A Comprehensive Guide to Compressors and Gas Compression

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Compressors & Gas Compression

• Categories and Types

• Compression Process

• Compressor Characteristics

• Key Design Parameters

• Calculation Methods

• Specification Data Sheet

• Selection Guidelines

• Control Systems

• Typical operating Problems

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Compressors & Gas Compression

• Positive Displacement

• Reciprocating (Piston, Diaphragm)

• Rotary Type (Screw, Lobe, Slidiong Vane

• Dynamic

• Centrifugal (Radial and Axial)

• Blowers

Categories and Types

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Categories and Types

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Centrifugal Compressor

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Axial Compressor

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Ranges of Application

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Compression Process

• Gas compression is a thermodynamic process where change takes place in the physical state of the gas

• Compression adds energy to the gas resulting in pressure- volume changes defined by ideal gas laws

• Compression take place under conditions defined:

• Adiabatic: no heat added or removed from systems

• Isothermal: constant temperature in system

• Polytropic: heat added or removed from system

• Compression of ‘real’ gases in ‘actual’ compressors deviate from conformance with ‘ideality’, usually significantly,

affecting compressor design.

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Compressors & Gas Compression

Compressor Characteristics

• Capacity/Head

• Performance

• Terminology

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Reciprocating Compressor

• Performance Diagram

• Terminology

• Piston Displacement

• Clearance Volume

• Volumetric Efficiency

• Pressure Ratio

• Rod Loading

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Compressors & Gas Compression

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• Performance Curves

• Terminology

• Operating Point

• Surge Point

• Stonewall

• Stability

• Turndown

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Centrifugal Compressor Performance

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Centrifugal Compressor Key Design Parameters

• Capacity

• Gas Properties

• Pressure Head

• Power

• Efficiency

• Multi-Stages

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• Flow Rates

• Normal

• Maximum

• Minimum

• Design Capacity

Capacity

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• Composition

• Contaminants

• Molecular Weight – MW

• Specific Heat Ratio – Cp/Cv

• Compressibility

Gas Properties

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10º C38º C66º

C93º C121º C

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Centrifugal Compressor

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100ºF = 560ºR: 560/549 = 1.02

100ºF = 311K, 549ºR = 305K: 311/305 = 1.02

PV = ZmRT/MW

P=100psia = 6.89 bar a T=100ºF = 37.8ºC = 310.9K

ρ = m/V = P(MW)/(ZRT)

= 6.89E5x34.27/(0.946x8314x310.9)

= 9.7kg/m³

= 0.61lb/ft³

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0.973

0.077 1.02

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0.8 8

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Centrifugal Compressor Key Design Parameters

• Available vs. Required Head

• Available Head is Compressor Related

• H(Available) = CV²/g

• C = Pressure Coefficient (≈0.55)

• Required head is System-Related

Head

H(Required

)

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For centrifugal compressors the following method is normally used:

• First, the required head is calculated.

Either the polytropic or adiabatic efficiency is used with the companion head.

Horsepower Calculation

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Where:

Z = Average compressibility factor: using 1 will yield conservative results

R = 1544/(mol weight)

T1 = Suction Temperature, ºR

P1, P2 = Suction, discharge pressures, psia

K = Adiabatic exponent, (N-1)/N = (K-1)/(KE_p) E_p = Polytropic Efficiency

E_A = Adiabatic Efficiency

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Compressors & Gas Compression

The polytropic and adiabatic efficiencies are related as follows:

From Polytropic Head:

HP = WH_poly/(E_p 33000)

From Adiabatic Head:

HP = WH_AD/(E_A 33000)

Where:

HP = Gas Horse Power BHP = Brake Horsepower W = Flow, Lb/min

BHP = HP/E_m

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Efficienc y

• Hydraulic Efficiency

• Adiabatic

• Polytropic

• Volumetric Efficiency

• Reciprocating

• Mechanical Efficiency

• Drivers

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Approximate polytropic efficiencies for centrifugal and axial compressors

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Temperature Rise

Temperature ratio across a compression stage is:

T₂/T₁ = (P₂/P₁)^(K-1)/K Adiabatic T₂/T₁ = (P₂/P₁)^(N-1)/N Polytropic Where:

K = Adiabatic exponent, C_p/C_v

N= Polytropic exponent, (N-1)/N = (K-1)/KE_p P₁, P₂ = Suction, discharge pressures, psia T₁, T₂ = Suction, discharge temperatures, ºR E_p = Polytropic efficiency, fraction

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Compressors & Gas Compression

Temperature Rise

The usual centrifugal compressor is uncooled internally and follows a polytropic path.

Temperature must often be limited to:

• Protect against polymerization as in olefin or butadiene plants

• At T > 230-260ºC, the approximate mechanical limit, problems of sealing and casing growth start to occur.

High temperature requires a special and more costly machine.

Most multistage applications are designed to stay below 250- 300ºC

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Compressors & Gas Compression

Temperature Rise

Intercooling can be used to hold desired temperatures for high overall compression ratio applications.

This can be done between stages in a single compressor frame or between series frames.

Sometimes economics rather than a temperature limit dictate intercooling.

Sometimes for high compression ratios, the job cannot be done in one frame. Usually a frame will not contain more than 8 stages (wheels). For many applications the compression ratio across a frame is about 2.5 – 4.0

The maximum head that one stage can handle depends on gas properties and inlet temperature. Usually this is about 2000 to 3400m for a single stage.

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Compressors & Gas Compression

Surge Controls

A centrifugal compressor surges at certain conditions of low flow.

Surge control help the machine to avoid surge by increasing flow.

• For an air compressor, a simple spill to atmosphere is sufficient.

