Investigation of diffusion pumps

Vacuum~volume41/numbers 7-9/pages 2061 to 2063/1990

0042-207X/90S3.00 + .00 © 1990 Pergamon Press plc

Printed in Great Britain

Investigation of diffusion pumps K S S a d y k o v and S A F i g u r o v , NPO "'Vacuummash", 420107 Kazan, USSR

The following processes, taking place in the diffusion pump, are under discussion experimentally and theoretically: vapour generation, working steam flow, backstreaming, and ejector stage functioning. Experimental and calculated data are presented.

4

I. Vapour generation Mineral oil boiling in oil-diffusion pumps is due to the scheme typical for all oil products. As the light components, which during vapour generation carry off some vapour of the heavier components, boil away, the heavier components of higher boiling temperatures and of less vapour pressure start boiling away. The technique, proposed by A M Tregubov, according to which oil fractions are represented as a continuous systemcontinuum, consisting of an infinitely great number of components, has been used for describing properties of these mixtures. Usually one proceeds from distillation curves owing to true boiling temperature (TBT) attaching to them any convenient property for making calculations which steadily changes with the mixture composition. For the experimental evaluation of data for building curves (TBT) the method of molecular distillation is employed. Properties of minerial oil BM-I which is the product of distillation of medical vaseline oil with pressure vapour P = 9.8 x 10 - 7 Pa at T = 293 K, flash point Tn = 513 K, and solidification temperature Tso~= 261 K have been studied in this work. Original oil was distilled into 9 fractions in the Claisen flask. Experimental estimation of equilibrium temperature of the boiling liquid with its vapour was carried out with the ebulliometer. Results of experimental studies are given in Figure 1. The curves presented may be described with the beam of lines, converging in a common pole. Beam equations will have the form: lg,°4=6.33

_(\T----43 103

)

1.54 • K(M)

(1)

where K(M) is the beam parameter. When making up equation (1) the results of CalingertDavis were used. For purposes of gas-dynamic calculations it is expedient to assume fraction molecular mass as a proof of the composition distribution over the curve (TBT). The diagram (1.103/T)= f(~/~mea,), based on experimental data, is given in Figure 2. Experimental points may be approximated with elliptical arc with the maximal approximation error equal to 65%.

2

10 z 8

g~6 4

1.9

2.0

2.1

2.2

1

2.3

2.4

1

~r_-~)-lO ~, ~Figure 1. Saturated vapour pressure of oil BM-I fractions vs temperature: l--initial fraction; 2--residue.

Approximation dependence equation is 103 "~1 / ~ - 1.39 0.24 - r ~ 43] #. . . . + = 1 0,14 0,22

(2)

here/~ is the current molecular mass of the fraction, # . . . . is the mean molecular mass of the oil. Combining the diagrams I/T H =f(la/p . . . . ) and lg P = f ( T n ) the surface of the third order is obtained with the following structural formula:

(lg p)2

a2

t-

b2

= 1

(3)

where a, b, c are constants. The equation (3) is given for the case of coincidence of the surface top with the beginning of coordinate axes in symmetry to the 0-1g P axis. Having made corresponding changes of coordinates, having solved the equation as to lg P and corn2061

K S Sadykov and S A Figurov." Diffusion pumps 1.4

o, TH=543 1.3

900 ~

5,3

12

1.1

700 o

1.0

1.9

2.0

2.1 1

2.2

Figure 2. Vapour molecular mass vs equilibrium temperature of liquid and vapour (#. . . . = 459; P = 400 Pa). pared the equation obtained with that of (1) we obtain: 4.71 + 0.19X/0.22- (PmPan - 1.34) z K(M) =

0.88-

0.22

#

400

~

]'=261

~o.8~f 0.9

T.- 261

J

I

I

1.4

1.6

1.8

I

2.0 kd:~k S k2-OK

I

I

2.2

2.4

Figure 3. 'Enthalpy-entropy' diagram in the temperature range from 261 to 543 K. - - - , boundary curves of the fraction # = 459; - - boundary curves of the fraction # = 427; . . . . , boundary curves of the fraction # = 637; Th, low-boiling component temperature; Tb, high-boiling component temperature.

1.34

]Amean

When plotting diagrams experimental data of liquid heat capacity have been used. Freezing (solidification) temperature of oil, T = 261 K, is accepted as the origin. Enthalpy h' and entropy S ' results are on the left boundary curve C ' dt; h" = C'dT+ o T o o ' when calculating it is accepted that: C' ~ Co; 6' = 0. Values of parameters are on the right boundary curve: S" =

S" = S ' + -

h"=h'+r;

r

T where r is the vaporization heat: r = I n 10' R - T 2d(lg P) dT It is accepted that: RT ~ 9 " - oa' = 7 ~ - ;

y=l

where ~9' and ~" are the specific volume on the left and right curves, correspondingly, and 7 = coefficient of compression. Taking into account (1) instead of (4) it has been obtained: r = l n 10" R • K(~)

.

