Confused about the difference between a single boiler, double boiler and a heat exchanger when it comes to coffee machines? This little guide should help! The water in a given coffee machine is used for two primary functions: extraction in the group head and steaming with the wand.
With a single boiler espresso machine, there is a single source of water (the boiler) for both of these activities. As a result, machines will typically have a brewing and a steaming mode since each activity takes place at a different temperature. To make steaming possible, the water in the boiler has to get much hotter than if you were to pull a shot. Since this tends to raise the temperature of the group head, a single boiler machine needs to cool down after steaming before a shot is pulled. Or in the reverse scenario, you'd have to wait for the water to heat up to the steaming temperature to begin that process.
Despite it being arguably the most efficient type of boiler-setup, a double boiler is by far the easiest to understand. The water for brewing and the water for steaming each have their own separate boiler where they get pumped water from the machine's water source. This means that the user can adjust the temperature for both of these activities independent of one another. Unlike a single boiler, there's no need to wait for the water to cool down or heat up before steaming and pulling a shot.
A heat exchanger boiler system also has a single boiler just like the style we previously spoke about. The water temperature in the boiler is kept at steam temperatures (higher than what you'd use to pull a shot). This water in the boiler can be used directly for the steam wand. But unlike a single boiler system, there is an additional tube within the boiler. When the barista pulls a shot, the water flows from the water source through this tube located in the boiler. As it flows through this tube, it reaches the appropriate temperature for brewing (around 190-200 degrees give or take) and flows into the group head for shot extraction. Any leftover water in this tube after the shot will most likely need to be flushed as it will keep getting hotter because of the temperature of the steam water. Because of how this process is designed, it means that you don't have direct control over the water temperature that's being used for brewing. Instead, the user changes the temperature of the water that's being used for steam which will indirectly modify the temperature of the brew water passing through the pipe.
There is no one best type of boiler. Each one is suited to it's own application depending on the machine type, use, cost and functionality.