CENTAC - Ingersoll Rand Productsmanualarchive.ingersollrandproducts.com/manuals/manuals/... · The...

INGERSOLL-RAND�

AIR COMPRESSORS

CENTAC�

Principles of Operation

CENTAC PRINCIPLES OF OPERATION

CST-TP0003 Rev 4� 1999 Ingersoll-Rand CompanyDate of Issue: 25 October, 1999

Copyright Notice

� 1996-1999 Ingersoll-Rand CompanyTHIS CONTENTS OF THIS MANUAL ARE PROVIDED “AS IS” AND WITHOUT ANY IMPLIEDWARRANTIES WHATSOEVER.

Ingersoll-Rand air compressors are not designed, intended, or approved for breathing airapplications. Ingersoll-Rand does not approve specialized equipment for breathing airapplications and assumes no responsibility or liability for compressors used for breathing airservice.


CST-TP0003 Rev.4� 1999 Ingersoll-Rand CompanyDate of Issue: 25 October, 1999

Table of ContentsIntroduction ________________________________________________________________1Dynamic Compressor Principles of Operation ____________________________________1

Characteristic Pressure versus Flow Plot ______________________________________3Surge ____________________________________________________________________5Choke and Stonewall _______________________________________________________7Effect of Air Density________________________________________________________8Effects of Changing Air Inlet Temperatures and Pressures _______________________9

The Centac Performance Curve _______________________________________________12Inlet Valve Control of Throttle Range __________________________________________15Bypass Valve Control of System Low Flow Demands_____________________________17Starting, Stopping and Un-loading Control______________________________________19Effect of Changes in Air Inlet Conditions when the Electronic Control System is used _20Function of the Inlet Guide Vane (IGV) _________________________________________20Basic Compressor Multi-Staging and Intercooling _______________________________22Typical (Actual) Centac Performance Curve _____________________________________23References ________________________________________________________________25


CST-TP0003 Rev.4� 1999 Ingersoll-Rand Company

Date of Issue: 25 October, 1999

1

IntroductionThe purpose of this manual is to provide Centac compressor maintenance servicepersonnel and interested others with a basic understanding of the principles of operation ofthe centrifugal air compressor. A general knowledge of the principles and their applicationin real situations is a requirement for the performance of quality operational maintenanceand service.This manual is intended for use as an information and training study document forIngersoll-Rand Company and its authorized Distributor service employees in support oftheir requirement to provide quality service. Others including new hire sales, technical andengineering personnel may find it useful.The booklet is a compilation of technical information presented in a form that does notrequire an engineering background to understand. For the purposes of demonstration andexplanation, much of the data presented is intentionally exaggerated.Recommended reading material listed under References in the index is for those whodesire to broaden their knowledge of CENTAC operating principles beyond the scope ofthis booklet.

Dynamic Compressor Principles of OperationCentac compressors are classified as dynamic compressors of the radial flow centrifugaltype. A Centac compressor is also classified as a constant pressure-variable volumemachine.The compression process in a dynamic compressor is quite different from that of thereciprocating or rotary screw type compressors. In a centrifugal compressor work is doneon the air by centrifugal forces as the air passes through a rotating impeller. The work inputtakes place as an increase in pressure and velocity (speed) of the air flow through theimpeller. The air flow subsequently loses velocity in the diffuser section. The diffuser is astatic component that guides the air flow as it leaves the impeller. The slowdown in the flowresults in an additional pressure increase. Figure 1 shows the velocity, pressure andtemperature changes that occur in a theoretical single stage centrifugal compressor. Abouttwo-thirds of the pressure developed occurs in the impeller with one-third produced in thediffuser.The terms used in describing the process of compression in the centrifugal compressorcome from centrifugal pump theory. Air can be considered as a fluid, and the samethermodynamic principles and terms apply to compressors as well as pumps. The term"head" is one of these terms. In the centrifugal compressor, the amount of work done inmoving one pound of air through the compressor is the "head”. The dimension of head isfoot-pounds per pound (ft-lbs/lb) and is a measure of work.The process that air undergoes in the centrifugal compressor (velocity slows, pressureincreases) can be re-stated as: If flow decreases, pressure increases. This re-statement istrue because it is a volume-of-air that has velocity. The volume measurement used is cubicfoot or cubic meter of air. A volume that is moving past a point on a per-unit time basis hasa rate. The term for rate is "flow" and its dimension is cubic feet per minute (ft3/min).


CST-TP000 Rev4� 1999 Ingersoll-Rand CompanyDate of Issue: 25 October, 1999

2

Figure 1. Pressure, temperature and velocity relationship in a centrifugal compressor

In a centrifugal compressor, at constant speed and constant inlet cfm, the amount of workrequired to move one pound of air (head) remains constant regardless of inlet conditions asa study of Table 1 reveals. The Items marked (*) on Table 1 are changes from the columnlabeled Design.

� Notice the horsepower changes when inlet air pressure changes.

