D. Heat transfer coefficient in the shelves of a freeze dryer

The aim of this study is the determination of the heat transfer in the shelves of a freeze dryer. The inner geometry of the shelves is a long rectangular duct, which guides the heat fluid specifically inside the shelf. The heat transfer from fluid to the inner wall of the rectangular duct based on the energy and heat balances for a stationary process and an uniform temperature distribution over the shelves is deduced. The calculation of the heat-transfer coefficient by convection within the channel of a plate is performed and provides a formula valid for all shelves with rectangular ducts. Measurements on a freeze dryer with 20 m(2) of shelves surface result in a heat-transfer coefficient of 214 W/m(2.)K at -45 degreesC up to 792 W/m(2.)K at 15 degreesC. The determined a value is realistic and is in the same order of magnitude compared with the values determined in earlier studies. This study is to be seen as a partial contribution to the complete study of heat transfer between heat transfer medium and product within the vials during freeze drying.