Operation principle of coffee machine principle of single boiler heat exchange

Since Luigi Bezzera launched the first commercial coffee machine with steam pressure in 1901, whether it is the Pavoni Gaggia or other machines that use steam or pistons as pressure, there has been only one boiler: the bottom of the boiler is the hot water for brewing coffee, and the top provides steam, but in order to make steam, the whole boiler must be heated to boiling, causing hot water near the boiling point to burn.