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Experimental Analysis of Heat and Mass Transfer in a Two Stage Jet Cum...F F

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Experimental Analysis of Heat and MassTransfer in a Two Stage Jet Cum

Packed Bed Deaerator

K. V. Suryanarayana1*, K. V. Sharma2, P. K. Sarma3, V. Dharma Rao4,D. M. Reddy Prasad5

*1Department of Chemical Engineering, Sri Venkateswara EngineeringCollege, Suryapet 508213, E-mail: [email protected].

2Faculty of Mechanical Engineering, Universiti Malaysia Pahang, 26300Gambang, Kuantan, Pahang DM, E-mail: [email protected].

3International Director, GITAM University, Visakhapatnam 530045,E-mail: [email protected].

4Department of Chemical Engineering, A.U College of Engineering,Visakhapatnam 530002, E-mail: [email protected].

5Faculty of Chemical Engineering, Universiti Malaysia Pahang, 26300Gambang, Kuantan, Pahang DM, E-mail: [email protected].

ABSTRACT: Experiments have been conducted to study the effect ofmass flow rate on heat and mass transfer coefficients in a two stagedeaerator. It has been observed that an increase in mass flow rate ofwater increases the heat and mass transfer coefficients in both thestages of the deaerator. The empirical correlations of Mayinger [7] forthe first stage and the equation of Rabas et al. [4] for the second stagepredicts the mass transfer coefficients satisfactorily in the experimentalrange tested. The diffusion of oxygen from deaerator feed waterobtained from experimental data can be used to estimate the masstransfer coefficients.Keywords: Deaerator, two stages, jet flow, packed bed, heat and masstransfer coefficients, oxygen stripping.

1. INTRODUCTIONBoiler feed water may contain significant amounts of dissolvedoxygen, which causes pitting and iron deposition. Deaerators areused to remove these dissolved and corrosive gases from boiler

International Journal of Material Research, Electronics and Electrical SystemsJanuary-December 2011, Volume 4, Number 1-2, pp. 91– 106

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International Journal of Material Research, Electronics and Electrical SystemsF F

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makeup feed water. The makeup water and the condensate areheated in the deaerator for removal of oxygen before entry into theboiler as even small amounts of dissolved gas can cause significantcorrosion. The high temperature of boiler feed water will furtherincrease the corrosivity due to dissolved oxygen, if left untreated.

In jet cum packed bed deaerator, the incoming water enclosedin a chamber is passed through a pipe which is located at the top ofthe deaerator and gets collected at a distance of 0.55m from the pipe,referred to as the first stage. The length of the second stage is 1.2mand packed with 38mm nominal size pall rings made of nylon. Boththe first and second stages are engulfed with steam at a pressure.Experiments are conducted at different flow rates of deaerator water.

Hasson et al. [1, 2] theoretically analyzed the jets of variousconfigurations such as fan spray sheet, uniformly thick sheet andcylindrical jet subjected to zero surface resistance. For Graetz numberless than 30, the local Nusselt number is constant for all the threejets. In this region, the Nusselt number of cylindrical jet is observedto be lower compared to the other two. For Graetz numbers greaterthan 100, the local Nusselt number of fan spray sheet is observed tobe higher compared to the other two types. The experimentalinvestigation revealed the heat transfer coefficients to vary between11630 and 23260 W/(m2K) and remained unaltered when steampressure is reduced from higher atmospheric to sub atmosphericpressure. The heat transfer coefficient decreases with increasing aircontent in steam. Nosoko et al. [3] conducted experiments on oxygenabsorption using a single column horizontal tube bank of 16mmdiameter and 284mm wetted length. They observed Sherwoodnumber to increase with increase in tube spacing from 2 to 5mmand then levels off at 10mm or higher. They concluded that thevolume of horizontal tube absorber to be 1/2.2 to 1/1.18 times lowercompared to vertical orientation for the same heat duty. Rapaset al.[4] presented the experimental data on mass transfer coefficients ofcounter flow pall-ring packed deaerator. The authors have takenthe data from full size deaerator operating at desalination plants,checked the mass transfer performance of two inch pall-ring packingand spray nozzle distribution system. The effects of antifoam and


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the stripping steam flow rate on the effluent oxygen concentrationare not considered. Ferro et al. [5] observed oxygen concentrationto decrease with increase in temperature of seawater at inlet ofdeaerator. Costa et al. [6] observed the performance of the oxygenstripping of the packed bed when the steam flow rate is low. Theyconcluded that maximum oxygen content in the outlet water to be20–30 ppb even in the absence of steam.

