Espresso! My Espresso!
How a Heat Exchanger ("HX") Works
by Randy Glass - Copyright 2009 - All rights reserved

      Heat Exchangers have been around for a long time. A basic one resides in your kitchen in the form of a double boiler. In an espresso machine the heat exchanger works on the same principle of taking a heat source and transferring the heat energy to another object. The boiler itself supplies the user with steam for stretching milk as well as hot water, but the heat exchanger helps create temperature-regulated water for brewing espresso. While the actual design parameters can be complicated in order to heat water to the correct temperature for brewing, the basics are quite simple.

      Above is a diagrammatic view of a boiler which is equipped with a heat exchanger. Numbers in parenthesis below refer to the diagram:

      (1) - The heat exchanger is a large metal tube which passes through the boiler.

      (2) - Most of the heat exchanger is submerged in the hot water...

      (3) - ... and a portion of the heat exchanger is exposed to the steam.

      The hot water in the boiler (2) is what is drawn off when hot water is dispensed through the hot water wand. The water is forced out by the pressure of the steam (3) through the hot water pipe (8). You may hear the pump run, but that is just refilling the boiler. When stretching milk, steam (3) comes off the top area of the boiler through the steam pipe (7). All water which comes through the brewhead (such as when brewing espresso) comes from inside the heat exchanger (1).

      The heat exchanger shown here is the typical design used for an E-61 brewgroup among others. These large brewgroups have to be heated because they are located on the face of the machine and not to the boiler. Some of these weigh nine pounds, and if they were not preheated their mass of metal would not allow a high enough temperature for good espresso brewing. To do this they use a thermosyphon. The hot water in the heat exchanger rises through convection, going up through the metal pipe (5) and into the brewgroup. The water cools as it loses heat energy to the brewgroup, and the cool water becomes slightly more dens and drops down through the lower pipe (4) and returns to the heat exchanger. As the water enters the heat exchanger, some heat energy is absorbed ("exchanged") from the hot water in the boiler and the process of circulating the water through the group continues this way.

      When the user begins a brew cycle, the machine's pump sends pressurized water through the injector (6) into the heat exchanger. The hot water contained in the heat exchanger (1) is then displaced and forced into pipe (4) and pipe (5), sending hot water through the brewhead and into the portafilter.

      The benefits of a heat exchanger machine include:

      - The ability to have as large of a boiler in any given amount of space inside the machine which holds a good supply of steam.
      - The ability to steam and brew espresso concurrently.
      - Fresh water is supplied for each pull since a minimum amount of water is held in the heat exchanger compared to a larger single boiler.
      - One heating element supplies all needs as opposed to two elements in a dual boiler machine
      - The relatively large volume of water in the boiler helps achieve consistency from pull to pull as well as allowing a significant number of shots to be pulled successively without a drop in temperature of the brew water.

      The main problem of a heat exchanger revolve around the fact that when the machine is sitting idling (on, but unused), the water in the heat exchanger can become overheated reaching temperatures near the boiling point when released from the brewhead. This is easily resolved by doing a "cooling flush" before pulling a shot. This is achieved by simply turning on the brew function and allowing some water to be released from the group before locking the portafilter for brewing espresso.