The Losses Of Turbomachines - Nathi
The Losses of Turbomachines11.Internal Losses: Losses which take place in the innerpassages of the machine and directlyconnected with rotor or flow of themedium and which are adding heat to theflow medium2. External losses: Losses which appear outside of the innerpassages of the casing and which do nottransfer the generated heat directly intothe flow medium1
Internal Losses Due to the inner losses,, the total energygy exchangeg betweenthe rotor and the flow medium is altered. In the case of the working machine more energy has to beexchangedhd In the case of the power machine less energy is exchangedthan byy a machine under the same condition but withoutlosses. The losses can be :– Specific Energy Losses– Volume/massVolume/mass‐flowflow losses2
A. Hydraulic Loss Zh Is a specific Energy Loss due to friction, separation,contraction, diffucion, eddy formation etc. while the flowpasses through the main flow passages from entrance todischarge flange of the machine In case of turbines the needed energy to overcome thehydraulic loss Zh is taken from the available spec. energy Y. Nm Yblade Y Z h kg In case of working machines, the impeller has to transfer inaddition to the spec. energy Y a spec. energy Zh Nm Yblade Y Z h kg 3
B. Disc Friction Loss Zr The surface of the rotor which does not form the main flowpassage is surrounded by a fluid medium While the rotor rotates, a friction is generated between thisrotor surface and its surrounding fluid medium. The neededpower to overcome this friction can be written as:N r ρ VZ rWhere: Zr disc friction spec. energy loss and is involved with ρV4
C. ReturnReturn‐FlowFlow Loss Za A return flow of already energy loaded medium may benoted in pumps.pumps Especially by those of the axialaxial‐flowflow type.typeN a ρ VZ aWhere Za the spec.spec energy lossinvolved and related to ρV. Na is very small for the design point and, thus it is normallyneglected. But it may increase greatly when discharge Vx is lessthat the design point discharge V.V5
D. Leakage Loss ΔV (Loss of Volume or mass‐flow Due to possible leakage, the volume V passing through thepressure flange of the machine differs from the volume V’ passingthrough the rotor vane channels.If ΔV is the total leakage, the volume flowingthroughtherotorchannels is:V ' V ΔVIn working machinesV ' V ΔVIn power machinesThe specific energy lossdue to leakage:Zl ΔVYbladeV6
Internal Power The total internal loss, ZiZi Zh Zr Za Zl Accounting for all internal losses, the power transferredbetween the rotor and the flow is defined as:N i ρ (V Δ V )Yblade ( N r N a ) ρ VY i Where Yi is internal specific work defined as:NiΔV 1 Yblade (Z r Z a )V ρV Yi Y Z iFor power machinesYi Yi Y Z iFor working machines7
External Losses All external losses due to friction in the bearings, sealingsand due to ffluid frictionfat the outside rotatingg surfacesfof the machine can be counted together as a power loss: N Nmkg m External losses also include losses from auxiliary equipments(oil pump, bearing lubrication speed regulators) since theyare mostlyy driven directlyy byy the shaft of the turbomachine The coupling power N which considers all internal andexternal losses isN N i N m ρ (V Δ V )Yblade ( N r N a N m )8
Sankey diagram For working Machines For Power Machines9
The Efficiency of the Turbomachine Definition of Efficiency:η energy outputpower output energygy inputpppowerinputp Different efficiencies can be defined considering differentlosses:A. Hydraulic Efficiency: Yη h Yblade 1 Y Y Zh 1 Hydraulic efficiency cannot be measured directly by test becauseNr and Na are also involved in the flow through and around therotor.10
Yη i YiB. Internal Efficiency: 1 ρ VY Ni 1 Considers all internal losses including the hydraulic losses. ForF steamtor a gas tturbinesbith velocitythel it andd geodeticd ti energycan be neglected: the internal efficiencyy can be determined byy measuringgtemperatures or enthalpiesΔ t adT D' T SΔ i adi D' i Scompressio n η i ΔtTD TSΔiiD iSexp ansionηi Δ t adT TSΔ i adiD iS DT D T S'Δii D i S'Δt11
Ni N C MechanicalC.M h i l EffiEfficiency:i 1ηm Ni Ni Nm 1 The mechanical efficiencyy considers onlyy the external losses.D. Overall Efficiency: (Often referred to as ‘efficiency’):For working machines :η ρ VYN ρ VY N iNiN η iη mFor power machines :η Ni NN η iη mρ VYρ VY N i The overall efficiency which includes all internal and externallosses can be determined by test.12
Nov 02, 2013 · the machine can be counted together as a power loss: kg Nm N m External losses also include losses from auxiliary equipments (oil pump, bearing lubrication speed regulators) since they are mostly driven d
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