The estimation of condensation heat transfer coefficient fromjets by inducting non-condensable gas into vapor region has beenanalyzed by many. Heat and mass transfer studies with non-condensable gas such as oxygen getting stripped from boiler feedwater is quite limited. Hence, it is proposed to conduct experimentsto estimate the heat and mass transfer coefficients from jet cumpacked bed deaerator at different mass flows rates and its impacton oxygen stripping from the boiler feed water.

2. FABRICATION OF THE EXPERIMENTAL SETUPThe experimental setup consists of 0.15m dia column, 0.55m lengthin first stage and 1.2m length of packed bed in second stage. A 200liters feed water storage tank, a steam jacket on the deaeratorwater inlet pipe for regulating its temperature and a pump forcirculating water are other accessories. In the steam circuit a pressureregulator and steam trap are connected to a buffer tank for removalof water droplets after steam expansion in the pressure regulator.A water bath of 25 liters capacity with a copper coil to regulate thetempe-rature of sample water to 30-40 ºC connected to DissolvedOxygen (DO) meter, flow components such as valves, flow meters,pressure gauges and thermocouples are provided. The process andinstrumentation diagram of the experimental setup is shown inFig. 1.

3. JET FLOWSThe estimation of heat transfer coefficients due to condensation ofsteam on water jets is of great significance as high heat transfercoefficients can be attained with these cylindrical jets. The analysisof the data is based on the following assumptions.


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3.1. Assumptions(a) The liquid jet moves in a medium of constant temperature.(b) Jet velocity is assumed constant, neglecting the interfacial drag

and reaction on the jet due to vapor initially at rest condensingon a moving liquid.

(c) The variation of properties such as CP, , k of water with tempe-rature is negligible.

(d) The temperature of water in the second stage is assumed toincrease linearly.

3.2. Estimation of Heat Transfer Coefficients-Stage 1The heat conduction equation for cylindrical jets can be written as

V1JT

z

∂∂

= α2

2

1J JT T

r r r

∂ ∂+ ∂ ∂

...(1)

Where V1, the jet velocity is estimated based on volumetric flowrate of water and the diameter of the jet. The variation of non-dimensional temperature with Graetz number for condensingvapors on laminar liquid jets for the condition of zero resistance atthe interface has been presented by Hasson et al. [1]

θ = 0.6915 Exp (–23.136/Gz) for Gz ≤ 10 ...(2a)

and θ = 1 – 8

Gzπ for Gz > 10 ...(2b)

where Gz = V1 D21 /α z

From the definition of dimensionless temperature, the tempe-rature of water at the end of first stage can be determined from therelation

TJ = TS – (TS – Ti)θ ...(3)The average Graetz number GzAV for the first stage can be

evaluated by integrating

GzAV =1

1

L

1

0

( )L

Gz z dz∫ ...(4)


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The average Nusselt number and heat transferred in the firststage can be estimated

NuAV = 5.784 for GzAV ≤ 10 ...(5a)

NuAV =( / )

1– 8/AV

AV

Gz

Gz

ππ

for GzAV > 10 ...(5b)

Q1 = h1 A1(TS – Ti) ...(6)where h1 = NuAV k/D1

3.3. Estimation of Heat Transfer Coefficients-Stage 2The energy balance equation for the deaerator comprising of thefirst and second stages can be estimated from the relation

QE = m CPL(To – Ti) = Q1 + Q2 ...(7)The heat transferred in the second stage Q2 is calculated using

