compressor basics

Los Angeles, June 1, 2007

Objectives

• Review basics of centrifugal compressors• Explore key terminology• Understand parameter relations• Performance capabilities of different

compressor technologies

Basics of Compressors

• The principles of increasing pressure for pumps and compressors are the same.

• A PUMP moves an incompressible fluid - a liquid. The volume of a liquid does not change with pressure and temperature.

• A COMPRESSOR moves a compressible fluid - a gas. The volume of a gas changes with pressure, temperature, and gas composition.

Centrifugal Compressor characteristic (performance) curve

Flow (ACFM)

d Surge

Stonewall

Centrifugal compressors don’t make pressure ratio

They make

Had (ft-lbf/lbm) = z * 1545 * T1 (°R) * k * P2 (k-1)/k - 1

M.W. k-1 P1

… And they do Volume Flow, not Mass Flow

Q = w * T * z

M.W. * P

• Q = volume flow• w = mass flow• MW = mole weight• P = absolute pressure• T = absolute temperature• z = compressibility

• Process Engineers are trained to calculate mass balances and therefore work in MASS flow (lbmm/hr, kgm/hr).

• Centrifugal compressors are designed to handle a given VOLUME flow.

• Difference between– “Standard” volume flow, and– “Actual” volume flow

Standard Conditions = 0 psig, 60 °F

Normal Conditions = 0 BAR g, 0 °C

Example: 1,000 lb/min of pure Methane @ 1,000 psia & 100 °F correspond to 339.2 acfm, or 23,616 SCFM;

1,000 lb/min of pure Methane @ 500 psia & 50 °F correspond to 632.7 acfm, or 23,616 SCFM

Key Terminology

• Volume Flow• Headrise (adiabatic and polytropic)

– Specific heat ratio (k)

– Compressibility (z)• Specific Speed• Mass Flow• Power• Mach Number• Surge, Stonewall or Choke

Headrise

• Head is the energy in ft-lbff (N-m) required to compress and deliver one lbmm (kgm) from one energy level to another.Head H = ft-lbff / lbmm ( (N-m/ kgm)

Headrise

• Reversible thermodynamic paths– Isentropic (adiabatic) = no heat loss– Polytropic = heat loss– Adiabatic and polytropic virtually the same for

single stage. Much different for multistage

Headrise Calculation

• Required Headrise = z* R*T1(r m-1)/m

– z = Compressibility Factor (approx. 1.0)– R = Universal Gas Constant (1545/MW)

– T1 = Absolute Suction Temperature of Gas

– r = Pressure Ratio– m = [(k-1)/k– k = Specific Heat Ratio of Gas (Cp/Cv)

– = Polytropic Efficiency

Headrise

The amount of energy required to compress a volume to the same pressure for a gas is much higher because the gas is at a much lower density than the liquid.

Polytropic vs. Adiabatic (Isentropic) Head

Had (ft-lbf/lbm) = z * 1545 * T1 (°R) * k * P2 (k-1)/k – 1

M.W. k-1 P1

Hp (ft-lbf/lbm) = z * 1545 * T1 (°R) * n * P2 (n-1)/n – 1

M.W. n-1 P1

n = polytropic exponent

T2 = P2 (n-1)/n p = k-1 * n Had = Hp

T1 P1 k n-1 ad p

Headrise

LIQUID

HEAD = 2.311 X P(Ft.) S.G.

Water P = 100 PSIHEAD = 231 Ft.P1 - 14.7 PSIA1T1 = 100°F

HEAD(Ft.) = 1545

M. W.(T1) K

K-1 -1

K-1 K P2

Nitrogen P = 100 PSIHEAD = 86,359 Ft.P1 = 14.7 PSIA1T1 = 100°F

231 Ft.

114.7PSIA

86,359 Ft.

