc gc
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
4
Review of Thermodynamic Principals
2nd Law of Thermodynamics
▪ In a cyclic process entropy will either stay the same (reversible process) or
will increase
Relationship between work & heat
▪ All work can be converted to heat, but…
▪ Not all heat can be converted to work
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
5
Common Paths for Heat and Work
Isothermal
constant temperature
ΔT = 0
Isobaric
constant pressure
ΔP = 0
Isochoric
constant volume
ΔV = 0
Isenthalpic
constant enthalpy
ΔH = 0
Adiabatic
no heat transferred
Q=0
Isentropic
(ideal reversible)
no increase in entropy
ΔS = 0
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
6
1st Law for steady state flow
Equation 1.19a (ΔH  ΔU for flowing systems)
2

u
g
Hˆ +
+ z = Q − W
2gc gc
For adiabatic, steady-state, ideal (reversible) flow (using WS as positive value)
2

u
g
Wˆ s = Hˆ +
+ z
2gc gc
P2
2

u
g
ˆ
=  V dP +
+ z
2
g
gc
c
P1
P2
P2
dP
Wˆ s   Vˆ dP = 

P1
P1
▪ The work required is inversely proportional to the mass density
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
7
Thermodynamics of Compression
Work depends on path – commonly assume adiabatic or
polytropic compression
Calculations done with:
▪ PH diagram for ΔH
P2
Ws =  VdP = H
P1
▪ Evaluate integral using equation of state
• Simplest gas EOS is the ideal gas law
• Simplest liquid EOS is to assume incompressible (i.e., constant density
with respect to pressure)
P2
P
dP 1 2
P −P
Ws = 
=  dP = 2 1
  P1

P1
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
8
Liquid vs. Vapor/Gas Compression
Can compress
liquids with little
temperature
change
ΔH for gas compression much
larger than for liquid pumping
GPSA Data Book, 13th ed.
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
9
Mechanical Energy Balance
Differential form of Bernoulli’s equation for fluid flow (energy per unit mass)
d ( u2 )
2
+ g dz +
( )
dP
+ d ( wˆ s ) + g d hˆf = 0

▪ Frictional loss term is positive
▪ Work term for energy out of fluid – negative for pump or compressor
If density is constant then the integral is straight forward – pumps
 ( u2 )
2
P
+ g z +
+ wˆ s + g hˆf = 0

If density is not constant then you need a pathway for the pressure-density
relationship – compressors
 ( u2 )
2
 dP 
+ g z +    + wˆ s + g hˆf = 0
 
P1 
P2
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
10
Pump equations
Pumping requirement expressed in terms of power, i.e., energy per unit time
Hydraulic horsepower – power delivered to the fluid
▪ Over entire system
  ( u2 )

P
Whhp = m ( −wˆ s ) = ( V ) 
+ g z +
+ g hˆf 

 2

( )
1
= V ( P ) + ( V ) ( g z ) + ( V ) g hˆf + ( V ) ( u2 )
2
▪ Just across the pump, in terms of pressure differential or head:
Whhp = V ( P ) or Whhp = V gH
Brake horsepower – power delivered to the pump itself
Wbhp =
Whhp
pump
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
11
Pump equations for specific U.S. customary
units
U.S. customary units usually used are gpm, psi, and hp

in3 
231 gal 
 gal   lbf 


Whhp hp  
2


ft  lb f / sec 
 min   in   sec   in  
60
12
550

 min   ft  
hp


 1  gal   lbf 
=

2
 1714   min   in 
Also use the head equation usually using gpm, ft, specific gravity, and hp
lbm 


ft
8.33719
32.174



2
gal 
 gal 

sec
Whhp hp  
ft


  o 
ft  lb f / sec   32.174 lbm  ft
 sec  
 min 
60
550

 min  
lb f  sec2
hp


 1  gal 
=
 ft  o 


39
58
min



Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)