Eq. (7)

Q2 = QE – Q1 ...(8)Heat transfer coefficient in the second stage is estimated using

Q2 and log mean temperature difference as the driving force in thesecond stage.

h2 = 2

2 LN

Q

A T∆...(9)

The theoretical heat transfer also be estimated from the heattransfer coefficients evaluated for the first and second stages givenby

QT = h1 A1(TS – Ti) + h2 A2(TS – TJ) ...(10)

3.4. Estimation of Mass Transfer Coefficients in First StageThe following assumptions are made in the analysis to evaluate thequantity of dissolved oxygen removed.

1. Jet is cylindrical in configuration.2. Diffusion occurs under non-isothermal conditions i.e. the

major resistance for diffusion is within the liquid.


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3. The resistance for diffusion of oxygen from the interface tosteam environment is negligible.

The component continuity equation can be written as

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Dπ

L1 ρL

( )dX t

dt= –kL,J CA π D1 L1 ...(11)

Further in the evaluation of X(t) in Eq. (11), the mass transfercoefficient, kL,J should be known apriory. The empirical correlationsof Mayinger [7] valid in the laminar-wavy, transition and turbulentflow regime for falling films are given by

ShJ = 2.24 × 10–2 Re0.8J Sc0.5

J ...(12a)valid in the range 12 ≤ ReJ ≤ 70 and ScJ ≥ 2.32 × 104/Re1.6

J

ShJ = 8.0 × 10–2 Re0.5J Sc0.5

J ...(12b)valid in the range 70 ≤ ReJ ≤ 400 and ScJ ≥ 1.82 × 103/ReJ

ShJ = 8.9 × 10–4 Re1.25J Sc0.5

J ...(12c)for Rej ≥ 400 and ScJ ≥ 1.47 × 107/Re2.5

J

Eq. (11) is rearranged and solved in conjunction with Eq. (12)for the initial condition t = 0, X = Xi to obtain

( )

i

X t

X= exp

1

–4 J LSh D t

D

δ

...(13)

where DL = (7.481 × 10–16 × TAV)/ (µL vA0.6 ).

3.5. Estimation of Mass Transfer Coefficients in Second StageThe mass transfer is governed by the rate equation given by

JA = kL,p ∆xLN ...(14)where ∆xLN is the log mean concentration difference and

expressed as

∆xLN =* *

, , 0, ,

* *, , 0, ,

( – ) – ( – )

( – ) /( – )i P i P P o P

i P i P P o P

x x x x

x x x x

...(15)


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Rabas et al. [4] have calculated the equilibrium concentration ofoxygen in water at the inlet and outlet of packed bed with thefollowing equations

*,i Px =

2 ,

( / )

(1 / ) / ( / ) /D C

e A N W A p i F fg i

P H

K M M M M s C T H x + + + ∆ ...(16)

*,o Px = A A

D C V

y M

P H M...(17)

Eqs. [15] - [17] are substituted in Eq. [14] to obtain an equationfor mass transfer coefficient as

kL,p =*

C

V

A, ,–i P o P

LN

x x

x∆...(18)

Another expression for determination of mass transfer coefficienthas been given by Costa. et al. [6]

,CL Pk =

2

'LM

C

NTU L

a A L...(19)

where NTULM = In ,

,

i P

o P

x

x

The mass transfer coefficients are evaluated using Eqs. (18) and(19) which are applicable to packed beds.

4. RESULTS AND DISCUSSIONThe temperature and concentration at the end of the first stage areevaluated using Eqs. (3) and (13) respectively and salient resultspresented. The increase in mass flow rate of deaerator waterincreases heat transfer coefficients in the first and second stages ofthe deaerator as shown in Fig. 2. The rate of increase is more in thefirst stage than in the second due to higher temperature potentialbetween steam and water. A comparison of the heat transferred fromboth stages estimated from energy balance Eq. (8) with valuesestimated with Eq. (18) is in good agreement as can be seen fromFig. 3. This validates the heat transfer coefficients obtained with Eqs.(7) and (9) for the first and second stages respectively.


