Boiling of natural sea water in falling film evaporators

Desalination, 18 (1976) 71-94

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

BOILING OF NATURAL SEA WATER IN FALLING FILM EVAPORATORS VICTOR C . VAN DER MAST 77ie Lpiolm Company, 7(X10 Portage 114., Portage, 4tlith . 49081

STANLEY M . READ A'.D LEROY A . BROMLEY* Department of Chemical Engineering and The Bodes a Pr! urine Laborator)-, Unt i e rsiry of California, 11erAele.r, Cahf. 994720 (U.S.A .)

(Rcceihcd October 10, 1975 : in revised form May 7, 1976)

SUMMARY

Under conditions normally encountered in falling film evaporators for sea water, the boiling mechanism is one of evaporation at the continuous liquid-vapor interface with little nucleation involved . In smooth tubes, the heat transfer coefficient for the inlet-section is considerably higher than for the rest of the tube, and thereby contributes significantly to the average overall heat transfer coefficient . especially in short tubes . in the case of natural sea water, evaporation at the liquid-vapor interface causes large interfacial instabilities . Inlet devices, such as vortex level control and flow distribution, induce instabilities at the inlet of the tubes, thereby causing increased waviness further down . Although these disturbances enhance heat transfer, they increase entrainment and the tendency for scale formation in fluted tubes . Spirally corrugated tubes have significantly better heat transfer characteristics than smooth tubes. Heat transfer improvement caused by surfactant addition in smooth tubes depends strongly on the temperature difference . SYMBOLS

Bo Bo,, CP , i

D AT en

- Boiling number gllif9G - Modified boiling number Bo,,, = Bo (p f /pq ) - Heat capacity of the liquid, BTU/Ib,,, °F Inside tube diameter, ft - Temperature difference between vapor temperatures in steamchest and inside tube. °F - Entrainment collected at the outlet of the tube, pph

* To whom

correspondence should be directed .

72 c tttr~ar F,,, Foul

G he hf,r h t„ 1t,,, t kt I Pr, q

Re, St TS AT

L' x

Y Pf fit; Pf Pg yg,out

V . C. VAN DER MAST . S. St. READ AND L . A. BROMLLY

Entrainment at the outlet of the tube : (en/F0 ,,,) 100% Total liquid ffowrate at the inlet of the tube, pph Total liquid flowrate at the outlet of the tube, pph Total mass flux, lb m lhr-ft` Two phase heat transfer coefficient, BTU/hr-ft' 'F Heat of vaporization (condensation) . BTU/ib m Average inside heat transfer . BTU/hr-ft' 'F Average inside heat transfer coefficients for section 1, BTU/hr-ft' 'F Thermal conductivity of the liquid, BTU/hr-ft' 'F Distance from the inlet of the tube (outlet of the nozzle), ft Nusselt number for boiling h 9 D/fi t Liquid Prandtl number_ ep . ji 1/k, Heat flux . BTU/hr-ft 2 Liquid Reynolds number, (I - x)GD/ft J Stanton number = (h 0/ep . ,G) Temperature in steam chest, =F Average overall temperature difference, - F Logarithmic mean temperature difference between steam chest temperature and inside temperature for section 1 . Average overall heat transfer coefficient for the total length of tube Local vapor mass fraction Martinelli parameter

- Distance from wall, ft - Liquid viscosity, lb./hr-ft - Vapor viscosity, lb,,,/heft - Liquid density . fbmift 3 - Vapor density, lb,,,/ft 3 - Vapor velocity at the outlet of the tube, ft/s

I. INTRODUCTION

Sea water conversion by evaporation accounts for about 85 ° ;, of all desalted water (1) . Among the distillation processes, the Vertical Tube Evaporator (VTE) system or a combination of VTE and Multi-Stage Flash (MSF) system accounts for 12% of all distilled water, the balance being produced mainly by MSF systems . Even though VTE systems appear economically competitive wiith the MSF, many questions still remain with respect to the reliability of VTE systems, particularly with respect to systems which incorporate enhancement techniques (2). In VTE systems, such as the Multiple Effect Flash (MEF) Evaporator,

for dropwise condensation on smooth tubes, or condensation on fluted or

BOILING NATURAL SEA WATER IN FALLING FILM EVAPORATORS

73

designed by Bromley and Read (3), the heat transfer surface accounts for close to 50% of the total capital investment . The brine tubes, in which sea water is allowed to boil, account for over 65% of the total tube area, the balance being preheater and condenser tubes . The performance of the brine tubes, therefore, strongly affects the economic competitiveness of VTE systems, as well as the reliable operation of eventual plants . The objective of this work has been to gain a better understanding of the boiling of natural sea water inside tubes, both with and without enhancement techniques . Based upon this understanding . the heat transfer performance can be optimized . more accurate performance predictions be made, and operating conditions for reliable trouble-free operation est . bushed . This worb is a continuation of the work on MEF systems reported before in this journal (3) . Therefore . only falling film evaporation sill be discussed here . Generally, the outside heat transfer resistance is relatively predictable, as . for filmwise condensation on smooth tubes . Often, the outside resistance is s mall as corrugated tubes . Emphasis in this investigation has therefore been placed on the inside heat transfer

