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On a paper or board machine, the drying process involves both heat transfer and mass transfer. Heat transfer is the movement of thermal energy due to a temperature difference. In multi-cylinder drying systems, the driving force for heat transfer is the temperature difference between the sheet and the dryer cans. This process is called contact drying, because the sheet directly contacts the dryers. Heat transfer occurs when the steam condenses inside the dryer can and gives up its latent heat to the inside surfaces. The heat energy from the condensing steam is then transferred by conduction through the dryer cans' cast iron shell to the sheet. Because the steam inside the dryer is saturated, the steam temperature is directly related to the steam pressure.