• For a hydrocarbon compressor, recirculation from discharge to suction is used.

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Compressors & Gas Compression

Surge Controls

There are many types of surge controls.

Avoid the low-budget systems with a narrow effective range, especially for large compressors.

Good systems include the flow/ΔP type.

The correct flow to use is the compressor suction. However, a flow element in the suction can rob excessive horsepower.

Therefore, sometimes the discharge flow is measured and the suction flow calculated within the controller by using pressure measurements. The compressor intake nozzle is also sometimes calibrated and used as a flow element.

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Compressors & Gas Compression

Compressor Calculation Method

• Define gas properties: MW, Cp/Cv, Z

¹

• Define inlet conditions: Temp & Press.

• Calculate gas flow rate: Normal and Design

¹

• Establish total discharge pressure.

• Calculate compression ratio and number of stages

• Define selection & polytropic efficiency

1. At inlet conditions

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Compressor Calculation Method

^cont’d

• Calculate heat capacity factor ‘M’

• Calculate ‘required’ polytropic head

• Calculate hydraulic gas horsepower

• Calculate discharge temperature

• Calculate total brake horsepower

• Estimate inter-stage cooling

requirement

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Compressors & Gas Compression

Compressor Calculation Example 1:

Calculate compressor required to handle a process gas at the following operating conditions: Inlet press and temp at 20 psia and 40ºF. Discharge pressure of 100 psia. Gas rate 2378 lb.mol/hr of the following composition and calculated properties:

Mol% Mol/h Mol.

Wt

∑Mt C_p ∑Cp T_{c (R} ∑Tc P_c ∑Pc Ethane 2 48 30.1 0.60 11.96 0.24 550 11 708 14 Propane 95 2259 44.1 41.9 16.55 15.70 666 633 617 587 Butane 3 71 58.1 1.74 22.50 0.68 766 23 551 17

Total 100 2378 44.24 16.62 667 618

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Compressor Calculation Example 1:

^cont’d

• Inlet flow:

Weight flow = 2378 x 44.24/60 = 1753 lb/min P_r = 20/618 = 0.0324, T_r = (40+460)/667 = 0.75

Compressibility factor Z = 0.97 (from generalized Z chart) Density (ρ)= (MW x P₁)/(10.73 x T₁ x Z)

= (44.24 x 20)/(10.73 x (40 + 460) x 0.97) = 0.17 lb/ft³

Inlet volume = 1753/0.17 = 10 310 ft³/min

Calculation:

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^cont’d

• Heat Capacity Factor

k = Cp/Cv = Cp/(Cp – 1.99) = 16.62/(16.62 – 1.99) = 1.137 M = (k-1)/(kη_p)

Assume η_p = 77%:

M = (1.137 – 1)/(1.137 x 0.77) = 0.156

Calculation:

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^cont’d

• Polytropic Head, H_p

Calculation:

= 0.97 x (1545/44.24) x (40 + 460)/0.156 x [(100/20)^0.156 -1]

= 30 988 ft

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^cont’d

• Discharge Temperature, T2 T2 = T1(P2/P1)M

= 500(5)0.156

= 643ºR

= 183ºF

• Gas Horsepower (GHP) & Brake Horespower (BHP) GHP = W . H_poly/(33000η_p)

= 1753 x 30988/(33000 x 0.77)

= 2140

BHP = 2140/0.98 = 2180 (Assume Mechanical Eff. = 98%)

Calculation:

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Example

Calculate the Brake Horsepower for the following Compressor:

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Example

Calculate the Brake Horsepower for the following Compressor:

Calculate Gas Mixture Properties

Composition: H2 = 65.6/(65.6+21.4) = 75.4 vol%

N2 = 100 – 75.4 = 24.6 vol%

Composition Mole% Mole Wt ∑MW mass% Cp ∑CP Hydrogen 75.4 2 1.51 18.0 14.3 2.57

Nitrogen 24.6 28 6.89 82 1.04 0.85

Total Gas Mix 100.0 8.40 11.04 3.42

Use Z = 1 for conservative

results

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Example

Calculate the Brake Horsepower for Compressor: Cont’d

Let’s look at the first stage:

First calculate Polytropic Head:

T₂/T₁= (P₂/P₁)^(N-1)/N

ln(T₂/T₁) = (N-1)/N ln(P₂/P₁) (N-1)/N = ln(T₂/T₁)/ln(P₂/P₁)

= ln(372/295)/ln(4400/2518)

= 0.416

H_poly = 1 x (8.314/8.4) x 295 x ((4400/2518)^0.416 -1) 0.416

= 183.4 kJ/kg

T₁ = 22ºC = 295K T₂ = 99ºC = 372K

P₁ = 2418 kPag = 2518 kPa a P₂ = 4300 kPag = 4400 kPa a

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Example

First stage:

(N-1)/N = (K-1)/(Kη_p)

→ η_p = (1.4 -1)/(1.4 x 0.416)

= 0.69

W = (107 000/22.414) x 8.4 = 40100kg/h = 11.14 kg/s Cp/Cv = Cp/(Cp-R/MW) = K

= 3.42/(3.42-8.314/8.4)

= 1.4

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Example

First stage:

Gas Horsepower = W . H_poly/η_p

= (11.14 x 183.4)/0.69

= 2960 kJ/s

= 3.0 MW

Similar for stage 2, 3 and Recycle:

GHP(stage 2) = 2.9MW GHP(stage 3) = 3.3 MW

GHP(recycle stage) = 1.0 MW

Total GHP = 3.0 + 2.9 + 3.3 + 1.0 = 10.2 MW

A good assumption for Mechanical Efficiency = 95%

BHP = 10.2/0.95 = 10.6 MW

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Compressors & Gas Compression

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