COMPRESSOR

Compressor is a mechanical device, which is used for increasing the pressure/head of gaseous fluid

The process of compressing the fluid requires that work should be expanded on it. Therefore the compressor must derive its motive power from an external source, called its drive.

USES OF COMPRESSOR To increase the pressure of various gases for process requirement. To increase the pressure of refrigerant in refrigeration and air conditioning

system To supply the compressed air in Gas Turbine for combustion of gases. To compress air for various purpose like

-Boosting I. C. Engines-Creating blast in Blast Furnace-Paint spraying-HVAC system-Driving a wide variety of pneumatic tools and Mining M/c,etc

TYPES OF COMPRESSOR

Types of centrifugal compressor

Horizontally split Compressor

Model Type MCL/2MCL/3MCL/DMCL Where MCL stands for Meridiane Compressori Libro Cast / Fabricated Casing Pressure up to 40 ata Services: Air/Ammonia/Propylene/Wet gas

Vertically Split Compressor

Model Types BCL/2BCL/DBCL/ BCLa/BCLb/BCLc/BCLd Where BCL stands for Barrel Compressori Libro Forged Casing ( Barrel) Pressure up to 700 ata Service: Syn Gas, CO2 etc

Types of Centrifugal Compressor

PCL Compressor

Model Type: PCL Suction & Delivery positioned sideways opposite each other Pipeline Application

SRL Compressor

Model Type: SRL Shroudless Low Pressure Services Several Shafts with over hang impellers Open Type Impellers Impeller Inlet is Coaxial & Outlet is tangential Very High Polytropic Efficiency

Type / Size Designation

Example: 2 BCL 306 a2 - Functional Feature (Compressor. Stages in series)BCL - Constructional Feature (Barrel Type)30 - Nominal Dia of impeller (30 cms)6 - No. of Impellers ( 6 impellers on rotor)a - Pr. Rating ( up to 200 ata)

Industrial Application of Centrifugal Compressor

Type of Plant - Gas CompressedFertilizers - Syn Gas, NH3.Air, Co2, Nitrogen, OxygenRefineries - Air, Wet Gas, HydrogenPetrochemical Plants - Propane, Propylene, ethyleneMetallurgical Plants - Oxygen, Air, Gas

PRINCIPLE OF CENTRIFUGAL COMPRESSOR

Kinetic energy of the gas is increased by centrifugal action of impellers. This kinetic energy is converted into pressure energy by a device called diffuser.Modern practice is to design the compressor so that about half the pressure rise occurs in the impeller and half in the diffuser.

ADVANTAGES OF CENTRIFUGAL COMPRESSOR

1. Occupy less space.2. Less maintenance cost.3. Higher volumetric capacity.

4. Less noise.5. Less initial investment.6. Contamination of oil with compressed gas is almost

negligible.

DISADVANTAGES OF CENTRIFUGAL COMPRESSOR

1. Minimum capacity limitation.2. Operation becomes unstable below around 50% of the design capacity

at design speed. This phenomenon is called SURGE.3. Developed head is less.

Multi - stage compression is required for higher pressure.

CONSTANT SPEED PERFORMANCE CURVE

SELECTION CRITARIA FOR COMPRESSORS

DESIGN EQUATIONS

DESIGN EQUATIONS

Q1 = W V1 = W / 1 = W z 1 R T / P1

144 ( 1 )Defines the volumetric flow under inlet conditions.

* *

**%

Head

%H

P

Head

Volumetric flow

Efficiency

Horsepower

Effi

ciency

Inlet flow, acfm

Dis

charg

e P

ress

psi

a

ReciprocatingCentrifugal

Axial flow

HP = Z m R T1 {K n P /(K - 1)} [{P2 / P1} (K - 1) / K n P - 1] ( 2 )

Polytropic Head Equation.

G . HP = W HP / 3300 nP ( 3 )Gas horsepower equation Q1 = Inlet flow, cfmW = Total mass thruput , lb / minV1 = Specific volume at inlet condition, cft /lbρ1 = Density at inlet condition, lb / cftZ1 = Inlet compressibility factor R = Universal gas constantT1 = Inlet temperature, absolute ( F + 460)P1 = Inlet pressure, psiaHP = Polytropic head, ft - lb /lbZ m = Mean compressibility factorK = Specific heat ratio, CP / Cv

P2 = Discharge pressure, psianP = Polytropic efficiencyG.HP = Gas horse power

Decrease in inlet temperature will cause increase in discharge pressure, density, mass flow and hence increase in horsepower, while other conditions remain same.

Decrease in inlet pressure will cause decrease in discharge pressure, density, and mass flow and hence decrease in horsepower, while other conditions remain same.

Increase in MW will cause increase in discharge pressure, density, and mass flow and hence increase in horsepower, while other conditions remain same.

Decrease in K causes increase in discharge pressure.