11 . BACKGROUND

In case of falling film evaporation of sea water, the boiling mechanism could be one of nucleate boiling or evaporation at the liquid-vapor interface, or a combination of these two mechanisms . Bergles and Rohsenow (4) have shown that in case of forced convection_ nucleate boiling can be rather easily suppressed . In case of nucleate boiling, the heat transfer coefficient increases rapidly with increasing temperature difference as explained by McAdams (5) . This dependence on temperature difference is frequently used as a means to detect the presence of nucleate boiling . As observed by Bromley and Read (3) existing correlations for falling film evaporation inside smooth tubes, as developed by Wright et al . (6), for example, do not satisfactorily predict heat transfer coefficients inside brine tubes . These correlations were developed for fresh water and do not take into account the entrance effects. Unterberg and Edwards (7) compared the stability of falling evaporating films of sodium chloride solutions and distilled water . While running down the outside of a smooth tube, sodium chloride solutions appeared more stable . In cases of liquid films running down corrugated surfaces with evaporation at the liquid-vapor interface, Jansen and Owzarski (8) observed that a sodium chloride solution replenishes the ridges whereas distilled water does not . Kays and Chia (9) . when evaporating failing films of fresh water and salt solutions in fluted tubes, observed considerably higher heat transfer coefficients for 3 .5 % NaCl solutions than for fresh :water . The fact that heat transfer coefficients



74

V. C. VAN DER MAST, S . M . READ AND L. A . BROMLEY

for natural sea water were even higher was explained speculatively by the tendency of natural sea water to foam . The coefficients for fluted or corrugated tubes are always considerably higher than for smooth tubes . Natural sea water has generally been considered as a mere salt solution . the simplest form of simulated sea water being a 3.5 °o NaCl solution . Horne (10) and Revak (II) indicate that natural sea water also contains surface active species . The concentration of these organic molecules decreases with larger depth of intake and with increasing distance from shore . The concentration depends on the marine "fertility" of the area but has seasonal fluctuations as well . Revak (11) experimentally determined the concentration of organic species in natural sea water taken from the same source as the sea water used in this work . Fatty acids were believed to be the rlain surface active species . Even though the total surfactant concentration may be as small as 0.5 ppm . or less . Van der Mast and Bromley (12) have shown how in falling film evaporators having evaporation at the liquid-vapor interface, small quantities of surfactants may have a significant effect on the interfacial instabilities . They present a model which indicates chat natural sea water enhances interfacial disturbances . This is in contrast with observations made for 3 .5','„ NaCl solutions which damp interfacial instabilities, as described by Unterberg and Edwards (7) and Janscn and Ow%,zarski (8) . Interfacial disturbances strongly affect heat t ransfer. as explained by Frisk and Davis (13) . Entrainment generation also depends strongly on the waviness of the falling liquid film, as shown by Hewitt and Taylor (14) . Wallis (15) and Van der Mast and Bromley (12). Increased waviness also increases the scaling danger when evaporating sea water in fluted or corrugated tubes, as shown by Van der Mast and Bromley (12) . III . EXPERIMENTAL

The bulk of the testing was done on a Single Ef fect Downlow • Evaporator, having a single smooth tube . Natural sea water and city water feed were used in the tests. A large number of runs were also made on an experimental 5-effect Multiple Effect Flash Evaporator, consecutively using smooth and spirally corrugated tubes . The test facility was part of the Bodega Marine Laboratory, located at Bodega Bay, California . Ocean water is continuously pumped to the Marine Laboratory . The sea water intake is in a rocky zone at a depth of about 6 ft . below the water surface . This area is continuously pounded with large incoming waves . The surrounding intertidal zone is covered with an abundance of plant and animal growth . The sea water feed to the desalination facilities was screened, filtered in a sand filter, acidified with H 2 SO,, to a p1-f of 4 .5, decarbonated in an atmospheric spray column and degassed in a packed vacuum column .



BOILING NATURAL SEA WATER IN FALLING FILM EVAPORATORS

75

Preheater

9 A,

m

C

I

I

t-7 I

4

I

1

Glass Section +

6

Collecting System

+

~_ For Flows l to T

,

5 Fig. 1 . Single etTect downflow evaporator . Flows: 1 . Condensate from cup 1 ; 2. Condensate from cup 2 ; 3. Condensate from cup 3 ; 4. Product steam : 5 . Liquid entering movable tube . 6. Brine out ; 7. Pretreated feed (natural sea water or city water) ; 8 . Dry saturated steam to chest ; 9. Drainage from cup in cone separator ; 10. Vent . instrumentation : a . Hewlett Packard 2850 series probes for Hewlett Packard 2801A quartz thermometer ; b. Brooklyn mercury thermometers - 51100 - F accuracy : c. Heist Gauge - 2 .5(100 psi accuracy ; d. Vortex level control and flow distributor ; e. Cyclone separator; f. Movable tube ; g . Cup at bottom of cone separator.



\\ entering the torte\ of it, level high control alkahntt) orascit\ sea and deswned \\ \% flu 12u,cd the bsmeasurement Bronilc_\ controls (no/lle) thickness and of the the Read at Clots heat The 0) the

76

\ . (- . \ \\ 1)I R \I \S1 . S \1 RI A1) ~\\1) I . \ . IIRtt\II I \

For sonic run, . Bodega Ba\ cit\

\\atc r \\a s

front local

\ \ells . Becaus e

city \\ate r

.t, pretreated in the same \\a \

. This \cater Is _encrated .8 milheyw\alents liter) . this .tter

Sr~r, k c Hec t ('I aporulor A schematic diagram of the single etlict evaporator s\ ,tent Is sho\\n in Fig I - A more detailed description I, ,riven by Van der \last (5) This evaporator consists of a ,utuvle tube . 4 tt . long . 1 .5 in . 0 .1) and 0-0 5 in

\\al l

tube is a smooth OLIN tube . alloy No . 605 . Composition- 2 Iron . 001",, phospho :ous . balance copp :r The thermal conductt\It\ is 140 9 BTL hr tt2 F A torte\ level control and flo \\ distributor. I N -,% .I,, Installed at the inlet of the tube .

