Condensing heat transfer and pressure drop characteristics of hydrocarbon refrigerants
Introduction
Traditional refrigerants CFCs and HCFCs are being gradually replaced by HFC refrigerants internationally. Although HFCs have zero ozone depletion potential (ODP), those suffer from high global warming potential (GWP), and hence they are not particularly attractive from the environmental view point.
New alternative refrigerants should not only have low ODP but should also have low GWP, be safe, be reliable, be less flammable, and be economical for being used in the existing facilities [1], [2]. Under these circumstances, hydrocarbon refrigerants (HC’s—e.g. propylene, propane, iso-butane, etc.) are being examined vigorously as alternative refrigerants due to their low cost, ease of availability and better mixing properties with general lubricants. But the developed countries like US have not yet adopted them due to their flammability except Europe [3]. According to James and Missenden [4], in the case of household refrigerators, the hydrocarbon refrigerant charge is so small (about half of the general CFC) that the possibility of explosion due to flammability is practically negligible.
However, before these refrigerants can be accepted by the refrigeration industry internationally, fundamental heat and mass transfer characteristics of these refrigerants need to be investigated for the optimal design of the heat exchangers and thereby, the refrigeration systems. Recently, the authors had performed an extensive study on the evaporation heat transfer and pressure drop characteristics of hydrocarbon refrigerants inside smooth tubes and published the results in a sister paper [5]. However, as a part of the global project, this paper presents the physics of condensation heat transfer and corresponding pressure drops of hydrocarbon refrigerants in smooth tubes. Although there are some studies available in the open literature [6], [7], [8] that deal with the fundamental aspects of numerical heat transfer, the information on the condensation heat transfer dealing with hydrocarbon refrigerants is practically absent. Therefore, the current study fills in this void by presenting the condensing heat transfer characteristics of hydrocarbon refrigerants. In doing so, the paper presents the fundamental experimental heat transfer data on the condensation heat transfer and pressure drop of hydrocarbon refrigerants, namely R-1270 (propylene, 99.5% purity), R-290 (propane, 99.5% purity), R-600a (iso-butane, 99.5% purity); and compares it against R-22 in smooth tubes.
Section snippets
Experimental apparatus
Fig. 1 shows the schematics of the experimental apparatus including basic air-conditioning and refrigerating system consisting of a compressor, a condenser, an expansion valve, an evaporator and a peripheral device such as an oil separator, a receiver, an accumulator and so on. The system also consists of two main flow loops: a refrigerant loop and a secondary heat source water circuit involving either evaporation or condensation loop. In the test section of the experiment, the condenser is a
Condensing heat transfer
To scrutinize the reliability of the experimental set-up, the heat balance between the refrigerant and the heat source water in the condenser was examined and the result is shown in Fig. 4. The heat capacity (calculated by Eq. (1)) is plotted on the x-axis, while the condenser refrigerant capacity (calculated by Eq. (2)) on y-axis of Fig. 4. It is evident from the figure that the two values agree with each other to within ±10% for all refrigerants and tube diameters.
Fig. 5 shows the
Conclusions
This study reports the condensing heat transfer and pressure drop data for R-22 and HC refrigerants that would be useful in the future designs of heat exchangers involving HC refrigerants. The local condensing heat transfer coefficient of all HC refrigerants were higher in smaller diameter tube and further, were generally higher by at least 31% than conventional R-22. The general trend indicated that the average condensing heat transfer coefficient increased with an increase of mass flux in
Acknowledgement
The authors are thankful to the financial support from Pukyong National University’s Research Abroad Fund (in 2004) for this research work.
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