Supercooling brine in a helical coiled heat exchanger for ice slurry production

: Experiments supercooling brine in heat exchangers to generate ice slurry hygienically and efficiently.

Abstract

Ice slurries are formed from a combination of ice and water containing freezing point depressant. Despite the proven potential uses, they remain underutilised. Improving the efficiency of slurry production could increase adoption in many industries. Applications requiring greater hygiene have also developed in food production and medicine. Currently, scraped surface generators are the favoured generation method; a large body of work has focused on finding an effective replacement. This work presents a new method, utilising plastic pipes to slow ice growth and reduce adhesion.

A helical coiled heat exchanger (HCHX) arrangement with nylon pipe was used to cool an internal brine flow until ice nucleated. Initial results showed that ice slurry above 15% could be produced, at refrigeration temperatures from -8 to -13°C. Refrigeration temperatures that are warmer than scraped surface generators, indicating potential improved efficiency. Changing the flow direction, coil orientation, and fluctuating the flow rate in the pipe, failed to increase ice production. A control system, designed to set flow rate of the fluid, maintained ice fraction above 10%, despite rising refrigeration temperatures. A comparison between aluminium, silicone rubber, PTFE and nylon HCHXs was conducted. PTFE produced the highest ice fractions and production rates, possibly as a result of its high contact angle (hydrophobic) surface. Further experiments with silicone rubber pipe cooled in a tube-in-tube heat exchanger were conducted. The pipe was flattened in order to help remove ice formed in the tube. Small improvements in ice production were observed.\\

Successful improvements noted in previous chapters were combined with a smaller HCHX. The fluid phase was recirculated in the heat exchanger to generate ice fractions up to 23.5% after 6000 seconds of operation. Possible designs for implementing this method in a working generator with a refrigeration loop were proposed. A working prototype of one of these methods would be required to confirm the improved efficiency over scraped surface generators. Nevertheless, this method does offer more hygienic ice slurry generation, which could be exploited in the food and drinks industry or medical cooling. Many remaining avenues of research still exist which could optimise ice production in plastic heat exchangers further.

Date of Award	25 Jun 2019
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Joe Quarini (Supervisor) & Michael Tierney (Supervisor)