rotating equipment: pumps, compressors, &...
TRANSCRIPT
Rotating Equipment: Pumps, Compressors, &
Turbines/Expanders
Chapters 2 & 9
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
TopicsFundamentals
▪ Starting relationships
• Thermodynamic relationships
• Bernoulli’s equation
▪ Simplifications
• Pumps – constant density compression
• Compressors – reversible ideal gas compression
▪ Use of PH & TS diagrams
▪ Multistaging
Efficiencies
▪ Adiabatic/isentropic vs. mechanical
▪ Polytropic
Equipment
▪ Pumps
• Centrifugal pumps
• Reciprocating pumps
• 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
Updated: January 4, 2019Copyright © 2017 John Jechura ([email protected])
Fundamentals
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Review of Thermodynamic Principals1st Law of Thermodynamics – Energy is conserved
▪ (Change in system’s energy) = (Rate of heat added) – (Rate of work performed)
▪ 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
o Enthalpy, H = U+PV, convenient for systems at constant pressure & flowing systems
4
E Q W = −
= + +2
ˆ ˆ2 c c
u gE U h
g g
= + +2
ˆ ˆ2 c c
u gE H h
g g
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
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
5
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Common Paths for Heat and Work
6
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, 2019Copyright © 2019 John Jechura ([email protected])
1st Law for steady state flowEquation 1.19a (ΔH ΔU for flowing systems)
For adiabatic, steady-state, ideal (reversible) flow (using WS as positive value)
▪ The work required is inversely proportional to the mass density
7
2
ˆ2 c c
u gH z Q W
g g
+ + = −
2
1
2 2
1 1
2
2
ˆ ˆ2
ˆ2
ˆ ˆ
s
c c
P
c cP
P P
s
P P
u gW H z
g g
u gV dP z
g g
dPW V dP
= + +
= + +
=
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Thermodynamics of Compression
Work depends on path – commonly assume adiabatic or polytropic compression
Calculations done with:▪ PH diagram for ΔH
▪ 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)
8
= = 2
1
P
s
P
W VdP H
−= = =
2 2
1 1
2 11P P
s
P P
dP P PW dP
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Liquid vs. Vapor/Gas Compression
Can compress liquids with little temperature change
9
GPSA Data Book, 13th ed.
ΔH for gas compression much
larger than for liquid pumping
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Mechanical Energy BalanceDifferential form of Bernoulli’s equation for fluid flow (energy per unit mass)
▪ 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
If density is not constant then you need a pathway for the pressure-density relationship – compressors
10
( )( ) ( )
2
ˆˆ 02
s f
d u dPg dz d w g d h+ + + + =
( )2
ˆˆ 02
s f
u Pg z w g h
+ + + + =
( ) 2
1
2
ˆˆ 02
P
s f
P
u dPg z w g h
+ + + + =
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Pump equationsPumping requirement expressed in terms of power, i.e., energy per unit time
Hydraulic horsepower – power delivered to the fluid
▪ Over entire system
▪ Just across the pump, in terms of pressure differential or head:
Brake horsepower – power delivered to the pump itself
11
( ) ( )( )
( ) ( )( ) ( )( ) ( )( )
= − = + + +
= + + +
2
hhp
2
ˆˆ2
1ˆ2
s f
f
u PW m w V g z g h
V P V g z V g h V u
( )= = hhp hhp or W V P W V gH
=
hhp
bhp
pump
WW
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Pump equations for specific U.S. customary unitsU.S. customary units usually used are gpm, psi, and hp
Also use the head equation usually using gpm, ft, specific gravity, and hp
12
=
3
fhhp 2
f2
f
in231
gallbgalhp
min in ft lb / secsec in60 12 550
min f
lb1 gal
1714 min in
t hp
W
=
m
2
hhpmf
2f
lb ft8.33719 32.174galgal sechp ft
lb ftmin f
1 galft
39
t lb / secsec 32.17460 550lb secmin
58
hp
mino
oW
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Static Head Terms
13
Fundamentals of Natural Gas Processing, 2nd ed.Kidnay, Parrish, & McCartney
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Pump ExampleLiquid 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?
14
GPSA Data Book, 13th ed.