Compressor

114.7PSIA

Headrise from the Impeller’s Point of View

• Head = C * N2 * D2

– C = Unit conversion constant

– N = Speed

– D = Impeller Diameter

– = Impeller head coefficient (.4 to .7)

Volume Flow

• Actual Flow – volume flow rate entering the suction flange– acfm, m3/hr

• Standard Flow – volume flow rate referenced to an established set of P, T conditions– scfm, Nm3/hr, MMSCFD

Specific Speed

• Ns = N * Q1/2

– N = speed– Q = flow– H = headrise

• Specific Speed drives impeller geometry and efficiency

Impeller EfficiencyImpeller Efficiency

SPECIFIC SPEED - Ns

Full Emission Impellers

Partial Emission Impellers

• Gas Horsepower (GHP)GHP = Head * Mass Flow

33,000 * Eff

• Brake Horsepower (BHP)

BHP = GHP + FHP (seal + gearbox losses)

Mach Number

• Acoustic Velocity

a = 223 * T11 * Z11 * k MW

• Relative Mach Number

MnRelRel = Inlet Velocity a

• Machine Mach Number

MnMachineMachine = U = D * Na 229*a

• Affects curve shape and range. Practical limit = 1.3

Surge & Stonewall

Flow (ACFM)

d Surge

Stonewall

• Surge is a system phenomena that is the result of flow separation caused by low gas velocity anywhere in a compressor stage.

• Surge is an oscillation of backflow and forward flow.

• Left to continue, Surge is a bad thing!

Stonewall or Choke

• Stonewall or choke flow is the maximum flow a given stage can handle.

• This value occurs when the ratio of the relative gas velocity to the acoustic velocity of the process gas is equal to 1.0, or Mach 1.

Parameter Relations

Had (ft-lbf/lbm) = z * 1545 * T1 (°R) * k * P2 (k-1)/k - 1

M.W. k-1 P1

Q (acfm) = ώ (lb/min) * 10.729 * T1 (°R) * z

P1 (psia) * M.W.

BHP (hp) = GHP + losses = Had (ft-lbf/lbm) * ώ (lb/min) + losses

33,000 * ad

Parameter Relations

• Assuming No Hardware Changes• Assuming Hardware Can Be Altered

No Hardware Changes

VARIABLE CONSTANT CHANGED CONDITIONS

P1 ACFM, T1, H, MW P2 , T2 , w , HP

MW P1, T1, H, ACFM P2 , T2 , w , HP

T1 P1, H, MW, ACFM P2 , T2 , w , HP

ACFM P1, T1, MW H , P2 , T2 , w , HP

With Hardware Changes

VARIABLE CONSTANT CHANGED CONDITIONS

P1 P2, w, T1, MW H , T2 , ACFM , HP

MW P1, P2, T1, ACFM H , T2 , w , HP

T1 P1, P2, T1, ACFM H , T2 , w , HP

ACFM P1, P2, MW, T1,

w , HP

Affinity LawsQ2 = N2 H2 = N2

Q1 N1 H1 N1

GHP2 = N2 3

GHP1 N1

GHP (hp) = ώ (lb/min) * Had (ft-lbf/lbm) = ώ (lb/min) * Hp (ft-lbf/lbm)

33,000 * ad 33,000 * p

New Inquiry: Required info: Q & H

– Flow (Q)– Suction pressure (P1)– Suction temperature (T1)– Compressibility (z)– Specific heat ratio (k)– Mole weight (MW) or Gas Analysis– Discharge pressure (P2)

Compressor Technologies

• Positive Displacement– Reciprocating (piston & diaphragm)– Screw (oil flooded and dry)– Rotary (liquid ring, sliding vane, lobe)

• Dynamic (Turbo)– Centrifugal– Regenerative– Axial

Compressor Types

Multi-st.Axial

Multi-st.Recip.

102 103 105 106

Multi-stagecentrif

Integrally GearedCentrifugal

Sundyne Sundyne

Rotary

Single StageRecip.

