Heat and mass transfer analysis on multiport mini channel shelf heat exchanger for freeze-drying application
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Sådhanå (2020)45:260 https://doi.org/10.1007/s12046-020-01496-x
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Heat and mass transfer analysis on multiport mini channel shelf heat exchanger for freeze-drying application G SRINIVASAN and B RAJA* Indian Institute of Information Technology, Design and Manufacturing, Kancheepuram, IIITDM Kancheepuram, Chennai 600 127, India e-mail: [email protected]; [email protected] MS received 25 January 2020; revised 24 June 2020; accepted 8 September 2020 Abstract. An experimental study on heat transfer characteristics of shelf heat exchangers, which are used in small-scale freeze dryers, is presented in this paper. The proposed heat exchanger consists of the multiport minichannel flow paths, and a comparison with a conventional serpentine is carried out based on uniformity of the product temperature achieved. The experimental results indicate that the mini-channel shelf has better temperature uniformity than the latter. Skimmed milk is used as the test fluid. In the freezing and drying process, the product temperature variation is minimal in the mini-channel heat exchanger, with a variation of 12.5 % and 25%, respectively, and the heat transfer coefficient is found to be from 140 to 267 Wm-2 K-1. The moisture content in the product reduces to 50 % in 2 hours, and the drying rate is higher at 0.032 kg hr-1 after 1 hour of the drying process. The redesign of the heat exchanger will be an essential tool to improve the performance with uniform temperature distribution on the product and to improve the product quality. Keywords. coefficient.
Freeze drying; multiport mini channel; serpentine channel; shelf heat exchanger; heat transfer
1. Introduction Food and pharmaceutical industries extensively use freezedrying for the manufacture of dry products with the residual moisture content less than 1%. The freeze-dried food product has the least thermal degradation and well retained the original flavor and aroma. A detailed review of the application of freeze-drying can be observed in the open literature [1–6]. Figure 1a, b and c show the schematics of the overall freeze-drying process in a P-T diagram, various stages, and isopropyl alcohol ramp-hold sequence. The process consists of three stages, namely freezing, primary drying, and secondary drying. In the freezing stage, the food product is kept on a tray and mounted on the top surface of the shelf heat exchanger. Heat exchange fluids like isopropyl alcohol, silicone oil, etc. are used as heat exchange fluids. It is necessary to freeze the product well below the eutectic or glass transition temperature [7]. It is needless to mention that the rate of freezing decides not only the size of ice crystals but also the mass transfer resistance and sublimation rate during the subsequent drying stage. The freezing process can be divided into three regions, namely pre-cooling, phase change, and sub-cooling. Precooling consists of super-cooling and nucleation. In super*For correspondence
cooling, the product remains in the liquid
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