ct214 eficiencia con variadores en bombas, ventiladores y compresores (1)

8/12/2019 CT214 Eficiencia Con Variadores en Bombas, Ventiladores y Compresores (1)

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3/32EDITION - mai 2008CT 214

Energy efciency:benets of variablespeed controlin pumps, fans and compressors

no 214

Jacques Schonek

A graduate of the ENSEEIHT engineering school with a doctorate inengineering from the University of Toulouse, J. Schonek took part inthe development of Telemecanique variable-speed drives from 1980 to1995.He was subsequently in charge of studies in the eld of HarmonicFiltering and then Electrical Distribution architectures.He is presently in charge of developing solutions for the Watersegment of Schneider Electric.


4/32Cahier Technique Schneider Electric n 214 / p. 2

Glossary

Energy efciency:Optimum use of electrical energy, includingreduced consumption and cost and improvedavailability.Variable speed drive (VSD):

A device used to control the speed of a machine.Frequency converter:

A device used to adjust the frequency of thepower supplied to a motor and thus control itsspeed.Soft starter:

A device used to limit the inrush current when amotor is started and to control its acceleration.Pump:

A device used to raise or move a uid.Centrifugal pump:

A pump in which the liquid is put into motion by acircular movement.Fan:

A device used to displace air.Compressor:

A device used to increase the pressure of avolume of gas.Flow rate:The quantity of a uid conveyed per unit time.Head:The pressure at a given point in a circuit,expressed as the height of a column of liquid.

Useful power:The power transferred to a uid drawing a certainquantity of energy per unit time. Also referred toas the output power.Mechanical power:The mechanical power transferred to a machine(pump, fan, compressor) such that it can delivera certain useful power to the uid. Also referredto as the shaft power.Electrical power:Power drawn by the electric motor driving themachine. Also referred to as the input power.Head loss:

Additional power that must be transferred to theuid to overcome the various forces that opposethe ow.Booster:

A device used to maintain a certain pressure in auid circuit, whatever the required ow rate.Water hammer:

A sudden variation of the pressure in a circuitfollowing an excessively fast drop in the ow ratedue to the closing of a valve or the stopping of apump.Cavitation:

A phenomenon involving the formation andsudden collapse of vapour bubbles in a pumpfollowing a drop in the inlet pressure of the liquid.


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Cahier Techniq e Schneider Electric n 214 / p 3

Energy efciency:benets of variable speed control inpumps, fans and compressors

Sommaire1. Introduction p. 4

2. Centrifugal pumps p. 5

2.1 General aspects p. 5

2.2 Fundamental characteristics p. 6

2.3 Operating point p. 8

2.4 Varying the ow rate at a xed speed p. 9

2.5 Variable speed operation p. 12

3. Fans p. 20


3.2 Fixed-speed operation p. 21

3.3 Variable-speed operation p. 23

4. Compressors p. 27


4.2 Variable-pressure operation p. 28

4.3 Variable-speed operation p. 28

5. Conclusion p. 29

Appendix 1: Bibliography p. 29

A large proportion of the electricity produced around the world is used toraise, move or pressurise liquids and gases with machines such as pumps,fans and compressors.

Given the increasing importance of controlling energy consumption, specialattention must be paid to the way these machines are operated and theenergy savings that can be achieved through variable speed control.

These different aspects will be dealt with in this Cahier Techniquepublication, both from the qualitative and quantitative standpoint. Variablespeed drives are among the front-ranking solutions proposed by SchneiderElectric to increase Energy efciency.



1. Introduction

Electrical energy consumed by pumps, fans andcompressors represents a signicant proportionof the electricity used around the world. It isestimated that in industrial processes andbuilding utilities, 72 % of electricity is consumedby motors, of which 63 % is used to drive uidow in pumps, fans and compressors.Numerous industrial sectors have pumping,ventilation and compression needs. Forexample:

In the Water sector, for lifting, irrigation,

distribution, treatment, etc.In the Oil and Gas sector, for extraction,transport, rening, liquefaction, etc.

In buildings, for heating, ventilation, airconditioning, etc.Traditional methods of controlling ow rate orpressure involve varying the effective cross-section of the pipe or circuit through which theuid ows.

b

b

b

Valves, taps and gates are the most commonlyused devices.However substantial energy savings can beobtained by using variable speed drives tocontrol the ow rate or pressure in pumps, fansand compressors as opposed to the abovephysical or mechanical means. In pumpingapplications, the most signicant savings areachieved with centrifugal pumps.The aim of this document is to describe howcentrifugal pumps, fans and compressors

function in different operating modes, andto quantify the energy savings that speedcontrol can generate. Other advantages of thistechnique in terms of Energy Efciency are alsoreviewed.