� Observe what happens when inlet air temperature changes or as different density airenters the machine.

� Take special note of how head remains constant as various conditions change.The head (work) remains constant, but the pressure developed will vary approximately asthe density of the inlet air. Air density varies with temperature, humidity, pressure, andweight of different gases. Table 1 examples use dry air (Humidity = 0%) for claritypurposes.

FTemperature ( )

27.827.326.826.325.825.322.822.321.821.320.820.319.819.318.818.317.817.316.816.315.815.314.814.3

psia

893846799752705658611564517470423376329282235188141 94 47 0

ft/sec

SUCTION IMPELLER RADIALDIFFUSER

DISCHARGEDIFFUSER

Pressure (psia)

F230210199188177166155144133122111100

8978 Velocity (ft/sec)

GRAPH OF THE RELATIONSHIP OF PRESSURE, VELOCITY AND TEMPERATURE




3

The terms "head" and "pressure" are used interchangeably when discussing a compressor.This is common terminology, but be aware that there is a marked difference between thetwo. While 25 psia is said to be the head the compressor is producing, this is not the samehead as the ft-lbs/lb.The volume of air that enters a centrifugal compressor has several interchangeable wordsto define it. The terms for the moving volume are capacity, flow or through-put.

Table 1

Item Design 1 2 3Inlet pressure (psia) 14.4 12.4* 14.4 14.4

Inlet temperature (�F) 60 60 100* 60

Molecular weight 29 29 29 20*

Relative Humidity (%) 0 0 0 0

Density (lb/ft3) 0.0749 0.0645 0.0695 0.0516

Total lbs/min. (W) 749 645 695 516

Discharge pressure (psia) 25.4 21.85 24.45 21.60

Discharge temperature (�F) 180 180 220 142

Head (ft-lbs/lb) 17,490 17,490 17,490 17,490

BHP Horsepower 509 438 471 350

Characteristic Pressure versus Flow PlotThe compression process diagram for a theoretical single stage compressor with abackward-leaning bladed impeller operating at constant speed is shown in Figure 2. Theideal characteristic pressure versus flow plot for the theoretical compressor is a straight linethat slopes downward to the right. Notice, in Figure 2, how the pressure decreases as flowincreases.The ideal characteristic plot is affected by various energy losses in a real compressor. Apressure versus flow curve for a real or actual compressor is shown in Figure 3. This plot isthe ideal curve minus the energy losses. The dotted line shows energy losses.Figure 3 illustrates that the actual curve is not usable over the entire range of zero-to-maximum flow. On the left of the actual curve, the pressure is shown to rise as flowincreases. This rise cannot occur since; to remain stable, pressure must decrease as theflow increases. The compressor is not usable in the part of the curve where pressure isshown to increase as flow increases.



4

Figure 2. Ideal characteristic pressure vs flow plot

Figure 3. Actual pressure plot

Ideal head plot

PSIA

INLET (ICFM)

ACTUAL HEAD PLOT

Ideal head plot

Losses

PSIA

INLET (ICFM)




5

On the right end of the actual curve, the pressure decreases in nearly a straight linedownward as the flow increases. For very small changes in flow in this area there will be alarge, change in pressure.A centrifugal compressor cannot function over the entire zero-to-maximum flow rangebecause of the limits imposed by the losses.The useful part of the actual pressure curve is between the left and right limit areas. Thepressure in this middle area will decrease as flow increases in a predictable and stablefashion. Figure 4 shows the actual curve with the unusable areas eliminated. The unusablearea on the left is where surge occurs and the unusable area on the right is where chokeoccurs.Figure 4 is a characteristic curve. It is the useful range curve, normally without the surgeand choke labels, that is commonly seen when centrifugal compressors are discussed.

Figure 4. Useful operating range

SurgeSurge occurs at the peak pressure of the characteristic curve and it exists because of theway losses affect ideal compression. When the compressor is in surge it is unstable andcannot perform a useful function. Surge is a momentary reversal of flow in a compressor.Figure 5 shows a graphic model of the phenomenon of surge. Assume a discharge valvealone controls flow for a compressor and flow is at point Q. When the discharge valve isclosed, the compressor is ingesting more flow than the discharge valve can pass. Pressurerises toward point P as the flow decreases. At point P, surge occurs and the operatingpoint jumps to point J with reverse flow. Reverse flow is on the left. The reversal of flow isaccompanied by a noise that may sound like "womp”.