12
Static Head Terms
Fundamentals of Natural Gas Processing, 2nd ed.
Kidnay, Parrish, & McCartney
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
13
Pump Example
Liquid propane (at its bubble point) is to
be pumped from a reflux drum to a
depropanizer.
▪ Pressures, elevations, & piping system
losses as shown are shown in the
diagram.
▪ Max flow rate 360 gpm.
▪ Propane specific gravity 0.485 @
pumping temperature (100oF)
▪ Pump nozzles elevations are zero &
velocity head at nozzles negligible
What is the pressure differential across
the pump?
What is the differential head?
What is the hydraulic power?
GPSA Data Book, 13th ed.
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
14
Pump Example
Pressure drop from Reflux Drum to Pump inlet:
P = −
g
z + ( P )piping
gc
( 0.485 )( 62.3665 )  ( −20 )
=−
+  −0.5 − 0.2
144
= 3.5 psi
Pinlet = 203.5 psia
Pressure drop from Pump outlet to
Depropanizer:
P = −
g
z + ( P )piping
gc
( 0.485 )( 62.3665 )  ( 74 )
=−
− 3.0 + 2.0 + 1.2 + 13.0 + 1.0 + 9.0 
144
= −44.7 psi
Poutlet = 264.7 psia
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
15
Pump Example
Pump pressure differential:
( P )pump = Poutlet − Pinlet
= 264.7 − 203.5 = 61.2 psi
Pump differential head:
gc ( P )pump gc ( P )pump
hpump =
=
g

g (  owater )
=−
(144 )
( 61.2 ) = 291 ft
( 0.485)( 62.3665)
Hydraulic power:
Whhp =
( 360 gpm)( 61.2 psi) = 12.85 hp
1714
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
OR Whhp =
( 360 gpm )( 0.485 )(291 ft ) = 12.84 hp
3958
16
Gas Compression:
PH Diagrams
Ref: GPSA Data Book, 13th ed.
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
17
TS Diagram
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
18
Thermodynamics of Ideal Gas Compression
Choices of path for calculating work:
▪ Isothermal (ΔT = 0)
 P2 
dP
Ws =  VdP = RT 
= RT ln 
P
 P1 
P1
P1
P2
P2
• Minimum work required but unrealistic
▪ Adiabatic & Isentropic (ΔS = 0)
• Maximum ideal work but more realistic
▪ Polytropic – reversible but non-adiabatic
• Reversible work & reversible heat proportionately added or removed along path
• More closely follows actual pressure-temperature path during compression
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
19
Thermodynamics of Compression
Ideal gas isentropic (PV = constant) where  = CP/CV
▪ Molar basis
( −1)/ 

  P2 
 
Ws = ( RT1 )
− 1
 − 1  P1 


▪ Mass basis
 P ( −1)/  
W
RT

 2 
Wˆ s = s = 1
− 1
M M  − 1  P1 


Can also determine discharge temperature
P 
T2 = T1  2 
 P1 
( −1)/ 
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
20
Thermodynamics of Compression
Calculation of  for gas mixture
xC

=
x C
i
p ,i
i V ,i
xC

=
x C −R
i
i
p ,i
p ,i
Use the ideal gas heat capacities, not the real gas heat capacities
Heat capacities are functions of temperature. Use the average
value over the temperature range
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
21
Example Calculation:
Ideal Gas Laws
For methane:
▪  = 1.3
▪ M = 16
▪ T1 = 120oF = 580oR
▪ P1 = 300 psia
▪ P2 = 900 psia
▪ R = 1.986 Btu/lb.mol oR
( −1)/ 
 (1.3)(1.986 )( 580 )  900 (1.3−1)/1.3 
RT1  P2 
 
Ws =
− 1 =
− 1 = 90 Btu/lb


M (  − 1 )  P1 
16
1.3
−
1
300
( )(
)  