.

The tube is divided into three equal section, and cups are Installed to collect the condensate from each section . thereh\ allo \\m u input in each section . Pretreated ,ca \\atc r point \\he n

.tier feed I, at or near it-, boiling \\ distributo r

Inlet of the tube The noiile t.onsnts of an inlet section . \\ filc h

Flt 2 \ Ie\v, of t\so nozzles used in the single-stage c\aporator



RiftI\(# \lit ItUt SI % 1\ 1tIK I\ IALI i\ti III M I\At'OKAtt)K>

77 -

and imparts to the flo\\ a \ortc\ motion, and a con\er_gin`-dt\erging - flash section . As the \\
*oe, to tile %~ .tll and tile 'ataor is at the center . thetcl) generatino annular I1o%% . Fhe t\\o different noiile, used in fill,, unit are ,ho\\n in Fig . 2 . The main differences sire the diameter of the throat tO 16 in and 0 1 to . re,peLti\ci\ ) and the type of le\ci control 'I lie Nina II throat noiile ha, t\%o sets of tangential Inlet hold . \thereas the other one has tangential Inlet ,lot, 1 - he,lots pro%ide for more continuous 110m control . There list , is less danger of plugging . Both nonle, \ \et c made of 1)olNsuitone . Since our main intete,t the holhnu ,tile of the tube . the outside resistance \\ . ti 11111111111/ 0.1 h\ ipit+tnottuegood ;lra)l,\\£,e io£t-densatton fha \\ J 5 done Using

Fig 3 Single-stage tnaporator.



78

V. C . VAN DER MAST, S . M . READ AND L. A . BROMLEY

1

S

the outside coefficient for dropwise condensation also exist . Since the outside coefficients are high (about 40,000 BTU/hr •ft 2 `F), even large errors in predicting the outside coefficient will still allow accurate determination of the order heat transfer resistances, if only the overall coefficients are known . A cone separator is attached to the outlet of the tube . It allows the measurement at the outlet of the tube of the liquid entrainment, the maximum liquid film thickness, and the liquid mass flux as a function of radial position . A more detailed description is given by Van der Mast (17) and Van der Mast and Bromley (12) . The evapo -ator shell is made up of standard pieces of 6° 1 . D . Corning Pyrex glass . This allowed inspection of the nozzle feed system_ the quality of the dropwise condensation and the flow conditions at the outlet of the tube . For some runs Neodol* was added to the cold pretreated feed . The surfactant was introduced just before entering the preheater with a Whitey metering pump (1140 rpm, 72 :1 gear ratio) . A diaphragm pump with these characteristics has one stroke per 3 .75 seconds . Since the feed has a residence time of about 15 minutes in the preheater at near boiling temperatures, the surfactant was assumed fully dissolved when entering the vortex level control and flow distributor . Further details of auxiliary equipment and procedures are given by Van der Mast (17) . A photograph of the main part of the actual single stage evaporator set-up is shown in Fig . 3.

Five effect MEFevaporator Fig . 4 gives a schematic diagram of the MEF evaporator used in this work . It consists of one upflow and four downflow stages . This evaporator resembles very closely some of the conceptual designs made earlier by Bromley and Read (3). Although Fig . 4 is almost self-explanatory, more detailed information regarding the operation of such evaporators is given elsewhere (3, 17) . For the downflow stages, thermally saturated liquid enters the tubes through vortex level control and flow distributors, which operate in the same fashion as described for the single effect nozzles . The nozzles used here were also made of polysulfone . They had two sets of inlet holes . The bulk of the data were taken with two smooth copper tubes per effect. Towards the end of the p Yorkshire Imperial metals were installed, one tube per dimensions are The nozzle dimensions used are given in Table 1 . The to shown in Table TL A cross section of the spirally corrugated tubes, perpendicular to the axis, is given in Fig . S. Some additional information about the spirally corrugated tubes :

Neodol 25-3A (Shell Chemical Company) is an ammonium salt of a sulfated primary alcohol containing three ethylene oxide groups . Average molecular weight is 436 . It has 12 to iS alkyl

groups . Commercially available as 60 wt % of ethanol-water solution .

BOILING NATURAL SEA WATER IN FALLING FILM EVAPORATORS

79

yK

' Or3rtni PT PT

is not to scale)

F P

Trnerrrocoue1es - Recorder - t•e r^c:ouc'es-° .jsrt:u"o" tr5tr„mer71 17 lowmerers Pressu •e C-__^y es-Mcrre'ers .