2. Centrifugal pumps

2.1 General aspects

Centrifugal pumps cover a very wide spectrumin terms of power, ow rate and pressure. Theyare used in a host of applications, notably in thewater sector. This is the most widespread typeof pump.The principle involves actuating an impellerthat transfers mechanical energy to the uid,which is then converted into potential energy(represented by the pressure) and kinetic energy(represented by the ow rate).Figure 1 shows the main parts of a simple

single-impeller centrifugal pump:the pump body, which comprises the inlet andoutlet manifolds,

the impeller, which is xed to the drive shaft.

b

b

Figure 2 shows a centrifugal pump driven by athree-phase asynchronous squirrel-cage motor,the most common type of electric motor used.These motors operate at a xed speed whenconnected directly to the power grid, but areperfectly suited to operation at variable speedswhen powered via a frequency converter.

Fig. 2. Centrifugal pump driven by an electric motor.

Outlet Outlet

Volute

Inlet

Impeller

Fig.1. The main parts of a centrifugal pump.

Fig. 3. Multi-stage centrifugal pump.

Different centrifugal pump congurations havebeen developed to cover a wide range of owrates and pressures. In particular, the pressurecan be increased by placing several pumps inseries. Figure 3 shows an example of a multi-stage pump.



2.2 Fundamental characteristics

A pumps fundamental role is to deliver a

certain quantity of uid in a given time at agiven pressure. The major parameters used inpumping are therefore ow rate and head .The ow rate (or discharge) Q represents thevolume of uid transported per unit of time and isexpressed in m 3/s.The head (H) represents the pressure at a givenpoint of the circuit, expressed as the height of auid in a vertical column (in m).Relationship between the head and thepressure: Pr = gHPr: pressure (Pa) : density of the uid (kg/m 3)g: acceleration due to gravity (9.81 m/s 2)

H: head (m)

In the case of water: = 1000 kg/m 3

A pumps Total Dynamic Head (TDH)represents the pressure differential the pumpcreates in the uid between the inlet and theoutlet, expressed as the uid column height. TheTDH varies according to the ow rate. The curverepresenting TDH as a function of ow rate ischaracteristic of each pump.There is a different TDH curve for every pumpdrive speed.The Maximum Total Dynamic Head (TDH max )sometimes refered to as shut off headcorresponds to the maximum pressure the pumpcan exert on the uid, at a ow rate of zero. Thiscorresponds to the maximum uid column height

the pump can maintain, as illustrated in gure 4 .

Fig. 4. Illustration: Maximum Total Dynamic Head.

TDH max



The useful power (Pu) transferred to the uid isgiven by the formula: P u = gHQ (in W)

The mechanical power (P) supplied to thepump takes into account the pumps efciency , i.e.

P=(1/ )P u =(1/ ) gHQ

The pumps efciency varies with the owrate. It is zero when the TDH or ow rate arezero. This is the case because no power istransferred to the uid.The nominal operating point, or Best EfciencyPoint (BEP), is dened as the point at which thepumps efciency is at its maximum.

Figure 5 represents variations in TDH, efciencyand power as a function of ow rate, for a typicalcentrifugal pump

P n

Q (m 3/s)0

0

0

0 Q n

00

T D H ma x

T D H

P

00

Hn

Fig. 5. Characteristic curves of a typical centrifugal pump.



2.3 Operating point

The distribution circuit to which the pump is tted

is characterised by:the water column height between the suctionpoint and the point at which the uid is used(total geometric height Z),

the head loss, corresponding to the additional

b

b

pressure that needs to be exerted on the uid to

overcome friction in the conduits. A simplied distribution circuit is represented ingure 6 .

Z

Fig. 6. Simplied distribution circuit.

0

H (m)

Q (m 3/s)0

Z

R

Fig. 7. Characteristic curve of a distribution circuit.

H: head at the pumpZ: water column height (static head)R: head loss (dynamic head)

The head loss R is proportional to the squareof the ow rate. This results in a curve thatis characteristic of the distribution circuit, asrepresented in gure 7 .

0

H (m)

0 Q (m 3/s)

PumpCircuit

Fig. 8. Operating point of a pump in a circuit.

The operating point of the pump tted to thecircuit is determined by the intersection of the twocurves characterising the pump and the circuit,as indicated in gure 8 .In this case, the useful power supplied by thepump to the uid (equal to gHQ) is proportionalto the shaded area.



2.4 Varying the ow rate at a xed speed

In most applications, the uid ow rate to besupplied varies over time, according to the needs

of users.When using a xed-speed pump, differentmethods can be applied.Some of the most common solutions are outlinedbelow.Use of valves downstream from the pumpThis aim here is to reduce the effective cross-section of the pipe downstream from the pump.This results in an increase in head loss in thecircuit, which translates into an increase in thepressure at the pump outlet and dissipation ofenergy in the uid.In gure 9 , point A is the operating point at thenominal ow rate Q n. Point B is the operatingpoint at the reduced ow rate Q r . The circuitsoptimal operating point at ow rate Q r would bepoint C. The shaded area therefore representsthe power lost with this type of operation.