 900 
T2 = ( 580 ) 

 300 
(1.3−1)/1.3
= 747°R  287°F
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
22
525 BTU/lb
440 BTU/lb
Enthalpy Changes:
𝑊 = ∆𝐻 = 525 − 440 = 85
Outlet temperature: 280°F
Compress methane from 300 psia &
120°F to 900 psia
𝐵𝑇𝑈
𝑙𝑏
Example Calculation:
Using a Simulator
Work Btu/lb
Outlet oF
HYSYS Peng-Robinson
86.58
281.8
HYSYS Peng-Robinson & Lee-Kesler
87.34
280.5
HYSYS SRK
87.71
281.2
HYSYS BWRS
87.04
281.1
HYSYS Lee-Kesler-Plocker
87.40
280.6
Aspen Plus PENG-ROB
86.61
282.4
Aspen Plus SRK
87.82
281.8
Aspen Plus BWRS
87.10
281.6
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
24
Multi-Stage Gas Compression
If customer wants 1,000 psig (with 100 psig & 120oF inlet)…
▪ Then pressure ratio of (1015/115) = 8.8
▪ Discharge temperature for this ratio is ~500oF
For reciprocating compressors the GPSA Engineering Data Book
recommends
▪ Pressure ratios of 3:1 to 5:1 AND …
▪ Maximum discharge temperature of 250 to 275oF for high pressure
systems
To obtain higher pressure ratios higher must use multistage
compression with interstage cooling
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
25
Multistaging
To minimize work need good interstage cooling and equal pressure ratios
The number of stages is calculated using
1/ m
P 
RP =  2 
 P1 
 m=
ln( P2 / P1 )
ln(RP )
To go from 100 to 1000 psig with a single-stage pressure ratio of 3 takes 2
(1.98) stages & the stage exit temp ~183oF (starting @ 120oF)
 1015 
ln

115  ln( 8.8 )

m=
=
= 1.98  m = 2
ln( 3 )
ln( 3 )
 P 1/m 
T2 = T1  2  
 P1  
( −1)/ 
 1015 1/2 
= (120 + 460 ) 
 
115
 

Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
(1.3−1)/1.3
= ( 580 )2.97
0.2308
= 746°R  286°F
26
Multistaging
Work for a single stage of compression
 P ( −1)/   (1.3)(1.986 )( 580 )  1015 (1.3−1)/1.3 

RT
1
 2 
Wˆ s =
− 1 =
− 1 = 204 Btu/lb


M (  − 1 )  P1 
16 )(1.3 − 1 )  115 
(




Work for two stages of compression (interstage cooling to 120oF)
▪ Intermediate pressure
Pint = P2P1 =
(1015)(115) = 341.7 psia
▪ Total work – 13% less work
1.3−1 /1.3
  1015 (1.3−1)/1.3  
1.3)(1.986 )( 580 )   341.7 ( )
(
Wˆ s =
− 1 + 
− 1 
 


16
1.3
−
1
115
341.7
( )(
)  


 
 
= 178 Btu/lb
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
27
Compression Efficiency
Compression efficiencies account for
actual power required compared to
ideal
▪ Isentropic (also known as adiabatic)
efficiency relates actual energy to
fluid to energy for reversible
compression
IS =
( H )S=0
( H ) fluid
 Wfluid =
Total
Energy
To
Device
WS =0
IS
▪ Mechanical efficiency relates total
work to device to the energy into
the fluid
mech =
Wfluid
Wtotal
 Wtotal =
Wfluid
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
mech
=
WS =0
mechIS
{
Minimum Energy for Compression
Total
Energy
Into
Fluid
Energy to Overcome Mechanical Inefficiencies – Goes
Into Fluid & Increases Temperature
Energy
Lost
To
Environment
28
Compressor Efficiency – Discharge
Temperature
GPSA Engineering Data Book suggests the isentropic temperature
change should be divided by the isentropic efficiency to get the
actual discharge temperature
P 
( T )S =0 = T1  2 
 P1 
So: ( T )act =
( −1)/ 
( T )S =0
IS
 T2,act = T1 + ( T )act
 P ( −1)/  
− T1 = T1  2 
− 1
 P1 

