Thermodynamics II Compressors

THERMODYNAMICS- II

University of Engineering and Technology

Lahore, Pakistan

by

Dr. Muhammad Mahmood Aslam Bhutta

Department of Mechanical Engineering

PakistanLahore

DR. Muhammad Mahmood Aslam Bhutta Lahore ,Pakistan

Thermodynamics- II

(ME-242)

Dr. Muhammad Mahmood Aslam Bhutta


Introduction

• Air compressors are more than 2000 years old; the first was the hand blow.


Introduction

• Today, reciprocating compressors, pumps, and engines are the most prevalent form of machinery in the world.


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Types of Compressors


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• Used for air and refrigerant compression (air-conditioning, cooling Systems)

• Cylinder volume reduces while pressure increases, with pulsating output

• Many configurations available ( inline, V ) Air /Liquid Cool

• Single acting when using one side of the piston, and double acting when using both sides

Reciprocating Compressor



Screw compressor

• Rotors instead of pistons: continuous discharge

• Benefits: low cost, compact, low weight, easy to maintain (less number of parts)

• Drawbacks: Small life, sensitive to foreign particles, costly maintenance,

• Sizes between 30 – 200 hp

• Types

• Lobe compressor

• Screw compressor

• Rotary vane / Slide vane

Rotary Compressor



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• Rotating impeller transfers energy to move the air

• Continuous duty

Centrifugal Compressor


• Designed oil free

• High volume applications > 12,000 cfm



It is a modern industrial machine that compresses gas with the reciprocating (back and forth) motion of a piston within a cylinder. It is often called a “recip”.”


Introduction

• Recips are also called Positive Displacement Compressors because the piston decreases the volume of the gas to directly increase its pressure.

• On the other hand, centrifugal compressors impart velocity to the gas that is converted to pressure.


Compressor CycleProcesses involved:

– Compression• To raise the pressure of gas to delivery pressure.

– Discharge• To supply the high pressure gas.

– Expansion• The gases left in the clearance volume expands to

atmospheric pressure.

– Intake• To take the fresh gas for increasing pressure


Compressor Cycle/P-V Diagram

P 2

P 1

S T R O K E

1IN L E T P R E S S U R E

R E C E IV E R P R E S S U R E

D IS C H A R G E

IN L E T

TDC BDC

Atmospheric pressure

VC VS

Delivery pressure


Compression Process

P 2

P 1

S T R O K E



D IS C H A R G E

IN L E T

2

TDC BDC

V1


Discharge ProcessP 2

P 1

S T R O K E



D IS C H A R G E

IN L E T

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Expansion Process

P 2

P 1

S T R O K E



D IS C H A R G E

IN L E T

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4


Intake Process

P 2

P 1

S T R O K E



D IS C H A R G E

IN L E T

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4


ReciprocatingCompressors


Use of RecipsRecips are used in:

– Onshore/offshore gas extraction from the gas fields

– Gas transmission through pipelines– Chemical processing

• Drying, mixing, Crude material refining

– Mechanical Processing• Tyres, Cutting, Surface preparation, cleaning, Air

pressure tools/ Impacters, Brake systems, air pressure bearing, air motors

– Power generation• To run turbines/engines


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• Efficiency at full, partial and no load

• Noise level

• Size

• Oil carry-over

• Vibration

• Maintenance

• Capacity

• Pressure

• Duty cycle

Comparison of Compressors


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Intake air filters( wet ,dry) cyclone paper filter,Intake Air Filters that prevent dust from entering a compressor. Dust

causes sticking valves, scratched cylinders, excessive wear etc

Inter-stage coolersInter-stage Coolers that reduce the temperature of the air before it

enters the next stage to reduce the work of compression and increase efficiency. They are normally water-cooled in large compressors.

After coolersAfter-Coolers with the objective to remove the moisture in the air by

reducing the temperature in a water-cooled heat exchanger

Main Components in Compressed Air Systems


• Moisture drain traps

Moisture drain traps that are used for removal of moisture in the compressed air.

• Receivers

provided as storage and smoothening pulsating air output - reducing pressure variations from the compressor


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• Significant influence on energy use

Energy Efficiency Opportunities

1. Location

2. Elevation• Higher altitude = lower volumetric

efficiency


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3. Air Intake

• Keep intake air free from contaminants, dust or moist

• Keep intake air temperature low

Every 4 oC rise in inlet air temperature = 1%

higher energy consumption



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4. Pressure Drops in Air Filter

• Install filter in cold location or draw air from cold location

• Keep pressure drop across intake air filter to a minimum



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5. Use Inter and After Coolers

• Inlet air temperature rises at each stage of multi-stage machine

• Inter coolers: heat exchangers that remove heat between stages

• After coolers: reduce air temperature after final stage

• Use water at lower temperature: reduce power



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• Higher pressure

• More power required for compressors

• Operating above recommended pressures(rated operating pressure)

• Waste of energy

• Excessive wear


6. Pressure Settings


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Reducing delivery pressure

less energy and reduced leakage rate



Segregating high/low pressure requirements

Pressure reducing valves no longer needed


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d. Design for minimum pressure drop in the distribution line

• Pressure drop: reduction in air pressure from the compressor discharge to the point of use

• Pressure drop < 10%

• Pressure drops caused by

• corrosion

• inadequate sized piping, couplings hoses

• choked line filter elements




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d. Design for minimum pressure drop in the distribution line



Typical pressure drop in compressed air line for different pipe size


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© UNEP 2006

7. Minimizing LeakageEnergy Efficiency Opportunities

• Consequences• Energy waste: 20 – 30% of output• Drop in system pressure• Shorter equipment life

• Common leakage areas• Couplings, hoses, tubes, fittings• Pressure regulators• Open condensate traps, shut-off valves• Pipe joints, disconnects, thread sealants


• Use ultrasonic acoustic detector

• Tighten joints and connections

• Replace faulty equipment

8. Condensate Removal

• Condensate formed as after-cooler reduces discharge air temperature

• Install condensate separator trap to remove condensate

7. Minimizing Leakage


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© UNEP 2006

9. Controlled usage

Do not use for low-pressure applications: Combustion air, agitation pneumatic conveying, air circulation, Use blowers instead.