H (m)

0 Q (m 3/s)0

B

A

Qn

C

Q r

Fig. 9. Varying the ow rate using a downstream valve.

Using a bypass circuitThe principle involves returning some of thepumped uid back to the source, using a bypassvalve. This allows the ow rate to be closelycontrolled but presents the drawback of lowenergy efciency.In gure 10 , point A is the operating pointrelating to the nominal ow rate Q n. The optimaloperating point for this circuit at a reduced owrate Q r would be point C. The bypass valvelocated downstream from the pump makespractically no difference to its operating point.The shaded area therefore represents the powerlost with this type of operation.This type of operation allows a low ow rateto be achieved without running the risk of

increasing the pump outlet pressure to anexcessive degree.

0

H (m)

0 Q (m 3/s)

A

Qn

C

Q r

Fig. 10. Varying the ow rate using a bypass valve.



Intermittent operationThis operating conguration is commonly usedto ll storage tanks such as water towers. Thepump is selected so that it operates at an optimallevel of efciency for the water height in thecircuit considered and the maximum ow raterequired. The pump is switched on during theperiods at which electricity is cheapest.The drawback of this method is that operating thepump at its maximum ow rate means the headloss in the circuit is also at a maximum.

Parallel pumpingWhen the ow rate in a circuit needs to vary to agreat degree, it is a good idea to place severalpumps in parallel. This conguration, whichis illustrated in gure 11 , makes it possible tooperate pumps as close as possible to theiroptimal level of efciency.

P1

P2

P3

Pa

Fig. 11. Pumps in parallel.

If, for example, three identical pumps areinstalled in parallel, the resulting TDH curve isplotted point by point by adding together thecorresponding ow rates at a given head.In a given circuit, there are therefore threepossible operating points, depending on thenumber of pumps in operation, as represented ingure 12 .

H (m)

0 Q (m 3/s)0

Q1 Q1+2 Q1+2+3

Fig. 12. Combination of identical pumps in parallel.

Note that Q 1+2 < 2 Q 1.This is due to the increase in system resistance(friction).


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2.5 Variable speed operation

The fundamental characteristics of a centrifugalpump are linked directly to its rotational speed. Ifwe consider the pump on its own (without takinginto account the water column height), at arotational speed N other than the nominal speed N n:

the ow rate Q is proportional to (N/N n),the total dynamic head TDH is proportional to (N/N n)

2,the power P is proportional to (N/N n)

3

Note that these pumps afnity laws areaproximation but are legitimate on a wide rangeof speed variation.

b

b

b

0

HMT

0 Q (m 3/s)

Nn

Nn x 0.8

Fig. 16. Characteristic curves of a centrifugal pump at

two different speeds .

Based on the characteristic curve at the nominalspeed, the characteristic TDH(Q) curve at adifferent speed can be plotted point by point,with the homologous points being located on aparabola, as shown in gure 16 .

P (W)

0 Q (m 3/s)0

Nn

Nn x 0.8

Fig. 17. Characteristic P(Q) curves of a centrifugal pump at two different speeds.

Similarly, the characteristic P(Q) curve can beplotted point by point, with the homologouspoints being located on a cubic curve, as shownin gure 17 .Varying the ow rate in a given circuitWe saw above that it is possible to vary the owrate of a pump operating at a xed speed using avalve placed downstream. This type of operationis illustrated in gure 9 .Figure 18 illustrates the power reductionachieved when the ow rate is varied by alteringthe pumps rotational speed. The useful powersupplied by the pump is proportional to theshaded rectangular areas, therefore one canobserve a signicant reduction in power in thevariable speed conguration.

H (m)

Q (m 3/s)0

0

H (m)

Q (m 3/s)0

0

Fig. 18. Variation of the ow rate at a constant speedand with variable speed.



Varying the rotational speed makes it possibleto use the pump at its highest level of efciencyat all times. In this case, the rectangular shadedareas are therefore directly proportional to thepower drawn by the pump. In this example,the way the power drawn varies is illustrated ingure 19 .

P (W)

0 Q (m 3/s)

H (m)

Q (m 3/s)0

0

0

Fig. 19. Variation of power with variable speed.