C

Condict .vnty

=ress..re Recorders Ce ;s

Goo, P.s

o

Fig. 4. Five effect MEF evaporator . Angles between length of pipe and flutes 30' Number of flutes 33 O.D. (outside ridges) - I .D. (inside ridges) about 0 .25' The spirally corrugated tubes were purchased in 5 ft . lengths, with a 12' smooth section on one end and a 6" smooth section on the other end of each tube. After installation, the effective length was : 1/4" smooth section at the top, 42" spirally corrugated section in the middle, 84' smooth section at the outlet . Runs were made with both filmwise and dropwise condensation . Cups located at the bottom of each effect made it possible to continuously measure the



80

V . C . VAN DER MAST, S . M . READ AND L . A . BROMLLY

TABLE I MEF EVAPORATOR DOSS NI Lt11V STAGES - NOZZLE DIMtE\SMO%S ' stage

Throat diameter inches

Outlet diameter inches

Lea~r~th inches

Sniovil: tubes Ci .313' inlet diameter s 0 .14$ 3 0.160 4 0 .160 S 0 .174

1 .174 1 .426 1 .426 1 674

3.90 4 23 4 .22 4-25

Spirally corrugated tubes 0 .719 inter Wamerer 2-5 0.180

1 .415

TABLE 11 MEF EVAPORATOR DOWFLOW crAGUS --- rUBE t)IMLNSIONS (Pall thickness 0 .035") Stage

Outer diameter inches

Smooth tubes 55.25" eflecrAe length -r 1 .25 3 1 .5 4 1 .5 5 1 .75 Spirally corrugated tubes 50.75" e(jectite (enptlr 2-5 1 .5 (nominal)

Fig. 5 . Spirally-corrugated tube cross section, perpendicular to axis .

81

BOILING NATURAL SLA WATER IN FALLING FILM EVAPORATORS

condensate rate on the evaporator tubes, from which the actual heat input can then be calculated . The shelf of the evaporator consisted of standard pieces of 6" I .D . Corning Pyrex glass which made visual observation possible at the inlet and outlet of the tubes, as well as the type of condensation at the outside of the tubes . 1 14 . fl -.Sr RESULTS AND DISCUSSION

The range of operating conditions studied in the single effect and four effect downflow esaporators is given in Table Ill . The conditions were within the range of interest in the distillation of sea water . Single c :fleet e t aporator Fluid dtuatttics As also reported elsewhere (12), large interfacial disturbances have been observed in falling film evaporation of natural sea water, when evaporation occurs at the liquid-vapor interface . No such large disturbances have been observed with city water, under similar conditions . The instabilities are believed to be caused by the presence in natural sea water of small quantities of surfactants . Since natural sea water enhances instabilities as they travel down the tube . inlet devices which induce larger instabilities at the inlet of the tube will generate increased waviness and entrainment at the outlet of the tube . This was actually observed . In case of a small throat nozzle (0 .16 in .) only an insignificant fraction of steam can enter the throat . There is a sudden drastic increase, therefore, in specific volume as the saturated liquid passes through the throat, caused by flashing . This causes larger instabilities than in the case of a large throat nozzle (0.3 in) where the change in specific volume is less severe. Test results are reported elsewhere (12. 17).

TABLE [it RA"C.E Of OPCRATTh( ; CONDITIONS

Liquid flow, pph/ft Steamside temperature, Heat flux, BTU/hr ft= Vapor velocity out, ft/sec. Neodol concentration in feed, ppm Type of liquid Concentration factor Type of condensation

Smooth tubes (Single effect and AfEF)

Spiro/I corrugated tubes (MEF)

254 to 1222 140 to 230 8.000 to 50.000 10 to 120 0 to 60 Sea water, city water 1 .0 to 4.0 Ftlmwise, dropwise

1312 to 1764 160 to 200 43 .359 to $0,100 57 to 122 ft/s 0 to 22 Sea water 1 .0 to 2.0 Filmwise, dropwise



82

V . C . VAN DER MAST, S . M . READ AND L . A. BROMLEY

TABLE IV E'TRAINMENT DATA - NO `tEOLX)L ADDttION

Run No.

Nozzle throat

Type liquid

large large large small small small small small small small

sea water sea water sea water sea water sea water sea water sea water sea water sea water sea water sea water sea water city water city water city water

small small small small small

Foist .

pph

342 210 200 201 169 113 211 185 96 67 169 140 161 133 89

V9 .

oat,ftls

43 37 46 14.84 40.49 119 7 25 13 39 24 57 15 40 105

Ettro „t, % 5.70 2.77 2 .9') 7.78 I 1 21 13.98 2.83 3 .86 1 .89 1 .67 4.22 5.67 2.37 2.88 8 .97

TABLE V ENTRAiNMENT DATA - `sEODOL . ADDED

Run no.

Nozzle throat

Type liquid

45 large sea water 47 large sea water 52 large sea water 53 large sea water 54 large sea water 55 large sea water 56 large sea water 57 large sea water 58 large sea water 59 large sea water 60 large sea water 61 large sea water 62 large sea water 65 large sea water 66 large sea water 68 large sea water 72 small sea water 74 small sea water 75 small sea water 76 small sea water 77 small sea water 93 small city water 94 small city water

Foist, pph

V9 . out .

ftfs

Ettro ut,

278

24

5.16

242 268 236 7?2

46 14 24 48 73 10 19 47 75 25 43 75 55 74 44 20 15 47 73 109 31 56

8.86 2.44 6.84 9.41 15.75 1 .33 2.96 6.26 9.76 3.84 10 .13 17.64 14 .44 18.06 11 .08 6.10 5.78 11 .05 12.48 19.95 7 .02 10.96

