14A-40

P. CORONEL, K. P. Sandeep, and T. Palazoglu. Food Science, North Carolina State University, 39 Schaub Hall, Box 7264, Raleigh, NC 27695

The use of coiled heat exchangers has increased in the past few years, especially for high temperature-short time processes due to the benefits they bring in terms of compactness, high heat transfer rate, uniform RTD and improved mixing (Dean effect) within the tube.

The focus of this study is to determine the overall heat transfer coefficient (U) of a helical heat exchanger when heating water and pseudoplastic CMC solutions (k=0.001 to 0.5 Pa-sⁿ) flowing at 4 to 7 gpm, at varying set point (SP) temperatures (90 to 150°C.)

Water and CMC solutions were pumped using a positive displacement pump through a helical heat exchanger (0.25m Coil dia., 0.0102m ID pipe, 39.64m length), a holding tube, and four tubular coolers, each 6.097m long. The helical heat exchanger used in these experiments operates by using pressurized hot water (which is heated by steam), circulating at 200 gpm through the system, and has an automatic temperature control. Experiments were conducted to investigate the variation of U with flow rate (4 to 7 gpm), product viscosity (0.001 to 0.5 Pa-sⁿ), and set point temperature (90ºC - 150ºC).

Preliminary results with water show that U in the helical heat exchanger varies from 3500 W/m²ºK at 4gpm and 90ºC to 9000 W/m²ºK at 7gpm and 150ºC. The corresponding values of U in the tubular coolers are much lower -- 1800 and 2500 W/m²ºK respectively. Comparisons with theoretical correlations that account for the Dean effect in coiled tubes are also being made.

The results of this study will aid engineers in designing efficient heat exchangers, will also help processors in designing a process that will result in rapid and uniform heat treatment with an existing system.