Taking H n as the TDH at the pumps nominaloperating point, one can dene the followingdifferent types of circuits:

Z = 0: circuit with head losses aloneZ = 0.85 H n: typical water supply

(the geometric height is apreponderant factor)

Z = 0.5 Hn: intermediate value

The top graph of gure 20 shows that to obtaina similar reduction in the ow rate from Q n toQ r , the decrease in pump speed will be differentdepending on the type of circuit. This results indifferent curves for power as a function of owrate, as shown in the bottom graph of the gure.The greater the decrease in speed, the greaterthe reduction in power.

b

b

b

P (W)

Q (m 3/s)0

H (m)

Q (m 3/s)0

0

0

Qn

P n

Hn

Q r

Fig. 20. Variation of power for different types of circuits.

Variation of power in different types ofcircuitsThe variation in the power drawn by the pumpas a function of the ow rate depends on thecharacteristics of the circuit to which it is tted.The parameter to be taken into account is theratio between the head at the pumps nominaloperating point and the head Z at a ow rate ofzero (see gure 7 ).

Z = 0Z = 0.5 HnZ = 0.85 Hn



At a ow rate of 80 % of the nominal ow rate,the power drawn at the nominal speed is equal to94 % of the nominal power.

At this same ow rate, the power drawn atreduced speed is equal to 66% of the nominalpower.

Electrical power at the nominal speed:

Power at reduced speed:

The difference in power consumption is 25.8kW,which represents an energy saving of 226 MWhper year for continuous operation, and thereforea saving of 11,300 per year, assuming a cost of

0.05/kWh.

P f P n1

mo t

---------- P Q( ) 100. 1------------ 0.94.= . = =. 98.9 kW0.95

P f P n1

mo t

---------- P Q( ) 100. 1------------ 0.94.= . = =. 98.9 kW0.95

P r P n1mo t

---------- P Q( ).= . 1

vs d

----------. 100.1------------ .= . 73.1 kW=0.66

0.931------------

0.97P r P n1mo t

---------- P Q( ).= . 1

vs d

----------. 100.1------------ .= . 73.1 kW=0.66

0.931------------

0.97

Example of a power reduction calculationLet us consider a motor-driven pump with aninstalled power of 100kW tted to a circuit withhead Z equal to half the pumps nominal TDH(Z = 0.5 . H n).We wish to compare the energy consumedat 80% of the nominal ow rate when thepump is driven at its nominal speed with thatconsumed at a reduced speed. A valve is placeddownstream to reduce the ow rate at thenominal speed.

Motor efciency: mot = 0.95 at the nominal speed

mot = 0.93 at 80% of thenominal speed

Variable speed drive efciency: vsd = 0.97

The power drawn in each case is presented bythe curves above and specied in gure 21 .

P (%)

0 Q (%)0

10094

66

20 40 60 80 100

Fig. 21. Power variation.



H (m)

0 Q (m 3/s)0

Hn

QnQn/2

Circuit Pump atreduced speed

Pump atnominal speed

Z = H n/2

Fig. 23. Characteristic curves of the pump and thecircuit.

Eco 8 softwareThis Schneider Electric software applicationmakes it possible, for general cases, to estimatethe energy savings that can be achievedby varying the speed, as opposed to moretraditional techniques such as throttling the owby downstream valves or the use of bypasscircuits.

Fig. 22. Eco8 software application.

Power reduction compared to intermittentoperation

The intermittent use of a xed-speed pump wasdiscussed above as a possible solution for the

adjustment of the average ow rate in a circuit.We will now provide an example of the savingsthat can be achieved by varying the speed.Let us consider a circuit with head Z at a owrate of zero is equal to half the pumps nominalTDH, i.e. Z = 0.5 H nThe head in the circuit as a function of ow rateis given by:

H = Z + k1 . Q 2 (1)

If Qn is the pumps nominal ow rate, we obtain:

Hn = Z + k1 Q n2 with Z = 0.5 H n

then k1 = (2)

Substituing k1 in quation (1) leads to :

H 0.5 Hn

1 + QQ

n

------2

= ( ( ) ) (3)H 0.5 H n 1 + QQn

------2

= ( ( ) ) (3)

Let the desired ow rate be equal to half thenominal ow rate. In the case of a xed-speedpump, this requires intermittent operation atnominal power, with a duty cycle of .

The average power required will therefore begiven by:

P avg = k () H n Q n

The coefcient k takes into account the pumpsefciency, which is assumed to be optimal at thenominal operating point (H n, Q n).When operating at reduced speed, the powerwill be given by:

P r = k . H . Q n/2

We can use the same coefcient k if we assumethat the pump operates at reduced speed at itsoptimal level of efciency.The head H can be calculated using equation (3)with Q = Q n / 2.

Thus, we obtain:

P r = (5 / 8) . k . () . H n . Q n

Therefore: P r = 0.62 . P avg

The use of a pump operating at reduced speedtherefore allows the power drawn to be reducedby almost 40 % in this example, without takinginto account energy losses in the motor andvariable speed drive.

