The basic knowledge of coffee machine the principle of single boiler heat exchange

Since Luigi Bezzera launched the first commercial coffee machine with steam pressure in 1901, whether it is the Pavoni Gaggia or other machines that use steam or pistons as pressure, there has been only one boiler: the bottom of the boiler is the hot water for brewing coffee, and the top provides steam, but in order to produce steam, the whole boiler must be heated to boiling, causing hot water near the boiling point to burn the coffee powder and lose the aromatic oil hidden deep in the coffee powder. And extract the coffee that is doubly bitter.

Because the water for brewing coffee is heated in a large boiler, the water will not be fresh if it is placed in the boiler for too long, and the temperature difference of hot water will increase due to the use of steam, because the different amount of steam also increases the temperature difference of hot water for brewing coffee.

In order to improve the temperature difference, in 1960, Ernesto Valente, in the world's first pump-powered FAEMA E61, changed the way the water used to brew coffee was heated, so that the water used to make coffee did not come into direct contact with the heat source, but used a method similar to water-separated heating.

In the middle of the boiler of FAEMA E61 (hereinafter referred to as the outer boiler) is a tube (hereinafter referred to as the inner pipe line), which uses a pump to introduce fresh cold water into the internal curved pipeline. When the pump pushes fresh cold water through the tube, the cold water will become hot due to the hot water in the outer boiler, but there are tubes on the outside, so the fresh water will not come into contact with the hot water of the outer boiler, and the inner water pipe is curved in shape. the aim is to make the cold water have enough time to come into contact with the external boiler and become a hot water temperature just suitable for brewing coffee.

(see the picture below, which is a bit simple.)

┌-steam

│ ┌-→

│ │

┌-┐ │ ^ │ water level

Cold water-┴ ┴-┴-┐┌-┴-┐

→-┬ ┬-┬-┐││┌-┬-┐│

└-┘ │ │└┘│ │ shower head

Pump │ └-- ┘ │

│ │

│ │

└-┘

Boiler boiler

Due to the circulation of hot water in the boiler, the temperature of each cup of coffee is very close, whether it is one cup an hour or a hundred cups at a time. In this way, it can not only reduce the temperature difference, but also take into account the freshness of the water, while the hot water in the outer boiler is only used to produce steam to make milk foam, not to brew coffee.

This is the principle of a single boiler heat exchanger. Today, more than 30 years later, the internal structure and principle of most Italian coffee machines are still very close to the FAEMA E61.

From the above explanation, we can infer that the larger the capacity of the external boiler, the lower the temperature difference in the hot water for brewing coffee, and the more it can reduce the temperature difference caused by the use of steam.