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Pump ExamplePressure drop from Reflux Drum to Pump inlet:
Pressure drop from Pump outlet to Depropanizer:
15
( )
( )( ) ( )
piping
inlet
0.485 62.3665 200.5 0.2
1443.5 psi
203.5 psia
c
gP z P
g
P
= − +
− = − + − −
=
=
( )
( )( ) ( )
piping
outlet
0.485 62.3665 743.0 2.0 1.2 13.0 1.0 9.0
14444.7 psi
264.7 psia
c
gP z P
g
P
= − +
= − − + + + + +
= −
=
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Pump ExamplePump pressure differential:
Pump differential head:
Hydraulic power:
16
( ) outlet inletpump
264.7 203.5 61.2 psi
P P P = −
= − =
( ) ( )
( )
( )
( )( )( )
pump pump
pump
14461.2 291 ft
0.485 62.3665
c c
o water
P Pg gh
g g
= =
= − =
( )( ) ( )( )( )hhp hhp
360 gpm 61.2 psi 360 gpm 0.485 291 ft12.85 hp OR 12.84 hp
1714 3958W W= = = =
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Gas Compression:PH Diagrams
17
Ref: GPSA Data Book, 13th ed.
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Thermodynamics of Ideal Gas Compression
Choices of path for calculating work:
▪ Isothermal (ΔT = 0)
• 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
19
= = =
2 2
1 1
2
1
lnP P
s
P P
dP PW VdP RT RT
P P
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Thermodynamics of Compression
Ideal gas isentropic (PV = constant) where = CP/CV
▪ Molar basis
▪ Mass basis
Can also determine discharge temperature
20
( )( )−
= − −
1 /
21
1
11
s
PW RT
P
( )− = = −
−
1 /
1 2
1
ˆ 11
ss
W RT PW
M M P
( )−
=
1 /
22 1
1
PT T
P
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Thermodynamics of Compression
Calculation of for gas mixture
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
21
, ,
, ,
i p i i p i
i V i i p i
x C x C
x C x C R = =
−
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
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
22
( )
( )( )( )( )
( )( )
( )
( )( )
− −
−
= − = − =
− −
= =
1 / 1.3 1 /1.3
1 2
1
1.3 1 /1.3
2
1.3 1.986 580 9001 1 90 Btu/lb
1 16 1.3 1 300
900580 747°R 287°F
300
s
RT PW
M P
T
440 BTU/lb
525 BTU/lb
Enthalpy Changes:
𝑊 = ∆𝐻 = 525 − 440 = 85𝐵𝑇𝑈
𝑙𝑏Outlet temperature: 280°F
Compress methane from 300 psia & 120°F to 900 psia
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
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
24
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
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
25
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
MultistagingTo minimize work need good interstage cooling and equal pressure ratios
The number of stages is calculated using
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)
26
( )
( )
1/
2 12
1
ln /
ln
m
P
P
P PPm
P
= =
RR
( )
( )
( )
( )
( )
( )
( )
− −
= = = =
= = + = =
1 / 1.3 1 /1.31/ 1/20.23082
2 1
1
1015ln
ln 8.81151.98 2
ln 3 ln 3
1015120 460 580 2.97 746°R 286°F
115
m
m m
PT T
P
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Multistaging
Work for a single stage of compression
Work for two stages of compression (interstage cooling to 120oF)▪ Intermediate pressure
▪ Total work – 13% less work
27
( )
( )( )( )( )
( )( )
( )1 / 1.3 1 /1.3
1 2
1
1.3 1.986 580 1015ˆ 1 1 204 Btu/lb1 16 1.3 1 115
s
RT PW
M P
− − = − = − =
− −
( )( )int 2 1 1015 115 341.7 psiaP P P= = =
( )( )( )
( )( )
( ) ( )1.3 1 /1.3 1.3 1 /1.31.3 1.986 580 341.7 1015ˆ 1 1
16 1.3 1 115 341.7
178 Btu/lb
sW− −
= − + − −
=
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Compression EfficiencyCompression 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
▪ Mechanical efficiency relates total work to device to the energy into the fluid
28
( )
( )0 0
IS
IS
S Sfluid
fluid
H WW
H = =
= =
TotalEnergy
ToDevice
EnergyLostTo
Environment
Minimum Energy for Compression
Energy to Overcome Mechanical Inefficiencies – GoesInto Fluid & Increases Temperature{ Total
Energy IntoFluid
0mech
mech mech IS
fluid fluid Stotal
total
W W WW
W = = = =
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
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
29
( )( ) ( )1 / 1 /
2 21 1 10
1 1
1S
P PT T T T
P P
− −
=
= − = −
( )( )
( )
( )
( )
1 /
2
101
IS IS
1 /
2
12, 1 1
IS
1
So:
1
1
S
act
act act
P
T PT T
P
PT T T T
−
=
−
−
= =
−
= + = +
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Example Calculation:75% isentropic efficiency
ΔS =0Work Btu/lb
ΔS =0Outlet oF
=0.75Work Btu/lb
=0.75Outlet 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
30
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
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.
