Compression
Chapter 9
Based on presentation by Prof. Art Kidnay
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Topics
• Introduction
• Fundamentals
 Thermodynamics of compression
 Multistaging
 Compressor Efficiencies
• Drivers
• Compressor Types
 Positive displacement compressors
 Dynamic Compressors
• Capacity and Power Calculations
 Capacity
 Power Requirements
• Comparison of Reciprocating and Centrifugal Compressors
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
2
Plant Block Schematic
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
3
Topics
• Introduction
• Fundamentals
 Thermodynamics of compression
 Multistaging
 Compressor Efficiencies
• Drivers
• Compressor Types
 Positive displacement compressors
 Dynamic Compressors
• Capacity and Power Calculations
 Capacity
 Power Requirements
• Comparison of Reciprocating and Centrifugal Compressors
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
4
Review of Thermodynamic Principals
• 1st Law of Thermodynamics – Energy is conserved
 (Change in system’s energy) = (Rate of heat added) – (Rate of work performed)
 System’s 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
o
Define Enthalpy, H = U +PV, to describe systems at constant pressure
• 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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
5
Common Paths for Heat and Work
• Isothermal – constant temperature, ΔT = 0
• Isobaric – constant pressure, ΔP = 0
• Isochoric – constant volume, ΔV = 0
• Isenthalpic – no heat or heat, constant enthalpy, ΔH = 0
• Adiabatic – no heat transferred, Q = 0
• Isentropic (ideal reversible) – no increase in entropy, ΔS = 0
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
6
1st Law for steady state flow
• Equation 1.19a (ΔH  ΔU for flowing systems)
u 2 g
H 
 z  Q  W
2gc gc
• For adiabatic, steady-state, ideal (reversible) flow (using WS as positive value)
u 2 g
Ws  H 
 z
2gc gc
P2
u 2 g
  VdP 
 z
2gc gc
P1
P2
Ws   VdP
P1
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
7
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 EOS is the ideal gas law
P2
Ws   VdP  H
P1
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
8
PH Diagrams
Ref: GPSA Data Book, 13th ed.
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
9
TS Diagram
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
10
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
11
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
12
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
13
Thermodynamics of Compression
• Assume ideal gas: PV = RT
• Choices of path for calculating work:
 Isothermal (ΔT = 0)
P2
P 
dP
 RT ln  2 
P
 P1 
P1
P2
Ws   VdP  RT 
P1
• Minimum work required but unrealistic
 Isentropic (ΔS = 0)
• Maximum ideal work but more realistic
 Polytropic - empirical
• Considers some heat loss and gas nonideality
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Thermodynamics of Compression
Work equations
• Isentropic (PVg = constant) where g = CP/CV
 g1  / g


RT1 g  P2 
 
Ws 
 1
M g  1  P1 


• Polytropic (PVk= constant) where k is empirical constant > g
 k1  / k




RT1 k
P2
 
Ws 
 1
M k  1  P1 


John Jechura – jjechura@mines.edu
Updated: February 9, 2016
15
Thermodynamics of Compression
• Calculation of g for gas mixture
xC

g
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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
16
Example compression calculation
Example
• Have sales gas (assume pure methane)
• Initial conditions: 40oF & 100 psig
• Desired outlet pressure: 400 psig
Compute work of compression on mass basis
• Using PH diagram
• Assuming ideal gas and adiabatic compression
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
17
Example Calculation – PH Diagram
H2 = 462 Btu/lb
H1 = 370 Btu/lb
WS = (H2 – H1) = 92 Btu/lb
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
18
Example Calculation – Ideal Gas Compression
• For methane:
 g = 1.3
 M = 16
 T1 = 40°F= 499.67oR
 P1 = 100 psig = 114.7 psia
 P2 = 1000 psig = 1014.7 psia
 R = 1.986 Btu/lb.mol oR
 g1 / g
 1.3 1.986  499.67   414.7 1.31/1.3 
gRT1  P2 
 
Ws 
 1 
 1   93 Btu/lb


M  g  1   P1 
16
1.3

1
114.7
 
 




John Jechura – jjechura@mines.edu
Updated: February 9, 2016
19
Discharge temperature
• For ideal gas compression
P 
T2  T1  2 
 P1 
 g1  / g
• For the example problem:
 414.7 
T2   499.67  

