18 May 2017

O.3.5.4 Turbo-compressors: Prototype tests of mechanical vapour re-compression for steam driers

Steam driers show especially for the food industry improved quality compared to the more common air drying. One of the main advantages of steam drying is the possibility to recover the excess product steam through mechanical vapour recompression (MVR) and use this heat as thermal energy source for the process steam. Effective steam driers have a specific energy consumption as low as 0.8 kWhthermal per kg of product steam. By applying MVR it is possible to reduce this number to 0.2 kWh/kg. In order to enable an effective and simple MVR-drier specialized atmospheric steam driers are available, which require a temperature lift of 25 Kelvin from the MVR. Consequently, the MVR system requires a pressure ratio of 2.5 when the process is operated at atmospheric pressure.
A prototype of a modified turbo-charger was tested in an experimental setup, with focus on cost effective and compact design. The electric drive capacity is 59kW and the DC-motor is directly mounted through the planetary gearbox on the impeller.
The main operation condition of the MVR-drier as well as the principal function of the turbo-compressor are explained in the present paper. Further, the accomplished experiments are described and the performance of the compressor is documented. It was possible to proof that the specific energy consumption of the drier can be reduced by 80% with the used technology. A simplified return of investment calculation shows that the system is paid back in one year. The technology has the potential to substitute fossil energy Carriers.