Indian Journal of Chemical TechnologyVol. 1, July 1994, pp. 214-220

An experimental study for predicting circulation rates in a reboiler tube

Mohd Kamila, S S Alam" & H Alia"Institute of Petroleum Studies and Chemical Engineering, Aligarh Muslim University, Aligarh 202 002, India

"Department of Chemical Engineering, Aligarh Muslim University, Aligarh 202 002, India

Received 4 November 1993; accepted 8 March 1994

An experimental investigation of heat transfer to boiling liquids under natural convective flowthrough a vertical tube thennosiphon reboiler has been carried out to study the effect of heat fluxand liquid submergence on circulation rates. Three. test liquids namely: water, methanol and ethyleneglycol were used to cover a wide range of boiling points and thennophysical properties. The testsection was an electrically heated stainless steel tube 25.56 mm i.d. and 1900 mm length. The uni-form heat fluxes in the range of 4.1 to 43 kW/m2 were employed while inlet liquid subcooling variedfrom 0.5 to 11.6°C. The liquid submergence levels were maintained around 100, 75, 50 and 30%.Circulation rates for all test liquids have been correlated in terms of dimensionless groups.

Vertical thermosiphon reboilers consitute one ofthe most widely used types of heat transfer unitsin chemical, petrochemical and petroleum indus-tries where significant capital investment is repre-sented by reboilers, evaporators and vaporizers.They owe their popularity to excellent heat trans-fer rates, mechanical simplicity and no expendi-ture of power to circulate the process fluid. Theprocess fluid entering the tubes of heat exchangergets heated and moves upwards due to densitydifference between the liquid and boiling twophase fluid. The boiling of liquids in vertical tubesof closed loop circulation systems as above, findsits application also extending to refrigeration,power plants and nuclear reactors. The processfluid entering the vertical tubes of heat exchangerreceives the heat from the heat flux supplied.When vaporization takes place, the specific vo-lume of the fluid is increased, resulting in its up-ward movement while the liquid is siphoned fromthe adjoining cold leg. Thus, a net flow throughthe circulation loop sets in. The prediction ofrates of heat transfer and thermally induced flow(circulation rate) is the primary requirement forthe design and efficient operation of the thermosi-phon reboilers. A number of studies have beenreported in literature to predict the rate of heattransfer, but very little information is available forthe estimation of circulation rates":". Most ofthese studies were over a limited range of var-iables for very few systems. Inspite of the indus-trial importance of the natural circulation boilingof liquids in vertical tubes where flow and heattransfer interact with each other, no systematic

study seems to have been conducted to obtain theexclusive effect of important parameters on circu-lation rates.

In the present investigation, an attempt hasbeen made to study the exclusive effect of heatflux and submergence on circulation rates in asingle vertical tube thermosiphon reboiler.

Experimental ProcedureThe experimental reboiler was made of two

long vertical tubes connected to a vapour-liquidseparator and condenser vessels forming a ther-mosiphon loop as schematically shown in Fig. 1.The stainless steel tube which served as test sec-tion, was 25.56 mm i.d. and 28.85 mm o.d. Outof a total length of 2100 mm, a section of 1900mm was tapped between two thick copper clampswhich were designed to provide electrical connec-tion to the tube with almost negligible contact re-sistance. The test section was electrically isolatedfrom the rest of the set-up by means of especiallydesigned flanges fitted at the lower end and theupper glass tube section. The stabilized powerwas supplied through a low voltage high currenttransformer. The energy input to the test sectionwas measured by calibrated precision typevoltmeter and ammeter. In order to monitor theheat transfer surface temperatures along the tubelength, twenty-one copper-constantan thermocou-ples were spot welded on the outer surface of thetube at intervals of 50 mm up to a length of 200mm from the bottom end and 100 mm over theremaining length. A copper constantan thermoc-ouple probe was placed in the view port to mea-