USEFULOPERATING RANGE

SURGE

CHOKE

PSIA

FLOW



6

Figure 5. Model of surge flow reversal

The flow reversal causes pressure to decrease along line J - K. Forward flow re-establishesas it crosses the pressure axis. The flow at point Q is larger than the flow that thedischarge valve can pass, so pressure starts to increase again along line Q - P. Whenpressure reaches point P, the surge process repeats. Figure 5 provides a way to visualizethe surge phenomenon. This graph in reality has large imperfections. The reality of surge isthat at the peak pressure, flow reverses, a noise is heard, pressure drops, a rapid increaseof positive flow occurs, flow again decreases, pressure again rises, surge is reached andthe process repeats.The terms "pumping" and "surge" are used interchangeably. Pumping is a very descriptiveword for the appearance of surge. As soon as the surge reversal occurs, the compressorcan re-establish positive flow. The re-occurrence of surge happens quickly and repeatsuntil such time as actions to un-load or otherwise stabilize the flow through the machinetakes place. This repeating process of surge has the name pumping because of the in-outaspect of the flow.An operational machine can experience "incipient surge" or "stall”. This is where a stageelement, impeller, or diffuser, experiences a flow separation. This separation is a stall thatis not severe enough to constitute a full stage surge. The stage can remain in the stallcondition. It is very near the point of full or complete surge when stall is occurring. A minorflow disturbance in this condition can trigger a stage or whole machine surge. Surgetriggering can occur in various areas in the impeller or diffuser.

FLOW

P

Q

J

K

L

O

CYCLIC SURGE MODEL (LARGE VOLUME)

FLOW +-

psia




7

The peak pressure point on the characteristic curve is called the "natural surge point”. Inmulti-stage machines, each stage has its own natural surge point. A controlled test thatcauses a compressor to reach its natural surge point checks the compressor's suitability foruse. Degradation of internal components is indicated from the test if the machine surges ata pressure that is less than a baseline reference. The compressor aerodynamics areassumed to be good if the natural surge test pressure remains the same as the reference.A "natural surge test" is usually done as part of an initial start-up to obtain on site baselinedata.The physical geometry (shape) of the parts in a specific compressor system determines itssurge point. Damage or foreign matter build-up in the compressor elements can changethe shape of these parts. A reduced surge point on an operational compressor is quiteoften the result of environmentally caused material build-up on the diffuser and impeller.The build-up alters the shape of the impeller or diffuser. Changing the physical shapechanges the shape of the characteristic curve. The new curve describes a new lower surgepoint. This new surge pressure may be near, at, or below normal set pressure and if it is,surge occurs when normal operation is attempted.Surge happens when the bypass valve malfunctions in a way that limits flow. It happenswhen the system demand decreases faster than the inlet valve can react to the decrease.When the inlet valve is improperly set up or adjusted, surge can occur. Any situation wherethe system pressure becomes greater than compressor discharge will likely result in surge.An example is the high side pulsation pressure from a reciprocating compressor feedingthe same header that a centrifugal is feeding.Surge is an abnormal operational event and, if allowed to continue, will eventually causedamage. This damage is in the form of over-worked thrust bearings, since on reversal offlow, thrust reverses. Pumping will "hammer" the thrust bearings over time. Damagecaused by overheating can happen with surge. When flow reverses, hot air (from the heatof compression) flows to the inlet side of the impeller. The hot air receives more heat whenit is re-compressed. The temperature cycle is self limiting, but very high temperatures arepossible. The occurrence of surge without pumping is not in itself a damaging event.Performance of a natural surge test or an occasional surge event will do no harm. It is thesituation where rapid recurring surge or pumping continues over a period of time wheredamage may occur.

Choke and StonewallThe area designated as choke is on the right side of Figure 4. Choke is what limits themaximum flow through the compressor. When in choke, the compressor physically cannotpass any more air volume due to the losses being encountered either in the compressoritself or in the discharge system. These losses can be considered, simply, as resistance toflow. Choke is a fixed physical limit of the compressor.The point of demarcation between the useful range and choke has the name "stonewall”.The name is appropriate since an attempt to increase flow above the stonewall pointresults in a situation where further flow increases stop as if a stone wall has been placed inthe flow path.The technical definition of stonewall is: That point on a characteristic curve when the flowvelocity reaches the speed of sound. Flow near the speed of sound results in a conditionwhere the characteristic curve is vertical or nearly so and any increase in flow stops. Thereis a stone wall in the path of flow.



8

In cases of compressor deterioration, or if non-specification parts are installed, it is possiblethat a lower stage in a multi-stage compressor may be in choke, which in turn starves thedownstream stages, causing surge or incipient surge.Choke is normally not associated with damage to the compressor as in the case of surge.Choke is an aerodynamic limit that causes no damage; however, if pressure becomesunstable in the choke situation, this could result in thrust bearing hammering as pressureloads rapidly lessen and increase.

Effect of Air DensityThe density of the inlet air affects the pressure developed by the compressor. Density of airis influenced primarily by temperature, pressure, and humidity. The dimension of density ispounds-per-cubic-foot at STP conditions. STP is the abbreviation for standard temperatureand pressure of air. The values used for STP in the gas compression industry are 60�F,14.7 psia, with a relative humidity of 0%. The STP temperature used by general scienceand industry is usually 68�FAs air temperature increases, the density decreases. The opposite is true for colder air. Asair pressure increases the density increases. Decreasing pressure results in decreasingdensity. The more water vapor that is in the air, the lighter a cubic foot of air will be. On avery hot humid day, the air is light and on a very cold dry day the air is heavy.

