Pressure and Flow

By Arno Holschuh, Bellwether Coffee

Any time you are called out to work on an espresso machine because “it’s not making coffee,” you are likely to enter into the world of fluid dynamics. Of course, there’s no need to get a degree in engineering to de-gunk a clogged group or replace a pump, but in both cases, you’re working to solve a problem that encompasses pressure and flow, the two bedrock concepts in fluid mechanics. Espresso extraction is highly susceptible to changes in either. The flow rate of water through the puck dictates your contact time and therefore is proportional to your extraction percentage. And pressure is what makes espresso espresso: a thick, quick-to-prepare coffee topped with a colloid of coffee and oils that we call crema. The fine grind and hard tamping creates a puck that’s hard for water to get through, which is whole reason pressure can build up to extract the oils out of espresso. So right away we see that flow restriction is used in espresso machines as a way of achieving a desired pressure.

Consider the hunt for a soft pre-infusion. The idea is to subject a puck of espresso to a small amount of pressure for some period of time, during which the puck will swell up and the grounds will become saturated, much like “blooming” in pour-over. Then, once the coffee is “ready,” the machine will allow a full 9 bars of pressure to be applied the puck to start extraction proper. The soft pre-infusion concept originally comes from lever machines, where the puck was initially exposed to boiler pressure (say 1.5 bar) before the piston delivers higher pressures (as high as 11 bar, decreasing to around 7 to 9 bar by the end of the shot). In that case, the machine uses two different pressurized systems to provide two different pressures.

Now think about those little flow restrictors in an espresso system (often called gicleurs). They exist to provide a soft pre-infusion, by reducing the flow rate of water onto the puck, they make the time period required for the pressure to build to 9 bars longer. It’s like you’re trying to blow up a tire to 35 psi, but there’s only a pinhole nozzle on the air compressor. Will it finally get to 35 psi? Yes, but not before you get angry at being stuck on the side of the road. And there are more sophisticated ways of using a flow restrictor to control pressure – consider the now widespread design of manual paddle groups that allow the barista to throttle the flow up or down by opening or closing a valve located in the heart of the group.

This can be a tricky dance, because the size of the opening in the flow restrictor is not in a straight linear relationship with pressure. Opening a valve just a tiny little bit can cause a big increase in flow, which will allow pressure to equalize on both sides of the valve. If you’ve ever made coffee on a manual paddle group and noticed that turning the paddle just a few degrees dramatically increased the dispensing pressure, you’ve seen this in action. Systems that use flow restriction to control pressure are finicky.

A more sophisticated way to address flow and pressure is the use of variable-speed pumps, as in the La Marzocco Strada EP. In that machine, an electric pressure sensor in the group tells the brain what the current pressure is. The brain compares that to what the pressure should be according to the pressure profile the barista is using, and it speeds the pump motor for that group up or down to hit the goal.

There is yet another way to control pressure: Spring-loaded valves. Think about an expansion valve. It is a spring set to a relatively high force – usually enough that it won’t compress at all unless it’s pushed aside by 12 bar of pressure. Once 12 bar is achieved, the spring will compress a teeny little bit, allowing a few drops of flow out of the valve, reducing the pressure to just below 12 bar (and then closing).

And this basic design of a spring-loaded valve can be used for the widely-sought goal of achieving a soft pre-infusion as well, albeit without managing flow. Old-fashioned progressive pre-infusion valves, which have been around for decades and are still in use in Kees van der Westen’s Speedster prosumer machine, actually do a bang-up job of absorbing the initial pressure of a pump like the coil spring in a car’s suspension; only when they’ve bottomed out at the end of their travel will the puck receive a full 9 bar.

There’s one more topic to cover, and it is unfortunately not another cool example of engineering. Pressure drop occurs when a hydraulic fluid, like water, is pushed through a pipe from point A to point B using pressure. Even in a straight piece of copper tubing, there is enough friction as the water molecules travel over the smooth inner surface of the tube that pressure will decrease a little bit. In a sense, this is just like the intentional pressure reduction created by using one of the techniques discussed above. But here it can really mess with your measurements of pressure.

The most common case of this occurring is when one tunes the pump pressure to 9 bar (using, I might add, a spring-loaded bypass valve on the pump to manage pressure). That 9 bar at the gauge does not mean you have 9 bar at the group. If the gauge is plumbed into the espresso dispensing system significantly upstream of the group (like directly into the heat exchanger or boiler), it might read significantly higher than the group, because the water will experience pressure drop as it travels from the point of measurement to the point of dispense. Or maybe the gauge reads lower, because the manufacturer put a skinny little capillary-style copper tube to connect the dispensing system to the gauge.

Basically, the only way to know your pressure at the group is to measure it at the group using a tool that allows roughly the same amount of flow as a puck of espresso. The best and original tool is a Scace, named after its inventor and one of the true patron saints of espresso mechanics, Gregory Scace. While they are not cheap, they are invaluable if you are trying to provide the best possible tuning of an espresso machine to your barista colleagues. And that, after all, is our role: to support those around us in the industry so that they can make the best coffee possible.