196 240 216 185 148 260 245 221 340 313 291 185 199 158 132 105 122 95

°•

o



BOILING NATURAL SEA WATER IN FALLING FILM EVAPORATORS

83

The stronger the interfacial disturbances, the larger also the entrainment generation. Some entrainment data are given in Table IV- At high vapor velocities, frictional forces start to dominate and differences between city water and sea water and between different types of inlet devices become less significant . Addition of surfactant (Neodol), in sufficient quantities to reach micellar concentrations dampens instabilities (12, 17). A foamy layer, adjacent to the liquid film at the wall, may be generated, however . At high vapor velocities this foamy layer tends to break up, causing large entrainment generation . Some entrainment data are given in Table V . Heat transfer

In case of nucleate boiling, the heat transfer coefficient increases rapidly with increasing temperature difference . To detect the contribution of nucleate boiling in the work, the inside heat transfer coefficients were measured in the top section of the tube, for increasing temperature differences . The top section was used, since in that section, the vapor velocity is lowest, and therefore the suppression of nucleate boiling would be least likely . A large throat nozzle was used at the inlet of the tube so that the same vapor velocity could be maintained at the outlet of section 1, even with increasing temperature differences, by decreasing the vapor velocity into the tube . As shown in Figs . 6 and 7, the average inside heat transfer coefficient in section I decreases with AT, which indicates that nucleate boiling does not contribute significantly to

4000

I

I

Data points show average vapor fraction in Section I

3000 C IH Cal C N C

2000 Runs 4-7 and 9-I1 Natural Seawater Large Throat Nozzle No Neodol Added Approx Flowrate 340 ppti I

1000 0

I0

20 AT, ('F)

Fig. 6 . Average inside heat transfer coefficient in section 1 as a function of the average overall temperature difference in that section .



V. C. VAN DER MAST, S . M . READ AND L . A . BROMLEY

84 4000

i

i

Data points show average vapor fraction in Section I

00189 1 00199 0 3000-

N L H

m

c~

WOO

00 017371 Runs 22-26 Natural Seawater Large Throat Nozzle No Neodol Added Approx Flowrate 400 ppH I

1000 O

20

10 AT I ('F)

Fig . 7 . Average inside heat transfer coefficient in section 1 as a function of the average overall temperature difference in that section .

I

4000

3000

. U O v

2000 CO

x

CN

0

1000



0

Runs 78-83 . Small Throat Nozzle No Neodu7 o Section 1 . Approx . Flow Rate 210 ppH V Section 2 . Approx Flow Rate 190 ppH - o Section 3 Approx Flow Rate 170 ppH • Total length of tube

I 0

10

20

30

AT, 'F Fig . 8 . Average inside heat transfer coefficient for the different sections and the total length of the tube as a function of the overall average .1T. Natural sea water .

the evaporation . The boiling mechanism is therefore assumed to be one of evaporation at the liquid-vapor interface . Fig . 8 shows the average inside heat transfer coefficient for the three different sections and for the total length of the tube, as a function of the overall average



BOILING NATURAL SEA WATER IN FALLING FILM EVAPORATORS

85

10 Large Throat Nozzle Sea Water

Small Throat Nozzle Approi mate 1 Sea Water 'City Water Flow gRpatqe (ppH

• 3511

0

•

o 0

1 (00

m

Schrock and Grossman Correlation

01 00001

0 001 so • 1 .5 x 10' 4 Xtt /3

001

Fig . 9. Comparison of the average inside heat transfer coefficient in section I with the Schrock and Grossman correlation .

I0 arge Throat Nozzle Sea Water



7

•

Sea Water

Cut

Appa„mate ater Flow Railif H)

1((((((( epa-tiI-ti a

Schrock and Grossman Correlation

01 00001

001

0001 So + 1 5 x (Q-4

X t ' /3

Fig . 10 . Comparison of the average inside heat transfer coefficient in section 2 with the Schrock and Grossman correlation .



V. C . VAN DER MAST, S . M . READ AND L . A . BROMLEY

86 to Large Thr,ot Nozzle Sea e4ater

Small throat Mutts Sea Wafer Ca WOW

Approximate Flow Rote (

N

s iI.;r

r

o

~~;dry solsont 'fl_- nti so

m 5 z

Schrock and Grossman Correlation

of 0.0001

k

0 001 80 + 1 .5 x to'

Fig- 11 . Compact Schrock and Grossman co

Oct

tisith the ion.

AT in each sectio . As can be seen, the average inside heat transfer c e

for section I is considerably higher than for sections 2 and 3 . as well as for the total length of the tube . It is also of interest to note that the average inside heat transfer coefficient in all sections, as a function of AT, goes through a minimum . The dependence on temperature difference is strongest for section 1 . The behavior shown in Fig. 8 is believed to be caused by entrance effects . Very little is understood about exactly what determines the magnitude of these effects and the tube length affected . The entrance effects may manifest themselves as far as 41 feet down the tube. Figs. 9, 10, and II give a comparison of the experimentally-determin average inside heat transfer coefficients in sections 1 . 2, and 3 with an existing correlation for local inside heat transfer coefficients : = 320 [Bo + 1 .5 (10

3

31

(1)