 114.7 
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
1.31  /1.3
 672.19°R  212.5°F
20
Thermodynamics of Compression
• If customer wants 1000 psig…
 Then pressure ratio of (1015/115) = 8.8
 Discharge temperature for this ratio is ~360oF
• For reciprocating compressors the GPSA Engineering Data Book recommends
 Maximum discharge temperature of 250 to 275oF for high pressure systems AND
 Pressure ratios of 3:1 to 5:1
• To obtain overall high pressure ratio must use multistage compression with
interstage cooling
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
21
Multistaging
To minimize work need good interstage cooling and equal pressure ratios in stages.
• The number of stages is calculated using
1/ m
P 
RP   2 
 T1 
ln  P2 / P1 
 m
ln R P 
• 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 @ 100oF)
 1014.7 
ln 

114.7  ln  8.8 

m

 1.98
ln  3 
ln  3 
 P 1/ m 
T2  T1  2  
 P1  
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
 g1  / g
 1014.7 1/2 
  499.67  
 
114.7

 

1.31  /1.3
 642.59°R  182.9°F
22
Topics
• Introduction
• Fundamentals
 Thermodynamics of compression
 Multistaging
 Compressor Efficiencies
• Drivers
• Compressor Types
 Positive displacement compressors
 Dynamic Compressors
• Capacity and Power Calculations
 Capacity
 Power Requirements
• Comparison of Reciprocating and Centrifugal Compressors
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
23
Compressor Efficiencies
• Number of definitions for compressor efficiencies
• Two major definitions
 Adiabatic efficiency (also known as the isentropic efficiency):
IS 
 H S0
 H actual

WS0
Wactual
 Polytropic efficiency:
P 
 g  1 / g
 k  1 / k
• Polytropic efficiency not necessarily 100% at reversible conditions
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
24
Why use polytropic efficiency?
• More closely follows the actual pressure-temperature path during the
compression
• In general P > IS
• Can use this to estimate the actual discharge temperature
 P2 
T2  T1  
 P1 
 1  g1 
 

 P  g 
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
25
Compressor efficiency example
• Continuation of example given in the beginning of this section…
• Actual work required is:
Wactual 
WS 0
IS
• Assume IS = 80% then:
Wactual 
93 Btu/lb
 116 Btu/lb
0.80
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
26
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
T2,S 0
P 
 T1  2 
 P1 
 g1  / g
 P2 
P 
T2,act  T1  T1  1 
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
 P  g1/ g 
 T2,S 0  T1  T1  2 
 1
P
 1 

