A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment.
Steam based, domestic espresso machines will normally have a heat exchanger. These coffee makers typically have a single boiler system. A heat exchanger is necessary because two different temperatures are required for foaming or steaming milk (for cappuccino and latte), where the temperature needs to be boiling in order to create steam, and for brewing espresso, where the temperature needs to be cooler so that the coffee isn’t burned. The heat exchanger adjusts the temperature of the boiler.
A heat exchanger is paired with a set of solanoid valves, which allows for the passage of cold water. This cold water brings the temperature down when the user switches from steaming to espresso brewing. In this way the user has to wait very little time between steaming milk and brewing espresso.
Apart from heat exchangers, other popular heating systems in domestic coffee machines would be the Thermoblock, which is a block made from some sort of metal that is heated by water and in turn water to the appropriate temperature for steaming or brewing espresso, as well as a double boiler system, where one boiler controls the steam temperature and another boiler controls the brew temperature.