Energy consumption in pumps friction losses

Transcription

1 Energy consumtion in ums friction losses In this secon article in a series on energy savings in ums, Hans Vogelesang, irector of Netherlans-base esign consultancy PumSuort, eals with some ractical asects of flui mechanics. In articular, he looks at the influence of ie iameter, bens an other features of iing systems on friction loss an energy consumtion. I n the first article in this series1, the concets of geoetic ifferential hea an ossible ressure ifferences in the suction an ischarge tanks were iscusse. Together these etermine the ifferential ressure to be elivere by a um to ensure the require flow. However, the introuctory article i not eal with the effects of friction losses, which will now be gone into more extensively. Effects of friction losses Friction losses refer to the ifference in ressure neee to overcome the ressure ro uring flow through ies (Figure 1). Such losses only occur as a result of ynamic movement cause by flow; consequently, the ressure ifference associate with this rocess is referre to as the ynamic ifferential ressure. Friction losses can only occur when flow actually takes lace. As was shown in the revious article, the um must rouce a sufficient ifference in ressure to brige the geoetic ifferential hea an ossible ressure ifferences in the tanks. This is known as the stationary ifferential ressure, referring as it oes to the ifference in ressure require for motionless fluis. Once a flui begins to flow through a ie it will therefore be necessary to overcome the extra ifference in ressure cause by friction losses. This will have to be rovie by the um, in aition to it overcoming the stationary ifferential ressure. The um s ifferential ressure will always be mae u of the sum total of the stationary an ynamic ifferences in ressure. As friction losses always reresent an absolute energy loss, they nee to be reuce as much as ossible, if energy is to be save. As exlaine reviously, a 1 bar ressure ifference at a 100 m 3 /h um caacity costs 4 kwh. A reuction in friction loss from 1 bar to 0.5 bar will result in the energy loss being reuce by 50%, thereby saving 2 kwh every hour. This is why it is ossible to achieve significant savings, esecially in (near) continuous oerations (365 ays er year, 24 hours er ay = 8760 hours er year!). What influences friction losses? It is common knowlege that friction losses have a significant effect on our aily lives. For instance, we are all aware that cars nee more energy an therefore more fuel at higher sees than at low sees. Luckily for us, car esigners have alreay one a great eal to limit this: they have their esigns teste in win tunnels an try to reuce rag as much as ossible by aating the esign. However, esite otimal esigns, friction still occurs an increasingly so at higher sees. Flow irection in f out The velocity of the flui is also of great influence in ie flow. So, just as a car esigner aats the esign to reuce rag, the um system esigner will have to o the same. How can this be achieve when testing in a win tunnel is not an otion? L f = - in Figure 1. During ie flow, friction loss causes the ownstream ressure to fall. out Fortunately, much research into friction losses has alreay been one. This knowlege an exerience has been lai own an ublishe in hanbook form. One examle is the much-use Crane Technical Paer TP-410 entitle Flow of Fluis through Valves, Fittings an Pie 2. This well-known English text / Elsevier Lt. All rights reserve WORLD PUMPS Aril 2008

2 gives the stanarize metho an etails neee to calculate ressure loss resulting from flow in ie systems. Designer s worlwie use Crane Technical Paer TP-410 as the stanar work in the fiel of flow in ies, valves an other in-line comonents such as bens an reucing ieces. It rovies not only the theory, together with grahs an formulas, but also ractical worklace examles an reaily useable resistance factors. The latter are base on extensive testing an enable recise calculations to be mae regaring which equiment to emloy in a articular system. TP-410 was originally ublishe in Nowaays, the more-recent eitions are eeme to be essential reaing for engineers, esigners, researchers an stuents in all areas of technology that eal with ie flow. A significant number of these conitions are, however, etermine by the mechanics of the rocess an cannot be altere. For examle, the volume an viscosity of the flui to be transorte are two factors that are largely reetermine. Extremely viscous fluis coul be heate beforehan, but the ownsie is the extra energy this woul consume. However, in the case of fluis that nee to be heate uring the rocess anyway, it oes make goo sense to o this before uming actually begins. Equation 5 in Box 1 shows how friction losses can be reuce when the recommenations below are ahere to. Reuce ieline length The length of a ieline is crucial, as Equation 5 shows. Friction loss can be reuce by esigning a system that significantly limits the transortation istance. Positioning a storage tank Reucing friction loss In all cases of ie flow a ressure ifference is require to overcome friction loss. Two factors lay a art in the ressure ifference. Firstly, there is the friction that results from the articles flowing along the rough inner ie surface an from the ifference in velocity of the iniviual articles. In the case of turbulent flow, the articles will aitionally all be moving in ifferent irections, causing extra turbulence losses. This is known as viscous friction an can be calculate using the Darcy-Weisbach equation (see Box 1). Seconly, there is the extra ressure ifference require to rouce a change in the flow (flow irection through bens an T-ieces, an acceleration through reucing ieces, non-return valves, etc.). How these friction losses can be calculate is shown in etail in Box 1. Any such calculations will be affecte by a large number of factors. If you wish to save energy then the friction losses must be ket as low as ossible by selecting the most favourable conitions for any articular esign. WORLD PUMPS Aril

