A common practice for designers of heat exchangers is to use the same traditional calculation tools for food with particles as for food without. The problem is that the presence of particles changes the flow and heat dynamics significantly in a heat exchanger. New research by Tetra Pak has investigated exactly how particles affect heat transfer.
The research shows that some conventional formulas can result in deviations by as much as 12°C between the calculated outlet temperature and the actual measured temperature of particulate food leaving a heat exchanger. To avoid this, the heat exchanger is typically over-dimensioned.
On the other hand, if the actual temperature is much higher than expected, the particulate food can be partially destroyed with loss of flavour and colour. In terms of investment costs, the heat exchanger may be over-dimensioned for its purpose, and this is a waste of money and space.
The research, which was carried out over more than two years at Tetra Pak’s Product Development Centre in Lund, Sweden, involved a group of about ten experts, including an external professor and research associates specializing in heat transfer at the University of Lund. Different kinds of particulate foods were tested, such as mango preparations, soup concentrates, chutneys and carrot slurries.
The findings clearly showed how the heat transfer coefficient varies in the presence of particles. As expected, the particles actually improve mixing in the tubes and therefore increase the overall heat transfer coefficient of the liquid phase.
A tool for optimizing process design
The influence of particles has now been quantified, and Tetra Pak has been able to devise a new heat-transfer coefficient formula for particles, and a new calculation tool for particulate food. The new tool is called PartCalc and has been validated by experimental data. It is now being used to help customers optimize their process design for particulate foods.
Tetra Pak’s experiments show that the carrier liquid temperature of a variety of particulate foods leaving a heat exchanger, as calculated by PartCalc, deviates by less than 3°C on average from the actual temperature. This is a considerable improvement on traditional calculations, and gives a good correlation with actual temperatures, thus allowing Tetra Pak to more accurately dimension heat exchanger systems for particulate foods.
The potential benefits to the food processing industry include:
» Improved food quality with assured food safety
» Reduced operating and maintenance costs
» Reduced product losses
» Reduced environmental footprint
“A key factor in process design is to minimize heating without compromising food safety. This demands knowledge about heat transfer in particulate foods,” says Helena Arph, Technology Specialist, Viscous and Particulate Foods at Tetra Pak in Lund, Sweden. She has been closely involved in the research work and presented papers on the findings.
