Managing the Brew Temperature of HX Espresso

14. 1. 4.
Managing the Brew Temperature of HX Espresso Machines •
Managing the Brew Temperature of HX
Espresso Machines •
Have a question or just want to talk espresso?
Check out the forum and its FAQs and Favorites [1]!
How I Stopped
Worrying and
Learned to
Love HXs
By Dan Kehn
Or, how to manage the brew temperature of your heat exchanger (HX) espresso
The E61 is one of the most popular espresso machine designs among home
enthusiasts. Named after the year the patent was approved (a year of an eclipse),
its gorgeously sculpted and highly polished group is recognized as a hallmark of
fine espresso machines. The design of the E61 group also has well-earned
reputation as the heart of an espresso machine that is forgiving of minor errors
in barista technique. However, knowing how to maximize this forgiveness
necessitates a certain understanding of how heat exchanger espresso machines
work, as will be presented in the next section. If you're already familiar with
these types of espresso machines, you may want to skip ahead to the main topic
of this how-to, Managing HX Brew Temperature [2].
Espresso Machines 201: Heat Exchangers
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Managing the Brew Temperature of HX Espresso Machines •
Thermosyphon circuit and heat exchanger
Unless you repair home appliances for a living, there's a good chance you don't
really know or care what's under the covers of your dishwasher. If you're like me,
you have a basic appreciation of what's going on. To the best of my
understanding, my dishwasher works something like this:
Hot water and soap go in,
A shower-marimba of hot water ensues for twenty or thirty minutes,
Dirty water is pumped out,
Clean dishes emerge.
This pretty much covers the full depth of my knowledge of our dishwasher's
inner workings. There's little motivation to learn more because I don't see any
practical use for acquiring the knowledge. In contrast, it is worth understanding
the principles behind how your espresso machine works because this knowledge
will guide your preparation techniques and that definitely has practical benefits
—better espresso!
Whatever the espresso machine design, if the goal is to produce espresso and
espresso-based milk drinks, they all must address the same requirements:
How to produce the coffee's optimal brew temperature water consistently,
accurately, and reliably
How to produce powerful, abundant steam for frothing milk.
And as a consequence of the first requirement, all designs must also consider:
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Managing the Brew Temperature of HX Espresso Machines •
How to get the group to the desired coffee brew temperature.
If you're new to the espresso machine market, you may want to read Espresso
Machines 101 [3] before continuing. It briefly introduces three of the most
common espresso machine boiler designs, namely single boiler, double boiler,
and the focus of this article, single boiler / dual temperature, which are
commonly called "heat exchangers" (HX).
An intuitive appreciation for the workings of a single boiler espresso machines
is easy. They have two thermostats, one for brew temperature and another for
steam temperature. Flipping a switch chooses which controls the heating
element. Dual boilers are even more obvious; they have one boiler for the
coffee's brew temperature water and another for producing steam. Heat
exchangers are an innovative, efficient design that defies such a brief
explanation. To understand them more fully, let's address each of the questions
above as they apply to such espresso machines.
In essence, these requirements collectively pose the question, "How do heat
exchanger espresso machines work?"
How to produce brew temperature water consistently, accurately, and
Heat exchanger espresso machines rely on a heavy, highly thermally conductive
metal group. Temperature-wise, think of the brew group as a chunk of brass
having almost the same weight as a bowling ball and the few ounces of water for
an espresso as an egg. This analogy really adds meaning to the notion of
"thermal mass," wouldn't you agree?
The heat exchanger, shown in the simplified schematic to the right as the tube
passing through the center of the boiler, is responsible for warming the
incoming fresh water to near the coffee's optimal brew temperature before it
reaches the grouphead. Once all that solid brass is at the desired brew
temperature, it acts as a dampener to either reduce the temperature of the
incoming water if it's a little too hot, or raise it if it's a little too cool. Your goal
is to get the group as close to precisely the desired brew temperature as possible
so it can "fine tune" the somewhat volatile temperature of the water exiting the
heat exchanger. This of course begs the important question that follows.
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Managing the Brew Temperature of HX Espresso Machines •
How to get the group to the desired brew temperature
The means by which the brew group arrives at the desired coffee brew
temperature depends on the espresso machine's design. Many commercial
machines rely on direct thermal conduction by attaching the group directly onto
the boiler. Other machines, like those we're considering in this article, use a
thermosyphon to circulate water from the boiler through the group, as shown in
the schematic to the right. As the water in the heat exchanger portion of the loop
warms (double lines), it rises and flows towards the group (red arrow). The water
then cools and descends towards the bottom of the group (blue arrow), returning
to the boiler where it reheats and repeats the circuit.
How to produce powerful, abundant steam for frothing milk
The boiler itself is only partially full of water (or partially empty, depending on
your point of view). The space above the water is filled with steam under
pressure. Thus the boiler serves two purposes: Providing steam from the top
portion for frothing milk and an easily regulated heat source to warm the water
passing through the heat exchanger.
The boiler pressure gauge indicates the amount of pressure, generally between
1.0 and 1.2 bar (one bar = 14.5 PSI). An espresso machine's boiler works under
the same principle as pressure cookers by increasing the boiling point of water
to higher than the 212°F one expects when heating water in a pan on the stove.
The table to the left shows the actual water temperature at common espresso
machine boiler pressures.
The entire brew pathway heats up to some percentage of the boiler temperature
as this "super hot" water circulates between the boiler and group. Since no
system has 100% thermal efficiency, the final grouphead temperature is
determined by its own heat loss and that of the copper tubing carrying water to
it. In the case of most U.S. espresso lovers, their target brew temperature is
around 201-203°F; for some countries overseas, the tastes are for a cooler
192°F. Manufacturers adjust the length and diameter of the tubing making up
the thermosyphon loop to increase or decrease the efficiency of the circuit to
arrive approximately at the desired brew temperature.
Looking at the schematic, notice that the pump feeds into the same
thermosyphon loop. To simplify the diagram, I've omitted valves and solenoids
14. 1. 4.
Managing the Brew Temperature of HX Espresso Machines •
that direct water through the group and highlighted the heat exchanger portion
of the circuit passing through the boiler. For most semi-commercial espresso
machines, the heat exchanger is little more than a fat tube that traverses the
boiler with part of it immersed in the super hot water and the rest exposed to
only steam. The heat exchanger's job is to conduct some of the boiler's heat to
the water being pumped towards the grouphead. One problem, however, is that
being surrounded by water at no less than 250°F, it won't be long before the
water inside the heat exchanger will also be super heated. Extracting an espresso
using this water will blast the coffee with blistering hot steam and assure an
extremely hot, bitter brew.
Enter the most important HX ritual you'll ever learn: The HX cooling flush. The
next section will introduce why, what, and how to ready an HX for brewing
espresso. The essentials are covered in the first few paragraphs. Experienced
baristas may wish to continue reading the rest of the section for a discussion of
the thermodynamics behind these efficient and fascinating machines.
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