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Investigation of performance characteristics in jet-sprayed desalination plants
Desalination, 56 (1985) 431--437
43]
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
INVESTIGATION OF PERFORMANOE CHARACTERISTICS DESALINATION PLANTS V.N. 31esarenko, V.M. Stchetinln, Institute ( USSR )
IN JET-SPRAYED
Far Eastern Polytechnical
During desalination of seawater and brackish water thin film evaporators characterized by some advantages, viz: high intensity of heat exchange when a thin layer of liquid is heated; a short period of its contact with the surface; a small consumption of energy during the process are used. However, their operating experience shows that when seawater is fed to the heat exchange surface both in vertically oriented and horizontally oriented heat exchangers disturbances of wetting the tubes with liquid films at the end part or on the lower rows can be observed due to insufficient spraying and evaporation of liquid [I] At the same time there is an opportunity to el~m~uate this disadvantage especially in vertical tube evaporators by feeding water being desalted in the form of sprays discharging from a sprayer placed inside the main tube through orifices of a small diameter. Investigations of main mechani~mA of the process under Jet spraying of the la~rface have been made on a pilot plant (fig. I ) @
with
three
sections
forming
a tube
inside
of which a duralumin
tube sprayer of 6xi.5 diameter is disposed. The sprayer is of a changeable type havimg 0.6 - 0.8 ~sa orifices of row and staggered arrangeaents
with
are
either
provided
60 - 1 20 i n n u m b e r a l o n g with
the
apparatus
the
capable
length.
Sprayers
of removing
a
film from the tube surface or without it. The total length is of 1.1 |a with a tube 26x2 I in diameter. Performance parameters of the pls~at varied in the following r = ~ e s :
heat f l o w - 0"10 3 - reflux intensity 2.5 -9 "'v -3 a2/ s; ~- liquid pressure in the sprayer 40-I 20 kPa.
432
The pilot plant allowed to make visual observations without heating when the working surface had been replaced by a glass tube when discharging from the orifices to the inner surface a liquid forms a solid thin film flow which differs from a free gravitation/al
one by forming film uniformly along the whole
height of the tube with the help of Jets outflowln~ from the sprayer. In this case one can see a steady rise of film thickness at the end part of the tube, this prevents from film breaking on some areas of the heat exchange surface.
4
Fig. I. Experimental plant. I - working area; 2 - sprayer| 3 -main 7 -
tank;
separator.
4 -
head
t~n~;
5 -
ejectors;
6 -
condensators;
433 The o v e r a l l p a t t e r n of l i q u i d f l o w B y be r e p r e s e n t e d as t h e s e t s o f spot m p i l l i n g formed by J e t i m p a c t s en t h e background o f a falling film flow which significantly effects on the c ~ = ~ e s of the nature ef je~ spilling. From a hydrodynamic standpoint jet spraying can be considered as a film flow spreading out in all directions from a critical
p o i n t i n which j e t s
are struck against the heating surface.
Spots
ef spilling are of parabolic shapes with a maximum velocity
R~.//s #0 i0
... ,....,"
4 ..~
~...,,,.
X u w
6
J
6
~ . f
2
2
0
0.#
0.2
0.3
0.~'
O~
0.6
0.7
0,8
0..¢
~'.0
~/','7
^ ReN Fig. 2. D e p e n d e n c e ~ N a n d ~ - ~ o n t u b e length. 1 -R~; 2 - . ~ ; d
= o
=
3-~--~.~
p =
approximately equals to jet velocity near a critical point. While jets move off the point of impact the velocity sharply falls to near zero when a film moves upward and up to the velocity of gravitational spilling when a film moves downward. Intensity of velocity decrease of liquid particles flowing upward and downward is different. By virtue of equality of the conditions acting on this film flow forces we have got equations enabling us to estimate velocity values of jet spilling with liquid moving upward
434
and
downward
~//
where
.I.
~-
2
the velocity at which jets outflow from the nozzles, m/s;
f6 m /2
f ____
9
/ j i
8 /
Fig. 3. CompariSon of h e a t t r ~ f e r coefficients under different conditions film feeding. I - jet spre~FiI~; 2 - vertical film flow; 3 - horisontal t h ~ film flow,
..,o-C
Of"- the average film thickness, m; ~the hydraulic resistance coefficient; - the distance between a critical point and a cross - section considered, m.