This correlation was originally developed by Schrock and Grossman (18) for upflow and modified by Wright (6) for downflow . The correlation is only valid for systems with fully developed temperature and velocity profiles . As can be seen, the actual average heat transfer coefficients for natural sea water are always higher than predicted, as was also observed by Bromley (3)_ The agreement is best for section 3, where the entrance effects are smallest and where the interfacial phenomena are becoming less important compared to the increasing frictional



87

BOILING NATURAL SEA WATER IN FALLING FILM EVAPORATORS

forces. It should also be noted that city water has a lower average inside heat transfer coefficient than natural sea water, as expected from the fluid dynamics study. The small throat nozzle also has higher average inside coefficients than the large throat nozzles, because of the interfacial phenomena . A tentative local heat transfer correlation was developed for boiling of natural sea water in downflow in short, smooth tubes, having a small throat converging-diverging nozzle at the inlet of the tube. The sea water entering the nozzle should be saturated . The correlation attempts to incorporate entrance phenomena, interfacial phenomena and single-phase type forced convection phenomena : .945 t, O .106Ft r O .4

St = 0 .4054

.

O_457 Bo

µ 0 .342

(2)

This correlation should not be used for tubes over 10 feet long . When synthetic surfactants are added to the feed in sufficient amounts, a foamy layer is generated adjacent to the liquid film at the wall . Figs . 12 and 13 shoe the average inside heat transfer coefficients for the three sections and for the total length of the tube as a function of the overall average temperature

4000

3000

2 M

c

Runs 72-77

1000

Ser7ll Throat Notice

o Section t

200 PPM

V Section 2

180 ppM

O Section 3

150 ppM

20 ppm Neodol _

• Total length of tube Ri,ns 78-83 Small Intact Nozzle No Neodol 0 Total length of tube

200 ppM

I 1

0

t0

20 AT, eF

00

10

30

20 AT, 'F

Fig. 12. Average inside heat transfer coefficients for the different sections and total length of the tube as a function of the overall a%erage AT. Natural sea water.

for

the

Fig. 13. Average inside heat transfer coefficients for the different sections and total length of the tube as a funciion of the overall average AT. Natural sea water.

for

the



V, C . VAN DER MAST, S . M . READ AND L . A. BROMLEY

88

difference . The e heat transfer coefficients first increase with temperature difference, then decrease . This is not caused by nucleate boiling, as might be suspected . Rather it is believed that the inside heat transfer coefficients are enhanced by the increasing vapor velocity which causes the liquid layer at the wall to accelerate because of the intimate vapor-liquid contact, and therefore causes a reduction in the laminar boundary layer thickness . At increasing temperature differences and correspondingly increasing vapor velocities, a limiting vapor velocity is reached, beyond which the foamy layer starts to be broken up rapidly, as was also manifested by the large amounts of entrainment collected at the outlet . As of the tube . This then causes a reduction in inside heat transfer coefficient illustrated in Fig . 13. the enhancement of the inside heat transfer coefficient by surfactant addition may be as low as 13" or as high as 105°/ ;, depending on temperature differenceA tentative local heat transfer correlation for boiling of water in falling foaming films, having saturated liquid at the inlet of the nozzle, was developed :

Nub = 10 [Bo + (1 .5 X;r Re°' 8 Pr! 3

2'3 )

10

-410_2283

(3)

This correlation should only be used for short (less than 10 feet) smooth tubes with a vortex level control and flow distributor at the inlet of the tube . The correlation is based on data with Neodol at near micellar concentration (about 15 ppm) .

Multiple effect flash evaporator : D in

ges 2-5

Figs. 14 and 15 show the heat transfer coefficients for filmwise condensation on smooth short copper tubes with Vortex level control and flow distributors at the inlet. The data as obtained in this MEF unit agree quite closely with the ones

1800 . 1600 cs 1400 h 4 1200 aN G

'~ x-1000 ZW ,. S ~~ 0<)Q 0 _-w

m 0

a.a s

600 400 200 0

100

'

1

I

1

I

Run Number 0 7 0 13 • 8 0 14 • 9 1•1 5 IV 10 016 oil a17 to 12

First-order least squares curve for five-effect MEF evaporator Some as previous correlation (3)1 I 1 I I ' I 1 I I I

120

140

160

180

Vapor Temperature

200

220

240

( •F )

Fig. 14. Heat transfer in 5-effect MEF evaporator operating on sea water with filmwise condensation on oxidized copper tubes.



BOILING NATURAL SEA WATER IN' FALLING FILM EVAPORATORS

Run Number 4A 18 • 19 • 20 • 21

89

Previous correlation (3)-.

n 22

• 23

a 24 ! 25 AL 26 -order least squares curve eve-effect MEF evaporator .

6 Vapor Temperature (•F ) Fig. 15 . Hint transfer in 5-etTect MEF evaporator operatin condensation on clean copper tubes.

