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UBC Theses and Dissertations

Improvement of venting procedures and energy consumption by modification of conventional steam retorts Bennett, Lee


The sterilization process for low acid canned foods requires accurate determination of the food product thermal characteristics as well as knowledge of the process equipment environment. Establishment of the retort vent schedule ensures that the temperature distribution within the retort is uniform through complete removal of air by purging with live steam. This has proven to be a successful procedure but is not without cost implications. Therefore investigation to quantify energy requirements and minimize energy costs associated with retort venting were studied. Energy utilization by a 4-crate vertical steam retort was investigated. Ballast loads of white beans in brine were processed in 300x407 cans to simulate production conditions. The steam energy use with a vent schedule of 7 minutes after the retort reached 107°C averaged 902 MJ per load. Losses due to venting accounted for about 16.0% of the total energy. The other major energy losses included 45.0% to heat the product, 13.9% for bleeders and blowdown and 9.34% to heat the retort shell and crates. These approximate percentages for each category would vary due to factors such as can size, process time and product characteristics. Initially, the recommended minimum guidelines pertaining to the steam spreader configuration and number of steam ports were followed. Later these were modified in an effort to enhance venting efficacy. These changes proved not to be successful as indicated in the temperature distribution plots and required come-up times in the test retort. The final modification focused on an increase in the steam supply line and steam control valve dimensions. It was hoped that the residual air in the retort vessel could be purged more effectively in terms of achieving a minimum vent schedule and come-up time. The largest spreader of 1.5 inches (38.1 mm) diameter significantly reduced venting and come-up times; however, an excessive amount of steam was expelled to the atmosphere through the vent outlet. An intermediate spreader dimension of 1.25 inch (31.7 mm) diameter proved to maintain the optimum conditions for steam utilization and venting efficacy as compared to the 1.0 and 1.5 inch spreaders. Investigations to determine the minimum vent schedule and come-up time were also carried out. Examination of the temperature distribution profile and energy balance equation revealed that the present vent schedule could be reduced. A reduction of the vent outlet port size was proposed in an attempt to increase back pressure in the retort system. It was believed that higher steam pressure would accelerate equilibration of temperature by reducing the amount of steam escaping with air. Experiments revealed that neither energy saving nor temperature distribution profiles were sufficiently improved to warrant further investigation.

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