0.5 H nQn

2



Variable speed parallel pumpingThe operation of the multi-pump congurationpresented above (see gure 11 ) can besignicantly improved through variable speed.

The most commonly used conguration involvesvarying the speed of one pump and operatingthe others at a xed speed. The use of avariable-speed pump allows the entire range tobe covered (H,Q), as illustrated in gure 24 .

H (m)

0 Q (m 3/s)0

Q1

Q1+2

Q1+2+3

QV

Fig. 24. Using a variable-speed pump.

The use of a variable-speed pump makes itpossible to maintain the pressure in the circuitat a set value. If the pressure drops or risescompared to the set value, an acceleration ordeceleration command is sent to the variablespeed drive. If the pumps maximum or minimumspeed are attained, one of the xed-speedpumps is started or stopped depending on thecase.The variable-speed pump makes it possibleto prevent large pressure differences, such asthose represented in gures 14 and 15.This system also allows a reduction in thenumber of motor starts and stops by preventingthe large pressure or ow rate uctuations thatoccur when a xed-speed pump is started orstopped. This reduces stress on the motors andthe risk of water hammer.

Comparison of different solutionsIn the following example, different options arecompared for ow reduction:

ThrottlingOne pump running at variable speed (lead

pump) and other pumps running at xed speed, All pumps running at variable speed

Let us consider a conguration with 3 identicalmotor-driven pumps in parallel, each one with apower at 100% ow equal to 100kW. The circuitstatic head Z is equal to half of the Total Head H n of the circuit (Z = 0.5 H n).

A comparison of the different options is madeat 70% of the total capacity, i.e. 210% of thenominal ow of one single pump.The pumps characteristics are represented ongure 25 , for different values of rotating speed.

b

b

b


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P (kW)

Fig. 25 : Pump and circuit characteristics.

With throttling, the power of each pump runningat full speed is reduced to 85 kW. The totalpower is then equal to 255 kW.The operation with one lead pump at reducedspeed is illustrated on gure 26 . As aconsequence, the ow of a pump running at fullspeed is increased to 130%, with an increaseof the absorbed power (around 7%). As the

requested total ow is 210% of the nominal ow

of one single pump, only one pump running atfull speed is necessary.The ow delivered by the lead pump is thenequal to 210 - 130 = 80% of the nominal ow.For this ow and head, the pump is running atapproximately 87% of its nominal speed, andthe absorbed power is around 66 kW.For this option, the total power is then equal to

107 + 66 = 173 kW.

P (kW)

Fig. 26 : Using one variable-speed lead pump.


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P (kW)

Fig. 27 : Using three variable-speed lead pumps.

With all three pumps running at the samereduced speed, their speed must be adjusted to85% of the nominal speed. The power of eachpump is reduced to 60 kW.The total power for this option is then equal to3 x 60 = 180 kW.

It can be concluded that the use of variablespeed brings a signicant total power reduction.In the example presented here, the lead pumpsolution has the better efciency, but the pumprunning at full speed must have a higher powerrating.



Other advantages of variable speedIn addition to the advantages alreadymentioned, the use of variable speed enablesgreater exibility in the design and running ofinstallations. In particular it allows:

The elimination of valves to adjust themaximum ow rate: if the pump is oversized,operation at reduced speed makes it possible toavoid the energy losses caused by a ow ratethrottle valve.

The reduction of noise and vibrations: the useof a variable speed drive means that the pumpdoes not have to be used for extended periodsat a xed speed that may cause resonance inpipes.

b

b

Less risk of water hammer and cavitation:these phenomena, which arise as a result ofrapid variations in pump speed, are avoidedthanks to the gradual acceleration anddeceleration made possible by the variablespeed drive,

The replacement of two-speed motors andother obsolete speed control devices that offerlow efciency.

Pump impeller life is related to impeller tipspeed. As such reducing motor speed willimprove impeller reliability

Speed control allows the pump to operateat high efciency. Operation away of BEP willreduce bearing and seal life.

b

b

b

b

Fig. 28. Schneider Electrics Altivar 61 range of variable speed drives.

To power such variable speed motors, SchneiderElectric offers the Altivar 61 range of variablespeed drives specially designed for pumpingapplications.

Additionnal pump control cards are availablefor even better control or for more complexapplications.



3. Fans

3.1 General aspects

Fans are machines designed to propel agaseous uid with a low compression ratio.They are therefore governed by the same uidmechanics laws that apply to centrifugal pumps,resulting in numerous analogies between the twotypes of machines.

There are many different fan congurations.Figure 29 presents two examples of these: acentrifugal or radial fan, and a propeller or axialfan.

Fig. 29. Examples of a centrifugal fan (left) and a propeller fan (right) together with their drive motors.

