Submitted to: Journal of Food Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2005
Publication Date: 6/10/2005
Citation: Christie, I.S., Patel, J.R., Toledo, R.T. 2005. Fluid to particle heat transfer coefficients in holding tube having noncircular cross section. Journal of Food Science. 70(5):E338-343. Interpretive Summary: Successful aseptic processing of food with solid particulates requires uniform heat treatment to ensure targeted bacterial destruction necessary for commercial sterility without overheating. The time the particulates spend in the holding tube and the heat transfer rates between carrier fluid and particles determine the accumulated lethality. A different design of a holding tube which narrows down particle velocity variation and yields a higher heat transfer rate would achieve the twin targets of faster heating of particulate food with less overheating of slow moving particles. The present study investigated a new design of holding tube with non-circular cross section. Numerical methods were used to estimate the point velocities and heat transfer coefficients in the holding tube. When compared with the conventional holding tube, higher heat transfer rates and narrower particle velocity variation were observed in experimental holding tube with non-circular cross section. The concept is useful in design of heat transfer equipments used for aseptic processing and ohmic processing.
Technical Abstract: Convective heat transfer coefficient (hp) between fluid and particle in continuous tube flow was compared for two configurations of holding tubes: one conventional having circular cross section (CHT) and the other having non-circular cross section (NCHT). A stream of plastic spheres was fed into the holding tube to simulate a food system with particulates. Live vegetative cells of Bacillus stearothermophillus were immobilized in aqueous gellan cubes and injected in the holding tubes with water as a carrier medium. The observed inactivation levels were compared with predicted values calculated from transient heat transfer equations solved by an explicit finite difference technique. Mean effective hp values ranged from 429 to 2501 W/m2.K for the NCHT and 362 to 1683 W/m2.K for the CHT at Reynolds number ranging from 6300 to 12750. The difference in hp levels between the two configurations increased with the flow rate and temperature. Although, no relation was observed between hp values and liquid to particle relative velocity, a trend between pipe Reynolds number and hp values was observed. The presence of large number of particles in the holding tube significantly influenced hp values.