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Experiments on direct contact condensation have been conduc-ted with steam-water by Genic [8] who presented a regressionequation for the estimation of liquid phase transfer units with kineticenergy parameter given by

CLNTU = 0.185 –1.48

LVF ...(20)

A comparison of the present data estimated for these parametersis in good agreement shown as Fig. 4 valid for the deaerator first stage.

The variation of experimental Nusselt with Reynolds number isshown in Fig. 5 along with the data of Kuznetsov et al. [9] who hasdone for upward gas-liquid flow in a tube filled with spheres. Experi-mental data with upward steam and downward water flow is notavailable in literature. The trend of the present experimental datafor both the types of deaerator is satisfactory when compared. Theincreasing trends of mass transfer coefficient shown in Fig. 6 forboth the first and second stages of deaerator with increase in massflow rate of water are similar to increases in heat transfer coefficientas can be seen from a comparison with Fig. 2. The variation ofReynolds number with ShJ × Sc–0.5

J for different operating conditionsin the first stage of the deaerator are shown in Fig. 7. The experimentalvalues are in good agreement with the values estimated with theequation of Nosoko et al. [3].

6. CONCLUSIONSThe following conclusions can be drawn from the present analysis.

(a) The increase in mass flow rate of deaerator water increasesthe heat transfer coefficients for both stages of the deaerator.

(b) The heat transfer coefficient is 10 to 7 times higher for firststage compared to second stage.

(c) The heat transfer coefficient in second stage containing pallrings has the values in the range of 200 – 500 W/m2K.

(d) The overall heat transfer estimated with heat transfercoefficients is in good agreement with the values calculatedfrom energy balance equation.

(e) The increase in flow rate of deaerator water increases themass transfer coefficients in the first and second stages.


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(f) The empirical correlations of Mayinger [7] predict theSherwood number satisfactorily for the first stage.

(g) The mass transfer coefficient in the second stage of jet cumpacked bed deaerator estimated with the theoretical equationof Costa et al. [6] is in good agreement with the presentexperimental values.

Figure 1: Process and Instrumentation Diagram of Deaerator Experimental Set-Up.

Note: The deaerator and the steam transfer line is lagged with glass wool insulation.


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Figure 2: Effect of Mass Flow Rate of Deaerator water on Condensation HeatTransfer Coefficient.

Figure 3: Comparison of Total Heat Transfer Estimated from Theory with That ofExperiment.


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Figure 4: Effect of Kinetic Energy Parameter on Liquid Phase Transfer Units inFirst Stage of Jet Cum Packed Bed Deaerator.

Figure 5: Comparison of Present Data with That of Kuznetsov et al. [9] withPorous Systems and Packing.


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Figure 6: Effect of Mass Flow Rate of Deaerator Water onMass Transfer Coefficient.

Figure 7: Effect of Reynolds Number on ShJ × ScJ( -0.5).


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NOMENCLATUREa packing area per unit volume, m2/m3.

A surface area for heat transfer, m2.

AC area of cross section for packed bed, m2.

CA Concentration of oxygen, ( = ρL X) kg/m3.

CP specific heat, J/kg K.

D diameter of the jet, m.

DL diffusion coefficient or diffusivity of oxygen in water, m2/s.

∆TF temperature drop or flashing at packing inlet, 0.3 to 0.6 °C.

∆TLN log mean temperature difference,

[(TS – TJ) – (TS – To)]/ln[(TS – TJ)/(TS – To)]

FLV kinetic energy parameter, 1 V

Ls

ρρ

.

g local acceleration of gravity, 2sm

Gz Graetz number

h heat transfer coefficient, W/m2K.

HC Henry’s law constant, N/m2/ppb

Hfg latent heat of condensation, J/kg.

JA molar flux of component A, kg – mol/m2s.

k thermal conductivity, W/mK.

Ke ratio of oxygen to nitrogen mass at packing inlet (≈ 1.75).

kL mass transfer coefficient, m/s.