 P2 
 
P
= T1  1 
( −1)/ 
−1
IS
  P ( −1)/ 

2
  
−1
P


= T1 1 +  1 

IS




Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
29
Example Calculation:
75% isentropic efficiency
ΔS =0
Work Btu/lb
ΔS =0
Outlet oF
=0.75
Work Btu/lb
=0.75
Outlet oF
Ideal Gas Equations
90
287
120
343
PH diagram
85
280
113
325
HYSYS Peng-Robinson
86.58
281.8
115.4
325.2
HYSYS SRK
87.71
281.2
116.9
325.2
HYSYS BWRS
87.04
281.1
116.1
325.1
HYSYS Lee-Kesler-Plocker
87.40
280.6
116.5
324.8
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
30
Polytropic Compression & Efficiency
Definition of polytropic compression (GPSA Data Book 14th ed.):
A reversible compression process between the compressor
inlet and discharge conditions, which follows a path such that,
between any two points on the path, the ratio of the
reversible work input to the enthalpy rise is constant. In other
words, the compression process is described as an infinite
number of isentropic compression steps, each followed by an
isobaric heat addition. The result is an ideal, reversible
process that has the same suction pressure, discharge
pressure, suction temperature and discharge temperature as
the actual process.
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
31
Polytropic Efficiency
Polytropic path with 100% efficiency is adiabatic & is the same as
the isentropic path
▪ Polytropic efficiency, p, is related to the isentropic path
P =
(  − 1) / 
(  − 1) / 
In general P > IS
Polytropic coefficient from discharge temperature
P 
T2 = T1  2 
 P1 
 −1 


  
 =
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
ln(T2 / T1 )
1
where  
1− 
ln( P2 / P1 )
32
Polytropic Efficiency
Actual work is calculated from the polytropic expression divided
by its efficiency
( −1) / 




RT
P
1

1
2
 
Wˆact =
=
− 1
p p M  − 1  P1 


Wˆ p
Note:
Wˆact =
Wˆ p
Wˆ S =0
=
p
IS
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
33
Why Use Polytropic Equations?
Polytropic equations give consistent P-T pathway between the
initial & discharge conditions
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
34
Effect of Internal Pressure Drops
Pressure drops at the inlet & outlet give an apparent inefficiency
of compression
▪ Assume isenthalpic pressure drops before & after the actual compression
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
35
Effect of Internal Pressure Drops
Pressure
Reductions
Before
Compression
psia
After
Compression
psia
Work
Btu/lb
Outlet
°F
Efficiency
0%
300
900
86.58
281.8
100%
1%
297
909
88.36
284.7
98%
5%
285
947
95.76
296.8
90%
10%
270
1000
105.7
313.0
82%
15%
255
1059
116.5
330.2
74%
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
36
Compression vs. Expansion Efficiency
Work to compressor is greater
than what is needed in the
ideal case
▪ Work to the fluid
IS =
( H )S=0
( H ) fluid
▪ Work from the fluid
 Wfluid =
WS =0
IS
▪ Total work to the device
mech =
Wfluid
Wtotal
Wtotal =
Work from expander is less
than what can be obtained in
the ideal case
IS =
( H ) fluid
( H )S =0
▪ Total work from the device

Wfluid
mech
W
= S =0
mechIS
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
 Wfluid = IS (WS =0 )
mech =
Wtotal
Wfluid