~",,,=°~
f6
'
I
~Q
6Q
'
I
/1 /
i J
io 9 8
~
Ii g_o
5O
~0
Fig. 4. Dependence of coefficient ~ o n
7O dO
the number of sprayed
435
o r i f i c e s . 1 -~M = 4.02 " 103 m2/s; tZ= 120; d = 0.6 ram;2 - ~ = = 3 . 2 • 103 m2/s; / Z = 9 0 ; ~ = 0 , 6 -,-;3 - ~ = 2,5 " 103 m2/s; =
6o;d=
o.6~
- orifices of staggered arr~-~ament ; /
- I', 2', 3, -
rows.
I n a n a l y s ~ - g t h e w o r k s on h e a t e x c h a n g e u n d e r j e t - s p r a y i n g conditions of the heat~ s u r f a c e i t had b e e n s t a t e d t h a t p h T s i oal conceptions of the above process are of contradictory natures. According to studies [2] the heat exchange coefficient values ~ remain constant along the whole length of the tube, this is conf i r m e d by t h e r e s u l t s o b t a i n e d i n t h e w o r k [ 3 ] , i n w h i c h i t was p o i n t e d a t a d e c r e a s e o b s e r v e d c~_ and i n v i e w o f t h i s i t was suggested to restrict the height of a tube surface in such heat exchanges . Our observations indicated that the average values of the heat exchange coefficient along the length of a tube falls at the expence of a steady rise of falling film liquid thickness, in this case spraying stability of the surface is defined by liquid pressure values in the sprayer, numbers, diameters and arrangements of orifices and the length of the sprayer as well. Fig. 2 gives processing of experimental data, it im elear from it that the value of the number~e/=~is very important for decreasing heat exc~-nge intensity along the tube length. The value of~ubstantially depends on a nozzle diameter of the sprayer. The heigk% of liquid in the sprayer and pressure of a liquid feeding to the sprayer influence the c b s ~ e s of the value. Erperiments showed that heat exchange in the film~ under jetsprays d conditions took place more intensively. This confirms the values obtained of the average heat exchange coefficients. If we compare them with those ~ 2 for thin film heat exchangers of other types under comparable conditions we can see that the values of the average heat exchange coefficients in vertical tube heat exchangers are 60 per cent higher and in horizontal tube heat exchsngers are 30 per cent higher than these values in other types of heat exchangers. (fig. 3) Very significant feature of the above-mentioned heat exchangers is the number of orifices in the sprayer (fig. 4). With increasing the number of orifices at the same diameters we can
436
obtain higher values of the heat exchange coefficient. Such functional dependence can be explained physically by high er uniformity of superimposing the heating surface with discharging jets, by intensive mixing a liquid in a film layer, this leads to heat exchange under more uniform heat transfer. In increasing reflux intensity in the ranges of the values investigated a rise of the coefficient ~ z occurs, but the range of its increments at fixed / ~ with larger heat flows drops. Due to th1~ film flow movement caused by Jets discharged a liquid cannot reject all heat supplied, this apparently results in
reducing ~ . EBtlmating hydrodynamic and heat characteristics of a Jeysprayed type heat exch--ger we may conclude that stability of spraying density by evaporated liquid in this heat exch--ger is much higher than in others, and intensity of heat e x c B A ~ e will permit to decrease gubstantially the sizes of the heating surface to get the same productivity. Observations indicated that stable spraying of the surface with liquid is determined by the value of liquid presaure in the sprayer, by numbers, diameters and arrangements of orifices as well as its length.
43~
REFERENCES
1. 2.
3.
I.M. P e d o t k i n , Intensification of the technological process. Kiev, High School, 1979, 344 p. V.S. Lipsm=~j R.K. Tkachuk, Flow and Heat tr-~fer to liquid under thin film spray-Jet generation in evaporators. Kiev, Tech-~ka, 30 (1979),pp. 20-24. B.Mo Borisov, Investigation on some regularities in spray-Jet desalination plants. Thesis, Shevchenko, 1969, p. 24.
