J. of Thermal Science Vol.11, No.2

An Experimental Study on Latent Heat Recovery of Exhaust Wet Flue Gas

Li Jia

Xiaoping Li

Jindong Sun

Deparment of Urban Construction Engineering, Beijing Institute of Civil Engineering and Architecture, Beijing 100044

Xiaofeng Peng Department o f Thermal Engineering, Tsinghua University, Beijing 100084

The experiment was conducted to investigate the heat transfer performance of wet flue gas in a vertical tube. The factors influencing the convective condensation of wet flue gas were experimentally investigated. The measured results indicate that the convective heat transfer of bulk flow and condensation heat transfer of vapor have significant contribution to the total heat transfer and the dominant transport mechanism is dependent upon the vapor fraction in mixture.

keywords: latent heat, recovery, exhaust flue gas. Introduction The convection-condensation heat transfer of gas mixture containing condensable gas is an important process in the chemical and power industry and has been extensively studied t~-3]. Understanding the phenomena and determining total heat transfer coefficient are of critical importance for gas mixture, since this kind of heat transfer process is comprehensively encountered in the utilization of waste or low temperature energy I41. Actually, This is an interesting research topic due to the existence of partial condensation in gas convection. Although the condensation of vapor from a vapor mixture having very little non-condensable gas in tubes has been studied for many years E5"6],the main interest of these studies is not to detect the effects of condensation of vapor on convection heat transfer, but to predict the effects of non-condensable gas on condensation heat transfer. For the gas mixture with low vapor fraction, few work has been published to indicate the influence of condensation on convection heat transfer in tubes, except a few reports on wet air phenomenon tTl. It is well known that the significant part of energy losses for boiler is the heat carried by exhaust flue gas to atmosphere. It consists of sensible and latent heat deposited in the vapor. The latent heat can be recovered Received 2001

by condensing the vapor in wet flue gas having water vapor volume fraction of 8%~30%. Studying the effects of condensation of water vapor on convection heat transfer of wet flue gas in a vertical tube is currently of practical importance and theoretical interest for saving energy.

Experimental System Fig.1 schematically illustrates the experimental system. Wet flue gases entered a vertical tube at a velocity of Ub and was cooled by counter-flow water in the cooling jacket. When the temperature at the wall surface is lower than the dew-point temperature of mixtures in the vertical tube, the vapor could condense and form a layer of liquid film with thickness 8 on the tube wall. The gas mixture contains dry flue gas and water vapor produced by natural gas combustion. The vertical test condenser was composed of an inner stainless steel tube of 0.038 m o.d. and 0.033 m i.d., surrounded concentrically by a steel cooling jacket. The length of the condensing section was 2 m. The test section was wrapped with thermal insulation material to avoid heat releasing to ambient environment. Condensate was collected by plexiglass elbow.

Li Jia et al.

An Experimental Study on Latent Heat Recovery of Exhaust Wet Flue Gas

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1 natural gas 2 boiler 3 flow-regulating valve 4 differential pressure meter 5 humidifier 6 computer 7 exhauster 8 inlet of cold water 9 outlet of cold water Fig.1 Diagrammatic sketch of vertical condensing tube system

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Fig.4 illustrates the comparison between latent and sensible heat transfer component in total heat transfer. The results show that the proportion of condensation heat transfer in total heat transfer increased rapidly and the role of convection heat transfer becomes less significant as vapor mass fraction increases. From these observations, a conclusion can be reached that the condensation heat transfer has great contribution to total heat transfer and even much higher than that of convection heat transfer. Apparently, if mass fraction of water vapor is higher than about 8% in the wet flue gas, condensation heat transfer dominates the transport process and the single-phase convection heat transfer was relatively less significant and even could be neglected. But for lower Re number, the situation would be different [8].

Results and Discussion

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The experimental results can be correlated as an empirical correlation as usually done. Two methods were employed to process the experimental data, one is the Wilson method and convective heat transfer correlation was obtained. The other is the Newton cooling equation. Fig.2 gives the comparison of experimental data obtained from correlation method and surface temperature method. According to Fig.2 the results of two methods are in good agreement.

400

/

300

¢

200 100 5

15

25

cg(~)

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Fig.3 Relation between Nu and mass fraction of vapor

120 O experimental

(Re=13000-15000,

/ 0 /

t w= 33.7°C~39.1 °C)

100 500 8O

tion

400 6O

300

40 20

200

~single-phase convection

100 20

40

60

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Fig.2 Comparison of theoretical and experimental results

The effect of mass fraction of vapor Vapor mass fraction of mixture is one of the important factors affecting condensation heat transfer and the heat transfer mechanism is actually dependent on the mass fraction of vapor. Figs.3 and 4 illustrate the heat transfer performance at different mass fraction of water vapor in the wet flue gas. Obviously, the N u number increases with the Cg - mass fraction of water vapor.

