Mixture Effects in Shell-and-Tube Condensers. Theory and Calculations
Abstract: Zeotropic mixtures have come into more frequent use as working fluids (refrigerants) in heat pumps and air conditioning equipment due to the phase-out of ozone-depleting substances. Substantial drop in performance for shell-side condensation in shell-and-tube condensers when replacing a pure fluid with a mixture has been reported in the literature. The aim with this work is to contribute to the understanding of the possible reasons and their relative importance for this performance drop. Factors investigated are mass transfer resistance and flow patterns, both in the gas phase and in the condensate phase.Three different calculation tools have been used in the investigations. Two are self-developed calculation programs. Both solve the transport equations in the vapour accurately, while one uses a simple assumption for the condensate phase, and the other solves the transport equations in the condensate accurately also. The third calculation tool is commercial CFD software. The first calculation tool has been used to simulate full-scale condensers. When comparing with experimental data, it turned out that the vapour-phase mass transfer resistance closest to the interface is not influential enough to explain the drop in performance. An additional assumption of imperfect condensate mixing is, however, influential enough to account for the drop in heat flux of sometimes up to more than 70%. The second calculation tool simulates a column with ten horizontal tubes. Calculation results show that mixing in the condensate by diffusion only is consistent with the performance drops observed. The resulting trends are similar to those obtained when simulating the full-scale condensers with the first calculation tool. The CFD software has been used in two different investigations. The first concerns the condensate flow in micro-scale on and between the tubes. The investigation was discontinued due to convergence problems. The other investigation concerns the vapour flow in the condenser. Flow field calculations have been carried out for a pure fluid and a binary mixture, and preliminary results show a significant difference in flow behaviour. However, with the calculated vapour flow field, the mixing of the condensate is less important, but the flow field of a mixture is important for the total heat flux. An adjustment to the inlet design resulted in an up to 24% higher heat flux. The resulting drop in performance for a mixture compared to a pure fluid is, however, not significant enough to account for the recorded performance drop. This leads to the overall conclusion that mass transfer resistance in the condensate is the most important factor to consider, but that gas-phase mass transfer resistance and flow patterns also need to be considered for mixtures.
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