Double Boiler Versus Heat Exchanger

A commonly question raised by many, “Should I get a double boiler or a heat exchanger?”.
In many cases, it depends on the user’s consumption level and expectation.

This is the double boiler.

doubler-boiler

A double boiler has two boilers on a single machine. Each boiler for a function. One is the brew boiler, set at a lower temperature. The other a steam boiler which is at much higher temperature.

By keeping them separate, the user will have full control over the brew temperature and is more consistent in the extraction.

This is the heat exchanger.

heat-exchanger

The heat exchanger has only one boiler, set at steam temperature. It has another smaller “boiler” inside, a heat exchanger rod that passes through.

When an extraction is done, the pump will push cold and fresh water from the water source and pass it through the heat exchanger. By thermodynamics and hydrodynamics, as the water travels through the heat exchanger rod, the water heats up. The water should heat to a therearound sweet spot in temperature, perfect for espresso extraction while it exits.

Since we know that Double Boilers have a dedicated boiler specifically for brewing espressos. This means that double boilers should have a stable straight-line temperature profile. Meaning, if I set the brew temperature running at 90 degrees. The temperature of the water throughout the extraction should maintain at around 90 degrees with little deviation from its set temperature.

Heat exchangers are expected to have fluctuations in the temperature during the extraction process. Initially, idling water in the heat exchanger will warm up to a higher temperature. As you do an extraction, cold water goes into the heat exchanger to warm up while the hot water exits. This displacement of cold water entering reduces the heat in the heat exchanger. What will happen is the temperature of your extraction should have a gradual reduction.

Here is an example of the heat profile comparing both systems

brew-profile

So the question is, which is better?

Shouldn’t double boilers be better with consistent and accurate temperature control? To answer that, we must first understand the history of all these heating systems and some science about coffee.

Double boilers were invented later than heat exchangers, and Italian manufacturers initially created them at the request of American cafes. As American coffee conglomerates were expanding quickly around the nation and globally, and they are always lacking trained baristas. They want the coffee machine to be more predictable, to make consistent shots of expected quality even from amateur baristas, thus the double boilers were invented. No more flushing extensively is needed, no more guessing if the temperature right. As the big cafes were using them, the name of the brands doing Double Boilers soon became popular globally. So what that means is Double Boilers were invented to improve consistency.

In some heat exchangers machines, they are equipped with a PID temperature controller and regulator. Some will ask whether adding a PID works or is useful in a heat exchanger. It does work by controlling the temperature of the steam boiler, which in turn influence how hot the heat exchanger in the boiler can become. Raising or declining the temperature does have an impact on the water used for brewing so it does influence the taste of the espresso as much as a double boiler. The difference is double boilers have a straight line temperature extraction, while heat exchangers have a declining heat profile. The deadband of temperature swing in a PID is also a lot more stable than using pressure controllers, which is normally known as the pressurestat.

Taste difference?

Is a straight line profile better in taste? Before that, we have to understand the definition of a good espresso? Who else drinks espresso faithfully in the world other than the Italians?

Here is their definition according to the Istituto Nazionale Espresso Italiano.

On sight, Italian Espresso has a hazelnut-coloured froth, verging on dark brown with tawny hinges. This cream has a very fine texture, which means that its mesh is tight and large or small bubbles are absent. The nose reveals an intense scent with notes of flowers, fruits, toasted bread and chocolate. All of these sensations are felt also after swallowing the coffee in the long lasting aroma that remains for several seconds, sometimes even for minutes. Its taste is round, substantial and smooth. Sour and bitter are well balanced and neither one prevails over the other. Astringency is absent or barely perceptible.

http://www.espressoitaliano.org/files/File/istituzionale_inei_hq_en.pdf

In short, the espresso should be well balanced in its acidity and bitterness, with a lovely blossom of floral or fruity notes, along nutty profiles. This may not appeal to everyone but this is generally how coffee is appreciated around the world for decades.

From many test taste, it’s found that heat exchanger releases a wider spectrum of characters that are closer to the criteria. Why is that so?

It’s long been known that the pressure profiling machines or spring lever espresso machines, produce the best-tasting espressos. These machines have a pressure that moves all the time. They are not stable, not producing straight-line temperature or pressure profiles. They are known to express a wider spectrum of flavours and tame the harsher astringents in espresso. These machines, don’t have a uniform extraction pattern, it’s dynamic and have a declining pressure profile.