43]
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
INVESTIGATION OF PERFORMANOE CHARACTERISTICS DESALINATION PLANTS V.N. 31esarenko, V.M. Stchetinln, Institute ( USSR )
IN JET-SPRAYED
Far Eastern Polytechnical
During desalination of seawater and brackish water thin film evaporators characterized by some advantages, viz: high intensity of heat exchange when a thin layer of liquid is heated; a short period of its contact with the surface; a small consumption of energy during the process are used. However, their operating experience shows that when seawater is fed to the heat exchange surface both in vertically oriented and horizontally oriented heat exchangers disturbances of wetting the tubes with liquid films at the end part or on the lower rows can be observed due to insufficient spraying and evaporation of liquid [I] At the same time there is an opportunity to el~m~uate this disadvantage especially in vertical tube evaporators by feeding water being desalted in the form of sprays discharging from a sprayer placed inside the main tube through orifices of a small diameter. Investigations of main mechani~mA of the process under Jet spraying of the la~rface have been made on a pilot plant (fig. I ) @
with
three
sections
forming
a tube
inside
of which a duralumin
tube sprayer of 6xi.5 diameter is disposed. The sprayer is of a changeable type havimg 0.6 - 0.8 ~sa orifices of row and staggered arrangeaents
with
are
either
provided
60 - 1 20 i n n u m b e r a l o n g with
the
apparatus
the
capable
length.
Sprayers
of removing
a
film from the tube surface or without it. The total length is of 1.1 |a with a tube 26x2 I in diameter. Performance parameters of the pls~at varied in the following r = ~ e s :
heat f l o w - 0"10 3 - reflux intensity 2.5 -9 "'v -3 a2/ s; ~- liquid pressure in the sprayer 40-I 20 kPa.
432
The pilot plant allowed to make visual observations without heating when the working surface had been replaced by a glass tube when discharging from the orifices to the inner surface a liquid forms a solid thin film flow which differs from a free gravitation/al
one by forming film uniformly along the whole
height of the tube with the help of Jets outflowln~ from the sprayer. In this case one can see a steady rise of film thickness at the end part of the tube, this prevents from film breaking on some areas of the heat exchange surface.
4
Fig. I. Experimental plant. I - working area; 2 - sprayer| 3 -main 7 -
tank;
separator.
4 -
head
t~n~;
5 -
ejectors;
6 -
condensators;
433 The o v e r a l l p a t t e r n of l i q u i d f l o w B y be r e p r e s e n t e d as t h e s e t s o f spot m p i l l i n g formed by J e t i m p a c t s en t h e background o f a falling film flow which significantly effects on the c ~ = ~ e s of the nature ef je~ spilling. From a hydrodynamic standpoint jet spraying can be considered as a film flow spreading out in all directions from a critical
p o i n t i n which j e t s
are struck against the heating surface.
Spots
ef spilling are of parabolic shapes with a maximum velocity
R~.//s #0 i0
... ,....,"
4 ..~
~...,,,.
X u w
6
J
6
~ . f
2
2
0
0.#
0.2
0.3
0.~'
O~
0.6
0.7
0,8
0..¢
~'.0
~/','7
^ ReN Fig. 2. D e p e n d e n c e ~ N a n d ~ - ~ o n t u b e length. 1 -R~; 2 - . ~ ; d
= o
=
3-~--~.~
p =
approximately equals to jet velocity near a critical point. While jets move off the point of impact the velocity sharply falls to near zero when a film moves upward and up to the velocity of gravitational spilling when a film moves downward. Intensity of velocity decrease of liquid particles flowing upward and downward is different. By virtue of equality of the conditions acting on this film flow forces we have got equations enabling us to estimate velocity values of jet spilling with liquid moving upward
434
and
downward
~//
where
.I.
~-
2
the velocity at which jets outflow from the nozzles, m/s;
f6 m /2
f ____
9
/ j i
8 /
Fig. 3. CompariSon of h e a t t r ~ f e r coefficients under different conditions film feeding. I - jet spre~FiI~; 2 - vertical film flow; 3 - horisontal t h ~ film flow,
..,o-C
Of"- the average film thickness, m; ~the hydraulic resistance coefficient; - the distance between a critical point and a cross - section considered, m.