MULTISTAGE COMRESSION

Multistage compression is used in centrifugal compressors to develop a higher head at a high efficiency/lower power requirement.

Interstage cooling is introduced to increase efficiency by approaching isothermal operation.

Interstage separator is used after the cooler to remove moisture/ condensate, which may cause damage to the machine e.g. high vibration, rotor unbalance, pitting, corrosion of impellers, and clogged diffuser passages, etc.

Pre

ssu

re

Volume

Adiabatic compression

Isothermal compression

Better mechanical balance is achieved by back-to-back arrangement of adjacent stages.

The shaded portion represents the work saved in multistage compression with intercoolers.

COMPRESSOR CASING & BEARING

Two types of casings are in use for multistage centrifugal compressors. Horizontally split (MCL type): used up to 40 ata. Barrel

type / vertically split (BCL): used above 40 ata. The compressor rotor is subjected to an axial thrust towards the suction

due to differential pressure between suction and discharge. To minimize this thrust a back-to-back arrangement of successive stages is provided in case of multistage compressors.

A balance drum absorbs the thrust partially (about 90%). Rest is absorbed by thrust bearing.

Journal bearing pads dampens the radial vibration of the rotor.

SEALS Labyrinth seals are used to reduce as leakage, between stationary and

rotating parts. Also to prevent reverse flow of gas between the stages and between

discharge and suction of each impeller, labyrinth seals are used. They are also used in balancing drum to minimize the gas leakage

between the two faces. A set of labyrinth seals are provided to prevent gas leakage between the

final discharge and intermediate stages. End seals are provided at both ends of shaft to prevent gas leakage from inside of the compressor to the surroundings.

If the gas is not expensive and non-hazardous labyrinth seals are provided at both ends.

If the gas is expensive/toxic/poisonous/explosive oil seals are provided along with labyrinth seals.

When oil seals are not advisable labyrinth seal is properly supplemented by inert gas sealing.

Labyrinth seals are made up of corrosion-proof, light alloy relatively softer than the material of shaft and impeller so to prevent any damage to the later.

MINIMUM LABYRINTH CLEARENCE

Compressor efficiency increases when clearance in inter-stage labyrinth is minimized. But when the rotor becomes slightly unbalanced or vibration occurs from another source, the seals will wear rapidly.

So if clearance is increased, efficiency will decrease and allowing greater leakage.

To prevent this, we specify that the minimum clearance is equal to the sum of

o Bearing clearance.o Rotor run out at the seal due to its weight.o A given amount of mass unbalance.

LUBE OIL/SEAL OILSYSTM

Lube oil is supplied at journal and thrust bearings and couplings at a definite pressure. Oil supply pressure may be increased in case of increase in bearing temperature.

A lube oil accumulator is provided to maintain oil pressure during oil pump changeover on auto.

Part of the lube oil is used as seal oil. Apart from preventing gas leakage seal oil also serves the purpose of seal cooling.

Seal oil flows at the outer side of the labyrinth seal whose pressure is always greater than the pressure of reference gas, which is at the inner side.

A part of the seal oil, which mixes with the reference gas, is known as sour oil. Sour oil is degassed before returning to the oil reservoir.

The system is provided with overhead lube oil tank to continue supply of oil when both oil pumps (online and standby) has stopped working, ensuring safety of machine parts.

A barring gear operated by oil pressure is provided to prevent sagging of the shaft due to heating or cooling and also ensure uniform heating/cooling, when the machine is not running.

WHAT CAUSES SURGE

Surging takes place due to sudden changes in process condition. These are as follows:

Reduced molecular weight. Higher suction temperature. Lower suction pressure. Higher discharge pressure. Lower speed of machine for variable speed machine.

If molecular weight of a constant speed machine is suddenly reduced so compressor must now deliver a greater Polytropic head to overcome same backpressure at discharge with low-density gas, so it will pump less gas. A compressor is said to be surging, when there is a sudden reversal of flow through the machine.

The head developed by a compressor depends on its speed. When the head or pressure in the system increases beyond the

UNSTABLE STABLE

SURGE LINE 80%

90%

100% SPEED

PR

ESS

UR

E R

ATIO INLET VOLUME

head developed by the compressor, the compressor will not be able to deliver the gas to the system. At this stage, the gas will try to flow back into the impellers and whereas the impellers try to blow the gas out. This causes an unsteady condition. This phenomenon is known as surge.

Trouble free operation can be expected if the compressor is operated at the right side of surge line Of any compressor as shown in figure.

SURGE AT CONSTANT SPEED

Surge taking place due to increase of delivery pressure.

FIG ( 2 )

SURGE UNDER CONSTANT PRESSURE CONDITION

Surge taking place when gas flow is reduced, hence speed also reduced to maintain system pressure

.

WHAT IS SURGE CYCLE

In fig (2) “ a “ is a normal operating point. When pressure increases, the point will move along the line.

