Intube condensation of halogenated refrigerants: a new predictive model

Many studies in the literature try to model the physics of condensation processes. There are handbooks and design manuals that supply reasonable recommendations for a designing procedure, but there is no agreement on what correlation is “best”. Besides, the air-conditioning and refrigeration industry has been facing an unprecedented transition in these years, due to the substitution of traditional synthesised refrigerants. A few of the new generation refrigerants display a working pressure much higher than the fluids they substitute for; this is the case, for example, of R410A, the major candidate for the substitution of R22. While these “high pressure” fluids can exhibit advantages when used as replacement refrigerants in place of lower pressure fluids, it is demonstrated that the use, with these new working fluids, of the traditional well established correlations for computing intube condensation heat transfer coefficients, may become quite inadequate, with unacceptable inaccuracies. The aim of a research project at Dipartimento di Fisica Tecnica of the University of Padova was to provide a new general model for the heat transfer coefficient and pressure drop during condensation of all halogenated refrigerants within all conditions encountered in modern refrigeration technology. There is agreement in the literature that the mechanisms of heat transfer and pressure drop are intimately linked with the prevailing two-phase flow regime. During condensation inside horizontal tubes, the two-phase flow may be dominated by vapour shear or gravity forces. While annular flow pattern is associated with high vapour shear, stratifying, wavy and slug flows appear when gravity is the controlling force. In a fully developed annular flow pattern, a thin liquid layer covers the inner surface of the tube, while the gas phase flows in the central core, and heat transfer is solely governed by vapour shear. In stratifying and wavy flows a thick liquid layer covers the lower part of the inner surface of tube, and a thin liquid film is present in the upper part; in this case heat transfer is mainly governed by the gravity force. At high flow rates and low qualities slug flow can be present, with interfacial waves that grow sufficiently in amplitude to block the entire cross section of the tube at some axial locations. In the open literature, several theoretical or semi-empirical condensation heat transfer models for annular and gravity dominated flows have been presented. Experimental heat transfer coefficients obtained by the present authors and other independent experimental data collected by different research workers, all pertaining to pure fluids or nearly azeotropic mixtures, have been compared against the predictions obtained by the most widely used semi-empirical procedures in Cavallini et al. (2000a). The comparison showed that quite a few of the available experimental points fall outside of the validity ranges given for the predicting procedures. This mainly happened with the so-called new “high pressure” refrigerants, such as R125, R32 and R410A. This meant that the available semi-empirical correlations were not able to predict the heat transfer coefficients for some fluids under extended operative conditions because of their limited validity ranges; even when the validity ranges were appropriate, quite a few procedures display unacceptable discrepancies when applied with the new “high pressure” fluids. The authors concluded that future work was certainly needed so as to extend the validity of existing correlations and/or to develop new predicting models. The new computing procedure for the heat transfer coefficient and pressure drop presented here has been developed to cover the most important different flow regimes that may occur in a horizontal tube during condensation, that is annular, stratifying, wavy and slug flow regimes. Different predictive correlations are suggested for three main two-phase flow regimes; these flow patterns can be singled out according with the proposed map. The model is tested against experimental data of both new HFC fluids, pure and nearly azeotropic mixtures, and traditional old-generation refrigerants. Predictions from this new procedure are compared both with present authors’ experimental data, and with a wide experimental data bank from independent authors; an excellent agreement in almost all cases is demonstrated. Heat transfer coefficients from present authors were obtained during condensation of refrigerants R22, R134a, R125, R32, R236ea, R407C and R410A in a 8mm inner diameter plain tube, carried out at a saturation temperature ranging between 30 and 50°C, and mass velocities varying from 100 to 750 kg/(m2s), over the entire vapour quality range.