ice-e info pack 6 pumps

8/13/2019 ICE-E Info Pack 6 Pumps

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Pumps

Medias transporting energy have to bepumped. This Info Pack covers mechanicalpumps.

Power consumptionThe theoretical power required for pumpingfluids is determined by:

Where:P is the power input in kW

is the density of the fluid in kg/m 3 Q is the flow rate in L/sg is gravity = 9,81m/s 2 H is the pressure difference (head) across the

pump in m As and g are constant, the powerconsumption is linear dependent on only twoparameters flow and pressure differenceacross the pump. So to minimize the powerconsumption the flow and pressure differencehave to be minimized.The flow is given by the required cooling.The pressure difference is determined bypressure drop in the distribution system whichconsist of

friction loss (dynamic head)height difference in the system(static head) in open systems

To calculate the electrical power consumptionthe efficiency of both the drive line (electricalmotor and if installed the VSD (Variable

Speed Drive) and the pump:

The amount of coolingto be pumpedIn general the fluids used in cooling systemscan be either phase changing refrigerants,such as ammonia and CO2, or subcooledliquids such as glycol or water. Subcooledliquids that does not change phase are oftenreferred to as secondary refrigerants. Whensecondary refrigerants contain an antifreeze,such as salt or glycol, the secondary

refrigerant is often called brine (even though itin scientific sense only covers the salt basedsolutions).

- Phase changing refrigerants obtain

heat as they evaporate at the coolingsurface when changing from liquid togas. In this setup the cooling surfaceis called an evaporator. Thetransition of liquid to vapour happensat a constant temperature meaningthat the evaporator has the sametemperature on the entire surface.The phase change requires a lot ofenergy meaning that a small amountof liquid can obtain a lot of heat. Asthe liquid evaporates it returns to thecooling plant as vapour where it

condenses and release the energy(heat) obtained during theevaporation. The liquid is thenpumped back to the cooling surfacewhere it evaporates once again.

- Secondary refrigerants obtain heatby simple temperature change of theliquid: The liquid enters a coolingsurface at a low temperature andreturns warmer, meaning that thecooling surface is coldest were theliquid enters and warmer where itexits. The increase of temperature inthe liquid is proportional to the heatremoved at the cooling surface. Theheated liquid is then pumped to acooling plant, cooled to the initialcold temperature and circulated backto the cooling surface.

Transporting heat using phasechanging refrigerants

As mentioned above, the phase change fromliquid to vapour takes up a lot of energy. Thespecific amount depends on refrigerant typeand to some extent the evaporatingtemperature. In general, the cooling capacitycan be calculated using the formula:

To minimize the

powerconsumption inpumps the flowand pressuredifference haveto be minimized

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Refrigerant pump(Grundfos)

Cooling water circulationpumps(Grundfos)


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Where: is the cooling capacity in kW is the mass flow of refrigerant in kg/sh vap is the heat obtained per kg refrigerant

that evaporates

Evaporating ammonia in a cooling surface at -30 C takes 925 kJ/L of ammonia thatevaporates. For comparison CO 2 only takes325 kJ/L, meaning that using CO 2, rather thanammonia, require bigger pumps. The floodedevaporators (cooling surfaces) used in suchsystems often have a circulation rate higherthan 1, meaning that the amount of refrigerantthat is applied exceeds the amount thatevaporates. Having a higher circulation ratemakes the heat exchanger more efficient andmust be considered when calculating thenecessary pump capacity.

Using the above example at -30 C and acirculation rate of 1,3 (meaning that anadditional 30 % of refrigerant is pumped), it ispossible to calculate the required volume flowat a cooling capacity of 100 kW:Using ammonia as refrigerant:

Using CO2 as refrigerant:

At a given cooling demand the pump capacityis determined by the circulation rate andcharacteristics of the specific refrigerant.

