A model for predicting the heat and mass transfer occurring inside a tube-in-tube counter current refrigerant cooled absorber (RCA) was developed. This particular heat exchanger has been used in a novel half-effect air cooled water ammonia chiller driven by low temperature heat (80 °C), whose key operating principle consists in utilizing part of the liquid refrigerant leaving the condenser to cool the solution in which the remaining part of the refrigerant is absorbed after leaving the evaporator. During such operation, water-ammonia solution (with ammonia mass fraction about 0.55 - 0.65) at low pressure (4.5-5 bar) flows in the RCA annulus, counter-current with respect to the refrigerant (nearly pure ammonia at about 8 - 10 bar) flowing in the internal tube. Different simulation runs have been performed in order to explore the impact of different designs on the overall UA value and pressure drops. The model turned out to be very sensitive to the mass velocities of the two streams; an aspect that it is amplified by the mutual influence between annulus and tube heat transfer coefficients. The model predictions have been compared against a few experimental data which were observed during the test of the chiller prototype. Experimental data were found in reasonable agreement with simulation results.
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