|Coffee & Espresso|
Heat Exchanger vs. Double Boiler
A common myth for newcomers to the espresso machine world is that one needs a double boiler machine to extract espresso and steam/froth milk simultaneously (at the same time). This myth is false. The following machines can extract espresso and steam milk at the same time:
Immediately below is a visual representation of the heat exchanger design with a thermosyphon grouphead. It is called heat exchanger because there is a transfer of heat between the steam/how water in the larger boiler vessel and the closed tube that passes through the boiler.
In the first diagram, the heat exchanger tube is the larger tube inside the single boiler with upside down 'V's. This is the water that is flash heated to make espresso - incoming cold water is mixed in with this hot water when the extraction starts. When not extracting espresso, this same water inside the closed loop system is heated in the heat exchanger and is pushed through to the grouphead which forces the slightly cooler water back to the boiler for re-heating. If the machine has a brew pressure gauge, this is one reason why the gauge will fluctuate and change while the machine is idle. This is why the brew pressure gauge should not be read while the machine is idling and is nothing to worry about.
In some cases, the larger boiler is laid down horizontal, and the heat exchange tube remains vertical. In some cases, the horizontal boiler incorporates a horizontal tube from inside to another. In most vertical boilers, the heating element is at the bottom of the boiler. There are a few cases where the heating element is on top of the vertical boiler with extensions internally to keep it under water. We have never seen a vertical boiler with a heating element on the side. On horizontal boilers, the heating element is usually on the side.
In some cases where there is no thermosyphon tubes into the grouphead, there is on one tube running to the grouphead. This grouphead is usually attached directly/welded to the boiler for passive heating by way of the metal.
Immediately, below is a visual representation of the double boiler design with a thermosyphon grouphead.
Espresso quality in the cup - The double boiler version has superior control over its heat exchange counterpart in the temperature control of the water in the boiler. This better temperature control allows the user to change the boiler water temperature to match better with the coffee blend being used. The superior temperature control offers a better extraction in the quality found in the cup. Therefore, 1st-line 'considers' the PID double boiler machine is a better choice for espresso-only consumption.
There have been some arguments made by some that the size of the coffee boiler on a double boiler espresso machine can determine the quality of the espresso in the cup. For example, there are some who argue that a smaller boiler will have faster turnover of the water, and this fresh water makes superior espresso. Although we were a believer in this camp before we carried double boilers, we no longer believe this theoretical model holds true. Our reasoning is that if the smaller coffee boiler theory held true, then a heat exchanger machine with a much smaller capacity would make extract superior espresso over a double boiler machine.
Steaming capacity - From our observations, the heat exchanger model is a more powerful steamer than its double boiler counterpart, If frothing or steaming milk for milk-based espresso beverages, the heat exchanger model is superior as the heat exchanger model seems to have more steaming power and more steaming capacity than its double boiler counterpart. The reasons are,
Energy Savings - The double boiler version allows the user to independently turn on/off the coffee boiler or steam boiler. If the user only operates the coffee boiler, electrical usage is only 600 watts for the coffee boiler heating element. The heat exchanger has an one on/off switch for the entire machine.
Delta (difference or offset) of Temperature between water temp in Boiler and Water temp at Grouphead
On average, the difference of the PID programmed temperature and the temperature of the water at the grouphead is about 20 degrees F. However, we have done a lot of testing with PID double boiler machines in several different environments, and we can say in our opinion with good certainty the following:
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