31
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
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
In general P > IS
Polytropic coefficient from discharge temperature
32
( )
( )
−
= =
−
1
2 122 1
1 2 1
ln /1 where
1 ln /
T TPT T
P P P
( )
( )
− =
− P
1 /
1 /
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Polytropic Efficiency
Actual work is calculated from the polytropic expression divided by its efficiency
Note:
33
( )1 /
1 2
1
ˆ 1ˆ 11
p
act
p p
W RT PW
M P
− = = −
−
0ˆ ˆ
ˆ p Sact
p IS
W WW == =
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Why Use Polytropic Equations?
Polytropic equations give consistent P-T pathway between the initial & discharge conditions
34
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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
35
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Effect of Internal Pressure Drops
Pressure Reductions
Before Compression
psia
After Compression
psiaWorkBtu/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%
36
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Compression vs. Expansion Efficiency
Work to compressor is greaterthan what is needed in the ideal case▪ Work to the fluid
▪ Total work to the device
Work from expander is lessthan what can be obtained in the ideal case▪ Work from the fluid
▪ Total work from the device
37
( )
( )0 0
IS
IS
S Sfluid
fluid
H WW
H = =
= =
mech
0
mech mech IS
fluid
total
fluid Stotal
W
W
W WW =
=
= =
( )
( )( )IS IS 0
0
fluid
fluid S
S
HW W
H =
=
= =
mech
mech mech IS 0
total
fluid
total fluid S
W
W
W W W =
=
= =
Updated: January 4, 2019Copyright © 2017 John Jechura ([email protected])
Equipment:Pumps, Compressors, Turbines/Expanders
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
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%
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
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Pump Classifications
40
Fundamentals of Natural Gas Processing, 2nd ed.Kidnay, Parrish, & McCartney
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Centrifugal Pump Performance Curves
41
Fundamentals of Natural Gas Processing, 2nd ed.Kidnay, Parrish, & McCartney
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Compressor TypesPositive displacement –compress by changing volume
▪ Reciprocating
▪ Rotary screw
▪ Diaphragm
▪ Rotary vane
Dynamic – compress by converting kinetic energy into pressure
▪ Centrifugal
▪ Axial
42
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Reciprocating CompressorsWorkhorse 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
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
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Reciprocating Compressors - Principle of Operation
Double Acting – Crosshead
Typical applications:
▪ All process services, any gas & up to the highest pressures & power
Single Acting - Trunk Piston
Typical Applications:▪ Small size standard compressors
for air and non-dangerous gases
44
Courtesy of Nuovo Pignone Spa, Italy
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Reciprocating Compressors - Compression Cycle
45
Suction
Discharge
1
1
2
2 3
3
4
54
5
p0
Pre
ssure
Volume
Courtesy of Nuovo Pignone Spa, Italy
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Reciprocating Compressors - Main Components
46
Pulsation BottlesCrosshead
Cast SteelCrankcase
Cast Iron
Distance Pieces
Slide Body
Connecting Rod
(die forged steel)
Forged
Cylinder
Crankshaft
Forged Steel
Cast
Cylinder
Main Oil Pump
Piston Rod
Rod Packing
PistonCylinder Valve
Counterweight
for balancing Ballast for
balancing ofinertia forces
Pneumatic
Valve
Unloaders
for capacity
control
Oil Wiper
Packing
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Reciprocating Compressors
47
https://www.youtube.com/watch?v=E6_jw841vKE
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Rotary Screw CompressorLeft 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.