The formula for the density of dry air is: Density = 1.325 X (Pb�T); where Pb is barometricpressure in inches of mercury and T is temperature in degrees Rankin (�R = �F + 459.7).

Dry air at 60�F weighs approximately 0.074 lbs/ft3. If the temperature increases to 100�F,the air will weigh 0.064 lbs/ft3.

One pound of dry air at 60�F has a volume of about 13.5 ft3. At 100�F, one pound of airexpands to fill 15.6 ft3.The way density affects dynamic compression may be explained by way of an analogy.Imagine attaching a string to a Ping-Pong ball and then spinning it around in a circle. Therewill be some tension on the string as the ball goes around. This tension can be viewed aspressure. Now imagine doing the same thing with a golf ball. The tension on the string willbe significantly higher. The tension is higher because the golf ball weighs more than thePing-Pong ball. The heavier spinning object exerts more tension or pressure. Similarly,denser (heavier) inlet air causes more pressure to develop. Less dense (lighter) inlet airdevelops less pressure. The density of air affects the compression ratio of the compressor.The compression ratio will increase with heavier air and decrease with lighter air.Density significantly affects how much work the compressor is doing at any given time. Ittakes more work to move heavier air.




9

As an example: Assume a compressor will transfer 10000 ft3 per minute at dry air inletconditions. At 14.4 psia and 60�F, the compressor will transfer {(10000 ft3/min.) X (1lb/13.5 ft3)} = 749 lbs/min. At 14.4 psia and 100�F it will transfer 695 lbs/min. Less work isdone on the 10000 ft3 in one minute at 100�F than at 60�F. At 60�F, the model'scompression ratio is (25.4 psia � 14.4 psia) =1.764. At the 100�F inlet condition, thecompression ratio is 1.698.To summarize the effect of inlet density changes: Higher density generally increasescompression ratio, increasing the discharge pressure. A higher density generally increasesthe amount of work required to move a fixed volume of air. Lowering density generally hasthe reverse effect.In relating the density effects to the characteristic curve; the curve will move downward(less pressure at same volume) with lower density air. The curve shifts upward with higherdensities (higher pressure at same volume).

Effects of Changing Air Inlet Temperatures and PressuresIn the graphs that follow, Figures 6, 7, and 8, no control valves are considered. Thecompressor is operating with a fully open inlet and with a fixed discharge resistance.Adding control valves alters the effect changing temperatures have on compressoroperation.

Figure 6. Effect of change in inlet air temperature

F20

INLET (ICFM)

DIS

CH

AR

GE

PSI

A

F60

F100

EFFECT OF CHANGE IN INLET AIR TEMPERATURE

1000



10

In Figure 6, effect of change in inlet air temperature, the 60�F curve is the normal curve.When inlet temperature increases to 100�F, the curve shifts down. Hotter air is less dense,pressure ratio is less, and for the same ft3/min flow, there is a reduced pressure. At thesame time, the amount of work required has decreased because there is less weight tomove. The volume remains the same, but because of its lower density, it weighs less andthe lbs/min flow has decreased. When flow is related to the weight of air, it is given thename "mass flow," for the air mass weight that is being compressed. With hotter air, thesurge point decreases.The opposite effect occurs when inlet temperature decreases. The curve rises and thesurge line moves to the left. The compressor's surge will be at a higher pressure withdecreased inlet air temperature. Pressure ratio will increase as the density of inlet airincreases and more work will be required to move the fixed heavier volume of air.

EFFECT OF CHANGES IN INLET AIR PRESSURE

INLET (ICFM)1000

14.4 PSIA

12.4 PSIA

(Mayfield Ky).

(Denver Co)

Barometric pressure change effect

DIS

CH

ARG

E PS

IA

Figure 7. Effect of decreased inlet pressure

Figure 7 describes the effect of changes in inlet pressure. As inlet pressure decreases,density decreases, discharge pressure decreases, and as with all reductions in density, thework required to move a fixed volume also decreases.Altitude (geographic location) has a very significant effect on the compressor. Weather-related barometric pressure changes have an effect but it is usually not significant. Figure 7uses the city of Mayfield KY at low altitude and the city of Denver CO at high altitude asexamples of how altitude affects the compressor. Geographical location (altitude) is a vitalconsideration in compressor design.




11

A multi-stage compressor has not been discussed yet, but consider what would happen ina two-stage machine that has air cooling between the first and second stage. Cooling watertemperature has a very significant effect on a compressor because of the largetemperature swings possible. The effect of high or low cooling water temperature, by itselfor combined with a high or low inlet air temperature on a multi-stage compressor, canmake the machine practically useless from an air production capability point of view. If thefirst stage has a low pressure due to hot summer air, the second stage will have a lowpressure capability because its inlet pressure is lower. Couple this low pressure with theeffect of high second-stage inlet temperature due to a malfunctioning cooler, and it shouldbe easy to see that the overall compressor output is in big trouble. Figure 8 is a graph ofwhat may happen in the event of these combined changes.