10. Compressor controls• Automatically turns off compressor

when not needed


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11. Maintenance Practices

• Lubrication: Check regularly

• Air filters: Replace regularly (before compression)

• Condensate traps: Ensure drainage

• Air dryers: Inspect and replace filters

(After compression)12. Control the irresponsible use of compressed air (unintended)

• Drying of clothes or hair

• Body cleaning/playing

(After compression)


Advantages of Reciprocating Compressors Over Centrifugal Compressors

• Lower capital investment• Higher pressures (up to 60,000 psi) • Variable loading without surge • Ease of installation• Design flexibility


Without Effective Monitoring

• Increased wear makes part life shorter than planned maintenance intervals– severe damage results

• Decreased wear makes part life longer than planned maintenance intervals


Basic Design

• A reciprocating compressor consists of the following major components:

• frame, • crankshaft, • crosshead, • distance piece,• pressure packing, • cylinder, • piston, • valves.• Of course, it also requires a source of power or a prime

mover.


Basic Design

• Horizontal balance-opposed compressor:


Major Components

• Frame• Crankshaft• Crosshead• Distance piece• Cylinder• Piston/piston rod• Valves


Frame

• Includes all static parts:– supports– cylinders– distance pieces– crosshead guides– crankcase– bolting– foundation


Frame

• Horizontal balance-opposed: cylinders are separated by 180º about the crankshaft

• Heavy and rigid• Designed to minimize vibration


Crankshaft

• Powered by a prime mover:– motor,– turbine, or– internal combustion engine


Crankshaft

• A flywheel mounts to the crankshaft to attenuate torsional forces.

• Not generally the source of recip malfunction.


Crosshead

• Sliding element with only one degree of freedom

• Converts rotational motion into translational motion.

• Together with the crankshaft and connecting rod it forms a classic slider-crank mechanism.


Pressure Packing

• Seal around the piston rod • Keeps pressure in the cylinder • Plastic or bronze• Higher pressure requires more rings of

pressure packing


Cylinder

• The housing in which the piston reciprocates to compress gas

• Valves at each end where gas enters and escapes

• Protected by wear bands (usually made of Teflon®) on the piston


Piston

• Reciprocates within the cylinder to compress gas.

• Uses compression rings to seal the gap between its circumference and the cylinder wall.

• Is protected by rider rings (or wear rings) which provide a riding surface within the cylinder.


Piston

• Can be single-acting or double-acting (performs work in only one or in both directions of travel).

• Double-acting pistons are the most common.


Piston Rod

• Connects the piston to the crosshead• Sealed within the crank end head by

pressure packing


Piston/Piston Rod


Valves

• Allow relatively unrestricted flow in one direction while blocking flow in the other direction

• Usually are automatic (open and close by differential gas pressure and springs)

• At least one intake and one discharge valve is needed in each cylinder.


Configurations

• Single-acting or double-acting– difference is whether piston compresses gas in

one direction or in both

• Lubricated or non-lubricated– non-lubricated if gas is sensitive to oil

contamination


Configurations (cont.)

• Single or multiple stage– gas is compressed in stages in multiple stage– to increase efficiency, the gas is cooled between

stages– intermediate cooling is called intercooling

• Single or multiple gas streams


Configurations (cont.)

• Two to ten cylinders per machine– even numbers because they are horizontal

balance-opposed to cancel vibration forces


Theoretical Compression Cycles

• Adiabatic Compression– All heat generated is retained in the gas (no

heat transfer).

• Isothermal Compression– Temperature of the gas is kept constant during

compression with heat removal devices.

• Actual Cycle is between the two.


Intercooling

• Intercooling in multistage compressors maximizes efficiency.– 1st stage is close to adiabatic

• Intercoolers between the stages bring the process closer to isothermal.– Isothermal compression requires less power.


Losses Affecting Efficiency

• Spring force in valves• “Sticktion” (valve friction)• Valve plate inertia• Pressure drop through valve• Leaking past rings• Packing leakage


Piston Rings

• Piston rings can wear out rapidly and fail if rider bands are not maintained.

• Worn piston rings, also diminish efficiency and can cause the machine not to function correctly.

Rider Rings

Rider rings/bands are provided to prevent contact of the compressor piston with the cylinder wall. Such contact could cause problems such as scuffing or more serious and costly problems such as piston and/or liner replacement. Rider rings/bands are normally made of a low friction material such as a Telfon(tm) blend but they can also be supplied in bronze and/or PEEK(tm) for harsh applications

with high temperatures.


Piston Rings

• Broken rings can be identified by looking at the vibration waveform (ac component) from the proximity probes.

• Normal waveform has small amplitude and is sinusoidal.

• Damaged rings will produce a waveform with spikes.


Transducer Mounting


Thank You

Thermodynamics II Compressors

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