48
Courtesy of Ariel Corp
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Rotary Screw CompressorsOil-free
First used in steel mills because handles “dirty” gases
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
Oil-injected
Higher throughput and discharge pressures
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
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
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
50
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Centrifugal compressors
Single stage (diffuser) Multi-stage
51
Bett,K.E., et al Thermodynamics for Chemical Engineers Page 226
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Centrifigual Compressor
52
Siemenshttps://www.energy.siemens.com/br/en/compression-expansion/product-lines/single-stage/stc-sof.htm
https://www.youtube.com/watch?v=s-bbAoxZmBg
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Centrifugal Compressor
53
Courtesy of Nuovo Pignone Spa, Italy
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Centrifugal vs. Reciprocating CompressorsCentrifugal
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)
ReciprocatingConstant 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
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
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, …
55
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Turboexpander Cutaway
56
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, 2019Copyright © 2019 John Jechura ([email protected])
Turboexpanders
GPSA Engineering Data Book, 14th ed.
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Methane Expansion – Isentropic vs. Isenthalpic
58
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Gas Turbine Engine
59
https://aceclearwater.com/product/case-study-ge-power-ducts/
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Gas Turbine Coupled to Centrifugal Compressor
60
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Industrial Gas Turbines
61
Ref: GPSA Data Book, 13th ed.
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
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
▪ Example: Dresser-Rand VECTRA 30G heat rate is 6816 Btu/hp·hr
Includes effects of adiabatic & mechanical efficiencies
62
( )=
2544Thermal efficiency
Btu LHVHeat rate,
hp hr
= =2544
Thermal efficiency 0.37326816
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Combustion
chamber
Compressor Turbine Load
Gas Turbine Engine
From: F.W.Schmuidt, R.E. Henderson, and C.H. Wolgemuth,
“Introduction to Thermal Sciences, second edition” Wiley, 1993
shaft shaft
Atmospheric air
Fuel
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
P1
P2
P3
P4
Gas Turbine Engine
63
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Te
mp
era
ture
T
Entropy S
P2 = P3
Patm = P1 = P4
1
2
3
4
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
Gas Turbine Engine
64
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
GT - Principle of Operation
65
Simple Cycle Gas Turbine
4
3
2
A B
Temperature °F
Entropy
1
Theoretical Cycle
Axial
Compressor
Combustion Chamber
H.P./L.P. Turbine
Fuel Gas
Air
Exhaust Gas (~950°F)
1
34
~1800 -2300°F
~650 - 950°F
Ideal Cycle Efficiency
Real Cycle
Centrifugal
Compressor
2
( )1 /
4 1 1
3 2 2
1 1id
T T P
T T P
−
− = − = −
−
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
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
66
Updated: January 4, 2019Copyright © 2017 John Jechura ([email protected])
Summary
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
SummaryWork 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
68
Updated: January 4, 2019Copyright © 2017 John Jechura ([email protected])
Supplemental Slides
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Reciprocating Compressors
70
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Propane Refrigeration Compressors
71
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Propane Compressors with Air-cooled Heat Exchangers
72
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Reciprocating Compressor at Gas Well
73
Courtesy of Nuovo Pignone Spa, Italy
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
2 stage 2,000 HP Reciprocating Compressor
74
Courtesy of Ariel Corp
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Oil-Injected Rotary Screw Compressor
75
Courtesy of Ariel Corp
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Two-stage screw compressor
76
Courtesy of MYCOM / Mayekawa Mfg
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Centrifugal Compressors – Issues Surge
• Changes in the suction or outlet pressures can cause backflow; this can become cyclic as the compressor tries to adjust. The resulting pressure oscillations are called SURGE
Stonewall
• When gas flow reaches sonic velocity flow cannot be increased.
Inlet Volume Flow
Rate
Pre
ss
ure
He
ad
Su
rge L
ine
Sto
ne
wa
ll L
ine
Centrifugal
Reciprocating
Axial
77
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Air & Hot Gas PathsGas 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
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EXHAUSTFRESH AIR
MECHANICALENERGY
COMPRESSION
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Air & Hot Gas PathsGas 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.
79
EXHAUSTFRESH AIR
MECHANICALENERGY
COMBUSTION
COMPRESSION
Updated: January 4, 2019Copyright © 2019 John Jechura ([email protected])
Air & Hot Gas PathsGas 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
80
EXHAUSTFRESH AIR
MECHANICALENERGY
EXPANSIONTURBINECOMPRESSION
COMBUSTION