The pressure differential created by the fan canbe expressed in the form of a uids geometricheight, as is the case for pumps. Figure 30 shows the rate at which head H and power P, inthe case of a centrifugal fan, vary as a functionof the ow rate Q, at a constant rotational speed.

0 Q (m 3/s)0

Qn

H

P

Fig. 30. Typical characteristic curves of a centrifugal fan.



In most cases, the outlet circuit exhibits nonotable pressure differential (the circuits inletand outlet are both at atmospheric pressure).The shape of the circuits characteristic curvecan be explained by head losses that areproportional to the square of the ow rate. Thecircuits H(Q) curve is therefore a parabolapassing through the origin. The operating pointof the fan tted to the circuit is determined by theintersection of the two curves characterising thefan and the circuit, as indicated in gure 31 .The area to the left of the characteristic curvespeak must be avoided, as it presents the riskof instability, which would lead to ow rate andpressure oscillations, as well as abnormal noiseand considerable mechanical stress.

H (m)

0 Q (m 3/s)0

Qn

Hn

Circuit

Fan

Fig. 31. Operating point of a fan in a circuit.

3.2 Fixed-speed operation

Different types of mechanical devices are usedto vary the ow rate of fans operating at a xedspeed.Devices placed downstream from the fanThe principle involves placing a device into theair conduit to control the circuits head losses.Depending on the size of the conduit, the devicecan be either a valve or a single or multi-leafdamper.

This method is the simplest way of varying theow rate, but its energy efciency is low. Thisis illustrated in gure 33 , which shows that theow rate can be varied by modifying the circuitscharacteristics. For the two operating pointsrepresented, the useful power values, which areproportional to the shaded rectangular areas,are very close. At low ow rates, a signicantproportion of the energy is dissipated in the uid.

Fig. 32. Example of a multi-leaf damper.

H (m)

Q (m 3/s)0

0

Fig. 33. Adjusting the ow rate with a device placeddownstream .



Devices placed upstream from the fanThe aim of these devices is to alter the fanscharacteristic curve, so as to displace theoperating point while leaving the circuitscharacteristic curve unchanged. Energyefciency is signicantly improved, because at

a reduced ow rate the fan does not producesuperuous pressure.Different technologies have been developed,including wicket gates, buttery valves, dampersand guide vanes.

With all these devices, the fans characteristiccurves are altered as indicated in gure 35 .The useful power values are proportional to the

shaded rectangular areas, and we can observea signicant reduction in power at a low owrate. This alteration of the characteristic curvesis accompanied by a smaller loss in efciencythan that caused by simply placing a damper atany point on the circuit.

Fig. 34. Examples of devices placed upstream: multi-leaf damper (left), guide vanes (right).

Other devicesWith propeller fans (axial), the ow rate

can be adjusted by varying the angle of the

vanes. Because of its mechanical complexity,this technique is only used for large fans. Thismethod is highly energy efcient

The ow rate can also be adjusted usinga bypass circuit, but this is uneconomical asenergy consumption is constantly at a maximum,regardless of the effective ow rate.Remarks:In many cases, fans are oversized to obtain amaximum ow rate that is greater than the usefulow rate. Dampers are therefore installed toreduce the ow rate or the speed of the air atthe outlet, and adjusted when the installation isput into operation. This results in constant head

losses that reduce overall energy efciency. Another drawback of xed-speed operation isthat the noise level is always at a maximum.

b

b

H (m)

Q (m 3/s)0

0

Fig. 35. Adjusting the ow rate with a device placedupstream.



3.3 Variable-speed operation

The fundamental characteristics of a fan arelinked directly to its rotational speed. If we

consider the fan in isolation, at a rotational speedN other than the nominal speed N n:the ow rate Q is proportional to (N/N n),the head H is proportional to (N/N n)

2,the power P is proportional to (N/N n)

3

Based on the characteristic curve at the nominalspeed, the characteristic H(Q) curve at adifferent speed can be plotted point by point,with the homologous points being located on aparabola, as shown in gure 36 .

b

b

b

H(m)

0 Q (m 3/s)0

Nn

Nn x 0.8

Fig. 36. Characteristic curves of a fan at two differentspeeds.

Similarly, the characteristic P(Q) curve can beplotted point by point, with the homologouspoints being located on a cubic curve, as shownin gure 37 .Figure 38 illustrates the shifting of the fanscharacteristic curves at different rotationalspeeds and the resulting ow rate in a givencircuit.Varying the rotational speed makes it possibleto always use the fan at its highest levelof efciency. This means that the shadedrectangular areas are directly proportional to thepower drawn by the fans.Speed variation is therefore the method thatoffers the greatest energy efciency. Figure 39 compares the variations in power resulting fromthe three main methods of varying the owrate: device placed downstream, device placedupstream, variable speed.