Transporting heat usingsecondary refrigerantsUsing secondary refrigerants, heat is obtainedas temperature in the fluid. This means that acooling capacity is determined by temperaturedifference, flow rate and the specific heatcapacity of the fluid. In general the coolingcapacity can be calculated using the formula:

Where: is the cooling capacity in kW is the mass flow of secondary fluid in kg/sCp is the specific heat capacity of the fluid in

t is the temperature rise of the fluid in kelvin

The factor 0,86 holds the Cp-value of waterand volume flow rather than mass flow. Thecooling capacity is proportional to both flowrate and temperature rise of the fluid. A givencooling capacity can be reached using either ahigh temperature difference, a high flow rate orcombination. Cooling systems often use highflow rates and low temperature differencesmeaning that bigger pumps and pipes areneeded whereas heating systems the energyis often transported with high temperaturedifferences meaning low flow and therebysmall pipes and small pumps.The formula above is used to calculate theflow rates in a 100 kW system with a

temperature difference of 6 K. The firstcalculation shows the required flow rateusing water as secondary refrigerantwhereas the second shows the flow rateusing a 50 % glycol mixture. In the second

calculation, the multiplication factor ishigher as the glycol mixture has a lowerspecific heat capacity.Water:

50% E-Glycol (-30 C):

Compared to the evaporating refrigerantsabove the flow rates are 10-40 timesgreater when using secondary refrigerants,meaning that pipes and pumps are verydifferent. The flow rates can be reduced byincreasing the temperature differences butas this result in a lowering of theevaporating temperature it will increase theenergy consumption of the cooling plant itis important to find the right balancebetween temperature and flow.

Type of pumps A lot of different types of mechanicalpumps exist such as

CentrifugalMembrane

GearImpeller

Only centrifugal pumps will be covered bythe following as this type by far the mostused in refrigeration systems.

Basic pump theoryThe pump curve

All pumps have a certain characteristicregarding flow and pressure. Thischaracteristic is described by a so-calledpump curve. Common centrifugal pumps

have pump curves similar to the one in thefigure 1.The blue line shows the pumpsperformance depending on pressure(vertical) and flow (horizontal): A centrifugalpump is not able to provide a high pressureat the highest flow rates and vice versa.The pump curve change, if the speed ofthe pump is changed. This is explainedmore thoroughly in the paragraphconcerning power consumption.

The system curveIn order to determine the so-called dutypoint the characteristics of the system itself

must be considered as well. This is oftendetermined by friction losses in pipes,valves, heat exchangers etc. and is

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Figure 1

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Figure 2


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described by a system curve shown on thefigure 2.The yellow line shows the systemscharacteristics on pressure (vertical) and flow(horizontal). The figure shows that thepressure required increases exponentiallywith increasing flow rates. A system withsmall pipes and narrow valves (high frictionlosses) has a steep system curve whereas asystem with big pipes and large valves has amore flat system curve.

The duty pointThe duty point of a system is where the pumpcurve intersects the system curve. The flowrate in the system and the pressuredifference across the pump is determined bythis point. This is shown in the figure 3.If other flow rates are required either thesystem curve, the pump curve or both must

be altered: When pumps with fixed speed areused it is not possible to change the pumpcurve meaning that different flow rates areobtained by changing the system curve. Thiscan be done using a control valve. Closingthe valve will make the system curve steeperand the new duty point will be at a higherpressure but less flow, whereas opening thevalve will make the system curve less steepand a new duty point will be at a higher flowrate but less pressure. This is illustrated onfigure 4.

Sys tem curv es in d i ff e ren ttypes o f sys t emsThe system curve is highly depending on thesystem design and the fluid to be pumped(viscosity)The static head (and hereby the energyconsumption) varies dependent on the type ofsystem.

Negative static headIn systems using evaporating refrigerantsfrom a liquid separator, there must be a statichead at the suction side to avoid cavitation(cavitation is explained in the nextparagraph). The principle of such a system isseen in figure 5.The liquid refrigerant inside the separator isat the saturation point meaning that a drop inpressure will cause the liquid to boil. Inside apump a lower pressure exist compared to theinlet and to avoid boiling inside the pump(cavitation), the pump is placed at the lowestpoint meaning that the refrigerant at thesuction side is subcooled by the pressure H1.