KAMYL et al.: CIRCUlATION RATES IN A REBOILER lUBE 215

------- ..-------~

Fig. I-Schematic diagram of the experimental set-up [I Testsection, 2 Copper clamps, 3 View port for inlet liquid, 4 Glass

tube section. 5 Vapour-liquid separator, 6 Primary condenser, 7Spiral coil, 8 Secondary condenser, 9 Liquid downflow pipe, 10Cooling jacket, II Wall thermocouple, 12 Liquid thermocoupleprobes, 13 Liquid level indicator, 14 Condenser downflow pipe,15 Bubbler, 16 Feeding funnel, 17 Auxiliary heater, 18 Rotame-

ters, 19 Centrifugal pump, and 20 Cold water tank 1

sure the inlet liquid temperature. The temperatureof the boiling liquid before entry to the vapour-li-quid separator was measured by another travers-ing thermocouple probe.

In order to check the heat balance and reliabi-lity of data, a series of runs with forced convec-tion was taken using water. The experimentallymeasured values of heat transfer coefficient withforced convection agreed well with those predict-ed using Gnielinski's correlation \0 and Sieder andTate equation, with correction suggested by Kernand Othmer" showing a maximum deviation of± 10 per cent.

During the start up of reboiler operation forconducting a series of runs, the test liquid wasboiled off for about 8-10 h to remove the lasttraces of dissolved air which was indicated by thedisappearance of the air bubbles in the bubbler.Then a desired heat flux was impressed and cool-ing water flow rate to the condenser was regulat-ed to give a maximum temperature rise consistentwith no loss of vapour due to inadequate conden-sation. The liquid level in the down-flow pipe wasll£ijusted by adding/draining requisite amount of&st liquid. The unit was then operated for suffi-cient time to attain thermal equilibrium. When thesteady state conditions were established, the read-

ings of ammeter, voltmeter, thermocouples androtameters were recorded. The liquid level in thedownflow pipe was observed and noted from thelevel indicator provided for the purpose. Similarprocedure was followed for other liquids. The ex-perimental data were generated at four differentlevels of liquid submergence and various heatfluxes. While changing over from one system tothe other, the circulation system was freed com .•pletely from the previous liquid.

The details of the equipment and proceduresare given else where 7.

Process fluids and parameter studied-The li-quids were selected to cover a wide range ofthennophysical properties such as density, visco-sity, latent heat of vaporization, boiling point andsurface tension which influence the developmentof flow and boiling process. The liquids selectedwere distilled water, methanol and ethylene glycol.Distilled water was prepared in the research la-boratory while all the other chemicals were of la-boratory reagent grade. Methanol was obtainedfrom Oualigens Fine Chemicals, a division ofGlaxo India Ltd (India), while ethylene glycol wasprocured from Ajax Chemicals- Timbi (India).Distilled water served as a basis for comparisonsince nearly all work reported in the literature in-cludes water. Ethylene glycol provided an exam-ple of a high boiling point, high density, high vis-cosity and high surface tension fluid. The range ofparameters covered during experimentation aregiven in Table 1.

Data processing-Based on thermal equilibriummodel as suggested by Saha and Zuber'? the cir-culation rates and liquid bulk temperature distrib-ution in the thermosiphon reboiler have been de-tcrmined by making a heat balance on the testsection. The rate of liquid circulation was deter-mined by making a heat balance over the non-boiling or sensible heating region of the tube asdiscussed earlier!". The effective boiling and non-boiling zones over the entire heated length weredetermined from the amount of net vapour gene-ration. This could be obtained by the vapour con-densed in the condenser. A heat halance aroundthe condenser gives:

Table I-Range of experimental parameters

System Heat flux Submergence ~ T,uba> 10-4 S, % °C

W/m2

Distilled Water

MethanolEthylene glycol

0.57-4.30.41-2.1

1.6-3.0

30,50,75,100

30,50,75,10030,50,75,100

0.5-4.61.0-3.7

5.8-11.6

e , 216 INDIAN J. CHEM. TECHNOL.,JULY 1994

M. = [F Co (Tc - Tel )], (A+C~(7~-Ty)]

Thus

.. , (1)

AM,ZB=--

:rrqd

and ZI\:B= L- ZB ... (3)The rate of liquid circulation caused by buoy-

ancy induced flow through the test section wasevaluated by making a heat balance over the non-hoiling section of the tube

