How do lever machines work? I know, no pump ... but ...

[DSLatte]

But how are these units heated up? Right now I am not so much excited about the lever as I am the prospect of a kind of heat exchanger type machine. How does the boiler work? How is it heated up and what is the role of steam? Most seem to have a sight glass on the side and some have a pressure gauge on top of that.

A diagram would help.

[HB]

Although the descriptions are written in German, the diagrams in this Poccina brochureare excellent, one of which I've excerpted below:


Diagram of a typical lever machine (see page 8)

Jim's Home Barista's Guide to Espresso introduces the history and basic designs, which explains the diagram above:

In 1901, the Italian inventor Luigi Bezzera came up with a workable solution. Pavoni manufactured these first espresso machines in 1905.

This machine was also steam powered. However, the steam does not come into contact with the coffee. Instead, steam pressure at the top of the boiler forces water at the bottom of the boiler through ground coffee. The coffee is held in a group consisting of a portafilter, a metal filter basket and removable brass mount, and a brew head into which the portafilter attaches. The piping and group were designed to act as heat radiators, so the temperature of the pressurized water dropped from 250°F (120°C) in the boiler to the correct brewing temperature at the grouphead. This brewing principle is still used in stovetop mochapots. Since the water was pressurized, the coffee could be ground finer than in a regular pourover brewer, reducing the minimum brewing cycle from about 4 minutes to 30 seconds. Espresso machines and their accompanying coffee grinders became the standard equipment for making coffee in Italy, Southern France, Spain and Latin America. In other parts of the world, it followed Italian immigrants who popularized it in each country they settled.

But technology moves on, and this method is no longer regarded as specifically espresso, although mochapots and other steam pressured brewers continue to be marketed under the name. In the 1920s through the 1940s, Italian engineers experimented with pumping devices to increase the brewing pressure. The first practical one was developed by Cremonesi in 1938 and manufactured by Achille Gaggia in 1946. It used a hand powered piston. On machines of this type, steam pressure in the boiler forces the water into a cylinder, but then it is pressurized further by a spring-powered piston to about 8 to 9 bar (120 to 135 PSI), or 8 to 9 times the pressure that had been developed by the steam machines. The spring that powers the piston is compressed by a lever forced down by the barista (Italian for barkeep)--the person making the coffee. As with the older generation machines, these lever groups are designed to cool the water from boiler to brewing temperature.

My own Espresso Machines 101 and Espresso Machines 201 cover the popular non-lever types of espresso machines.

[HooHaw]

About the Gaggia Achille. If that machine is supposed to be a HX, how will that prevent the grouphead from overheating?

[HB]

Recall that a regular lever-type machine like the one pictured above is using steam boiler water (>250F) to make espresso. As the water enters the group, it cools because the grouphead's metal is cooler than brew temperature. But its ability to act as a heatsink only lasts for a few shots; once it is at or above brew temperature, the final brew temperature will be too high.

Espresso Machines 202 explains how heat exchangers work. The brew water begins at room temperature and is flash heated as it passes through the steam boiler (pathway from pump to group indicated by red arrow):


Heat exchanger (and thermosyphon to warm group)

The Poccina brochure has a similar picture of a heat exchanger on page 9 on the right. Looking carefully, you will see that the heat exchanger coils around the heating element, entering to the left of the boiler and exiting at the grouphead. The heat exchanger of most of the machines that I've used had a simple straight copper pipe and the length / diameter is "tuned" to the desired brew temperature range; the designer can add or remove turns in the coiled equivalent to accomplish the same result. The grouphead doesn't overheat because it isn't acting as a heatsink, rather its purpose is to attenuate the inherent temperature swing of the water exiting the heat exchanger.

[HooHaw]

Yes... of course...

[DSLatte]

I'm looking at the break away diagram with the German labels and a few questions come to mind.

The heating coils in the bottom appear to be bubbling. Are they a source of steam? Could this system be considered a kind HX system in reverse, so that the steam routed through the coils heat the brew water?

Or, are the coils a simple heating element?

Also, I see space above the water below the piston in the brew head. Isn't this a problem? Would careful technique prevent air from forming there? It seems to me that air is compressable and so interferes with what ever advantages might be gained by actually feeling head pressure at the lever.

[DSLatte]

I'm looking through the PDF manual linked above for the Poccino. Let me see if I got this right ...

The "Das Boiler-Pumpdruck-System" is most like the basic Single Boiler system as is found in my Rancilio Silvia. The "Das Warmetauscher-System" is a HX system as is found in the Quickmill Andreja or the Pasquini Livia 90.

The illustrated lever system is maybe a little of both since you don't have to change the boiler temperature to get steam. Is that right? You don't have to activate a steam thermostat to get steam in a lever machine? It's always there?

[HB]

The illustrated lever system is maybe a little of both since you don't have to change the boiler temperature to get steam. Is that right? You don't have to activate a steam thermostat to get steam in a lever machine? It's always there?

Yes x 3. Pretty neat, isn't it? Lever machines are unique in that they can serve espressos OR cappuccinos without waiting, which Silvia and similarly designed single-boilers cannot. Of course levers cannot pull shot-after-shot without overheating, but for 2-3 drinks, they have a number of advantages over heat exchangers. It's because lever machines generally have lots of style and are well suited for an individual drinker, they're occasionally referred to as "estate" espresso machines -- although in the case of the Olympia Cremina, I think a "Zurich Banker" espresso machine is more appropriate.

[HooHaw]

What happens if one bolts a half-inch iron plate between the grouphead and the boiler of a Pavoni Pro or Elektra A Leva? Wouldn't that decrease that grouphead's tendency to overheat because of the added thermal mass?

[HB]

Keep in mind that the lever machines we're discussing are ideally suited for individual use, i.e., in less than 15 minutes you'll have two or three drinks from a cold start. That's not something needing "fixing" in the designer's eyes. If continuous use is important, I would begin with another design, say something like this:


(Image courtesy of Kees van der Westen)

More directly to your question, decreasing the transfer of heat across the grouphead bridge will slow the initial heatup and larger thermal mass will defer the ultimate overheating, but at the expense of the early shots running cold. My answer to this speed-versus-capacity dilemma is to have more than one type of machine, while others employ lever tricks like unplugging the machine, applying a cool towel to the grouphead, etc. Sometimes I use the lever for a few weeks in a row, sometimes it sits idle for a month. It was a gift from my wife and I consider it a treat to use; when not in the kitchen, it is in our living room:

[srobinson]

Or at my house...thanks for the loaner.

[HooHaw]

The Mirage would be nice... but I can only afford one and the Mirage isn't even close.