P (W)

0 Q (m 3/s)0

Nn

Nn x 0.8

Fig. 37. Characteristic P(Q) curves of a fan at twodifferent speeds.

H (m)

0 Q (m 3/s)

0

Fig. 38. Varying the ow rate by varying the speed ofthe fan.

P (W)

0 Q (m 3/s)0

Fig. 39. Power versus ow rate for the differentmethods.

DownstreamUpstream

Var. speed



P (%)

0 Q (%)0

100

85

90 100

77

50

46

18

Fig. 40. Variation in power drawn.

Examples of power reduction calculationsLet us consider a centrifugal fan with a nominalpower of 100kW.The fan is slightly oversized, which means thatthe maximum ow rate in the circuit must beadjusted to 90% of the fans nominal ow rate tolimit the speed of the air at the circuits outlet.In a 24-hour cycle, a ow rate of 90% is requiredover a 12-hour period (daytime), and a ow rateof 50% is required over the remaining 12-hourperiod (night time).We wish to compare the different adjustmentmethods.Motor efciency:

mot = 0.95 at the nominal speed mot = 0.94 at 90% of the nominal speed mot = 0.89 at 50% of the nominal speed

Variable speed drive (VSD) efciency: vsd

= 0.97

The power drawn by this fan using the differentadjustment methods is illustrated by the curvesshown previously in gure 39 and detailed ingure 40 .The general power calculation formula at thenominal speed is as follows:

b

b

b

P f P n1

mo t---------- P Q( ).= .P f P n

1 mo t---------- P Q( ).= .

The energy consumed is calculated bymultiplying the power by the operating time, foreach period (day and night): 12 hrs/day x 365days = 4,380 hrs/year, assuming that the fan isrunning constantly.

Assuming a cost of 0.05/kWh, the use of

variable speed allows annual savings of 19,600compared with control using a device placed

downstream, and 5,100 compared with controlusing a device placed upstream.The Eco8 software application presented aboveallows this type of calculation to be performedfor all cases (selection of the engine power,control of the upstream or downstream ow rate,

denition of the operating speed).

Setting: kW at 0.9 x Q n kW at 0.5 x Q nDownstream 105 89

Upstream 80 48

Variable speed 84 21

At reduced speed, the formula takes into accountthe efciency of the variable speed drive:

The table below shows the results of the powercalculation for the different methods, in kW:

P r P n1

mo t---------- P Q( ).= . 1

vs d----------.P r P n

1 mo t---------- P Q( ).= . 1

vs d----------.

Setting: kWh

Downstream 852 947

Upstream 562 484

Variable speed 460 661

DownstreamUpstreamVar. speed



H (m)

0 Q (m 3/s)0

Q1+2

Hn

1+2

Q1

1

Fig. 41. Characteristic curve of two fans operatingin parallel.

H (m)

0 Q (m 3/s)0

Qn

Hn

Nr

1+2

Q r

1+2Nn

1

Fig. 42. Operation of two fans at reduced speed.

Fans operating in parallelGaseous uids can be propelled at high owrates by placing fans in parallel. Generally,identical fans are used. The resultingcharacteristic curve is obtained from the sum ofthe ow rates at equal pressure, as illustrated ingure 41 .We can see that in a given circuit, as a result ofthe quadratic increase in head loss as a functionof ow rate, the resulting ow rate with two fansis not double the ow rate obtained with a singlefan.

Flow rate Q 1 represented in gure 41 can beobtained either by operating fan 1 alone atits nominal speed, or by operating both fanstogether at reduced speed. This type of operationis illustrated in gure 42 , which represents thecharacteristic curve of both fans operating at thesame time at the reduced speed N r.



Fig. 43. Schneider Electrics Altivar 21 range of variable speed drives.

In the example represented, the reduced speedis equal to approximately 2/3 of the nominalspeed N n. Each fan therefore absorbs powerequal to (2/3) 3 times the nominal power P n of afan.The total power is therefore given by:

P t 2.23

.P n 0.6.= (3

) .P n----------

Operating two fans at reduced speed thereforebrings about an energy saving of 40% comparedwith a single fan operating at the nominal speed.

The energy saving achieved is even greater witha multiple-fan conguration operating under ahead that is very low or even zero.

For example, let us consider a congurationwhere six fans with nominal unit power P n areplaced in parallel, with no signicant outletpressure.

To obtain a ow rate equal to half the maximumow rate, three fans can be operated at thenominal speed or all six fans can be operated athalf the nominal speed.

In the rst case, the power will be:

P 1 3 P n.=

In the second case, the power will be:

P 2 6.34

= P nP 14

-----------=.= 12

.P n(3

)---------- ----------

Therefore, this is another example of energyconsumption being substantially reduced whenvariable speed drives are used to adjust fans.