Q=:rrdZI\:Bq=mCdTs-I;.I) (4)

... (2)

:rr d ZNB qm=

C) (Ts - 7;))

The vapour fraction of two-phase mixture atthe exist of test section:

Xi = My/m ... (6)

The test liquid temperature distnbution alongthe non-boiling section of the tube were compu-ted by a linear relationship as givn below:

7; = 7;1 +(IS - TLJ) -~.. ( )Z '" 7NB

where Z~ZNB

r-----.-- ...- .-- ..-- .....---------,

114 5 - 100-Q .o.Tsub

~ 57300 33a 144410 46f-'Cl 25241.0 44tI. 380750 4·5

~ 43373{) 3~

112

108

106.u,~

I- 104

102 081",

II

I

100/

I --0I

98--Tw--- --h

970 0.4 OB 12 16 20 2.4

Z.rn

Fig. 2-Variation of wall temperature along the tube lengthwith heat flux as parameter for water

The variation of saturation temperature in theboiling section was computed taking into accountthe effect of hydrostatic head corresponding tovarious levels of liquid submergence studied.

The heat transfer coefficient at thermocouplelocations were calculated as

1/= q( Tw - 7; - 6. Tw)

Where the temperature drop across the wallthickness, 6. Tw, between the thermocouple beadand the inside surface was estimated using con-duction equation.

... (8)

(5)Results and Discussion

The wall temperature distribution along theheated length of test section was obtained fromthe experimentally measured values of surfacetemperatures at 21 locations on it. The typicalrepresentative plots of this distribution with heatflux as parameter is shown in Figs 2 and 3. Thisdistribution was indicative of effective lengths ofboiling and non-boiling zones in the reboiler tube

21G

m

208

204

200

U 196.-~

192

188

184

5=100q Alsub

o 21305·0 9·8C) 25241·0

1~~[4 29516.0 1 .•

---Tw

- ----- TL

180

08r : ...._---I

IIIII

I

I

176 L __ ~ __ ~_~ __ ~ __ ~ __ ~

o 0.4 0.8 1.2Z.rn

1.6 2.0 v,

Fig. 3-Variation of wall temperature along the tube lengthwith heat flux as parameter for ethylene glycol

KAMIL et al.: CIRCULATION RATES IN A REBOILER TUBE 217

0·5ATsub5

W 30 08 - 1·7

OJ • 50 10 - 27A 75 07 - 1·40 100 0.0 12

0·2

III 010

E 006

O·OL.

0·02

8 10 204, kWI m2

40 6080

Fig. 4- Variatiion of circulation rate with heat flux for water.

0.30 s, 30• 50• 750100

A Tsub08- 210.9- 15H -1.71..1,- 18

0.20

DiD008

0.06\/I-

0.02

0.013 20 308 10

Q,kW/m2

Fig. 5-Variation of circulation rat with heat flux for metha-nol

and was helpful in verifying the ZNB obtainedthrough the equilibrium model-P:".

The circulation of fluid through a closed loopthermosiphon system is established due to the dif-ferential head existing between the cold and hotlegs. The hydrostatic head in the cold leg (downflow pipe) of a thermosiphon reboil~r dependsupon the liquid submergence, the maxun~m ~alueof which in the present study equal the liquid le-vel corresponding to the top end of the test see-tion. This has been termed as 100% submergence

1/1-

5o 50V 75.100

A Tsub

50- 6279 - 8968 - 81

0.20

C\...·0.10E _0.08 -

0.06

0.0L.

0.0 2 L--- 1-_.-.L_-L----L--l.--1...-L....--.J

10 20 L.O 100q, kW/m2

Fig. 0- Variation of circulation rate with heat flux for ethy-lene glycol

(S = 100). The other values of submergence variedindependently were 75, 50 and 30%. The hydro-static head int he hot leg (Reboiler tube) is pro-vided by (he column of fluid consisting of two-phase mixture whose quality changes with boilingand vapour generation as the fluid flows upwards.The rate of circulation, therefore, depends uponliquid submergence, heat flux, inlet liquid sub cool-ing, vapour fraction and all those parameterswhich are involved in the frictional resistance ofthe circulation loop.