Figure 8. Combined effect of high and low temperature air and cooling water

COMBINED EFFECT OF CHANGES IN COOLING WATERAND INLET AIR TEMPERATURES

DIS

CH

AR

GE

PS

IA

INLET (ICFM) 1000

+F0 air

F40 water

Normal95 F air

waterF80

110 F+

air

water110 F



12

The Centac Performance CurveThe Centac performance curve is an expansion of the centrifugal compressor characteristiccurve. A thorough knowledge of the Centac performance curve will provide a useful tool inthe actual operation and maintenance of the Centac. Figure 9 is a stylized Centacperformance curve.

Figure 9. Stylized CENTAC performance curve

STYLIZED PERFORMANCE CURVE

Capacity (ICFM)

Capacity (ICFM)

Design point

Surge

Unloaded

100%

100%

100%

Rated hp

PSIA

BHP

Load

or su

rge l

ine




13

A design point appears on the performance curve. A Centac compressor is guaranteed toproduce the design point flow and pressure. For example: a machine may be built with adesign point of 100 PSIG at 1800 ICFM. The impellers and diffusers will be configured forthis specification and the head will be fixed by this configuration. The design point andhead determine what minimum horsepower driver needs to be used.The set of graphs that make up the performance curves include the brake horsepowerversus capacity chart and the discharge PSIG versus capacity chart. The horsepowercurve moves up and down with the effects of changing density.The function of the Centac compressor is to provide oil-free compressed air at a specifiedpressure over a range of user-flow requirements. Controls allow the CENTAC to perform itsfunction. Figure 10 is a stylized sketch of a single-stage centrifugal compressor that hasthe valves necessary for control. This stylized Centac compressor is the model fordiscussions that follow.

Figure 10. Stylized single-stage compressor with fixed position inlet and bypass valves

Figure 10 corresponds with Figure 9 and shows the compressor running at design inletconditions of 95�F inlet air temperature, a humidity of 0% and a pressure of 14.4 psia at thefixed open inlet valve entrance. In this condition, as long as inlet conditions don't change,the compressor will supply the design point flow at the design pressure. The bypass valvein this example is fixed at fully closed and the full amount of air entering the compressor isbeing discharged into the system.

SYSTEM

DIFFUSER

INLET VALVE

DRIVE SHAFT

IMPELLER

BYPASS VALVE

RADIAL



14

If the system flow demand decreases, as it will when the user shuts a system air valve, thepressure will rise as flow decreases. The pressure increase follows the performance curveas flow decreases. As additional valves in the system close, the flow decreases and thepressure rises higher and higher until the pressure reaches the surge point. At the surgepressure, the compressor will experience pumping until such time as the system is able toaccept more flow. Opening system valves causes flow to increase and pressure todecrease along the performance curve, re-establishing the design flow and pressure.The addition of a discharge pressure sensor and a device to open and close the inlet valvein proportion to the discharge pressure provides the means for constant pressure-variablevolume-compressor control. Figure 11 shows the stylized compressor with a controlsystem. The control system includes a pressure sensor, a controller and an inlet valveactuator.

Figure 11. Stylized single stage compressor with controlled inlet valve

VALVEACTUATOR

INLET

PRESSURESENSOR

PROPORTIONALCONTROLLER

SYSTEM

DIFFUSER

INLET VALVE

DRIVE SHAFT

IMPELLER

BYPASS VALVE

RADIAL

CONTROLLER




15

Inlet Valve Control of Throttle RangeThe control system shown in Figure 11 will provide for control of the discharge as shown inFigure 12. In Figure 12, the compressor is operating at the minimum closure point, at 60%of capacity. As long as inlet conditions do not change, the system will be supplied with 60%capacity at design pressure as long as necessary. If a valve in the system is opened, theflow will increase and the pressure will decrease. The pressure decrease will be sensedand an open signal will be sent to the inlet valve from the controller. The valve will begin toopen. Additional flow can pass through the inlet valve with a larger opening. The systemwill accept exactly as much flow as needed and then pressure starts to rise. The controllercauses the valve to stop opening when the design pressure (or set pressure) is reached.The system now is using increased flow at design pressure with more horsepower beingused. This process repeats up to the pressure and flow point where the inlet valve is fullopen. Any additional flow demand by the system after the inlet valve is full open will resultin decreasing pressure and eventually choke will occur if system demand increases highenough.If the system requires less flow, the pressure increases as flow decreases and the inletvalve is sent a signal to close. The reduction in flow process can continue until theminimum valve closure point is reached. The inlet valve is arranged so that it will notrespond to any increases in system pressure when at the minimum closure point. Theprocess of opening and closing of the inlet valve in response to the system demand istermed modulation. The inlet valve is said to be modulating when it is opening and closingto meet system demand. The capacity (flow) that the inlet valve can control between designpoint and near surge is termed the compressor throttle range.Figure 12 illustrates inlet valve modulation with the series of dotted lines above the inletvalve minimum closure point. Notice that the dotted lines have the exact shape as theuppermost line. The movement of the inlet valve causes the performance curve to shift.The curve shifts because the density of the inlet air between the inlet valve and the impellerchanges as inlet flow decreases or increases.



