**1. Introduction**

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Nowadays, the ozone depletion and global warming potential of commonly used refrigerant have been considered as a major environmental matter. The next generation of refrigerants is obliged, not only to be environment friendly, but also to provide high efficiency [1]. Therefore, considerable attention has been focused on the application of natural refrigerant. However, safety issues have been established, such as special demands or suitable applications for refrigerants which include high working pressure, flammability, or toxicity.

Propane is a natural refrigerant that has no ODP and low GWP. It is also a non-toxic chemical and has a suitable thermodynamics and a transport property which are almost similar with those of HFC refrigerant. Other advantages of propane include compatibility with most materials used in HFC equipment and miscibility with commonly used compressor lubricant. The HFC systems such as R22 one can use propane without major changes. Nonetheless, propane has a high flammability that meets the safety demands of refrigerants in design and operation. It means that the propane refrigeration systems should work with minimum refrigerant charges and zero refrigerant leakage.

The question now is: "Could the refrigeration systems avoid leakage?" The answer comes from the difference between the design and the actual, which is a challenge. Mobile air conditioning and commercial refrigerant systems are two types of those that have the largest amount of leakage. The estimated leakage rates of these systems are 7% and 15-20% of the total charges, respectively [2]. Although the leakage percentage improves every year through the develop‐ ment of technology, it is still high and could not possibly reach the ideal case in the near future. The fact clearly shows that the designs of propane systems currently need a decrease in the

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amount of refrigerant charges to as minimal as possible. And one of the best solutions at present is the decrease in the size of the heat exchanger. Compact heat exchangers using minichannels and microchannels have more efficient heat transfer performance than conventional types, due to their higher heat transfer area per refrigerant volume. Given that, the systems could reduce the refrigerant changers but still keep the coefficient of performance. However, since various researches observed the many differences of heat transfer and pressure drop of refrigerants in the mini/microchannels and conventional channels [3, 4]. The studies on these characteristics of propane in mini/microchannels and valuable information will be provided for researchers in related fields.

In fact, in the past few decades, numerous interesting researches were published in literature such as the studies of [5-9]. Nevertheless, the following chapter does not attempt to review all the available literature but those most focused on the particular heat exchanger design only or the consideration on the heat transfer and pressure drop separately. The intention is to rather present a basis on heat transfer and pressure drop of propane in minichannels, simultaneously under the variation of mass fluxes, heat fluxes, saturated temperature, and tube diameter. The phenomenon will be explained thoroughly by its mechanism that shows some differences observed in the conventional channel. Furthermore, the development of heat transfer coeffi‐ cient and pressure drop correlations are demonstrated. The content is believed to bring up the general understanding, as well as useful information on heat exchanger designs to the readers.
