1. Int In e t rnal Losses: tak ta e k place plac in the inner passages adding heat to the flow medium 2. External losses:

Transcription

1 he Losses of urbomachnes 1. Internal Losses: Losses whch take place n the nner passages of the machne and drectly connected wth rotor or flow of the medum and whch are addng heat to the flow medum 2. External losses: Losses whch appear outsde of the nner passages of the casng and whch do not transfer the generated heat drectly nto the flow medum 1

2 Internal Losses ue to the nner losses, the total energy exchange between the rotor and the flow medum s altered. In the case of the workng machne more energy has to be exchanged In the case of the power machne less energy s exchanged than by a machne under the same condton but wthout losses. he losses can be : pecfc Energy Losses olume/mass flow losses 2

3 A. Hydraulc Loss Z h Is a specfc Energy Loss due to frcton, separaton, contracton, dffucon, eddy formaton etc. whle the flow passes through the man flow passages from entrance to dscharge flange of the machne In case of turbnes the needed energy to overcome the hydraulc loss Z h s taken from the avalable spec. energy. blade Z h m kg In case of workng machnes, the mpeller has to transfer n addton to the spec. energy a spec. energy Z h blade + Z h m kg 3

4 B. sc Frcton Loss Z r he surface of the rotor whch does not form the man flow passage s surrounded by a flud medum Whle the rotor rotates, a frcton s generated between ths rotor surface and ts surroundng flud medum. he needed power to overcome ths frcton can be wrtten as: ρ r Z r Where: Z r dsc frcton spec. energy loss and s nvolved wth ρ 4

5 C. Return Flow Loss Z a A return flow of already energy loaded medum may be noted n pumps. Especally by those of the axal flow type. ρ a Z a Where Z a the spec. energy loss nvolved and related to ρ. a s very small for the desgn pont and, thus t s normally neglected. But t may ncrease greatly when dscharge x s less that the desgn pont dscharge. 5

6 . Leakage Loss (Lossof olume or mass flow ue to possble leakage, the volume passng through the pressure flange of the machne dffers from the volume passng through the rotor vane channels. If s the total leakage, the volume flowng through the rotor channels s: ' + In workng machnes ' In power machnes he specfc energy loss due to leakage: Z l blade 6

7 Internal Power he total nternal loss, Z Z Z h + Z r + Z a + Z l Accountng for all nternal losses, the power transferred between the rotor and the flow s defned as: ( ± ) blade ± ( r + a ) ρ ρ Where s nternal specfc work defned as: 1 ± ρ Z + Z blade ± ( Z + Z ) r a For For power machnes workng machnes 7

8 External Losses All external losses due to frcton n the bearngs, sealngs and due to flud frcton at the outsde rotatng surfaces of the machne can be counted together as a power loss: m m kg External losses also nclude losses from auxlary equpments (ol pump, bearng lubrcaton speed regulators) snce they are mostly drven drectly by the shaft of the turbomachne he couplng power whch consders all nternal and external losses s ( ± ) ± ( + ) ± ρ + m blade r a m 8

9 ankey dagram For workng Machnes For Power Machnes 9

10 he Effcency of the urbomachne efnton of Effcency: energy energy output nput power power output nput fferent effcences can be defned consderng dfferent losses: A. Hydraulc Effcency: h blade ±1 ±1 ± Z h Hydraulc effcency cannot be measured drectly by test because r and a are also nvolved n the flow through and around the rotor. 10

11 B. Internal Effcency: ±1 ±1 ρ Consders all nternal losses ncludng the hydraulc losses. F t t b th l t d dt For steam or a gas turbnes the velocty and geodetc energy can be neglected: the nternal effcency can be determned by measurng y y g temperatures or enthalpes ' ' ad ad t t n compresso ' ' exp ad ad t t anson 11

12 C. Mechancal leffcency: ±1 ±1 he mechancal effcency consders only the external losses. m ±. Overall Effcency: (Often referred to as effcency ): m For For workng machnes : power machnes : ρ ρ ρ ρ m m he overall effcency whch ncludes all nternal and external losses can be determned by test. 12