Heat Exchanger vs. Dual Boiler

There are essentially two "styles" of espresso machine. Heat Exchanger, and Dual Boiler.

Heat Exchanger Machines

Heat exchanger machines have a single boiler which is used for creating steam and heating water for brewing espresso. Steam is pulled from the top of the boiler. Water for brewing is heated with a heat exchanger, which is essentially a tube running through the boiler. Cold water is pumped into the tube, absorbing heat as it makes its way to the espresso on the other side. Ideally, it has hit the target brew temperature at that very moment.

Brew temperature is adjusted by changing the pressure of the boiler. Since temperature and pressure are directly related, raising the pressure of the boiler raises the temperature of the water coming out of the heat exchanger.

Technology exists to ensure temperature stability under constant use, most notably the e61 grouphead. The problem is that if the machine sits idle for any length of time, the temperatures of the water in the heat exchanger and boiler equalize. A steam boiler at a common setting of 1.33 bars will run somewhere around 250 degrees F (depending on atmospheric pressure). Pulling a shot of espresso under these conditions would mean subjecting coffee to temperatures well above boiling...not good for any blend. In order to work around this flaw, skilledbaristas develop flushing routines to bring the brew water back down to the target temperature. The amount of flushing required is dependent on idle time, and varies from machine to machine.

Dual Boiler Machines

Some espresso machines employ two (or more) boilers. The steam boiler is controlled by a pressurestat, and is designed for creating steam for milk-based drinks, and as a hot water source for tea and americanos. The brew boiler is controlled by a thermostat or PID, and is used only for brewing espresso.

Dedicating boilers to steam production and espresso brewing allows finer control and better temperature stability. La Marzocco espresso machines use a saturated group, which keeps the group and portafilter hot at all times. Since brewing water is never super-heated, the barista does not need to worry about burning the espresso: the procedure for making a drink is the same regardless of idle time. Additionally, since steam boiler pressure no longer dictates brew temperature, the two can be adjusted independently: thebarista no longer has to sacrifice milk texture for espresso flavor.

The La Marzocco GB5 takes temperature one step further: it uses a PID controller to maintain brew temperature, and uses pre-heated water to fill the brew boiler.

The PID controller uses complex algorithms to keep the temperature exactly at the set-point by operating the heating element at the proper intensity. If you're interested you can read about the differences betweenPID and thermostat temperature control here.

Since the brew boiler does not contain any empty space, any water exiting the boiler must be replaced. In fact, the water coming into the boiler effectivly pushes the brewing water out through the puck of grounds. On almost all machines, this inlet water is cold, causing temperature fluctuations especially under heavy use. The GB5 fixes this problem by running the inlet line through the steam boiler first, which preheats the water. They even went so far as to mix a bit of cold water back in so that it's not too hot.