3 Box 1. Flow calculations for comletely fille circular ies Velocity calculation The following formula is use to work out the average velocity of a flui: ν = Q/A [Equation 1] where ν = velocity (m/s); Q = volume of flow (m 3 /s); an A = internal crosssectional area of the ie (m 2 ). Friction loss calculation Friction loss is mae u of two factors: the ifference in ressure require to overcome the viscous friction an the ifference in ressure require to change the kinetic energy at oints of irectional change (bens, etc.) an acceleration (reucing ieces, etc.). a) The ifference in ressure require to overcome the viscous friction can be calculate using the Darcy-Weisbach equation: = ½ρν2 (λl/) [Equation 2] where = ressure ifference ue to friction in N/m 2 (1 bar = 10 5 N/m 2 ); ρ = flui ensity (kg/m 3 ); ν = velocity (m/s); λ = friction factor (imensionless number, also exresse as f); L = ie length (m); an = ie internal iameter (m). The friction factor λ can be etermine from Mooy s iagram (Figure 2) an is base on the Reynols number an the relative roughness of the insie of a ie. The imensionless Reynols number, Re, is calculate as follows: Re = ν /ν [Equation 3] where ν = kinematic viscosity in m 2 /s (1 cst = 10-6 m 2 /s); an ν an are as reviously efine. Because the Reynols number is eenent on both viscosity an velocity, the friction factor will be mainly etermine by the flui s viscosity, the flow regime (laminar or turbulent) an the roughness of the insie of the ie. b) Using the following formula it is ossible to calculate the extra ifference in ressure require to rouce a change in the flow (change of flow irection an/ or velocity) in each searate comonent (ben, reucer, etc.) extra = ½ρν 2 k [Equation 4] where extra = extra ifference in ressure (N/m 2 ); k = resistance coefficient for a articular comonent (imensionless number, also exresse as ζ). c) The total friction loss in a ie that has several fittings, bens, attachments an the like is calculate using the formula below, by aing the ressure ifference in Equation 4 to that cause by viscous friction: f = ½ρν 2 [(λl/) + k] [Equation 5] where f = total friction loss (N/m 2 ); k = the total of the resistance coefficients of all the bens, etc.; an the other arameters are as reviously efine. close to the system coul result in the ie length being halve, thereby cutting energy losses by 50%. Pie iameter selection Accoring to Equation 5, the flui velocity will have a quaratic effect on friction losses, reucing friction loss by a quarter when the velocity is halve. The reverse will of course also aly: ouble the velocity an the ifferential ressure will increase fourfol. The esigner shoul therefore concentrate on limiting the velocity in the ie. Note that the rate shoul not be too low, as this woul require the ie iameter to be isroortionately large for a articular volume an the cost of the ies unnecessarily high. In its System Efficiency hanbook 3, Euroum recommens that when etermining ie iameter an energy-efficient choice shoul be base on the following criteria, which aly to fluis with a maximum viscosity of 100 centistokes (cst) an a ensity ranging between 700 an 1200 kg/m 3 : Discharge ies: velocity between 1.5 an 2.0 m/s; Suction ies: velocity between 0.75 an 1.25 m/s. By emloying these esign criteria it will be easy to select the correct ie iameter for a secific volume of flow. Sticking to these recommenations will rovie a number of benefits: not only will energy be save but it will also result in other imrovements, such as revention of cavitation an vibration an reuce incience of water hammer an erosion. However, um users shoul note that it will not generally be in the interests of contractors to fit an energyefficient system. If it is left to them, a smaller ie size will be installe for cost-cutting reasons, as all the shut-off valves, non-return valves an filters will then be cheaer too. This ractice is iametrically oose to the user s interests, ue to the increase friction losses an the corresoningly higher energy costs. It is therefore imortant when commissioning work to raw u clear an etaile secifications. Use hyraulically smooth ies Mooy s iagram (Figure 2) shows that the friction factor for turbulent flow is etermine by the Reynols number an the relative roughness of the ie. The relative roughness is the ratio between the actual roughness an the ie iameter. For a given level of finish, the relative roughness will ecrease as the ie iameter increases, that is, the ie becomes hyraulically smoother. Larger iameter ies can therefore be mae from concrete with a rough finish. Smaller iameter ies shoul be mae of materials with a smooth finish. It is imortant to take into consieration that the roughness will increase over the course of time ue to corrosion, scale eosits, etc. Avoi short-raius elbows Using long-raius elbows will ensure that the ifferential ressure require to steer the irection of flow can be lower than when using shar, rightangle connectors, such as elbow joins or Y-attern T-ieces that are wele together at right angles. As the raius is increase, the resistance coefficient (k or ζ) cause by the ben will be reuce. Avoiing short-raius elbows can therefore contribute to a reuction in energy loss WORLD PUMPS Aril 2008