To control the speed of fans, Schneider Electricoffers the Altivar 21 range of variable speeddrives, which is particularly well suited toheating, ventilation and air conditioning (HVAC).



4. Compressors

4.1 General aspects

A compressors fundamental role is to increasethe pressure of a gas from the suction pressureto the discharge pressure.

Air and gas compression has numerousapplications in industry. These include:

The production of mechanical energy fordifferent types of actuators (screw guns,pneumatic jacks, etc.),

The production of industrial gases throughliquefaction (nitrogen, oxygen, natural gas, etc.),

Air conditioning and refrigeration,

b

b

b

Aeration in wastewater treatment facilities.Requirements in terms of ow rate and pressurevary greatly, which is why different technologieshave been developed. The most common are:

Centrifugal compressors,Screw compressors,Piston compressors,Vane compressors,Turbochargers.

b

b

b

b

b

b

Fig. 44. Example of a screw compressor.

In general, the torque developed increases withthe speed, but the starting torque can sometimesbe high, as is the case with piston compressors.

As we saw above with pumps and fans, thecompressors operating point depends on thecharacteristics of the uid circuit, as illustratedin the following gure (for a centrifugalcompressor).

Pressure

0 Q (m 3/s)0

Qn

P n

Circuit

Compressor

Fig. 45. Operating point of a centrifugal compressor.



4.2 Variable-pressure operation

To cater for uctuations in system demand,all compressors are coupled with a pressureregulation device. The main adjustmentmethods, which are suited to the characteristicsof different technologies, are recalled below:

Starting and stopping the compressor.Intermittent operation is suited to low-powerdevices that can be started frequently withno unacceptable drawbacks. During phasesof operation, the compressor operates at itsoptimum level of efciency.

Recirculating or evacuating excess ow. Thisoperating method is only applicable to low-power

b

b

systems, because of its low energy efciency.Partial or total load shedding by shutting

the compressors suction ap or valve. Thecompressor operates at reduced or zero load,thus reducing the electrical power drawn.

Placing several units in parallel. The numberof compressors in operation can be adjusted ondemand.

Varying the compressor speed. This method isthe most energy efcient.

b

b

b

4.3 Variable-speed operation

Varying a compressors speed is a methodwell suited to most technologies. It presents anumber of major advantages:

Gradual start up: no current peaks, reducedmechanical stress,

Precise pressure regulation: the compressorsspeed and therefore ow rate can be adjustedon demand, which reduces the amplitude ofpressure uctuations and the size of the buffertanks required,

Optimum efciency: operating without headlosses in the circuit allows energy losses to bereduced.Speed variation is also well suited tocompressors operating in parallel. In general,only one of the compressors is driven at variablespeed, while the others operate on an on-offbasis.The gure below shows typical power variationcurves, when using different ow controlmethods.

b

b

b

P (%)

0 Q (%)0

100

20 40 60 80 100

2040

60

80

Fig. 44. Comparison of compressor control methods.

ModulationOn-Off Var. Speed



5. Conclusion

The use of variable speed in hydraulic machinessuch as pumps, fans and compressors is thekey way of reducing energy consumptionin numerous industrial and commercialinstallations.The energy savings are particularly signicant ifreduced ow rates are used on a frequent basis.The sums invested in variable speed drivesare paid back very quickly and subsequentlygenerate considerable savings.

As well as energy savings, variable speeddrives bring many other advantages to theseapplications. Thus, mechanical constraints,such as water hammer, cavitation and torque

surges, are greatly reduced by the gradual andcontrolled acceleration and deceleration ofthe motor. The life expectancy of equipment istherefore extended. In addition, the managementof the process is signicantly improved andsimplied, because it is possible to nely adjustthe uid ow rate and pressure.

An in-depth discussion of the operation ofpumps, fans and compressors was beyond thescope of this document. Nevertheless, the keyprinciples have been presented, together withexamples that illustrate the extent of the energysavings that can be achieved through the use ofvariable speed drives.

Technical booklets of the International WaterOfce, Limoges, France

"Entrainements lectriques vitesse variable"(Variable speed drives), Promthe, SchneiderElectric

"Improving pumping system performance", USDepartment of Energy

"Systmes de ventilation" (Ventilation systems)technical guide, Hydro-Qubec, Montreal,Canada

b

b

b

b

"Systmes de compression et de rfrigration"(Compression and refrigeration systems)technical guide, Hydro-Qubec, Montreal,Canada

"Energy Efciency Guide Book", Bureau ofEnergy Efciency, India

"Energy savings with electric motors anddrives", AEA Group, UK

b

b

b

Appendix 1: Bibliography


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