In most cases, there was only one boiler inside the espresso machine. However, nowadays, it is more common to see espresso machines with a central steam boiler and a brewing group boiler. There has also been a shift in materials used; traditionally, boilers were always made from copper and brass. But now, more stainless steel boilers are being used in espresso machines.
Boilers in espresso machines also need to be sealed properly. In the past, it was common to have an open boiler on one or two sides. These sides were sealed off with a boiler gasket and a boiler flange. Sometimes, they were used in combination with boiler rings to evenly distribute the pressure across the flange.
There are also critical components like the safety valve. This is the last line of defense in ensuring the espresso machine operates safely. The boiler safety valve opens up when the pressure inside the boiler exceeds a certain level. Additionally, the vacuum valve helps release any "false" air trapped in the boiler during heating. The vacuum valve will close once the pressure inside the boiler reaches the necessary level.