The fluid circulation rates through the reboilertuber were computed from heat balance as dis-cussed above. A typical variation in the values ofrates with heat flux has been represented by Figs4-6 for water, methanol and ethylene glycol, re-spectively. In the above figures, it is observedthat, the circulation rate increases linearly with qon logarithmic plot. The straight lines get shiftedto lower values of m as the submergence is re-duced from 100% to 30%. However, the slope oflines is almost same for a liquid. The slightchange in slopes and scatter in data may be dueto the variation in vapour fraction and inlet liquidsubcooling in the above plots. At a given sub-mergence, the liquid head in the cold leg remainsunchanged while an increase in heat flux shiftsthe point of boiling incipience towards tube inlet.Thus the saturated boiling occupies a large lengthof reboiler tube resulting in a higher vapour frac-

218 INDIAN J. CHEM. TECHNOL.,JULY 1994

0·30

0·['0 ~---------------,

0·20

0·10

UI 008

C7I~ 0·06E

0·04

0·02 Q ATSUbA 14441·0 07 - 1.11

• 25241.0 0.0 - 27

0 380740 0.0 - 1.7

001L-_~_~L-~~~~~-L __ ~20 30 50

5100

Fig. 7-Vananon uf circulation rate with submergence forwater

O·/,O

0·30 A

0.20

~

~ 0.1001~E 0.08

0.06

qA 12289·0• 16599·0G 21305.0

Alsub1.1-1·8D.9-2·20.9-1.7

q

A 251101·0• 195150

ATsub

6·8- 8·15·7 - 7·1

0.30

0.20

III

~0.10~E-o.oe

0.06

0.04

Fig. 9- Variation of circulation rate with submergence forethylene glycol

tion. The average density and hence the head oftwo-phase mixture gets reduced. The differentialhead responsible for fluid circulation, thus, getsenhanced with heat flux, explaining the increaseof m with q. As the submergence is lowered, theliquid head gets decreased while the vapour frac-tion increases due to the enhanced effect of satu-rated boiling in the tube. However, the differentialhead causing circulation becomes smaller thanthat at higher value of S. This explains the shiftingof lines to lower values of m with S in Figs 4-6.The effect of S on m has also been shown inFigs 7-9 wherein an increase in m with S is ob-served. The data points are found to lie almost instraight lines. As a result of increase in the circu-lation rate with submergence, the change of liquidtemperature along the tube length is diminished,and it requires a longer length of tube to attainsaturation value. This seems to be the reason whythe points of fully developed saturated boiling getshifted to larger Z at higher values of S. In the ex-perimental runs the inlet liquid subcooling couldnot be regulated to the desired value and theyprogressively increased on decreasing the heatflux as submergence was varied from 100 to 30per cent. The values of vapour fraction alsochanged in various runs. This seems to be respon-sible for the scatter of data and changes in theslope of lines.

It has been observed that circulation ratethrough the reboiler tube depends on the wall

Fig. 8- Variation of circulation rate with submergence formethanol

KAMIL et al.:CIRCULATION RATES IN A REBOILER TUDE 219

o Wat~r• M~ttlQnol• EthylA,"~ glycol

100

5

Ca'?culat~d ( Re X 163)

Fig. IO-Comparison of calculated (Rex 10-') and those pre-dicted by Eq. (10)

heat flux, liquid level in the cold leg, in let liquidsubcooling and vapour fraction for a given liquid.The linear variation of m with q and S in Figs 4-9(log-log plots) suggest that a functional relation-ship of the following form may be used to correl-ate the data.

... (9)

The effect of heat flux, inlet liquid subcooling andvapour fraction has been included in Eq. (9) asPeB, x.; and Xu, respectively.