Wtotal = mechWfluid = mechISWS =0
37
Equipment:
Pumps, Compressors,
Turbines/Expanders
Updated: January 4, 2019
Copyright © 2017 John Jechura (jjechura@mines.edu)
Pump & Compressor Drivers
Internal combustion engines
▪ Industry mainstay from
beginning
▪ Emissions constraints
▪ Availability is 90 to 95%
Electric motors
▪ Good in remote areas
▪ Availability is > 99.9%
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
Gas turbines
▪ Availability is > 99%
▪ Lower emissions than IC engine
Steam turbines
▪ Uncommon in gas plants on
compressors
▪ Used in combined cycle and
Claus units
39
Pump Classifications
Fundamentals of Natural Gas Processing, 2nd ed.
Kidnay, Parrish, & McCartney
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
40
Centrifugal Pump Performance Curves
Fundamentals of Natural Gas Processing, 2nd ed.
Kidnay, Parrish, & McCartney
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
41
Compressor Types
Positive displacement –
compress by changing volume
▪ Reciprocating
▪ Rotary screw
▪ Diaphragm
▪ Rotary vane
Dynamic – compress by
converting kinetic energy into
pressure
▪ Centrifugal
▪ Axial
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
42
Reciprocating Compressors
Workhorse of industry since 1920’s
Capable of high volumes and
discharge pressures
High efficiency – up to 85%
Performance independent of gas MW
Good for intermittent service
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
Drawbacks
▪ Availability ~90 to 95% vs 99+%
for others, spare compressor
needed in critical service
▪ Pulsed flow
▪ Pressure ratio limited, typically
3:1 to 4:1
▪ Emissions control can be
problem (IC drivers)
▪ Relatively large footprint
▪ Throughput adjusted by variable
speed drive, valve unloading or
recycle unless electrically driven
43
Reciprocating Compressors - Principle of
Operation
Double Acting – Crosshead
Single Acting - Trunk Piston
Typical applications:
▪ All process services, any gas & up
to the highest pressures & power
Typical Applications:
▪ Small size standard compressors
for air and non-dangerous gases
Courtesy of Nuovo Pignone Spa, Italy
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
44
Reciprocating Compressors - Compression
Cycle
5
4
Pressure
1
p0
3
2
Volume
Suction
1
2
3
4
5
Discharge
Courtesy of Nuovo Pignone Spa, Italy
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
45
Reciprocating Compressors - Main
Components
Pulsation Bottles
Crankcase
Cast Iron
Crankshaft
Counterweight
Forged Steel for balancing
Crosshead
Cast Steel
Ballast for
balancing of
inertia forces
Slide Body
Distance Pieces
Pneumatic
Valve
Unloaders
for capacity
control
Forged
Cylinder
Main Oil Pump
Connecting Rod
(die forged steel)
Oil Wiper
Packing Piston Rod
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
Cast
Cylinder
Rod Packing
Piston
Cylinder Valve
46
Reciprocating Compressors
https://www.youtube.com/watch?v=E6_jw841vKE
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
47
Rotary Screw Compressor
Left rotor turns clockwise, right rotor
counterclockwise.
Gas becomes trapped in the central
cavity
The Process Technology Handbook, Charles E. Thomas,
UHAI Publishing, Berne, NY, 1997.
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