0

i

5

I

I

I

15 cg(~) 25

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Hg.4Relafionofcondensafionhem~ansferand he~ vansfer~differentvaporfracfion (Re = 11500) Compared with the results in reference [8], one might conclude that in transition region the condensation heat transfer is one order of magnitude larger than that of single-phase convection heat transfer when mass fraction of vapor exceeds 20%. Whereas for turbulent flow the

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Journal of Thermal Science, Vol.11, No.2, 2002

condensation heat transfer will be one order of magnitude larger than that of convection heat transfer when mass fraction of vapor is about 8%. Mass fraction of vapor as well as Re number of wet flue gas greatly affect the latent heat transfer. The effect of wall and water temperature Figs.5 and 6 illustrate the Nu-Re relation at different wall temperatures. Obviously, the wall temperature alters the condensation contribution to the total heat transfer in a vertical tube. When the wall temperature is high, the temperature at the film surface would be accordingly high and bulk gas is in superheated situation. The vapor concentration at the interface is very small and mass transfer becomes less significance. The lower wall temperature produces lower vapor concentration in the gas at the interface of liquid film and induces larger concentration gradient in the wet flue gas.

tube. With the inlet temperature of cold water increasing, condensation of vapor from the wet flue gas would become weak and less latent heat recovery of exhaust flue gas would be obtained. Fig.7 shows the relation of Nu varying with Re at different inlet temperature of cold water. With the Re increasing, the difference between Nusselt number at different ts becomes larger. 120 100

--II-- 2

80 6o

A

3

X

4

4o 2o

3000

5000

7000

9000

Re 30

Fig.7 Relation between Nu and Re under different inlet temperature of cold water (1 t~= 18.8°C, 2 t~= 30.9°C, 3 t~=45.7°C, 4 ts=50.6°C)

25

20

lOO

lo

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,

0

2000

lateint

,

i

i

3000

4000 /le

5000

80 6000

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Fig.5 The comparison between latent and sensible heat transfer (tw= 43.84°C, Cg= 13%)

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0.2

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0.4

0.6

3

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0.8

1.0

Z/L

70 60

20 o.o

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-----Amsensible] Fig.8 Relation between Nu and dimensionless length under different inlet temperature of flue gas (1 t g = 6 0 ° C , 2 t g = 6 5 ° C , 3 tg=70°C, 4 tg=75°C)

50 4O 3O 20 lO 0

2000

I

L

|

3000

4000

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Fig.6 The comparison between latent and sensible heat transfer (tw= 37.4°C, Cg= 13%) Figs.7 and 8 are Nu-Re relations at different cold water temperature and different position in the vertical

Effect of wet flue gas temperature The relation of Nu number with the dimensionless length at different inlet temperature of flue gas was given in Fig.8 (tw=37°C, mass fraction of vapor Cg is 13%). Nu number decreases as the dimensionless length increases. The higher inlet temperature of gas mixture enlarges the temperature difference of sensible heat transfer and the vapor temperature of gas mixture simultaneously. At higher temperature of gas mixture, the variety of inlet temperature of gas mixture has larger effect on Nu than that at lower temperature.

Li Jia et al.

An Experimental Study on Latent Heat Recovery of Exhaust Wet Flue Gas

Conclusions 1. Vapor condensation in wet flue gas is a complex process of heat and mass transfer. When mass fraction of vapor in wet flue gas is about 8 % ~ 2 8 % , the heat transfer of wet flue gas differs from single-phase convection heat transfer of humid air and condensation heat transfer of vapor containing small amount of non-condensable gas. 2. With the increase of mass fraction of vapor, condensation heat transfer dominates the transport process and the single-phase convection heat transfer could be neglected. 3. At the higher temperature of mixture gas, the variety of inlet temperature of mixture gas has larger effect on Nu than at the lower temperature.

Acknowledgement This work was supported by science and technology key project of Ministry of Education (Contract No.00129) and National 973 Program (Contract No.2000026301).

References [1]Nusselt, W. Die O b e r f l a c h e n k o n d e n s a t i o n

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Wasserdampfes. Z. Ver. Dt. Ing., 1916, 60:541--546 [2] Jacob, M. Heat Transfer in Evaporation and CondensationII. Mech. Engng., 1936, 58:729--740 [3] Carpenter, F, Colbum, AP. The Effect of Vapor Velocity on Condensation Inside Tubes. In: Proceedings of a General Discussion of Heat Transfer. IME/ASME, 1951 [4] Jia, L, Li, X P, Sun, J D. The Experimental Research on Water Vapor Condensation of Wet Flue Gas in a Vertical Tube. In: Proceedings of International Conference on Energy Conversion and Application. Wuhan, 2001. 481-484 [5] Cossmann, R, Odenthal, H P, Renz, U. Heat and Mass Transfer During Partial Condensation in a Turbulent Pipe Flow. In: Proc. of 7th Int. Heat Transfer Conference. 1982. 53--58 [6] Gnielinski, V. New Equations for Heat and Mass Transfer in Turbulent Pipe and Channel Flow. Int. Chem. Eng., 1976, 16: 161-- 173 [7] Fuji, T, Nagata, T, Shiniato, K. Condensation of Water Vapor and Heat Transfer from Humid Air to Horizontal Tubes in a Bank. Refrigeration, 1982, 57 (658): 787--798 [8] Jia, L, Peng, X F, Yah, Y. Effects of Water Vapor Condensation on the Heat Transfer of Wet Flue Gas in a Vertical Tube. International Journal of Heat and Mass Transfer, 2001, 44 (22): 4257--4265