16

Figure 12. Inlet valve throttle range modulation curves

If air entering the inlet valve is at a constant density, the air after the inlet valve will be atsome other density. As the inlet valve closes from full open in response to the decreasedsystem demand, the flow through the valve is being limited. As the valve closes, the airpassing through it increases its velocity and the pressure after the valve will show acorresponding decrease; the volume of entering air is being limited. With less dense inletair, the pressure developed by the compressor will decrease. The flow decreases becauseit is being limited by the inlet opening.The function of the inlet valve is to control inlet flow and inlet pressure in the throttle rangeof the performance curve.Study Figure 12. When the inlet is at minimum closure (minimum throttle), the surge pointis nearly at the design pressure line. If the compressor has a controlled inlet valve with noother controls, as system flow demand reduces below the minimum closure position, themachine will surge. A surge caused by closing the inlet more than the minimum closurepoint is called a throttle surge.

Dis

char

ge p

ress

ure

(PSI

A)Po

wer

at c

oupl

ing

(BH

P)

Inlet valve minimum closure

Min. throttle

Surge Design point

Unload

100%

100%

100%

Rated hp

Capacity (ICFM)

Capacity (ICFM)

60%

60%

ThrottleRange




17

A control system that has just an inlet valve, while providing service in the throttle range,doesn't allow the air system to take advantage of the full 0%-to-100% flow range. Flowsbetween 60% and 100% can be utilized with inlet-only control and the effects of changinginlet temperature and pressure would be partially controllable. An inlet-valve-only controlsystem is not very useful since low system flows and changing inlet conditions cannot beaccommodated.

Bypass Valve Control of System Low Flow DemandsTo make available the full range of flow to the system, keep the machine out of surge andaccommodate inlet air changes, a bypass valve is added. Figure 13 illustrates the bypassvalve control configuration.The minimum throttle position of the inlet valve maintains the flow required to keep thecompressor out of surge. As system flow demand decreases below the inlet minimumthrottle position, pressure will start to rise. The control system responds to this rise inpressure by opening the bypass valve, venting off excess flow.The bypass valve uses system pressure like the inlet valve for control. A major differencebetween the inlet and bypass valve function is that the bypass does not control inlet densityor flow. It simply vents excess flow in the 0%-to-60% range.A disadvantage of operating in bypass is that vented air represents wasted energy. Thecompressor will always be compressing whatever volume the minimum closure of the inletvalve allows.So far in this discussion we have a compressor that has a controlled inlet and bypassvalve, it can provide a system with compressed air at design pressure and flow from 0%-to-100%. It can also accommodate some of the effect of changing inlet conditions; however,additional controls are needed for starting, stopping, loading and unloading thecompressor.



18

Figure 13. Inlet and bypass control valve configuration

VALVEACTUATOR

BYPASS

VALVEACTUATOR

INLET

PRESSURESENSOR

PROPORTIONALCONTROLLER

SYSTEM

DIFFUSER

INLET VALVE

DRIVE SHAFT

IMPELLER

BYPASS VALVE CHECK VALVE




19

Starting, Stopping and Un-loading ControlStopping presents no problem. Simply turning off the power supply (electricity, steam, orother) to the driver will result in the compressor slowing down and coming to a stop. Whenstopped, there is a very real possibility that the compressor will reverse rotate from areverse flow caused by the pressurized volume remaining in the system. An attacheddischarge check valve (see Figure 13) will prevent the pressurized air volume in the systemfrom back-flowing through the machine. Reverse rotation cannot be tolerated because itwill damage Centac compressor bearings. The check valve also serves to prevent a largevolume reverse flow in the event of surge.Starting presents a problem. If the compressor is started with the inlet valve at theminimum throttle position, there will be a heavy starting load on the compressor driver.Flow would increase along with the increasing speed of the motor but the accelerationunder load forces are likely to cause damage to compressor or driver components. Thehigh starting amperage in this type of start-up affects the electrical distribution system.A control function that causes the inlet valve to open to no more than about 5% to 20% onstart is desired to limit starting load. If flow is limited on start, the starting electrical andacceleration loads are limited.The compressor needs to have a small flow on start. This flow keeps the compressor frombecoming a vacuum pump and prevents startup-throttle surging. Running the compressorin a vacuum has two detrimental effects: With no flow path, the impeller will impart energyto the stagnant air in the compressor, heat will be generated, and over time this couldcause damage. The bearings are designed for a minimum load, and running under vacuumconditions puts stress on the bearings. To correct these problems on start, the controlopens the inlet valve a small amount and at this time the bypass valve is fully open to ventoff the starting flow.See Figure 12, unloaded position, in the upper graph: Once started, the compressor isoperating at the unloaded or start position that corresponds to the minimum openingposition of the inlet valve. To load the compressor into the system, the inlet valve initiallyopens to the minimum throttle point. The bypass valve begins to close once the inlet is atminimum throttle. Depending on system pressure, the bypass will modulate, and when itreaches fully closed, the inlet will modulate on system pressure.In Figure 12, the slanted dotted line from the unloaded capacity point to the surge point iscalled the load or surge line. The flow and pressure, in a well tuned control system,increases very close to this line as the inlet valve opens. The bypass valve is open at thetime of loading and all flow vents out the bypass, keeping the machine from surge onloadup.With the inlet and bypass controls properly adjusted, the compressor will provide 0%-to-100% of system demand at designed pressure, and allow for compressor start, stop, loadand unload.