As the liquid enters the cooling surface(evaporator) it evaporates and most of itreturns to the liquid separator as vapour. Inthis system there is a liquid column (H1) atthe suction side of the pump and a smallerliquid column (in the evaporator) at thedischarge side (H2). The static head isdetermined by the difference in H1 and H2.In this case the static head is negative as H1is bigger than H2. In others words gravity

helps the pump thus requiring lesspressure from the pump. When the statichead is negative, the system curve startsbelow 0 as shown in figure 6. As thestatic pressure is negative, the totalpressure required by the pump is thepressure from friction loss, minus thestatic head.

Secondary refrigerantsCooling systems using secondaryrefrigerants are either closed or opensystems. In closed systems the fluid issealed from the surroundings throughoutthe system. In open systems the fluid isin contact with the atmosphere thusmaking it an open system. This is thecase in systems using cooling towers orlarge storage tanks that are notpressurized.

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Initial dutypoint

New dutypointOpeningvalve

New duty pointClosing valve

Figure 4

H

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Figure 6

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Figure 7

Gasreturn tocompres

Liquid injectionfrom expansionvalve

Figure 5

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Figure 3


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No static head (closedsystems)In closed systems the liquid columns arealways the same on suction and pressuresides of the pump. This mean that the onlypressure the pump should overcome is thefriction loss independent of different levelsin the system. Such a system is shown onfigure 7.

The secondary refrigerant is pumped into acooling surface where it obtains heat, andthen circulated to a cooling plant beforeentering the pump again. The staticpressures before and after the pump areidentical meaning that the system curve inclosed systems always starts in 0 asshown in Figure 8.

As the system curve starts at 0 the totalpressure required from the pump is thepressure drop from friction losses.

Positive static head (opensystems)In open systems there might be differentstatic pressures dependent on the levels ofthe system. The example in figure 8 showsan open system with a cooling tower. Thepump circulates water from a receiver to acooling surface and afterwards into acooling tower at the roof. At the coolingtower the pressure of the water is equal tothat of the atmosphere making it an open

system. As the liquid column at thepressure side of the pump (H2) is higherthan the column on the suction side (H1),the static pressure in this system is positiveand determined by the pressure differencebetween H1 and H2. In this case gravitycounter acts and the pump must providethe extra pressure to overcome gravity.When the static head is positive, thesystem curve starts above 0. This is shownin figure 9.

NPSH and CavitationCavitation occurs when a liquid turns tovapour and implodes back to liquid insidethe pump. This occurs when the pressureof the liquid is close to the saturationpressure. In this case a pressure drop atthe suction side of the pump could causethe fluid to evaporate and create bubblesinside the pump. As pressure rise throughthe pump the vapour will turn back to liquidand the bubbles will collapse. Cavitationcan often be recognized by a distinct soundas if the pump was pumping gravel.Cavitation reduces the pumps capacity andefficiency, and in most circumstances theimploding bubbles will cause excessive

wear on the pump impellers. There is a

high risk of cavitation in systems usingevaporating refrigerants, as these arevery close to the saturation pressure.Secondary refrigerants (e.g. water andbrines) at low temperature on the other

hand, are very far from the saturationpressure and will only cavitate at a verylow pressure (which can be made is aninlet valve is being closed!).

The pressure drop inside a pumpdepends on3 the design of that specificpump and because of this different pumpstend to cavitate at different pressures andflow rates.

The minimum suction pressure requiredfor a specific pump can be read from thepumps NPSH-curve.

NPSH stands for net positive suctionhead and accounts for the balance inpositive (gravity and absolute pressure)and negative pressures (friction losses) ata certain flow rate. Effectively, NPSH isthe needed minimum difference of thepressure at the suction side of the pumpand the fluids saturation pressure and allpumps require a certain NPSH not tocavitate. The required NPSH is found inthe pumps specifications and a curve for aspecific pump could be like the one thefigure 10.

