Experimental analysis of bubble mode in a plate-type absorber

doi:10.1016/S0009-2509(02)00089-1

Chemical Engineering Science

Volume 57, Issue 11, June 2002, Pages 1923-1929

https://doi.org/10.1016/S0009-2509(02)00089-1 Get rights and content

Abstract

An experimental analysis of ammonia–water absorption was performed in a plate-type absorber. The flow of water and ammonia gas was performed in the bubble mode. The experiments were made to examine the effects of solution flow rate and gas flow rate on the performance of the absorber. It was found that the increase of solution flow rate rarely affected the mass transfer, but improved the heat transfer. As the gas flow rate increased, slugging occurred in the bubble mode and influenced the thermal boundary layer. Finally, the results were converted into dimensionless numbers to elucidate physical phenomena and plotted as Sherwood number versus Reynolds number for mass transfer performance and Nusselt number versus Reynolds number for heat transfer performance.

Introduction

Due to the ozone depletion problem associated with the use of the CFC and HCFC refrigerants, absorption heat pumps and refrigeration systems have taken an increasing interest in the recent years. More and more, they are regarded not only as environmentally friendly alternatives to CFC-based systems, but also as energy efficient heating and cooling technology (Berlitz, Satzger, Summerer, Ziegler, & Alefeld, 1999).

The absorber is a major component in the absorption refrigeration systems. The absorber greatly affects the overall system performance (Selim & Elsayed, 1999). Falling film modes and bubble modes have been recommended to enhance heat and mass transfer performance in ammonia–water absorption systems (Christensen, Kang, Garimella, & Priedeman, 1996). Falling film modes provide relatively high heat transfer coefficients and are stable during operation. However, falling film modes have wettability problems and need good liquid distributors at the inlet of the liquid flow. Bubble modes provide not only high heat transfer coefficients but also good wettability and mixing between the liquid and the vapor. However, the bubble modes require vapor distribution. Generally, accomplishment of vapor distribution is easier than liquid distribution. Recently, bubble modes were recommended strongly for ammonia–water absorption systems because the low wettability in the falling film modes is critical to the performance of the system (Christensen et al., 1996). Over the last 10 years, ammonia–water bubble mode has been extensively investigated both numerically and analytically (Elperin & Fominykh, 1996; Herbine & Perez-Blanco, 1995; Kang, Christensen, & Kashiwagi, 1998; Merrill & Perez-Blanco, 1997). However, few papers have been found on the experimental analysis. Merrill, Setoguchi, and Perez-Blanco (1998) tested three compact bubble absorbers developed for generator–absorber heat exchange absorption cycles (GAX). Results show that enhancement techniques are effective in reducing absorber length and increased tube diameters may increase absorber performance. Sujatha, Mani, and Srinivasa Murthy (1999) carried out experiments on a vertical tubular bubble absorber working with HCFC22-DMF. The experimental values of pressure drop, heat and mass transfer coefficients were compared with the results from the numerical model.

In the present study, the plate-type absorber was operated in the bubble mode and the effects of variables on heat and mass transfer performance were studied.

Section snippets

Experimental apparatus and procedure

The experimental apparatus was made to examine the heat and mass transfer performance. The size of the plate in the absorber was $0.112×0.264×0.003 m^{3}$ and pre-experiments were performed to retain laminar flow and good wettability even at low solution flow rate. Three types of plates were tested: smooth plate, hair lined plate treated by laser, and plate treated by sand paper. Laminar flow and good wettability were retained well in the plate treated by sand paper, so it was settled as the

Analysis

The heat transferred to the coolant can be expressed as the following heat transfer equations (McCabe, Smith, & Harriott, 1993): $Q_{c} =UA Δ T_{lm},$ $Δ T_{lm} = [T_{sol,in} −T_{c,out}]−[T_{sol,out} −T_{c,in}] ln [(T_{sol,in} −T_{c,out})/(T_{sol,out} −T_{c,in})] .$

The generated heat by absorption process can be estimated from the enthalpy difference. $Δ H_{gen} =(H_{sol,out} −H_{sol,in})+(H_{c,out} −H_{c,in}).$

The absorption rate is expressed as the following equations using the overall mass transfer coefficients, K. The mass transfer in the vapor phase is assumed

Effect of solution flow rate on heat and mass transfer

In this study, heat and mass transfer coefficients were measured by experiments for countercurrent absorption processes in a plate-type absorber that was operated in the bubble mode. Fig. 4 shows the effects of the solution flow rate on mass transfer coefficient. Increase of solution flow rate resulted in a little increase of the mass transfer. The mass transfer resistance in the water is greater than that in the interface between water and ammonia gas. Increase of solution flow rate enhanced

Conclusions

Bubble mode for ammonia–water absorption in the plate-type absorber was studied. The following conclusions were drawn from the present experimental studies:

(1)
As the solution flow rate increased, mass transfer performance increased slightly but heat transfer performance increased fairly.
(2)
As the gas flow rate increased, mass transfer performance increased. For the heat transfer performance, bubble mode showed good performance with increase of gas flow rate.
(3)
Though ammonia gas was absorbed well and

Notation

$A$	area, $m^{2}$
$D$	diffusivity, $m^{2} / s$
$g$	gravitational acceleration, $m / s^{2}$
$H$	enthalpy, J/s
$h$	heat transfer coefficient, $W /(m^{2} K)$
$K$	overall mass transfer coefficient, m/s
$k$	thermal conductivity, $W /(m K)$
$L$	length of plate, m
$m$	mass flow rate, kg/s
$Nu$	Nusselt number
$Q$	heat transfer rate, J/s
$Re$	Reynolds number
$r_{H}$	hydraulic radius, m
$Sh$	Sherwood number
$T$	temperature, K
$U$	overall heat transfer coefficient, $W /(m^{2} K)$
$V$	velocity of fluid, m/s
$W$	absorber width, m
$x$	concentration

Greek letters
$γ$	kinematic viscosity, $m^{2} / s$
$μ$	dynamic viscosity, $kg /(m s)$
$ρ$

Acknowledgements

This study was supported by research grants from the Korea Science and Engineering Foundation (KOSEF) through the Applied Rheology Center (ARC), an official ERC, at Korea University, Seoul, Korea.

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T. Berlitz et al.
A contribution to the evaluation of the economic perspectives of absorption chillers
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Combined heat and mass transfer during bubble absorption in binary solutions
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Performance of a packed bed absorber for aqua ammonia absorption refrigeration system
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