3600 3200

e l c t+ t t l t l e t e Runs t-6 Runs on five-effect MEF evaporator, made in 1970 Runs 2T-31,35 Runs made in 1971 RunNumber •

2 3 4

2400

•

5

w - 2000

•

c

W U w CO ~ tt

2800

on sea water with filmwise

0

27 + 28

to 29

o m

1600

2

0 0

1200

• 30 A 31 0 35

O

a

800

i

0 100

I I I t t t i 1 I 120 140 160 180 200 220 240 Vapor Temperature (°Fi

Fig. 16. Dropwise condensation in 5-effect MEF evaporator using sea water feed .

reported earlier by Bromley and Read, as obtained from an 8-effect experimental unit (3). Natural sea water was used in both cases . The dropwise condensation data obtained in this work are somewhat higher than the data reported earlier (3), as shown in Fig . 16. It is believed that this is caused by better venting of the noncondensibles and by the excellent dropwise condensation in this work . Comparison of the single effect and 5-effect dropwise data also shows generally good agreement. If anything, the heat transfer coefficients obtained in



90

V . C . VA\ DER MAST, S . M . READ A\13 L % . HRONILEI

TABLE VI AVERAGE C%FRALL HEAT TRANSFER COFFFICI %IS FOR SPIRALLI-CORRU( .A1F1) TUBES 1\ MIFF ORATOR NATUR U NF A

Stage Run

AT F

F, ppli

pph

III ppni

U RTL' izr fa

6002 601 1 607 .6

0 17.4 0

2 .551 .3 2 .529 3 3 .032.2

no.

no.

Tipe Fs condensattwr F Film Film

200 .0

1 180

2

I V

200.2

I

Drop%% use

200.0

154 IF 7

678 -. 675 .1 693 .6

I V VI

Film Film Dropwise

151 0 1838 182 .2

198 21 3 18.9

600 .2 601 .1 6076

5222 521 .1 515.6

0 19.5 0

2.345.9 '_2165 2,8986

I V

Film Film

1602 161 5

24 2 203

522 3 521 .1

4502 456 .1

0 22.5

VI

Dropwlse

162 .3

23 .3

515 .6

438.6

0

1,791 1 .9283 1,9902

3

4

1-% , %P-

w ATFR .Ve odeel

F

the single effect unit are somewhat lo""er than in the 5-effect unit . This is believed to be caused by the fact that generally the drop rise conden,ation in the single effect unit was of a somewhat lesser quality than in the multiple effect unit . Also. since the outside of the tube was divided up into three equal sections . the _rowing droplets on the tube wall %%ere swept away at longer intervals only, causing increased droplet size and larger outside resistance . The vapor used in the single effect unit (boiler steam) would also he expected to contain larger quantities of non-condensibles in spite of good venting than the vapor in the downflow stages of the MEF evaporator . which was obtained from evaporation of acidified (pH 4 .5) and degassed sea water . These noncondensibles have a strong effect on the outside heat transfer coefficient . Towards the end of the project, spirally-corrugated tubes were installed in the downflow stages of the MEF unit . The inlet nozzles were designed in such a way that a tangential flow is generated in the same direction as the spiral grooves . Some test results are shown in Table VI . The outside of the tubes was cleaned on stream with SO_ . As can be seen the heat transfer coefficients are quite high compared to existing literature data (9, 19) . It is believed that because of the spiral corrugations both the inside and outside heat transfer coefficients have been enhanced . The outside coefficients are considerably higher than for smooth tubes because of drainage of the condensate into the valleys . The inside coefficient has been enhanced by the vortex-type flow which is set up by the grooves, and because of thin film evaporation from the ridges on the inside of the tube . As explained earlier, natural sea water has a tendency to enhance instabilities and to drain the liquid away from areas with higher rates of evaporation . Since liquid on the ridges at the inside of the tube has the highest rate of evaporation .

BOILING NATURAL SI A WA ILR IN FALLING FILM EVAPORATORS

91

2 ft

2-V2 ft

3-1/2 ft

4-1/2 ft

Fig . 17 . CaSO1 scale in stage 5 at distances of 2 ft, 2A ft . 34, ft, and 4 ft from entrance .

V . C_ s

AN DER MAST . S. M . READ AND L . A. BROMLEY

the liquid will be drained from these ridges, causing high heat transfer coefficients because of thin film evaporation, but enhancing the scale danger . It has been observed by other workers (9 . 19) that natural sea water has higher heat transfer coefficients than either 3.>°.,, NaCI solutions or de-ionized eater. This has often been associated with foaming phenomena . Natural sea ssater (after acidification and de-aeration) without surfactant addition w as never observed to foam in this work . Addition of Neodol to the cold pretreated feed to the evaporator added in the same way as described for the single effect unit did not significantly change the heat transfer coefficients for spirally corrugated tubes under conditions as shown in Table VI . Very little foam was also observed inside the evaporator . Dropssise condensation does not significantly enhance the heat transfer coefficients for corrugated tubes as shown in Table VI . Since all the condensate is drained into the grooves, each individual drop has to grow until it has reached a large enough size to either drain into the grooves or be touched by the liquid in the grooves . This differs significantly from dropwise condensation for smooth tubes . where even small drops are regularly swept away by higher-tip droplets, leaving an essentially bare surface . Because of the fluid dynamics of natural sea water . a potential scale problem corrugated tubes, as shown in Fig . 17. In some runs CaSO 4 a to form about 21 feet from the entrance, but on the ridges only . The valleys stay entirely scale free . The scale thickness on the ridges increases, going further down the tube . It is of interest to note also that the smooth section at the outlet of the tube . where the concentration ratio is highest and liquid flow rate smallest, is entirely scale free . This indicates that the scale danger is considerably higher for enhanced tubes than for smooth tubes . This has to be taken into account when selecting tube design . tube loading, and heat flux . V . CONCLUSIONS

