Ultra fast freezing of warm product using cryogenic slush.

When the immersion in liquid nitrogen of a warm product doesn’t produce a quick enough freezing process, we sometime need a faster method. One solution consists of replacing pure liquid nitrogen by a cryogenic slush; a suspension of solid particles in liquid nitrogen. The calefaction in slush is significantly different than the calefaction in pure liquid nitrogen and this impacts the heat transfer. The purpose of this study is to quantify the heat transfer improvement we can get from a liquid-nitrogen-based slush. Several mixtures of liquid nitrogen with water ice crystal (H2O), carbon dioxide snow (CO2), solid nitrous oxide (N2O) and solid nitrogen (N2) have been tested at lab-scale. The slush was tired in a Dewar and the heat transfer was measured with a temperature probe fitted in a copper ball. The results show that the heat transfer can be significantly enhanced (doubled) when using the right slush.