PURE NH₃ AND CO₂ HEAT TRANSFER AND PRESSURE DROP IN HORIZONTAL SMOOTH TUBES FOR NH₃ TWO-STAGE AND NH₃/CO₂ CASCADE REFRIGERATION SYSTEMS: A REVIEW

Abstract

This paper presents a review of di®erences and similarities of in-tube heat transfer and pressure drop between ammonia (NH 3 ) and carbon dioxide (CO 2 ) from the perspective of the design of heat exchangers for NH 3 two-stage and CO 2 =NH 3 cascade refrigeration systems. The focus is on di®erences in thermophysical properties and thus di®erent characteristics of heat transfer and pressure drop. A brief summary of published literatures about CO 2 =NH 3 cascade refrigeration systems is provided and literature review of available correlations and developed correlations are presented for°ow boiling and condensation heat transfer and pressure drop. Because of large deviation of calculated values with exiting correlations from measured results, a new correlation to predict°ow condensation heat transfer coe±cients was developed based on experimental results for CO 2 at À15 C. From comparison of measured and predicted values, it is shown that some correlations, previously published in open literature, can be used to calculate°ow boiling heat transfer coe±cients for NH 3 at À20 C, if a°ow pattern can be appropriately determined for a°ow condition. Also, it is presented that existing correlations can predict well the heat transfer coe±cients for CO 2°o w boiling at À15 and À30 C. It is shown that some correlations can predict pressure drop relatively well for NH 3 and CO 2 two-phase°ow. The NH 3 and CO 2°o w evaporation heat transfer and pressure drop characteristics at À40 C are compared with predicted values.