Elsevier

Applied Thermal Engineering

Volume 37, May 2012, Pages 430-437
Applied Thermal Engineering

Effect of the NH3–LiNO3 concentration and pressure in a fog-jet spray adiabatic absorber

https://doi.org/10.1016/j.applthermaleng.2011.11.067Get rights and content

Abstract

This paper presents the effect that both the ammonia concentration in an ammonia–lithium nitrate solution and the absorber pressure have on the adiabatic absorption of ammonia vapour by droplets generated by a fog-jet injector. The injector ensemble is located at 205 mm from the bottom of the absorber. The solution has an ammonia mass fraction varying from 0.419 to 0.586 and the absorber pressure varies from 429 to 945 kPa. This is representative of the operating conditions for conventional absorption chiller cycles, but also for advanced cycles such as those with a booster compressor located in series between evaporator and absorber. This leads to a higher than common pressure in the absorber. Results show approach to equilibrium factors higher than 0.83, being the mean value of the experiments 0.9. The absorption ratio obtained was between 0.008 and 0.07. The increase in pressure and inlet subcooling increases the absorption rate, whilst the increase in the ammonia mass fraction increases the approach to equilibrium factor, decreasing the absorption rate.

Highlights

► Experimental study of spray adiabatic absorber using fog-jet injectors. ► NH3–LiNO3 as absorbent reporting a wide range of pressures and concentrations. ► Approach to equilibrium factor always higher than 0.83 and an average of 0.9. ► Absorption ratio up to 0.07, increasing with pressure, decreasing with concentration. ► For every pressure the approach factor increases linearly with inlet subcooling.

Introduction

Currently the absorber size significantly impacts the overall size of single-effect absorption machines due to transferring heat and mass at the same time between two phases with a small difference in temperature and concentration [1]. There are several configurations of the absorber, being the falling film the most common type in commercial absorption chillers. The main problem with this configuration is the bad liquid distribution/surface wetting, e.g. [2], hence not using efficiently the available interchange surface of the absorber. Other types of absorbers are bubble absorbers and adiabatic absorbers [3].

The adiabatic absorber separates the processes of heat and mass transfer. Before the poor in refrigerant solution enters the adiabatic absorber, it is subcooled in an external conventional heat exchanger operating with a single-phase flow, which allows reducing its size and cost [4]. The mass transfer process takes place adiabatically inside the absorber [5]. Usually liquid molecular diffusion limits the absorption rate [4]. In order to reduce the penetration length of the absorbed vapour into the liquid, the solution can be dispersed as small drops. When the drops start absorbing vapour their temperature rises, slowing the absorption rate. If the residence time in the absorber is large enough, the adiabatic equilibrium is reached at the outlet of the absorber. In order to increase absorption, part of the rich in refrigerant solution at the outlet of the absorber is mixed with the solution coming from the generator, downstream it is subcooled and recirculated through the absorber, so that the adiabatic absorption is repeated. This way, the equilibrium concentration corresponding to the inlet temperature of the external cooling flow can be approached, namely, the diabatic equilibrium. The ratio of mass flow rates of recirculated solution and of solution that goes to the generator, namely the recirculation ratio, can be in the range of 4–6 using the ammonia–lithium nitrate solution, for a reasonable approach to the diabatic equilibrium conditions [6].

The absorption of vapour by a sprayed liquid drop can be considered adiabatic. It has been studied theoretically for individual and independent spherical droplets by different authors [7], [8], [9], [10], [11], [12], among others. All of them consider resistance to heat and species flow only inside the droplet. Some of them [7], [8], [9] are analytical models, starting from the Newman model [7] where the mass transfer of a spherical droplet is considered with no surface heat release by absorption and no motion inside the droplet, coinciding with null Peclet number. Ref. [8] included the internal liquid recirculation driven by the viscous shear from the external vapour flow under free falling conditions, resulting in infinite Peclet number. Ref. [9] added the absorption heat dissipating inside the droplet, using thin mass and thermal boundary layers, but no internal motion. The other studies, used here as examples, are based on numerical models that solve the simultaneous heat and mass transfer inside a spherical droplet of a H2O–LiBr solution [10] and of a NH3–LiNO3 solution [11], [12], both including at the same time internal motion for low Reynolds numbers and heat release at the surface. There a discussion on the suitability of the Newman model has been included, as well as a discussion of previous published works. These studies shed light on the process of absorption, but as indicated by some authors [13], they cannot reflect the complexity of the cascade of sheet and ligaments disintegration into different droplet sizes followed with droplet collisions and resulting bouncing and oscillations, at the same time absorbing vapour. The few authors that have carried out experimental studies relating to refrigerant absorption are: [4], [14], [15] with the H2O–LiBr solution and [16] with the high viscosity LZB™ lithium-based liquid absorbents. All of them use multi-disperse sprays, excepting [15] that uses free falling large single size droplets. The results of these works anticipate the potential of adiabatic absorption. In the two first works, the experimental results were explained with the Newman model [7]. In addition, a numerical simulation [11] has demonstrated that this analytical model is able to predict the ammonia vapour absorption by a lithium–nitrate solution spherical drop, from 60 to 100 μm. Other experimental studies about adiabatic absorbers have been carried out with flat fan sheets and conical sheets [17], [18] using the H2O–LiBr pair. In these studies, the very low pressure of the H2O vapour phase preserves the sheet integrity for a large path length in the absorption chamber before break-up. The only experimental study with NH3–LiNO3 solution as absorbent in adiabatic absorbers was carried out with flat fan nozzles at the same experimental facility than this work [19]. The results show a high performance of the adiabatic absorber, reaching approach to equilibrium factors from 0.81 to 0.89, using a constant ammonia mass fraction of 0.45 and an absorber pressure varying from 300 to 400 kPa. One can conclude that the experimental data showing the behaviour of the ammonia vapour absorption by ammonia–lithium nitrate solution sprayed droplets should be extended.

