Basic absorption model vs. experimental results. Factors affecting the facility performance

Подпись: to [°C] Fig. 3. Comparison of basic model and experimental results. (a) Ideal and experimental cooling capacity vs. tG (b) Ideal and. experimental COP vs. tG image655

Fig. 3 illustrates the comparison of experimental performance parameters with those obtained through the basic model, Eqs. (14) and (15), for 220 test points. The differences observed help identifying the influence of components performance, serving as a diagnosis. In the following, the causes of deviation from the ideal conditions are explained, evaluated and included in the model.

5.1. Influence of components performance. Modified basic model.

The evaporators in this facility layout show low performance [8]. Because of this, part of m ref

supplied to them is not evaporated, but overflows. Therefore, in order to measure the performance of evaporators (fraction of evaporated refrigerant), the cooling capacity obtained if all refrigerant is used in the evaporators Qref is compared with QE exp:

Пе = ; Qref = mref • (hEin — hEo) (21)

Qref

Energy balance in the generator yields:

m = m — m

ref weak strong (22)

The obtained average nE is 50% which is considerably low. With the current design, the excess of refrigerant is not recirculated to evaporators, and therefore it goes to the solution reservoir inside the absorber. Beside this, it has been detected that the distribution of mref (m f and m f) is not symmetrical in some cases, and therefore one of the evaporators runs dry. This situation also explains the low value of qexp and its tendency is not consistent with rise in temperature (Fig. 3). Corresponding values of COPexp show obviously the same behaviour.

Taking into account the evaporators limitations explained above, the modified cooling capacity will be:

4E, mod 4Ei ‘ ПE (23)

The facility performance is also affected by the solution heat exchanger efficiency phex and consequently is considered in the analysis. Beside this, a noticeable difference was identified between solution temperature at generator outlet and separator outlet, as the generation process continued and the refrigerant separation kept on in the path followed by the solution. The temperature drop associated ranged from 4°C to 10°C. The heat transfer associated is called qG.

Then, considering the real efficiency of the solution heat exchanger and the heat transfer losses in the system generator-separator, the modified generation power will be:

qG, mod = qG, i + qhex. i (1 — Vhex ) + qG-sep (24)

As a result, the modified COP becomes:

COPmod = ^mo1 (25)

qG, mod

Подпись:
Results obtained incorporating the specific behaviour of components to the basic model can be appreciated in Fig. 4. The predicted cooling capacity and COP shows good agreement with experimental results. This indicates that the main causes of deviation have been included in the model, thus verifying the preliminary diagnosis.

(a) (b)

6. Conclusions

A facility representing a single effect adiabatic LiBr/water chiller implementing adiabatic absorption facility has been operated under different conditions, corresponding to both design and off-design operational conditions. Performance parameters have been experimentally determined for every test condition.

The differences observed between ideal and experimental results helped to diagnose and identify the influence of components performances on the overall performance of the facility. The

evaluation of the ideal and real cooling powers allowed detecting a low performance of evaporators caused by both dry operation and overflow. Another influence factors which causes the deviation from ideal behaviour are the solution heat exchanger efficiency and the heat transfer losses in the system generator — separator.

Taking into account the particular layout and operation features tested, a good agreement with experimental performance parameters and those obtained through a modified basic absorption model has been achieved. This model incorporates a quantified loss and/or efficiency separating from ideal and has been fitted to experimental data. Experimental results demonstrate the operational possibilities and flexibility of the design, showing a great potential for further work.

Acknowledgements

The financial support of this study by the Ministry of Education, Science and Technology through CLIMABCAR project DPI 2003-01567, TRANSMACA project DPI 2002-02439 and MINICOM project (FIT 0204-2004-68 and FIT 020100-2003-233), is greatly appreciated. The authors express their gratitude to the technicians of Universidad Carlos III de Madrid Mr Manuel Santos and Mr Carlos Cobos for their invaluable help in this work.

References

[1] Venegas M., Izquierdo M., Rodriguez P., Lecuona A. Heat and mass transfer during absorption of ammonia vapour by LiNO3-NH3 solution droplets, Int. J. Heat Mass Transfer, 47 (12-13) (2004) 2653­2667.

[2] Venegas M., Rodriguez P., Lecuona A., Izquierdo M. Spray absorbers in absorption systems using lithium nitrate-ammonia solution, Int. J. Refrigeration, 28 (4) (2005) 554-564.

[3] Arzoz D., Rodriguez P., Izquierdo M. Experimental study on the adiabatic absorption of water vapor into LiBr-H2O solutions, Applied Thermal Engineering, 25 (5-6) (2005) 797-811.

[4] Warnakulasuriya F. S.K., Worek W. M. Adiabatic water absorption properties of an aqueous absorbent at very low pressures in a spray absorber, Int. J. Heat Mass Transfer, 49 (9-10) (2006) 1592-1602.

[5] Elperin T., Fominykh A., Orenbakh Z. Coupled heat and mass transfer during nonisothermal absorption by falling droplet with internal circulation, Int. J. Refrigeration, 30 (2) (2007) 274-281

[6] Wang L., Chen G. M., Wang Q., Zhong M. Thermodynamic performance analysis of gas-fired air-cooled adiabatic absorption refrigeration systems. Applied Thermal Engineering. 27 (8-9) (2007) 1642-1652.

[7] Gutierrez G., Venegas M., Rodriguez P., Izquierdo M., Lecuona A. Experimental characterization of a single stage LiBr-H2O absorption test rig. In Proc. ECOS 2006, Vol. 3, Crete, Greece, July 12-14 (2006) p. 1331-1316.

[8] Gutierrez G., Zacarias A., Venegas M., Rodriguez P. Cooling power evaluation of a water lithium bromide absorption test rig. In Proc. ECOS 2007, Vol. 2, Padova, Italy, June 25 -28 (2007) p. 1183-1190.

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