The NPSH-curve shows the minimumNPSH required at different flow rates for aspecific pump. Higher flow rates tend torequire higher NPSH, as the pressure lossinside the pump increases with the flow.In order to avoid cavitation, the pressureof the fluid on the suction side of thepump must exceed the saturationpressure according to the NPSH-curve.Note that dynamic pressure losses mustbe considered as well.

As stated before in pump circulatedsystems more liquid refrigerant is pumpedinto the distribution lines to theevaporators than evaporated. If problemsarise like too little refrigerant supplied toan evaporator one could jump to theconclusion just to raise the flow into thesystem. But due to the shape of the

NPSH curve one could end up havingcavitation in the pump.

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H2

H1

Figure 8

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Static head

Figure 9

NPSH(m)

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Figure 10


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Energy consumption

As mentioned previously the theoreticalpower consumption of a pump is given by

As long as the inlet (secondaryrefrigerants) or evaporation (directsystems) temperature is not changed achange in the cooling (or heating) demandis adjusted by regulating the flow. Atreduced flow the pressure loss from frictionis also reduced meaning that the pumpspower consumption should decrease.When fixed speed pumps are used, theflow is adjusted using valves meaning thatthe pressure will increase or decrease asthe flow is adjusted. By adjusting the flowrate using valves, the system is adjusted to

fit a certain pump rather than adjusting thepump according to the system. This isillustrated on figure 4.The figure shows how adjusting valves willchange the flow rate when a certain pumpis used.

Instead of changing the system pressuredrop, the pump curve could be altered byadjusting the speed of the pump. Lookingat the system curve with the valve open, itis clear that less flow is possible at muchlower pressure levels. This means that theduty point with the valve closed could havebeen met using a lot less pressure thus

reducing the energy consumption. This isillustrated in figure 11 where a new pumpcurve (at low speed) meets the systemcurve with the valve open.

The figure shows how the new duty point isreached by changing the pump curveratherEquations of affinityChanging the speed of a centrifugal pumpaffects pressure, flow and energyconsumption according to the equations ofaffinity shown below (index b = after anda = before the change):

The ratio of the flow changes linear to theratio in speed:

The ratio in head and power consumptionare squared and cubed, meaning that thepressure is reduced to at half speed andthe power consumption to 1/8:

Because of this variable speed controlledpumps are particular suited for systemswhere flow and pressure are fluctuating.Control modes for variable speed pumpsIn order to exploit the reduction in energy

consumption fully, the components in thesystem must be able to cope with lowerdifferential pressures at low flow rates.This is not always possible and becauseof this most variable speed pumps can becontrolled in either constant pressure orproportional pressure modes.

Constant pressureConstant pressure means that the pumpwill keep a certain pressure according to aspecified set point and adjust the speed tomaintain the pressure at this set point asthe flow rate fluctuates.Figure 12 below shows the duty points

using constant pressure control mode.

Proportional pressureIn proportional pressure, the pressure inthe system is reduced proportional to theflow rate. This control mode means higherenergy savings at low flow rates as thepressure is reduced even further. Theinstaller chooses two set points for thepressure levels: One pressure level at thehighest flow rate and another at zero flow.The pump will reduce the speed so thatthe pressure never exceeds the set point.The duty points are at the same pressureuntil the pump reaches full speed at acertain flow rate. At higher flow rates, dutypoints will follow the pump curve at fullspeed.The pump will adjust the pressureproportional according to these two setpoints.

Figure 13 shows the duty points usingproportional pressure control mode.The pump adjusts the speed automaticallyto vary pressure at different flow rates asdetermined by the two set points.Proportional pressure is the best way toreduce the energy consumption of a pumpin systems with fluctuation flow rates.

The pump adjusts the speed automaticallyto vary pressure at different flow rates asdetermined by the two set points.Proportional pressure is the best way toreduce the energy consumption of a pumpin systems with fluctuation flow rates.The pump adjusts the speed automaticallyto vary pressure at different flow rates asdetermined by the two set points.Proportional pressure is the best way toreduce the energy consumption of a pumpin systems with fluctuation flow rates.