20

Effect of Changes in Air Inlet Conditions when theElectronic Control System is used

The electronic control system uses a motor amperage signal (MinLoad) to establish theinlet valve minimum closure (minimum throttle) point. This signal reflects the driverhorsepower where (Amps X Volts X Phase factor X Power factor = Watts) and (746 watts =1 Horsepower). If volts, phase factor and power factor are constant, the amperage isproportional to horsepower. In reality, power factor is variable with motor load and thisupsets the direct amperage to horsepower proportionally. Amps are nearly proportional tohorsepower and for normal control purposes this nearly proportional is adequate. Sincehorsepower is directly proportional to flow at constant pressure, the throttle surge point canbe related to motor amperage.The inlet valve responds to a signal based on system pressure as does the electro-pneumatic scheme, but an amperage level at which surge will occur is entered as a setpoint. If load on the driver nears this surge set point, the inlet valve will stop closing and thebypass will start to open to vent excess flow. The major difference between the electro-pneumatic, LLR control, and the electronic control is that, in the electronic system the inletvalve will stop closing at a specified amperage (�. horsepower � flow) rather than a fixedpercent open position. The effect of this is the same as re-adjustment of the LLR fortemperature-related growth of the compressor.The electronic controller causes the minimum throttle position to be a continuouslychanging position of the inlet valve. The effect of changing inlet density is automaticallycompensated for in the low flow region. This automatic compensation results in less wastedhorsepower when system demand is in the low flow region of the throttle range.In the high flow region of the throttle range, where overload is possible with increased inletair density, an amperage (flow) signal prevents the inlet valve from opening to the pointwhere the motor is overloaded. If the inlet valve was full open when the density changeoccurred, the inlet valve would close until the flow (horsepower) was reduced below theoverload point.The name given to the minimum throttle point control function in the Centac standard CMCelectronic control is "MinLoad”, (Minimum Load). The driver overload protective function iscalled "MaxLoad”, (Maximum Load). The unload position of the inlet valve control functionis called "Inlet Valve Unload Position".Refer to the Centac Microcontroller CMC manual for detailed information of this system.

Function of the Inlet Guide Vane (IGV)An inlet guide vane arrangement is used instead of an inlet (butterfly) throttle valve in someapplications. These applications are, normally, those processes that use air as a rawmaterial for an end product or where precise control of the flow is required. An air-to-nitrogen gas process or a pharmaceutical fermentation process are examples.




21

Figure 14. Example IGV characteristic curve

Each angle of the inlet guide vane produces a unique surge curve. The shape of thecharacteristic curve changes with vane angle in the IGV. This makes the IGV very unlikethe standard inlet valve. Figure 14 illustrates an example IGV characteristic curve.The detailed theory of operation of the inlet guide vane is complex. In the simplest of terms,the guide vane provides spin to the entering air as well as limiting flow. The spin given tothe air results in less work being required by the impeller. Without spin, the air enters theimpeller from an axial position (straight in) and the impeller must do work to change thedirection of the air from an axial to a radial flow. With spin imparted by the inlet guide vane(IGV), work is still required, but less is needed. The IGV provides an increase in theefficiency of the overall compressor.Each degree angle of the IGV vanes establishes a new curve of a different shape. Thisresults from the aerodynamic effects of the spin given to the inlet air.The IGV's primary use is in the degree of accuracy and stability that can be obtained whenthe compressor is operated under conditions where control of the pounds-per-hour(lbs/hour) mass flow is a requirement. IGVs offer a larger and more precisely controllablethrottle range than the standard inlet valve.



