Understanding the reasons behind why one machine is operated one way versus another is rooted in their group and boiler designs. This is a short summary of how these different machines operate under the covers and how it affects their usage.

Espresso and milk, they're natural friends, right? Certainly that's true for our palates, but as fate would have it, they don't have an all too friendly relationship in terms of their preparation. To state the obvious, this is because espresso is brewed at one temperature and milk is frothed at a much higher one. This simple fact reveals the puzzle at the root of the espresso machine designer's dilemma: How to deliver these two vital components reliably and quickly? It becomes all the more difficult because espresso demands of temperature control (the initial temperature of the brew water) and temperature stability (the temperature of the brew water during the shot) are surprisingly exacting.

The video below explains the differences between two common types of boilers for high-end espresso machines, heat exchanger and double boilers:
From Heat Exchanger vs. Double Boiler Espresso Machines

The remainder of this post briefly introduces the three types of espresso machine designs:

Single boiler



This type of espresso machine has two temperature settings controlled by a pushbutton. When released, the brew thermostat controls the boiler temperature. When pressed in, the steam thermostat controls the boiler temperature. Since the boiler serves both purposes of brewing and steaming, you must wait while it transitions from brew to steam temperature.

Single boiler, heat exchanger



This type of espresso machine has a creative solution to the two-temperature problem: The boiler is kept partially filled to allow for a layer of steam. Since the water is under pressure, its boiling point is higher, just like in an old-fashioned pressure cooker. A copper tube passing through the boiler called a "heat exchanger" is responsible for flash heating fresh water from the reservoir to near final brew temperature. But how does it deliver the desired temperature with any hope of accuracy? This requires a little understanding of the underlying design of the E61 brew group and how its temperature is affected when coupled with a heat exchanger. Let's cover the essentials and see how this affects your preparation routine.

A popular brew group design is called the E61 (presumably owing to the year the patent was granted, the year of an eclipse). One of this design's unique contributions to temperature control and stability is the way it circulates hot water through the group, relying on the natural rise in water as it warms. If you've ever swam in a lake or pool at night, you have surely noticed that the top layer of water is much warmer than the layers below. This group takes advantage of this law of physics in order to circulate water from the boiler towards the grouphead. The water heats the group and cools, returning to the bottom of the boiler and the cycle repeats.

Weighing nine pounds and made of solid brass, this group delivers temperature stability the old fashioned way: By way of overwhelming thermal mass. Temperature-wise, think of the E61 brew group as a bowling ball and the few ounces of water for an espresso as an egg. Get all that solid brass to the desired brew temperature and we won't need a physicist to know what the water's final temperature will be. Got the picture?

However that's only part of the story. The rest involves the heat exchanger. When making espresso with a single boiler espresso machine like the one described earlier, there's no need for a heat exchanger since the boiler water is already at the appropriate brew temperature. For those who only make espresso, life can't be much easier. As was mentioned earlier, the problem begins when you want to quickly prepare two things requiring two different temperatures of water, namely espresso and frothed milk.


Read about the E61 patent and see the detailed interior schematics

Remember the natural circulation of the E61's thermosyphon? Alas, this is where conundrum begins: In a heat exchanger espresso machine, this system will circulate steam temperature water (~257F) through the group, not brew temperature water (~202F). Given enough time, the flash-heated water that ultimately arrives from the heat exchanger brew group will be considerably hotter than brew temperature, despite the dampening effect of the brass grouphead.

Thus the heat exchanger / E61 combination requires a "cooling flush" to reduce the temperature of the group to a more reasonable brew temperature. A natural question you might ask yourself is, "How much water is needed to cool the brew group?" The short answer is somewhere between four and six ounces, maybe a little more, and probably not less. Most home baristas apply a simple trial-and-error approach to judge if the cool down flush was correct by first drawing water through the group until the steam and sputtering subsides and then tasting the subsequent shot. If the espresso is sour, too much water was flushed, if it is bitter, too little was flushed. The good news is that once you've done the initial cool down flush, you can pull shot after shot of espresso at a reasonable pace and get good temperature control and stability. If the machine is idle for several minutes do you need to repeat the cool down flush (see How I Stopped Worrying and Learned to Love HXs for more details). The boiler temperature is controlled via a pressurestat; it allows the boiler pressure to drop 0.2 bar before it activates the heating element. That represents a temperature swing of only 4F.

