How does the Machine Work?

In general, commercial espresso machines function because of three main factors: pressure, temperature and volume. A pump-driven, semi-automatic commercial espresso machine has a boiler which stays full at some constant volume, meaning the inflow goes in at the same rate as it leaves. This idea is called the Law of conservation, Q_in = Q_out and Q=VA where Q is the flow rate, V is the volume, and A is the area. Pressure is generated by the usage of a pump. The pump is attached directly to the plumbing pipes or to a holding reservoir that is periodically filled. The pressure acting on the boiler remains at a constant rate, usually 1-1.2 bars. The other important pressure is related to the machine gauge pressure. This pressure remains at about 4 bars when a shot is not being pulled, however, when a shot is being pulled the temperature of the water in the tube increase to equilibrate with that of the boiler and the pressure increases as a result of this. The brewing pressure when a barista pulls a shot needs to be between 8-10 bars. If it is not, there is a problem!

A shot will pour out in rounds of 4.5-5oz which is the equivalent of pouring three shots. 1 shot of espresso is typically 1-1.5 oz. The time required to pull a shot varies with the machine programming, but should be between 18-26 seconds. Typically a delicious, rich, strong, shot will run for 25 seconds.

The Governing Laws of Physics and the Espresso Machine

The commercial espresso machines, and others, are a perfect example of conservation of heat, work and energy of a system. There are three laws of thermodynamics which in summary make the assumption that energy is neither created nor destroyed and is always conserved. These laws are applicable to the espresso machine because heat and energy are never lost from the system, they are merely transferred around to heat, pump energy, equalize pressure, control volume and flow, the timing of the espresso and so on. One might make the argument that once the consumer consumes the coffee, it has been lost from the system; this is true, but the espresso remains in the universal system, which is justified by these laws.