Pump-style Espresso Machines
This type of machine is a little fancier, but is still fairly simple to operate.
Let's start by seeing how the machine is put together.
The reservoir holds the cold water used in the espresso machine. It is not pressure-tight or heated, and it is removable.
The pump draws water out of the reservoir and pumps it into the heating chamber at high pressure.
The heating chamber is a sturdy, stainless-steel structure with a heating element built into a groove in the bottom. The resistive heating element is simply a coiled wire, very similar to the filament of a light bulb or the element in an electric toaster, which gets hot when you run electricity through it. In a resistive element like this, the coil is embedded in plaster to make it more rugged.
The heating chamber also contains a one-way valve that lets water into the chamber from the pump, but not back into the pump from the chamber.
The porta-filter is the removable part of the machine the holds the ground coffee. Inside the basket is a small removable screen into which the ground coffee is packed. On the bottom of the basket are two spouts where the espresso comes out.
The steam wand is used to heat and froth milk for use in various espresso drinks. This wand is connected to the heating vessel. When the user puts the valve in the steam position, steam from the heating vessel is released out of the wand and into the milk.
The control panel in this machine contains the on/off switch, two indicator lights and a control valve. One of the lights indicates that the machine is on, and the other indicates if the heating chamber is up to the proper temperature. The valve is used to start the flow of water through the coffee in the porta-filter or to start the flow of steam from the steam wand. It also engages one of two micro-switches that control the pump and heating element.
Now let's take a look at what happens when you make a shot of espresso.