L length, m.'L superficial mass velocity of flowing liquid, kg/m2s.

M molecular weight, kg/kg – mol.


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m mass flow rate of water, kg/s.

NTULH number of liquid phase transfer units for heat transfer,

ln [(TS – Ti)/(TS – TJ)]

NTULm number of liquid phase transfer units for mass transfer.

Nu Nusselt number.

PD deaerator pressure, N/m2

Q heat transfer, W

r radius of the liquid jet, m

ReJ Jet Reynolds number, / LΓ υ .

s mass flow rate ratio of stripping steam to the inlet water.

ScJ Schmidt number of the liquid jet, υL/DL.

ShJ Sherwood number of the liquid jet, kL,J δ/DL.

t residence time, s

T temperature, K

TAV fluid properties evaluated at the average temperature,

{(To + Ti)/2}, K.

vA molecular volume of oxygen, m3/kg–mol.*

V volumetric flow rate of water into the deaerator, m3/s.x mole fraction of O2 in water.*x equilibrium mole fraction of O2 in water.

X mass fraction of 2O in water, ppb.

X* equilibrium mass fraction of O2 in water, ppb

y mole fraction of O2 in steam.

V Velocity of liquid, m/s.

z coordinate along the jet.


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Greek Symbolsα thermal diffusivity, m2/s.

Γ volumetric flow rate per jet perimeter, m2/s.

δ Nusselt film thickness, (3υL Γ/g)1/3, m

θ non-dimensional temperature at the end of the first stage.

υ kinematic viscosity, m2/s.

µ viscosity, kg/ms.

ρ density, kg/m3

SubscriptsA Oxygen or non-condensable gas.

AV average.

E energy balance

H heat transfer

i inlet

J jet

L liquid

M mass transfer

N2 nitrogen

o outlet

P packed bed

S saturation

T theoretical

V water vapor

W water

1 first stage

2 second stage


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REFERENCE[1] Hasson D., Luss D. and Peck R., “Theoretical Analyses of Vapor Condensation

on Laminar Liquid Jets”, Int. J. Heat and Mass Transfer, 7, 1964, pp. 969-981.[2] Hasson D, Luss D, Navon U., “An Experimental Study of Steam Condensation

on a Laminar Water Sheet”, Int. J. Heat and Mass Transfer, 7, 1964, pp. 983-1001.

[3] Nosoko T., Miyara A., Nagata T., “Characteristics of Falling Film Flow onCompletely Wetted Horizontal Tubes and the Associated Gas Absorption”,Int. Journal of Heat and Mass Transfer, 45, 2002, pp. 2729-2738.

[4] Rabas T. J. Inoue S. and Shimizu A., “An Update on the Mass Transfer ofCounter Flow, Packed Deaerators Containing Pall-Ring Packing”, Desalination,66, 1987, pp. 91-107.

[5] Ferro E., Ghiazza E., Bosio B., and Costa P., “Modeling of Flash and StrippingPhenomena in Deaerators for Seawater Desalination”, Desalination, 142, 2002,pp. 171-180.

[6] Costa P., Ferro A., Ghiazza E. and Bosio B., “Seawater Deaeration At VeryLow Steam Flow Rates in the Stripping Section”, Desalination, 201, 2006,pp. 306-314.

[7] Mayinger, F., “Strömung und Wärmeübergang in Flüssigkeitsgemischen”,Springer Verlag, Berlin, 1982.

[8] Genic’ S.B., “Direct-Contact Condensation Heat Transfer on DowncomerlessTrays for Steam-Water System”, Int. J. Heat Mass Transfer, 49, 2006, pp. 1225-1230.

[9] Kuznetsov V.V., and Dimov S.V., “The Influence of the GeometricalParameters of a Porous Medium on Two-Phase Filtration”, 4 th Symp. MultiphaseTransport in Porous Media: Prpc. ASME Winter Annual Meeting, Nov. 27–30,1993, pp. 207–223.

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