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New dutypoint

Initial dutypoint

Figure 11

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tpoint

Flow @max speed

Figure 12

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etpoint

Flow @max speed

Setpoint

Figure 13


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The work associated with this information pack has been carried out in accordance with the highest academic standards and reasonable endeavours have been made to achieve the degree of reliability andaccuracy appropriate to work of this kind. However, the ICE-E project does not have control over the use to which the results of this work may be put by the Company and the Company will therefore be deemedto have satisfied itself in every respect as to the suitability and fitness of the work for any partic ular purpose or application. In no ci rcumstances will the ICE-E project, its servants or agents accept liability howevercaused arising from any error or inaccuracy in any operation, advice or report arising from this work, nor from any resulting damage, loss, expenses or claim. ICE-E 2012

Nomenclature Not updated

A cross sectional area ofentrance, m 2 b thickness of door frame, mg acceleration due to gravity, 9.81

m s -2 Kf,L Correction factor, dimensionlessH height of entrance, mI Infiltration rate, m 3 s -1

t time, sTo, T i temperature outside and inside

colds store, oC

V volume of air within the room,m 3

Greek letters

i, o, avg density inside andoutside cold store andaverage, kg m -3

PED=Pedestrian doors

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Fixed speed

Prop. pres.Highest

flow rate

Figure 14

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Figure 15

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Systemcurve

Pumpcurve

Figure 16

systems trouble can occur when pipedistances (and hereby friction loss)between a pump and the different valvesare varying. In such applications it isimportant to register the availablepressure at the most critical points in the

system and control the pump speedaccordingly. It can be very complicated tofind the lowest possible working pressurein a system, and as the cycle betweenmax and min cooling demand often is oneyear, a change of the setup might causetrouble at a much later point. Because ofthis, it is often recommended to use aconstant pressure control mode and onlyuse proportional pressure with caution.However the benefits using proportionalpressure should be exploited in newsystems or at system rebuilds, where thecharacteristics of pipes and valves areconsidered anyway.

For more information, please contact: Lars Reinholdt ([email protected])

Figure 14 shows two of the duty pointsusing a fixed speed pump (blue) comparedto the proportional pressure control mode(green).

At the duty point at around half of the

highest flow rate, the pump slows downusing proportional pressure, meaning thatthe pressure in the system is actually lessthan half compared to a fixed speed pump.

As the pump slows down, the valves in thesystem will open further to provide theproper flow rate. The proportional pressurecontrol mode actually affects the systemand force the valves to open meaning thatthe system require less pressure at a givenflow rate thus making the system curveless steep compared to a fixed speedinstallation.

Ideally the flow should be adjusted bypump speed alone meaning that all dutypoints would be on the system curve at thehighest flow rate as illustrated on figure 15.

The figure shows a system with no valvesmeaning that the system curve is static andthe required pressure at a given flow isalways the same. This is not the case inmost applications where pumps provideflow for several circuits each controlled bya valve. In these applications the systemcurve will change depending on thedemand which means that the pump mustprovide a pressure just above the requiredin order to detect the fluctuations andadjust the speed accordingly. This can beoptimized using the proportional pressurecontrol mode and the two set points areadjusted to an exact fit of a given system.

Figure 16 shows a proportional pressuresetup where the pressure is kept justabove the required at all flow rates.

In this setup the pump will provide apressure just high enough to detectfluctuations in the system as valves openor close. This control strategy will forcevalves to open as much as possible andavoid energy consuming pressure lossesyet provide sufficient pressure in allsituations. Energy wise, the best control strategy inmost systems is a combination ofcontrolling both valves (system curve) andpump speed (pump curve) so that thepressure is reduced along with the flowrate. But it can be difficult to find the correctratio if a systems characteristics is not verywell known. A low working pressure cancause valves to lose their function andhereby cause inadequate cooling. In big

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