H2O–LiBr and NH3–H2O are the best-known working solutions for single-effect absorption cycles [20]. H2O–LiBr is commonly employed for air-conditioning purposes due to its overall favourable performances [1]. For industrial refrigeration, NH3–H2O is the most common solution. NH3–LiNO3 is a promising alternative for cooling purposes that has been studied among others by [21], [22]. Single-effect absorption using this solution offers higher coefficients of performance (COP) and a lower investment cost and size than NH3–H2O, as it does not require a rectification tower, e.g. [22]. One main advantage with respect to the H2O–LiBr solution is the possibility of spraying, using but the high pressure difference between generator and absorber, typically between 0.5 and 1 MPa.

The single-effect absorption cycle with an integrated in-series booster compressor offers reducing the hot water temperature needed for driving the cycle by up to 25 °C, taking advantage of a simpler hardware than with complex cycles, at the cost of additional small electricity consumption for the low compression ratio compressor. From another point of view, if the same hot water temperature is chosen, lower temperature cold production is possible offering the same COP. Among other issues this allows drying the conditioned air in humid climates, either on a continuous basis or intermittently [23]. The higher pressure in the absorber for this cycle makes that the inlet concentration in ammonia rises [23], increasing the margin to crystallization, reducing the viscosity and simultaneously increasing mass diffusivity.

The aim of this work is to show the experimental performance of an adiabatic spray absorber operating in a single-effect (SE) absorption cycle under variable conditions, using an effective spray injector. For that the solution concentration at the inlet of the absorber and the absorber pressure are varied substantially. This allows also including in the study the performance of the adiabatic spray absorber for different compression ratios of the hybrid cycle with a low pressure booster.

Section snippets

Experimental setup

The experimental facility constitutes a test rig for absorption machine components. It is formed by a thermochemical compressor for a SE absorption machine, i.e. the generator, the single-pass absorber, the absorber heat exchanger and the solution heat exchanger, as main components (see Fig. 1). In the facility, there is neither condenser nor evaporator, as would exist in a complete absorption machine. The ammonia vapour produced in the generator, being the refrigerant in this solution, flows

The approach to adiabatic equilibrium factor

The approach to adiabatic equilibrium factor Fad is the ratio of the change in mass fraction (concentration) achieved at the outlet of the adiabatic absorber over the change when reaching adiabatic equilibrium at the outlet:Fad=Xa,oXa,iXeq,adXa,i

Xa,o is the ammonia mass fractions of the solution measured experimentally at the outlet of the absorber by means of density and temperature of the solution measured by a Coriolis and a thermoresistance Pt-100 transducers, respectively:Xa,o=f(ρa,o;Ta,o

Results and discussion

When the experimental setup was operated under constant conditions the experimental data were recorded every 8 s during a period of time of 300 s, in order to obtain every experimental point. Fig. 3 shows the time evolution of the variables used in this study for one point. The standard deviation around a constant value is very small, demonstrating that the results are truly steady state. The conditions under which the adiabatic absorber was tested are summarized in Table 2. 59 points were

Conclusions

The following conclusions can be drawn from the experimental study of the ammonia vapour absorption by droplets of lithium nitrate solution in an adiabatic absorber using a fog-jet injector based on an ensemble of solid cone injectors forming a fog-jet spray:

  • The approach to adiabatic equilibrium factors Fad are higher than 0.83, indicating a high performance of the absorber with an injector height of 205 mm from the bottom of the absorber under a wide range of operating conditions.

  • Fad increases

Acknowledgements

The financial support of this study by the Spanish Ministry of Education and Science research grants ENE2005-08255-C02-02, and ENE2009-11097 and project CCG07-UC3M/ENE-3411 by the Local Government of Madrid and UC3M, are greatly appreciated. The authors want to acknowledge the help of Mr. Manuel Santos and Mr. Carlos Cobos in the laboratory work.

References (29)

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