Courtesy of Ariel Corp
48
Rotary Screw Compressors
Oil-free
Oil-injected
First used in steel mills because handles “dirty”
gases
Higher throughput and discharge pressures
Max pressure ratio of 8:1 if liquid injected with
gas
High availability (> 99%)
▪ Leads to low maintenance cost
Volumetric efficiency of ~100%
Small footprint (~ ¼ of recip)
Relatively quiet and vibration-free
Relatively low efficiency
▪ 70 – 85% adiabatic efficiencies
Relatively low throughput and discharge
pressure
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
Has two exit ports
▪ Axial, like oil-free
▪ Radial, which permits 70 to 90% turndown
without significant efficiency decrease
Pressure ratios to 23:1
Tight tolerances can limit quick restarts
Requires oil system to filter & cool oil to 140oF
Oil removal from gas
Oil compatibility is critical
Widely used in propane refrigeration systems,
low pressure systems, e.g., vapor recovery,
instrument air
49
Dynamic Compressors
Centrifugal
▪ High volumes, high discharge pressures
Axial
▪ Very high volumes, low discharge pressures
Use together in gas processing
▪ Centrifugal for compressing natural gas
▪ Axial for compressing air for gas turbine driving centrifugal compressor
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
50
Centrifugal compressors
Single stage (diffuser)
Multi-stage
Bett,K.E., et al
Thermodynamics for Chemical Engineers
Page 226
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
51
Centrifigual Compressor
Siemens
https://www.energy.siemens.com/br/en/compressionexpansion/product-lines/single-stage/stc-sof.htm
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
https://www.youtube.com/watch?v=s-bbAoxZmBg
52
Centrifugal Compressor
Courtesy of Nuovo Pignone Spa, Italy
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
53
Centrifugal vs. Reciprocating Compressors
Centrifugal
Constant head, variable volume
Ideal for variable flow
- MW affects capacity
++
Availability > 99%
+ Smaller footprint
- ηIS = 70 – 75%
CO & NOx emissions low
- Surge control required
++
Lower CAPEX and maint.
(maint cost ~1/4 of recip)
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
Reciprocating
Constant volume, variable pressure
Ideal for constant flow
+ MW makes no difference
- Availability 90 to 95%
- Larger footprint
+ ηIS = 75 – 92%
Catalytic converters needed
++ No surge problems
++
Fast startup & shutdown
54
Turbines & Turboexpanders
Utilize pressure energy to…
▪ Reduce temperature of gas & (possibly) generate liquids
▪ Perform work & provide shaft power to coupled equipment
Similar principles to a centrifugal compressor except in reverse
Most common applications:
▪ Turboexpander: NGL recovery
▪ Gas turbine: power to drive pumps, compressors, generators, …
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
55
Turboexpander Cutaway
https://www.turbomachinerymag.com/expander-compressors-an-introduction/
Video (LA Turbine. 2:54 minutes): https://www.youtube.com/watch?v=f5Gz_--_NBM
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
56
Turboexpanders
GPSA Engineering Data Book, 14th ed.
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
Methane Expansion – Isentropic vs.
Isenthalpic

Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
58
Gas Turbine Engine
https://aceclearwater.com/product/case-study-ge-power-ducts/
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
59
Gas Turbine Coupled to Centrifugal
Compressor
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
60
Industrial Gas Turbines
Ref: GPSA Data Book, 13th ed.
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
61
What is “heat rate”?
Heat rate is the amount of fuel gas needed (expressed heating
value) to produce a given amount of power
▪ Normally LHV, but you need to make sure of the basis
Essentially the reciprocal of the thermal efficiency
Thermal efficiency =
2544
Btu(LHV )
Heat rate,
hp  hr
▪ Example: Dresser-Rand VECTRA 30G heat rate is 6816 Btu/hp·hr
Thermal efficiency =
2544
= 0.3732
6816
Includes effects of adiabatic & mechanical efficiencies
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
62
Gas Turbine Engine
Gas Turbine Engine
From: F.W.Schmuidt, R.E. Henderson, and C.H. Wolgemuth,
“Introduction to Thermal Sciences, second edition” Wiley, 1993
Fuel
P2
P1
Compressor
Combustion
chamber
shaft
P3
shaft
Turbine
P4
Atmospheric air
Load
Combustion
products
Assumptions
To apply basic thermodynamics to the process above, it is necessary to make a number of
assumptions, some rather extreme.
1) All gases are ideal, and compression processes are reversible and adiabatic (isentropic)
2) the combustion process is constant pressure, resulting only in a change of temperature
3) negligible potential and kinetic energy changes in overall process
4) Values of Cp are constant
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
63
Gas Turbine Engine
P2 = P3
T
Temperature
Patm = P1 = P4
3
2
4
1
Entropy
S
wS = -∆h = -CP∆T
(9.1 and 1.18)
Note the equations apply to both the compressor and the turbine,since
thermodynamically the turbine is a compressor running backwards
Neglecting the differences in mass flow rates between the compressor and
the turbine, the net work is:
wnet = wt – wc = CP(T3 – T4) – (T2 -T1)
Since (T3 – T4) > (T2 – T1)
(see T – S diagram)
Since wnet is positive work flows to the load
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
64
GT - Principle of Operation
Theoretical Cycle
Temperature
°F
Fuel Gas
Combustion
Chamber
~650 - 950°F
3
2
1
Air
~1800 2300°F
3
Exhaust
Gas (~950°F)
Real Cycle
4
4
2
Centrifugal
Compressor
Axial
Compressor
1
H.P./L.P. Turbine
Simple Cycle Gas Turbine
A
B
Entropy
Ideal Cycle Efficiency
P 
T −T
id = 1 − 4 1 = 1 −  1 
T3 − T2
 P2 
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
( −1)/ 
65
Modeling Gas Turbine with Aspen Plus
Basics to tune model
▪ Combine heat rate & power output to determine the fuel required
▪ Determine the air rate from the exhaust rate
▪ Adjust adiabatic efficiencies to match the exhaust temperature
▪ Adjust the mechanical efficiencies to match the power output
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
66
Summary
Updated: January 4, 2019
Copyright © 2017 John Jechura (jjechura@mines.edu)
Summary
Work expression for pump developed
assuming density is not a function of
pressure
Work of compression is much greater
than that for pumping – a great portion
of the energy goes to increase the
temperature of the compressed gas
Need to limit the compression ratio on a
gas
• Interstage cooling will result in
decreased compression power
required
• Practical outlet temperature limitation
– usually means that the maximum
compression ratio is about 3
There are thermodynamic/adiabatic &
mechanical efficiencies
• Heat lost to the universe that does
affect the pressure or temperature of
the fluid is the mechanical efficiency
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
68
Supplemental Slides
Updated: January 4, 2019
Copyright © 2017 John Jechura (jjechura@mines.edu)
Reciprocating Compressors
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
70
Propane Refrigeration Compressors
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
71
Propane Compressors with Air-cooled Heat
Exchangers
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
72
Reciprocating Compressor at Gas Well
Courtesy of Nuovo Pignone Spa, Italy
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
73
2 stage 2,000 HP Reciprocating Compressor
Courtesy of Ariel Corp
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
74
Oil-Injected Rotary Screw Compressor
Courtesy of Ariel Corp
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
75
Two-stage screw compressor
Courtesy of MYCOM / Mayekawa Mfg
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
76
Centrifugal Compressors – Issues
Surge
• When gas flow reaches sonic
Axial
Centrifugal
Stonewall Line
Stonewall
Surg
e Lin
e
pressures can cause backflow; this
can become cyclic as the
compressor tries to adjust. The
resulting pressure oscillations are
called SURGE
Pressure Head
• Changes in the suction or outlet
Reciprocating
velocity flow cannot be increased.
Inlet Volume Flow
Rate
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
77
Air & Hot Gas Paths
Gas Turbine has 3 main sections:
A compressor that takes in clean outside air and then compresses it through a series of rotating and
stationary compressor blades
FRESH AIR
EXHAUST
COMPRESSION
MECHANICAL
ENERGY
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
78
Air & Hot Gas Paths
Gas Turbine has 3 main sections:
A combustion section where fuel is added to the pressurized air and ignited. The hot pressurized
combustion gas expands and moves at high velocity into the turbine section.
FRESH AIR
EXHAUST
COMBUSTION
COMPRESSION
MECHANICAL
ENERGY
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
79
Air & Hot Gas Paths
Gas Turbine has 3 main sections:
A turbine that converts the energy from the hot/high velocity gas flowing from the combustion chamber
into useful rotational power through expansion over a series of turbine rotor blades
FRESH AIR
EXHAUST
COMBUSTION
EXPANSION
COMPRESSION
TURBINE
MECHANICAL
ENERGY
Updated: January 4, 2019
Copyright © 2019 John Jechura (jjechura@mines.edu)
80