Styles of Heat Exchanger Systems
myth for newcomers to the espresso machine world is that one needs
a double boiler machine to extract espresso and steam/froth milk
at the same time. This myth is false. Please
read Heat Exchanger (HX) vs Double Boiler (DB) prior to reading this page.
Below are four generic representations of typical heat exchanger
systems. Each diagram is a cylinder, representing the boiler- either
horizontal orientation or vertical. The blue in the boiler represents
the water level. The black line, with the yellow sleeve on it represents
the auto fill sensor.
The yellow tube(s) that run the length of the boiler are the heat
exchanger tubes. In layman's terms- the water (blue) is heated to
the temperature needed for steam and hot water. (around 150º-170º)
The water passing through the heat exchanger tube is significantly
As this cold water passes through the pipe that is submerged in
the hot water, it absorbs some of the excess heat found in the water.
This is know as the heat exchange.
In the diagram found directly to the right, we see a very
basic HX system.
The boiler is filled around half way. The auto fill sensor ensures
that it does not over or underfill. Cold water enters the HX at
the bottom, and rises upward. As it rises it is heated to the proper
brew temp- this is controlled by the speed at which the water flows
through the HX. Slower, and it will absorb more heat, faster and
it absorbs less.
Immediately after exiting the top of the HX, the water is directed
toward the grouphead where it will make contact with your coffee
This style of HX is found on such espresso machines
as the Vibiemme Domobar.
Next, pictured directly below, is the next most common HX
type. This style is very similar to that which is found
in the Vibiemme with the only difference being the orientation of
the boiler. This style- mounted horizontally is found on such machines
as the Isomac Tea and Millennium.
This orientation is preferred in some cases because of the fact
that the HX tube is almost always 100% covered. In large volume
situations, drawing large amounts of hot water or steam can cause
the boiler level to drop. Although it refills quickly, this variation
in the level of hot water will effect the end temperature of the
heat exchanger. By having the HX pipe mounted lower and 100% submerged,
you avoid these fluctuations.
The next HX type is represented below, with the dual heat
exchanger pipes, running vertically. It is that which is
found on larger commercial machines, such as the Elektra Barlume.
This style is used on machines with multiple groupheads. Each grouphead
will have it's own respective heat exchanger.
Like the HXs above, this works by flowing water up from the bottom,
flash heating it and exiting it out the top. Because the boiler
size is so much larger for machines with multiple groups, horizontal
orientation is used in nearly 100% of cases. Also, with the boiler
being larger the level of water will not drop as drastically when
steaming or pulling hot water.
Lastly is the
HX style found in the Bezzera and Pasquini Espresso Machines. (pictured
to the right.)
This style is unique to Bezzera and Pasquini. The heat exchanger
and boiler both have their own style.
The HX here uses a rounded tube. The water enters and exits at the
top. The inside, opposite the openings is rounded off, and causes
the water to swirl back up inside the HX tube and exit out the top,
next to where it entered.
The second unique aspect is the shape of the boiler. These boilers
have a plate that sticks outward from the top of them. This is used
as a heatsink to help transfer heat to the grouphead.
Because Pasquini and Bezzera do not use an E61 grouphead, or a
thermosyphon to heat the group they had to devise a way to heat
the grouphead to the proper temperature. The boiler design does
just this- As the water in boiler is heated, the heat soaks into
the boiler walls, up onto the plate, and down into the grouphead.
Important Note about most
HX machines, and all machines with auto-refill probes:
The auto refil probe works by sending 5 volts into the boiler. The 5 volts enter through the probe, and grounds out when it makes contact with water. When
the ground connection is made, the auto refill stops. If water level drops, the probe is no longer grounded to the water.
Refill starts, and goes until the probe makes contact with the water.
IF YOU USE DISTILLED OR REVERSE OSMOSIS WATER - this water sometimes
is purified too much. The probe will not sense that it has made
contact with water. If this happens, the boiler will overfill and
spray water out of the safety valve located on the top of the boiler. This is due to RO and Distilled water being TOO PURE. We recommend
cutting this water with 20% tap water, wich will increase the ammount
of minerals in the water to the point were the probe will sense