The heat-exchanger system is efficient and with a little practice and attention to detail, delivers excellent temperature control. The E61 design is time-tested and widely recognized as one of the hallmarks of great espresso machines. However, of the three machines represented in this introduction, the heat exchanger espresso machine requires more attention to proper operation in order to produce an ideal espresso, however it isn't difficult to get the hang of the routine. If you wish to really refine your barista skills, there is of course abundant help and suggestions on the Internet.

One other point about the E61 design worth noting: It includes a preinfusion feature that can improve the quality of the extraction by allowing the pressure to build more slowly in the beginning of the pull. This slower build-up gives the puck a chance to expand and thereby reduce the occurrence of fissures that would otherwise lead to channeling. Channeling is the quick passage of water through the coffee, which produces a thinner, under-extracted espresso. When it occurs, you'll usually see sudden appearances of blond streaks in the stream of espresso and sometimes even pencil-sized holes in the coffee puck. The E61 has a reputation for being more "forgiving" because its preinfusion helps reduce the occurrences of channeling.

The E61 also allows for the diversion of some of the initial higher-temperature water away from the puck. The preinfusion and initial high-temperature water diversion are accomplished with the help of a small chamber located directly below the grouphead. If you are looking directly at the machine, it is the long silver cylinder below where the lever that engages the pump attaches. When the lever is in the up position, the pump starts and fills the grouphead. A spring-loaded valve leading to the preinfusion chamber allows a portion of the initial water to enter and thus reduces the overall pressure to around 4-6 bar until the chamber fills. Once this chamber is filled, the pressure raises to the full nine bar for the duration of the shot.

Double boiler



This type of espresso machine has separate boilers dedicated to each purpose and of course independent temperature controls. The particulars of the boiler and group design sometimes differs among double boilers. For example, the brew group design of the La Spaziale S1 has the grouphead directly attached to the boiler so heat transfer is via direct conduction, not water circulation as with the E61 brew group. Since the boiler is at brew temperature and it is an integral part of the group, the grouphead never overheats as is the case with the E61 / heat exchanger combination. The grouphead and portafilter may require a couple of blank shots, however, to bring them up to the final brew temperature. Otherwise the first and second shot will be cool by a few degrees.

A valuable addition to this 101 Topic under the section on Single Boiler would be an entry for the type of lever espresso machine that does not use a heat-exchanger but draws brew water from the bottom of the boiler and steam from the top. These machines do not need to transition between brew and steam.

Regards
Timo

I did a short intro and how it works when I did the Buyer's Guide to the Gaggia Achille, excerpted below:

cannonfodder wrote:Lever Espresso Machine - What is the big deal?

For those of you that are not members of the Lever Machine World Domination Plot (LMWDP), a bit of lever machine history and operational overview may help you to understand the passion these machines evoke.

Back in the late 1800 to early 1900's, the first pressure brewing methods were emerging. These pressurized percolators earned the moniker of 'espresso brewed coffee' due to the reduced percolation time and individual serving capability. The coffee was brewed 'expressly for you' one cup at a time. These early espresso brewers used steam to generate the brewing pressure. Unfortunately, that steam not only accelerated the percolation of coffee but burnt the coffee due to the high temperatures and direct contact of steam with the coffee grounds.

In the 1930's Sr. Cremonese patented the screw piston mechanism. This allowed for manual pressurization of the percolation group. The high pressure steam no longer came in contact with the ground coffee which greatly improved the taste of the coffee. In 1938 Achille Gaggia applied for a patent for the rotative screw piston group.

Mr. Gaggia never perfected the screw piston group. The design, while an improvement on the older steam pressure machines, still had many mechanical flaws. His next idea was to use a spring powered vertical driven piston. The spring would provide constant pressure to the group piston. Achille Gaggia discovered that using a consistent and fine grind in conjunction with the spring powered piston he could percolate a 'short black' in 15 seconds. The increased pressure also created a reddish brown froth; crema was born. On August 8, 1947 the spring assisted 'espresso machine' was patented, laying the foundation for what we now call espresso.

