Semi Automatic Espresso Machine Buying Guide
When purchasing a semi automatic machine there are several points to consider. The heating technology, the portafilter, the group.
First a short note about pumps. You will see many machines advertise that they have 15 or 18 bar pumps, this for the most part, should be ignored. The vast majority of espresso machines use the same stock pump made by a company called Ulka out of Italy. These vibration pumps are very reliable and hardly ever fail. The maximum pressure is meaningless since espresso is best extracted at around 8-10 bars at the group. The pressure is controlled using an Over Pressure Valve which "bleeds" water away reducing the pressure.
this is a picture of the pump that the majority of espresso machines use. An Ulka EX5.
Some Semi-commercial machines use rotary vane pumps. The advantages of a rotary pump over a vibration pump are that they are quieter, more robust and and the brew pressure can be controlled by adjusting a built in screw.
A thermoblock is basically a heating element that "flash heats" water. Water is drawn from the cold water reservoir and passed through the heated thermoblock and delivered on demand. Thermoblock steaming is accomplished by raising the temperature of the device even higher and having the pump delivery small bursts of water that are again passed through the thermoblock and heated into steam.
The pros of thermoblocks are that they heat up very quickly and espresso or steam production is not limited since the thermoblock is always hot.
The major con of thermoblock systems is that because of the small thermal mass of the device the temperature is quite unstable. Extraction temperature from a thermoblock system can vary widely and the steam produced is generally quite "wet".
Semi-automatic machines that use thermoblocks are Breville, Krups, Cuisine Art, Delonghi and most other lower end machines.
Single boilers are typically made out of aluminum, stainless steel or brass. The boiler serves two purposes; espresso extraction and steam production. The way this boiler functions is it draws water from the cold water reservoir inside and heats it to a set temperature. When steam is requested usually by pressing a steam button, the heating element heats the water further boiling it and creating pressure inside the boiler. The steam is released by opening a valve and travels through your steam wand. It is all mechanical and the pump is not needed during steam production.
The pros of single boilers are temperature stability for espresso extraction and improved steaming because of "dryer" steam
The cons of single boilers are downtime between switching from espresso extraction and steam production (this is the time it takes for the water to boil and then cool)
this is an example of a brass single boiler from a Rancilio Silvia.
Examples of machines with single boilers are Gaggia, Lelit, Rancilio
Heat Exchangers are larger boilers typically made of copper or stainless steel. A heat exchanger is filled about 2/3's of the way with water and is constantly at boiling temperature. As the water boils and turns in steam vapor the pressure inside the boiler rises. Typically HX boilers are set to around 1 bar of pressure, this is controlled by a pressurestat. The release of the steam is done mechanically by opening a valve and the built up pressure releases through your steam wand.
The water for espresso is taken from the cold water reservoir and passed through a tube running inside the boiler and heated via conduction; because of the large thermal mass of the boiler and the boiling water, the temperature stability is exceptional using this method.
Since the pump is not required for steam production a heat exchanger can draw espresso and provide steam at the same time.
The pros of heat exchangers are the ability to steam and draw espresso simultaneously, as long as the boiler is hot you can make an unlimited amount of espressos.
The cons of heat exchangers are that the price of such machines is higher and you have to perform a "cooling flush" if the machine has been idle for more than a few minutes because the water left inside the heat exchange tube will be too hot for espresso extraction.
Examples of machines that use heat exchanger boilers are Rocket, Vibiemme, Pasquini and Bezzera
Quite simple, there is one boiler for espresso extraction and one boiler for steam production. The steam boiler works just like an HX steam boiler being filled about 2/3's of the way so it becomes pressurized.
The pros of double boilers are the ability to steam and draw espresso at the same time, the ability to control the temperature of each boiler independently, some double boiler machines have PID devices that control the temperature of the brew boiler.
The cons of double boiler machines are again the price tends to be quite high, they take longer to heat up than other configurations, the water inside the espresso boiler can become stale in low volume environments and they also tend to be quite large.
Examples of machines that have double boilers are Vibiemme and Dalla Corte