4 Reuce bens; avoi arches The number of bens use can often be ket to a minimum by esigning the system carefully. It is also always imortant to avoi having high oints in ielines where ockets of air coul collect, which woul be extremely ifficult to get ri of. Any such arches are unnecessarily isrutive, as any air ockets will interrut the flow. This in turn will cause a comlete loss of the sihon action an i n c re a s e t h e re s s u re ro. Use free-flow valves Shut-off valves shoul be built into the system at various oints to allow for the rocess to be stoe an starte, an for secific arts of the system to be isolate for reairs or maintenance. By using valves of a tye that fully retracts from the line of flow as is the case with a gate valve the ie flow will not be obstructe, thus minimizing friction loss. However, if the valve remains in the line of flow as is the case with a butterfly valve or a globe valve then there will be some isrution to the flow, even when the valve is comletely oen, resulting in extra friction loss. The resistance coefficient is usually much lower with a gate valve than with a butterfly valve or a globe valve. The same hols true for other valve tyes too, such as non-return an foot valves. The resistance coefficient with a hinge valve will of course be much lower than with a full-lift valve. A significant contribution to energy saving can therefore always be mae by using those free-flow valves that have the lowest ossible resistance coefficient (k or ζ). Minimize ressure loss throughout All in-line equiment, such as heat exchangers, filters an static mixers, will require a ifference in ressure to overcome the friction losses. If there is a choice of roucts then this ifference in ressure shoul be the eciing factor when making your selection. For instance, a late heat exchanger that is more exensive than a ie heat exchanger is efinitely worth investing in if the rice ifference can be offset later against a reuction in energy losses. What savings oes the ifference make? Flui velocity the velocity of the flui through a ie greatly affects friction loss, which can be reuce by keeing the velocity own. Within aroriate limits, a larger ie iameter shoul be use for a given volume, as iscusse above. Take the ex amle of a quantity of water that has to be ie 100 metres at 100 m 3 /h. The ie has five bens (with a stanar raius of 1.5 iameter) an one gate valve. Using stanar steel ies a choice of ifferent iameters can be mae, as illustrate in Table 1. The energy loss is base on efficiency rates of 75% for the um an aroximately 90% for the rive. Energy costs are base on oerating 24 hours a ay, 365 ays er year (8760 hours) at a resume cost of 0.10 er kwh. This table clearly shows that Euroum s recommenations lea to efficient energy use. It can easily be aate to suit any iniviual situation. Conclusion Energy can be save by fining ways to reuce friction loss at the esign stage. It is of crucial imortance to limit the velocity by choosing ies with the correct iameter. It will frequently be necessary to choose moreexensive comonents with a lower WORLD PUMPS Aril

5 LAMINAR CRITICAL TRANSITION TURBULENT COMPLETE TURBULENCE, PIPES Friction Factor = f = 64/R Relative Roughness ( ) L D h L = SMOOTH PIPES Re - Reynols Number = = = Figure 2. Mooy s iagram can be use to etermine the friction factor (source: Crane TP-410M). friction loss. However, the initial extra outlay can usually be quickly recoue as less energy will be use when the system is in oeration. On to of that, the um can be smaller an cheaer, since the require ifferential hea is lower because of the reuce friction losses. It is imortant to realize that the contractor builing the system will not benefit from any savings that you, the user, are going to make from lower energy costs. Take care to avoi a situation in which the oosing interests of the two arties coul lea to higher friction losses an greater energy consumtion. Equally, false economies shoul be avoie with any investments. References [1] H. Vogelesang, An introuction to energy saving in ums, Worl Pums, No. 496, , (2008). [2] Crane Valve Grou, Flow of Fluis through Valves, Fittings an Pie, Crane Technical Paer TP-410M (metric eition) [available from Engineere Software; [3] System Efficiency A guie for energy efficient rotoynamic uming systems, Euroum, (2006). CONTACT Hans Vogelesang PumSuort (NL) Charley Toorostraat RE Henrik Io Ambacht The Netherlans Tel: Fax: info@umsuort.nl Table 1. Flow an energy loss ata for stanar commercial ies* Nominal iameter Velocity (m/s) Friction loss (bar) Energy loss (kw) Energy costs ( /year) ND ND ND ND ND ND ND ND *For the flow of water at a caacity of 100 m 3 /h. About the author Hans Vogelesang is irector of PumSuort, an ineenent consultancy for the esign of um systems in Henrik Io Ambacht, The Netherlans. He is also a lecturer in um engineering at several eucational institutes. This article was originally ublishe in Dutch in Fluis Processing Benelux WORLD PUMPS Aril 2008