At point “ b “ compressor cannot overcome head and back flow starts and no discharge.

System pressure falls and corresponds to point “ c “. Once pressure falls the compressor starts discharging and the point will

move to “ d “ on speed curve. Again i will build up pressure to reach normal operating point“a”. This is

one surge cycle. Depending upon speed of compressor and its mechanical strength, it can

withstand 100 - 2000 surge before thrust bearing fails.

PR

ESS

UR

E

UNSTABLE STABLE

SURGE LINE 80%

90%

100% SPEED

FLOW (Q)

110%

a

*

d

b*

*c

UNSTABLE STABLE

SURGE LINE 80%

90%

100% SPEED

PR

ESS

UR

E

FLOW (Q)

110%

*

a

*d

b*

* c

EFFECT OF SURGE

When a machine is operated under surge conditions for a long time, The pressure force can damage the internals of machine. It absorbs approximately 10% more powe. Excess power is converted into heat, causes excessive temp. rise inside the compressor, if conditions are severe enough, can damage shaft & balance piston labyrinth. This will affect the rotor balancing and overload thrust bearing. If thrust bearing fails the rotor will shift axially and impeller will rub against stationary parts of casing causing severe damage.

STONE WALL EFFECT

The upper limit of capacity is determined by the phenomenon of “ STONE WALL“.

Stonewall occurs when the velocity of the gas approaches its sonic velocity somewhere in compressor.

Sonic barrier is generally reached at the eye of the compressor. Shock waves result which restrict the flow,causing a choking effect, rapid

fall off in the discharge pressure for a slight increase in volume throughput.

Stonewall is usually not a problem when compressing air and lighter gases, however in compressing gases heavier than air, problem become more prevalent.

PERFORMANCE OF CENTRIFUGAL COMPRESSORS WITH VARIABLE SPEED

The performance of centrifugal compressors, at speed other then design, are such that the capacity will vary directly as speed, the head developed as square of the speed, and the required horse power as the cube of speed.

With variable speed, the compressor easily can deliver constant capacity at variable pressure, variable capacity at constant pressure, or a combination of variable capacity and variable pressure.

By varying speed, the centrifugal compressors will meet any load and pressure demanded by the process within the operating limits of the compressor and the driver.

FACTORS AFFECTING DECISION OF CONTROL

The type of control used depends first on compressor driver. For

Percent inlet capacity

Perc

en

t hors

epow

er

Unthrottled

Suction throttling

Adjustable guidevane

Speed controlSurge limit line

100%

10

0%

turbine driven compressors, normal control is accomplished by varying the speed. Speed control is more efficient than throttling the flow at constant compressor speed since, by artificially creating resistance, an unrecoverable loss of power results.For motor driven compressors, the control can become more intricate. Common methods are:

1. Speed control. 2. Inlet throttling.3. Adjustable inlet guide vanes. 4. Antisurge control

CRITICAL SPEED

Every compressor/turbine shaft has a particular length, diameter weight, volume, contour of shaft or configuration of the shaft, material of construction and mountings.

For the above factors the compressor/turbine shaft itself has particular frequency one or more than one, called the natural frequency of the shaft.

For practical reasons mass centre of the shaft never coincides with the geometric axis. As a result, when the shaft rotates by any driving force, then there will be a transverse force whose direction of rotation at the same speed as the shaft with a particular frequency. This frequency is called rotor frequency or shaft frequency.

At a particular RPM when the natural frequency of the rotor or shaft, coincides with the rotor or shaft frequency, then there will be a resonance, causing excessive vibration and hence tending to deflect the shaft. That RPM or speed is known as critical speed.

There may be one or more than one critical speed. Operating speed must be beyond the range of critical speeds.

ANTISURGE CONTROL

Purpose of antisurge control is to maintain a minimum flow through the compressor so as to avoid conditions leading to surge.This can be achieved by providing at the discharge

1. A bleed valve (for cheap/non - toxic gases)

2. A recycle valve (for costly/toxic gases)

The parameters taken into account for antisurge control are:

SO RT

Margin of safety

Flow input

Hp

* P

s

Surge control line

Recycle trip line

Surge limit lineSafety

on

1. Suction pressure 2. Discharge pressure 3. F low through the machine 4. Speed of rotation

Hp = Polytropic head Ps = Suction pressure

Surge limit line represents the locus of all points where flow is minimum and pressure is maximum.

Control action starts at a point located at the design margin of safety from the surge limit line. Locus of all such points is called surge control line.

Distance of operating point from the SCL is known as deviation. It has a positive value at the right side of the SCL, negative at the left side.

If the combined PID response of the controller is not able to maintain a minimum margin of safety, the open loop Recycle Trip response opens the antisurge valve in rapid, repetitive steps.

If unanticipated circumstances do produce a surge, Safety On response redefines the surge limits to stop the surge after a single cycle.

RT o

utp

ut

Time

Slow closing

Quick opening