Lever espresso machines harken back to that original spring piston design of Achille Gaggia. While the modern day pump driven machine is easier to use, it loses that Old World charm. You do not purchase a lever espresso machine to produce a hassle free drink. You purchase one to become one with the process. The machine must become an extension of yourself and for that brief moment you are reconnected with history.

cannonfodder wrote:Modern Day Lever Espresso Machines

Modern day lever piston espresso machines use three different water supply brew systems and two different piston power designs.

Spring assist and manual lever.

There are two different piston powered systems. The spring assisted lever uses a coiled spring to regulate the extraction pressure of the piston. When the lever is depressed, the piston is cocked. You simply release the lever and the spring delivers the needed pressure for your extraction. The Elektra Microcasa a Leva is one of the most popular spring assisted lever machines.

The second power system is, well, you. The full manual lever relies on the operator to supply the needed pressure to the lever for extraction. These systems require much more time to learn. The operator has to apply steady pressure to the lever during the extraction. The difficult part is training yourself to apply the same amount of pressure for every shot. The La Pavoni Europiccola and the Gaggia Achille are two of these full manual systems.

Water delivery methods.

The most common type of water delivery is the steam pressure driven. In these systems, a single large boiler supplies both brew water and steam power. The machine relies on steam pressure to force water up a brew group supply line and into the group piston chamber. In order for that to work, the water in the boiler must be hot enough to generate steam. Most of these machines operate around .8 bar, or roughly 250F.


Hydraulics diagram from the Olympia Cremina manual

Since the brew water is well beyond the target brew range (195-205F) the water must be cooled prior to the extraction. On these machines, the group acts as a heat sink, leaching heat from the brew water as it enters the piston chamber. This is not a good way to regulate the brew water. Every time you pull a shot, the group absorbs more heat. Most of these will only allow for 3-4 shots before they overheat. Extended idle time also results in an overheated group because the grouphead is directly attached to the boiler. Common machines in this category include the La Pavoni Europiccola, Olympia Cremina and Elektra Microcasa a Leva.

Gravity feed (open boiler) water delivery systems can avoid these overheating problems. A gravity system uses a boiler placed above the grouphead. When the lever is lifted, gravity pulls the water down into the group piston. Because steam pressure is not required to move the water, these systems can operate at much lower temperatures. The water in the boiler can be kept at or slightly above the target percolation temperature. The disadvantage of these systems are two fold:

1. Because the boiler is located above the grouphead, most of the machines mass is located high off the counter. That tends to make them a bit top-heavy and prone to tipping over if the base is not sufficiently large.
2. These systems generally have one boiler, the brew boiler. Because these are run at brew temperatures, there is no steam for creating milk drinks.

The La Peppina and Mini Gaggia/Minimoka are popular gravity fed open boiler machines.

Heat exchanger is the third water delivery method. A heat exchanger uses a high pressure (usually 1 bar and up) boiler with a brew water supply line running through it. Most commercial lever machines use a large steam boiler with one heat exchanger per group supplying brew water. As the cold water passes through the heat exchanger tube it is flash heated to brew temperature. The resulting brew water can be adjusted up and down in temperature by the dwell time. The longer the water sits in the heat exchanger, the hotter it gets. The majority of commercial machines use a heat exchanger system. Below is a heat exchanger hydraulics diagram for a pump-driven espresso machine:


For more on how a heat exchanger works see Espresso Machines 202

Commercial lever heat exchanger machines are plumbed into a water supply. The lever machine's heat exchanger uses the mains water pressure to force the brew water through the heat exchanger and into the brew group. This system gives the barista much more control over the brew water temperature. In the event the grouphead overheats, the barista can cool the unit by pulling a long flush of cool water through the group. These machines are intended for all day use.

It's good to logically separate machines by boiler type (or pump/lever). But the real question people usually want to know when they ask me about espresso machines is: What criteria is the most important to take note of and compare from machine to machine?

I think you could assume you were comparing machines of equivalent boiler and type.

Any ideas?

Would that dispersion be a machine design element or could it vary by machines of the same model?

So that's my problem!!! Actually I added the gicleur and typically use lower doses as a rule. I also found that lowering my brew pressure to roughly 8.5 gives me the best results and now I understand why.

Thanks!