I . The bo ling mechanism . as generally encountered in falling film evaporators for sea water, is one of evaporation at the liquid-vapor interface . 2 . Natural sea water, in falling film evaporators, has a tendency to enhance interfacial disturbances in contrast with 3 .5', NaCl solutions . These instabilities enhance the inside heat transfer coefficient . 3 . Inlet devices, which induce instabilities at the uch as Vortex level control and flow distributors, further enhance heat transfer . These nozzles also evenly distribute the liquid on the tube wall, generating proper wetting even at low liquid loadings . In case of smooth tubes at a concentration factor of 1, tube loadings as low as 0 .3 gpm per inch of tube diameter could be allowed at a emperature of 240°F. without any scale problem . At a concentration factor



BOILING NATURAL SEA WAFER IN FALLING FILM 1-VAPORATORS

93

of 4, and brine temperature of 150 F, no scale was formed at tube loadings as low

as 0.15 gpm per inch of tube diameter . 4 . In case of smooth tubes, the heat transfer coeflicient for the inlet-section

is considerably higher than further down the tube . This section contributes significantiv to the average overall heat transfer coefficient in case of short tuh ,:s, 5. In smooth tubes, heat transfer enhancement

by

Neodol addition to the

feed is very much dependent on temperature difference . The enhancement coefficient ranges from 13 to 105", . .

of inside

6 . With respect to spirally-corrugated tubes, the inside heat transfer coefficient

is enhanced because

of a cortex-type floss inside the tube and because

of thin

film evaporation from the ridges . The scaling danger has increased and this should be accounted for in tube design and operating conditions such as concentration factor . maximum brine temperature and tube loading .

ACKNOIvLEDGMIENT This pork was supported by the Water Resources Center

of

the University

of California.

RI I LRENULS 1 . N,len A . EL-R%Mt , ASD C. F. t os. .nos . Dewltini,' Plants /tit eniori . Report Vo . 5. College

of Business Administration . Unn of Haccaii . Honolulu Contract No . 14-30-3286 OWRT . 1975-

2 . ALA'- D . K_ LAIRD, Water Resource, Center . Desalination Report No . 57, Unn . of California, 3.

Saline Water Conversion Research, 1974 . L . A . BROMLEl a\D S M_ READ . Multiple Effect Flash (FIEF Etaporator)_ Desalination. 7 (1970) 343--391 .

4. A. F . BPRGLES AND W . M . RoItsL ow, The Determination of Forced-Convection SurfaceBoiling Heat Transfer, Trans AS3IE. J Heat Transfer, (1964) 365-372 . 5 . W. H . McAnAsts, Hear Trantnaccion . 3rd ed ., McGraw-Hill, New York, N .Y , 1954. 6. R . M . WRICHr. G . F . SOSIE R %ILI t, R. L. Sat A\I) L . A . BROMLEY, Dosvnilow Boiling of Water and n-Butanol in Uniformly Heated Tubes, Cheni . Errgr. Prot,. Stnrp . Series, 61 .

57 (1965) 220 . 7. W . UvEERUERG

%%I)

D K.

EDI% ARDs, Esaporation from Falling Saline Flow . : m . Inst . CF'ern . Envy. J., I I (1965) 1073 .

Water Films in

Laminar Transitional S . G . JANSEN AND P . C . OwZARsKi . Boiling Heat Transfer in Falling Film Evaporator, with Corrugated Surfaces, OJhce of Saline IL titer, Res . Des el. Progr. Rept . No 693, 1971 . 9. D. D. KAYS A%D W. S CHIA, Devclopment and Application of Mechanically-Enhanced Heat Transfer Surfaces, ASAIE Papet No . 7l-HT-40 . 10. R . A_ HORNE, .Marine Chenastrv, Wiley, New York, N .Y , 1969. II . T . T REVAK, Identification of Natural Surfactants in Sea Water, itl.S thesis. Umv . of

California . Berkeley 1973 . 12. V . C. VAN DER MAST AND L . A. BROMLEY, Interfacial Phenomena in Falling Film Evaporation of Natural Seawater, Am. Inst. Client. Big. J., in press. 13 . D . P . FRi,K AND E. J. DAVis, The Enhancement of Heat Transfer by Wascs in Stratified Gas-Liquid Flow, hit. J. Heat Mass Transfer, IS (1972) 1537-1552 . 14. G . F. HEW ITr AND N . S. HALL-TAI LOR, Annular Tiro-Phase Flow, Pergamon Press, London .

First Edition,

1970 .

94

V . C. VAN DER MAST. S. M . READ AND L . A. BROMLEY

15 . G . B. WALLLS, One-Dirnensuwnal T'.oc Flmr, McGraw-Hill . New York, N .Y., 1969 . 16. 0. G . WtLKLNS, Drop%ise Condensation Phenomena, Ph. D . thesis, Un:s . of California, Berkeley, 1973 . 17 . V . C. VAN DER MA.tir. Forced Convection Boiling of Seawater, Pit. D. the%hr, Univ. of California, Berkeley . 1975 . 18, V . E. SC14ROCK AND L . M4 . GRoSSMA\ . Forced Convection Boiling in Tubes, Nucl . Scl . F~tg ., 12 (1962) 474-481 . 19 . L . G . ALEXANDER AND H . W . HOFFMAN . Performance Characteristics of Advanced Evaporator Tubes for Long-Tube Vertical Evaporators, Office of Saline Water, Res . Detel. Prug. Reps_ Vo. 644, 1971 .