 g1  / g
1
IS
 T2,act
  P  g1/ g

2
  
1
P


 T1 1   1 

IS




27
Topics
• Introduction
• Fundamentals
 Thermodynamics of compression
 Multistaging
 Compressor Efficiencies
• Drivers
• Compressor Types
 Positive displacement compressors
 Dynamic Compressors
• Capacity and Power Calculations
 Capacity
 Power Requirements
• Comparison of Reciprocating and Centrifugal Compressors
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
28
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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
29
Topics
• Introduction
• Fundamentals
 Thermodynamics of compression
 Multistaging
 Compressor Efficiencies
• Drivers
• Compressor Types
 Positive displacement compressors
 Dynamic Compressors
• Capacity and Power Calculations
 Capacity
 Power Requirements
• Comparison of Reciprocating and Centrifugal Compressors
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
30
Compressor Types
Two basic types:
• Positive displacement – compress by changing volume
 Reciprocating, Rotary screw, Diaphragm, Rotary vane
• Dynamic – compress by converting kinetic energy into pressure
 Centrifugal, Axial
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
31
Compressor Pressure and Volume Ranges
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Reciprocating Compressors
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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Reciprocating Compressors - Principle of Operation
Double Acting – Crosshead
Single Acting - Trunk Piston
 Typical applications:
 Typical Applications:
 All process services, with any gas and up
to the highest pressures and power
 Small size standard compressors for air
and non dangerous gases
Courtesy of Nuovo Pignone Spa, Italy
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
35
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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
36
Reciprocating Compressors
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Propane Refrigeration Compressors
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Propane Compressors with Air-cooled Heat Exchangers
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Reciprocating Compressor at Gas Well
Courtesy of Nuovo Pignone Spa, Italy
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
2 stage 2,000 HP Reciprocating Compressor
Courtesy of Ariel Corp
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
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)
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Rod Packing
Oil Wiper
Packing Piston Rod
Piston
Cast
Cylinder
Cylinder Valve
42
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.
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Rotary Screw Compressors
Oil-free
Oil-injected
• First used in steel mills because handles
• Higher throughput and discharge pressures
“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%
• Has two exit ports
 Axial, like oil-free
 Radial, which permits 70 to 90% turndown
without significant efficiency decrease
• Pressure ratios to 23:1
• Small footprint (~ ¼ of recip)
• Tight tolerances can limit quick restarts
• Relatively quiet and vibration-free
• Requires oil system which filters and cools
• Relatively low efficiency
 70 – 85% adiabatic efficiencies
• Relatively low throughput and discharge
pressure
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
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
Rotary Screw Compressor
Courtesy of Ariel Corp
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Oil-Injected Rotary Screw Compressor
Courtesy of Ariel Corp
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Two-stage screw compressor
Courtesy of MYCOM / Mayekawa Mfg
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Dynamic Compressors
• Two types
 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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
48
Centrifugal compressors
• Single stage (diffuser)
• Multi-stage
Bett,K.E., et al Thermodynamics for
Chemical Engineers Page 226
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
49
Two-stage compressor
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Centrifugal Compressor
Courtesy of Nuovo Pignone Spa, Italy
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
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.
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
52
Axial
Centrifugal
Reciprocating
Inlet Volume Flow
Rate
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
Stonewall Line
Surg
e Lin
e
Pressure Head
Surge
Gas Turbine – Centrifugal Compressor
Axial
Compressor
Centrifugal
Compressor
Exhaust
Gas
Low Pressure
Gas
Air
Fuel Gas
Combustion
Turbine
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
High Pressure
Gas
Industrial Gas Turbines
Ref: GPSA Data Book, 13th ed.
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
59
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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
60
Gas Turbine
Courtesy of Nuovo Pignone Spa, Italy
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
64
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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
65
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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
66
GT - Principle of Operation
Theoretical Cycle
Fuel Gas
Combustion
Chamber
~650 - 950°F
3
2
1
Air
~1800 2300°F
Axial
Compressor
Exhaust
Gas (~950°F)
Temperature
°F
3
Real Cycle
4
4
2
Centrifugal
Compressor
H.P./L.P. Turbine
Simple Cycle Gas Turbine
1
A
B
Entropy
Ideal Cycle Efficiency
id= 1- (T4-T1)/(T3-T2)
= 1 – (P1/P2)(g-1)/ g
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
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
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
68
Topics
• Introduction
• Fundamentals
 Thermodynamics of compression
 Multistaging
 Compressor Efficiencies
• Drivers
• Compressor Types
 Positive displacement compressors
 Dynamic Compressors
• Capacity and Power Calculations
 Capacity
 Power Requirements
• Comparison of Reciprocating and Centrifugal Compressors
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
69
Centrifugal Compressors vs. Reciprocating Compressors
Centrifugal
Reciprocating
Constant head, variable volume
Constant volume, variable pressure
Ideal for variable flow
Ideal for constant flow
- MW affects capacity
+ MW makes no difference
++Availability > 99%
- Availability 90 to 95%
+ Smaller footprint
- Larger footprint
- ηIS = 70 – 75%
+ ηIS = 75 – 92%
CO & NOx emissions low
Catalytic converters needed
- Surge control required
++ No surge problems
++Lower CAPEX and maint.
++Fast startup & shutdown
(maint cost ~1/4 of recip)
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
70
Topics
• Introduction
• Fundamentals
 Thermodynamics of compression
 Multistaging
 Compressor Efficiencies
• Drivers
• Compressor Types
 Positive displacement compressors
 Dynamic Compressors
• Capacity and Power Calculations
 Capacity
 Power Requirements
• Comparison of Reciprocating and Centrifugal Compressors
John Jechura – jjechura@mines.edu
Updated: February 9, 2016
71