Heat and pressure can play a similar effect. The higher the heat or temperature, the more energy it brings to the coffee during extraction, the more bitter and less sour the espresso gets. If the heat is hitting too high, it easily diminishes the delicate flowery and fruity aroma. So with a declining heat or pressure profile, it draws out a wider spectrum of flavours from the two ends. At the same time, balancing the bitterness and sourness as it doesn’t stay at one point.

It is actually not a bad idea for a machine to have a declining heat profile found in heat exchangers. During extraction, the first few seconds will have ample oil from the beans extracted, this is the part that gives the flavour with the least astringent. The second phase of the extraction, normally after the 15th second, is more about adding volume to make a bigger espresso than flavours. From that point on, colloid from the lipid oil in the coffee starts to run out. These colloids have an effect of being receptors to the bitterness, and every bit more without the colloids increases the amount of astringent which can overwhelm the flavours in the coffee. As such, a declining temperature profile that brings less hot water and less energy to draw the astringents in the last part of the extraction may do very well for a dark roast.

However, if your coffee is of a lighter roast, or you choose to use a lot of coffee for your extraction. Increasing the temperature, having a straight-line profile, might not be a bad idea. As the heavy dose of coffee you use will mean an increase of colloids to have the stamina to withstand a longer extraction before releasing the astringents.

Does it mean anything about which heating system is more superior in taste? Not really. It still depends on the coffee,  and your preference. If we really have to decide which should produce a better-tasting espresso, I believe the odds should favour the heat exchanger. This should come as a big surprise as many assume that double boilers, being more expensive, should mean being more capable of making better espressos.

Water

Water is the primary component in an espresso.  To Italians, many believe that fresh water is essential to a good espresso. Many experienced baristas know that fresh water, release a wider spectrum of flavours in the extraction.

Where does the double boiler draw water for making its espresso? From its brew boiler. Water in the brew boiler has little movement and the same water gets reheated from the day it operates till the last time of its service. One can’t drain the boiler that easily, particularly for double boilers. To prevent a buildup of bacterial or microbial from nesting, most boilers are made of copper, which has natural anti-microbial properties. Double boilers made of stainless steel will require drainage of boiler more regularly.

For heat exchangers, it only draws fresh water from the water network or water tank for its extraction.

Maintenance

A heat exchanger only has one boiler to maintain. With one set of heating element, over-heating safety sensor, overpressure safety controller, low water sensor, safety valves, etc.

With a double boiler, the number of parts to maintain is almost doubled, and maintaining them will be both challenging and expensive. Chance of having something malfunction will likewise be increased. Doing basic things like draining a boiler in a double boiler, is certainly more work. As such, descaling, a necessary cleaning procedure, is also a lot more complicated that requires professional assistance.

A double boiler will have a longer water circuitry. This means that troubleshooting will need to look for a wider coverage, and will be more tedious to find out which part has failed. In any case, it will definitely be more costly to maintain a double boiler.

Startup time

Having two boilers, the double boilers will, of course, take a longer time to heat up.

A heat exchange E61 espresso machine can get properly heat up in 20 minutes; a double will require at least 30 minutes.

Usage

Both are used almost in the same way. Except that you will need to flush some hot water out of the heat exchanger if it had been idling for some time. You will also need to flush some hot water out of the double boiler as you may want to clear the residual water in the grouphead and make sure that the valves and rods are uniformly hot enough. Many Italian heat exchanger models are designed in a way to be “cooler” than being too hot and require minimal flushes.

So who needs a Double Boiler?

Double boilers do have an advantage. There’s less difference in temperature between cups, so you are likely to have more consistent shots. When one draws a lot of hot water from the steam boiler, it also won’t affect the brew temperature as much. This might be something important to a large cafe where importance on consistency is above all. However, it’s worthy to note that in coffee making, maintaining consistency is hardly an achievable feat. As the condition of your beans changes all the time from deterioration effected by light, heat, humidity and oxidising. Particularly if you are in an environment where usage isn’t heavy. There are other things that are hard to be constant; Like the roasters can’t get it accurately consistent in their roast or the condition of their roaster. The farmers can’t get the same grade and size constant. The absorption of nutrient, the temperature and watering of the coffee plantation is irregular, fermentation and processing are hard to maintain constant, the condition of storage remains the same throughout the seasons, and much more.

To some cafes or baristas, they also feel more confident to look at exact numbers than to guess if they have done it right. This can be done via the PID temperature controller.

The main difference between a good coffee and a bad one, still depends on the barista.

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