Heat capacity changes as separate fractions of the liquid boiled away have been determined by a diagram similar to that of DavisL Figure 3 is an 'enthalpy-entropy' diagram, built according to the equations stated above. Having estimated the value of the derivative d h " / d S " we obtain: dh'

4

dr

dh"

dh"/dT

dT

dS"

dS"/dT

dS" 1 dr tdT T dT

dh"

dS,,>0 2062

dS'

I dr

if ~ + - ~ > T - -

dT r T2 r

~

Real values of these magnitudes given that d h " / d S " > 0 if T > 360 K i.e. in the range of working parameters of oil-diffusion pumps. For most liquids at the isentropic expansion from the boundary curve the process proceeds in the range of damp vapour, for the oil under test it proceeds in the range of overheated vapour. 2. Flow of working steam Pump nozzles are calculated in approximation of boundary layer. Nonviscous 'nucleus' of the flow is calculated using equations from the natural system of coordinates 2. Using Stepanov-Mangler and Stjuartson transformations successively for the given system of equations, we obtain equations that completely correspond to the boundary layer equations in compressible liquids. Calculation of the boundary layer flow is made with automodel solution in accordance with the degree of the speed on the outer boundary layer. Pressure distribution over the cross-section of two nozzles has been studied experimentally with help of Pitot tubes. Nozzle dimensions and configurations are given in Figure 4. Figure 5 is the comparison of calculated and experimental data. It is necessary to note their good coincidence. Free jet investigation was made with pneumometric, thermoanemometric and electronic-beam methods. Calculation of jets, discharged from the sonic nozzle with no boundary conditions is solved according to the Euler equations. In these jets parameters depend only on the index value of the adiabata x. Parameter change analysis as measured experimentally, shows that these parameters are characterized by insufficiently high sensitivity to the change of x in the whole range from 1 to 5/3. Mixing of the pumped gas with the working jet and backstreaming were investigated with pneumometric diagnostics (pressure sounds). The calculated data are in accordance with the experimental results.

K S Sadykov and S A Figurov: Diffusion pumps 90o+ 50"

L

-I

/

No nozzte

~

/

45°+-30'

Dimensions (mm) O

D1

Dz

L

L

1

45034 101 °46 103-°sT 92-087 48 °62

2

62 °4

92046 94 -Os7 52-074 40 °6z

Figure 4. Configuration and dimensions of tested nozzles.

source, a one-stage model, a pump as a whole and a heated nozzle of the inlet stage were employed to measure backstreaming. Comparison of backstreaming during the work of only one inlet stage and the serial pump without any oil deflector shows that the contribution of the first stage to the total flow is 85%. If the pumps works without any oil deflector and if the nozzle of the inlet stage is heated by 25 K higher than the temperature of steam pipeline, the value of backstreaming decreases by 20%. This means that even if backstreaming takes place at the expense of condensate vaporization from nozzle walls, the value of backstreaming is not higher than 20%, i.e. it is significantly less than 90%, indicated in the work of Duval 4. However it is difficult to treat the results of this experiment unequivocally, because as a result of heating the nozzle walls, the structure of the flow may change, e.g. at the expense of total enthalpy rise. So it is clear that condensate vaporization of the pump does not prevail over the appearance of backstreaming. Weight measurements were made with five rings of equal cross-section to determine backstreaming distribution by the radius of inlet section. These results are compared with the results obtained with the quartz resonator 5 and with the calculated data, obtained with the Monte-Carlo method.

K • 1,054

0.8

/ -

0.6 --

~

1

/ .

Rekp 1 1,5000

2

2,,ooo

~

3 20000

eo ~ - ~ r ~

4 15000

-

4. Ejector stage To describe the work of the ejector stage, equations have been obtained on the basis of one-dimensional gas dynamics. Unlike usual ejection equations, irregularity of flows due to low Reynolds numbers as well as vapour condensation on the walls of the mixing chamber are taken into consideration. As result of calcualtions the dependance of rows of parameters on the condensation has been obtained.

Acknowledgements °zi o

-I lo

I 20

1 30

I 4O

I" 50

Figure 5. Calculated (at ~ = 1.054) and experimental distributions of the magnitude P'o/Po over nozzle cross-section. Lines I, 2--nozzle 2; lines 3, 4--nozzle 1.

3. Backstreaming Experimental investigation of backstreaming from the inlet pump stage was made by weighing. For determining the main

The author is indebted to B L Paklin, A K Rebrov and P A Skovorodko for their assistance in mastering calculation methods.

References F P Boynton and A Thomson, J Comput Phys, 3, 379 (1969). 2 p A Skovorodko, Proc Au-Union Conf. Institute of Thermophysics, Novosibirsk, 2, 143 (1980) (In Russian). 3 D R Hartree, Proc Camb Phil Soc, 33, 11 (1937). 4p Duval, Vak Tech, IM, 99 (1982). 5 M A Backer, J Phys E, Scient lnstrum, 17, 774 (1968).

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