VOLUME FLOW

Positive Displacement vs Dynamic

Positive Displacement Dynamic

Volume Volume

Positive Displacement Compressors

• Types Include– Reciprocating– Screws– Sliding Vane & Liquid Ring– Rotary Lobe

Reciprocating Compressor

• Similar to an automobile engine

• Compresses a given volume of gas through

the use a reciprocating piston

• Positive displacement compressors increase

the pressure of a gas by operating on a fixed

volume in a confined space.

Horizontal, Balance Opposed, Double Acting, Reciprocating Compressor

PISTON & RIDER RINGS

VALVE DESIGNMOTOR-OVER MOUNTING

SEGMENTED TEFLON PACKING

EXTERNAL VALVES

VARIABLE CLEARANCE HEADS

FORCE FEED CYLINDER LUBRICATION

CRANKCASE LUBRICATION

CROSSHEAD

CYLINDERS

Diaphragm Compressor

Reciprocating Performance

• Piston Type: flow to 3,000 cfm, pressure to 30,000 psi, compression ratio to 20:1 (3:1) per stage , power to 15,000 HP, efficiency +/- 90%– special designs to nearly 20,000 cfm at

low suction pressures• Diaphragm Type: flow to 100+ cfm, pressure

to 30,000 psi, compression ratio 20:1 per stage, power to 150 HP

Screw Compressor Hierarchy

SCREW COMPRESSORS

Oil Free Oil Flooded

Dry LiquidInjected

Medium Pressure Air & Gas

High Pressure Gas & Refining

Screw CompressorTop View

Screw CompressorSide View

Rotary Liquid Ring

Rotary Sliding Vane

Rotary Lobe (Roots) Blower

Four distinct “pockets” of gas are moved from the suction todischarge in each revolution of the driving shaft.

Operating Principle

Rotary Compressor Performance• Screw

– Pressure to 350 psid, 4:1 compression ratio dry, 15:1 compression ratio flooded, flow to 10,000 cfm, max efficiency 75%

• Liquid Ring

– Pressure to 175 psig (29” Hg Vacuum), 5:1 compression ratio, flow to 17,000 cfm, max efficiency 50%

• Sliding Vane

– Pressure to 50/100 psid, 4:1 compression ratio, flow to 6,000 cfm, max efficiency 70%

• Lobe (Roots Type)

– Pressures to 20 psid, 2+:1 compression ratio, flow to 25,000 cfm, max efficiency 70%

Centrifugal Compressors

• Dynamic Machines

• Impeller uses centrifugal force to add

velocity to gas

• Diffuser reduces the velocity changing the

energy from velocity to pressure

Single Stage, Overhung, Centrifugal

Multistage Centrifugal Compressor

Regenerative Compressor

Axial Compressor

Competitors

• Centrifugal– AC, Atlas Copco, Cooper, Demag, Dresser-Rand

Man Turbo (Sulzer + Borsig), York

• Reciprocating– Ariel, Dresser, GE, Neuman & Esser, Sulzer

Burckhardt

• Diaphragm / Regenerative– Burton-Corblin (Periflow)

• Screw– Mycom, Howden, Kobelco, Roots

Centrifugal Compressors

• Dynamic Machines

• Impeller uses centrifugal force to add

velocity to gas

• Diffuser reduces the velocity changing the

energy from velocity to pressure

Sundyne MultistageMulti-pinion

IG Compressor Staging Arrangements

Integrally Geared TechnologyBenefits and Features

• Almost 40 years experience• 1600+ process gas installations• Compact designs-reduced space• Lower installation costs• Fewer rotating components• API 617 specification• Participation on API sub-committee• Proven and accepted equipment• Optimized specific speed-higher efficiencies• Proven centrifugal reliability

Applicable API Standards

• 613 Special Purpose Gear Units• 614 Lubrication, Shaft Sealing ...• 617 Centrifugal Compressors• 618 Reciprocating Compressors• 619 Rotary Type PD Compressors• 670 Vibration, Axial Position and ...• 672 Packaged Integrally Geared,

Centrifugal Plant & Instrument Air

compressor basics

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