At the beginning of the primary drying phase, sublimation of the ice takes place at the surface. As the process continues, the subliming surface withdraws into the product, and the evolving vapour must be conducted through the previously dried outer layers. This means that the drying process depends on the speed of vapour transfer and removal as well as on the necessary heat of sublimation. The heat required for sublimation is supplied to the product by convection and thermal conduction and in a small part by thermal radiation. Apart from heat transfer by thermal conduction and radiation, it is most important that the heat transfer by convection is optimized. It must be taken into account, however, that due to the reduction of pressure in the drying chamber, convection will practically cease at a pressure below 10-2 mbar. This is why, as a function of the required sublimation temperature, the pressure in the drying chamber is adjusted during primary drying to the highest permissible value.