22

Basic Compressor Multi-Staging and IntercoolingInherent mechanical and aerodynamic limitations require the use of multiple-stagecompressors to achieve design flows and pressures. A single-stage high-flow, high-pressure dynamic compressor is not practical due to impeller speed-related stress, andother mechanical difficulties related to obtaining the required head. Centac uses 1, 2, 3, 4,and occasionally more stages, depending on customer need. The most common machinesbeing built have 2 or 3 stages. Each stage in an ideal multi-stage compressor will do thesame work. To do the same work, the stage pressure ratios are the same since the lbs/hrflow goes successively through each stage. The calculations below demonstrate how toestimate the interstage pressures of a multi-stage compressor operating at a specifieddischarge pressure.Pressure in psia is used in the calculations. If PSIG is used, a multiplier of 0 will enter theequation.Assume the inlet pressure is 14.4 psia and the desired discharge pressure is 114.7 psia.

Overall compressor pressure ratio = Pratio = Pout � Pin and 7.9653 = 114.7 psia � 14.4

Stage factor = Poverall ratio ( 1/# stages)

2 stage calculation:Compressor ratio = 7.9653, Stage factor = 9653.7 = 2.82231st stage in X stage factor = 1st stage out, 14.4 X 2.8223 = 40.64 psia2nd stage in X stage factor = 2nd stage out, 40.64 X 2.8223 = 114.7 psia

3 stage:Compressor ratio = 7.9653, Stage factor = 3 9653.7 = 1.9971

4 stageCompressor ratio = 7.9653, Stage factor is 4 9653.7 = 1.6799




23

Typical (Actual) Centac Performance CurveFigure 15 is a performance curve for a typical 2CV31M3 compressor. Arrowed labels havebeen added to Figure 15 for instructional purposes. The slope of an actual Centac curve issteeper than the model curve shapes used in earlier sections of this discussion and is atypical general shape for actual compressors. The curves are based on a compressor thatis operating at design conditions. These conditions are given on the form as ambientconditions. Pb = Barometric pressure and is the pressure based on height above sea level(altitude) where the compressor would operate at. P1 = Inlet pressure, called flangepressure, is the pressure that exits at the compressor inlet flange downstream of the inletvalve. The inlet valve is normally attached to the inlet flange.

Figure 15. Typical actual performance curve



24

The difference between Pb and P1 is the pressure drop in the inlet pipe system betweenatmosphere and the compressor flange. T1 = atmospheric (inlet) air temperature. Rh =relative humidity and is a measure of the water vapor in the air. Water vapor contributes toinlet density. The model 2CV31M3 is inter-cooled, and as a result, condensate will beproduced. As condensate drops out of the air flow the air density will change in directproportion. Stages 2 and 3 will respectively receive a smaller percentage of water vapor intheir inlet air because of the condensate drop out in the coolers. The 1st stage will do morework than down stream stages per ft3 of air because of the percentage of inlet water vapor.Tw = cooling water temperature and is the temperature of the water entering the coolers.Tw is the basis for determining what the inlet temperatures in the 2nd and 3rd stages willbe as well as how much condensate will drop out.The standard CENTAC design parameters are Pb = 14.4 psia, P1 = 14.1 psia, T1 = 95 F,Rh = 60% and Tw = 80 F. Any one or all of these parameters may be different in a specificdesign.The difference in discharge psia between the surge pressure and the design point pressureis called the rise to surge pressure. A high rise to surge in an actual machine normallyindicates that the compressor will have a corresponding larger throttle range. A largethrottle range is normally desirable.The BHP (brake horse power) curve shows how the horse power will be consumed fromthe natural surge point to the choke point. It is important to understand that the curvesrepresent a compressor with its' inlet open 100%. The curves show how the pressure andBHP will react as flow is increased or decreased by changes in flow demand by the systemthe compressor is feeding.The sloping line labeled butterfly throttle shows the horse power decrease or increase asthe inlet throttle is opened 100% or closed to the minimum throttle point. The dotted lineshows the power consumption when an IGV is utilized. An IGV throttle, except at thedesign point, causes less horsepower to be utilized for the same flow. There is greaterdifference between the IGV and butterfly BHP control in the low flow area; there is nodifference in BHP at the full open position of either valve. For users who normally operateat near design flow an IGV has little practical use. Where continued operation near thethrottle surge point is anticipated an IGV provides power consumption advantages as wellas its' precise flow control capabilities.




25

ReferencesThe following references were consulted in the preparation of this manual.

1. Gibbs, C. W, Compressed Air and Gas Data, Ingersoll-Rand Company,Woodcliff Lake, NJ., 1971

2. Moore, R. L, Control of Centrifugal Compressors, Instrument Society of America,Research Triangle Park, NC., 1989

3. Brown, R. N, Compressors selection and sizing, Gulf Publishing Company,Houston, TX., 1986

4. Rollins, J. P, Compressed Air and Gas Handbook, Compressed Air and gas Institute,New York, NY, 1973

5. Heald, C.C, Cameron Hydraulic Data, Ingersoll-Rand Company, Woodcliff Lake, NJ.,1988

CENTAC - Ingersoll Rand Productsmanualarchive.ingersollrandproducts.com/manuals/manuals/... · The...

Documents

Transcript of CENTAC - Ingersoll Rand Productsmanualarchive.ingersollrandproducts.com/manuals/manuals/... · The...