~",,,=°~
f6
'
I
~Q
6Q
'
I
/1 /
i J
io 9 8
~
Ii g_o
5O
~0
Fig. 4. Dependence of coefficient ~ o n
7O dO
the number of sprayed
435
o r i f i c e s . 1 -~M = 4.02 " 103 m2/s; tZ= 120; d = 0.6 ram;2 - ~ = = 3 . 2 • 103 m2/s; / Z = 9 0 ; ~ = 0 , 6 -,-;3 - ~ = 2,5 " 103 m2/s; =
6o;d=
o.6~
- orifices of staggered arr~-~ament ; /
- I', 2', 3, -
rows.
I n a n a l y s ~ - g t h e w o r k s on h e a t e x c h a n g e u n d e r j e t - s p r a y i n g conditions of the heat~ s u r f a c e i t had b e e n s t a t e d t h a t p h T s i oal conceptions of the above process are of contradictory natures. According to studies [2] the heat exchange coefficient values ~ remain constant along the whole length of the tube, this is conf i r m e d by t h e r e s u l t s o b t a i n e d i n t h e w o r k [ 3 ] , i n w h i c h i t was p o i n t e d a t a d e c r e a s e o b s e r v e d c~_ and i n v i e w o f t h i s i t was suggested to restrict the height of a tube surface in such heat exchanges . Our observations indicated that the average values of the heat exchange coefficient along the length of a tube falls at the expence of a steady rise of falling film liquid thickness, in this case spraying stability of the surface is defined by liquid pressure values in the sprayer, numbers, diameters and arrangements of orifices and the length of the sprayer as well. Fig. 2 gives processing of experimental data, it im elear from it that the value of the number~e/=~is very important for decreasing heat exc~-nge intensity along the tube length. The value of~ubstantially depends on a nozzle diameter of the sprayer. The heigk% of liquid in the sprayer and pressure of a liquid feeding to the sprayer influence the c b s ~ e s of the value. Erperiments showed that heat exchange in the film~ under jetsprays d conditions took place more intensively. This confirms the values obtained of the average heat exchange coefficients. If we compare them with those ~ 2 for thin film heat exchangers of other types under comparable conditions we can see that the values of the average heat exchange coefficients in vertical tube heat exchangers are 60 per cent higher and in horizontal tube heat exchsngers are 30 per cent higher than these values in other types of heat exchangers. (fig. 3) Very significant feature of the above-mentioned heat exchangers is the number of orifices in the sprayer (fig. 4). With increasing the number of orifices at the same diameters we can
436
obtain higher values of the heat exchange coefficient. Such functional dependence can be explained physically by high er uniformity of superimposing the heating surface with discharging jets, by intensive mixing a liquid in a film layer, this leads to heat exchange under more uniform heat transfer. In increasing reflux intensity in the ranges of the values investigated a rise of the coefficient ~ z occurs, but the range of its increments at fixed / ~ with larger heat flows drops. Due to th1~ film flow movement caused by Jets discharged a liquid cannot reject all heat supplied, this apparently results in
reducing ~ . EBtlmating hydrodynamic and heat characteristics of a Jeysprayed type heat exch--ger we may conclude that stability of spraying density by evaporated liquid in this heat exch--ger is much higher than in others, and intensity of heat e x c B A ~ e will permit to decrease gubstantially the sizes of the heating surface to get the same productivity. Observations indicated that stable spraying of the surface with liquid is determined by the value of liquid presaure in the sprayer, by numbers, diameters and arrangements of orifices as well as its length.
43~
REFERENCES
1. 2.
3.
I.M. P e d o t k i n , Intensification of the technological process. Kiev, High School, 1979, 344 p. V.S. Lipsm=~j R.K. Tkachuk, Flow and Heat tr-~fer to liquid under thin film spray-Jet generation in evaporators. Kiev, Tech-~ka, 30 (1979),pp. 20-24. B.Mo Borisov, Investigation on some regularities in spray-Jet desalination plants. Thesis, Shevchenko, 1969, p. 24.