The values of indices m1, m-, m, and m4 'andthe constant C 2 were determined by the regres-sion analysis using the data for all the test liquidstogether and the following equation resulted:

Re= 2.72 (PeB)093 (S)OX3 (Xtt)On82 (Ksuht0395

A comparison between the values of Re pre-dicted by Eq. (10) and those obtained from theexperimental data for all the systems in Fig. 10shows a maximum error of ± 25%.

ConclusionThe circulation rate Increases with heat flux

and decreases as the submergence is loweredfrom 100% to 30%. An empirical correlation hasbeen developed to predict the circulation rates.The form of the proposed equation is fairly gen-eral, however, its applicability to other systemsmay have to be substantiated with more data. Theresults may be used in uniform wall temperatureproblems if the heat flux profile is known. Thecorrelation obtained by Ali and Alam 1 had based

on their own experimental data was checkedagainst the data of the present study. The resultof comparison showed the maximum mean devia-tion (40%), as they had neglected the effect of animportant parameter, Xtt in their correlation.

Nomenclaturea cross sectional area of heated tube m2

C heat capacity J/kg °CC heat capacity of cooling water J/kgOC

cd inside diameter of heated tube mF condenser flow rate kglsh heat transfer coefficient W/m20Ck thermal conductivity W/moCL total heated length mm circulation rate kgls/1.1L liquid flow rate from separator kglsMv liquid flow rate from condenser kglsq heat flux W/m2S submergence 0/0T temperature °C(K)TCI inlet temperature of cooling water to

condenser °CTC2 outlet temperature of cooling water from

condenser °CTu inlet temperature of liquid to the tube °CT, saturation temperature of liquid °CT, condensate temperature in condenser

vessel °C!:l.i: degree of subcooling, ( Ts - TL ) °CZ distance along the test section m

Greek lettersA = latent heat of vapourization J/kgP = density kg/rn'fA = dynamic viscosity Ns/m 'o = surface tension N/m

(10)

Subscripts8 boilingL liquidNB non-boiling08 onset of boiling

saturationv vapourw wall

Dimensionless groupsK,ub = Subcooling number, (1+ Pt.!:l. T..Ublp, T.,)

PeB =Peeler numberfor boiling [qPL CJ APs kd 01 g(PL - Ps)05]

Re= Reynold's number, (d m/a fA)

X" = Lockhart-Martinelli parameter,

References1 Ali H & Alam S S, Int J Heat Fluid Flow, 13(1) (1992)

86.2 Beaver P R & Hughmark G A, A IChE J, 14 (1968) 746.3 Fair J R, Pet Refiner, 39 (1960) 105.

220 INDIAN J. CHEM. TECHNOL., JULY 1994

4 Hallinan K P & Viskanta R, lnt J Heat Fluid Flow, 6(1985)4.

5 JohnsonAE, Chem Eng Prog Symp Ser, 52(1956)37.

6 Johnson 0 L & Yukawa Y, Chem Eng Prog, 75(7) (1979)47.

7 Kamil M, Studies on a vertical tube thermosiphon reboil-er, Ph.O. Thesis, Aligarh Muslim University, Aligarh,1992.

8 Shellene K R. Sternling C V, Church 0 M & Synder N H.Chem Eng Prog Symp Ser, 64 (1968) 102.

9 Smith J V, Chem Eng Prog, 70 (1974) 68.10 Shah R K & Johnson R S, Correlations for fully developed

turbulent flow through circular and noncircular channels.

6th Nat Heat Mass Transfer Conf. I.I.T. Madras, HMT-126-81. 075-D96 (1981).

11 Kern 0 Q & Othrner 0 F, Forced convection experimentsand correlation. Vol. I. edited by Jacob M (John Wileyand Sons, New York), 1964,543.

12 Shah P & Zuber N, lilt J Heat Mass Transfer, 21 (1978j415.

13 Kamil M, Ali H & Alam S S, Predicting the onset of nu-cleate boiling in a vertical thermosiphon reboiler, Proc3rd World Conf on Experimental Heat Transfer, FluidMechanics and Thermodynamics, Hawaii, USA, Oct 31-Nov 5 (1993) 1232.

14 Kamil M, Ali H & Alam S S, Control France C. 40(2)(1994) 19.