Water Flow within Brew Boiler and Temperature Stability

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pizzaman383
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#1: Post by pizzaman383 »

With all the new double boiler espresso machines coming out it's time to bring up a topic that has a big impact on temperature stability - water flow within the brew boiler.

I have VBM DD v2 which has the old, now replaced large brew boiler. I've done quite a bit of experimenting and have now achieved very good temperature consistency within a shot. To make a long story short, I have investigated the impact of changing PID parameters, length of machine preheat, preheating the water entering the brew boiler, using different shape heating elements, eliminating the flow restrictor in the group head feed line, and changing the water injection into the brew boiler. All of these had some impact but the ones that made the most impact were the ones that changed the water flow within the brew boiler.

It's kind of funny (not) that the last thing I tried had the biggest impact. Changing the location, direction, and speed of the water feed into the brew boiler instantly changed the temperature stability within a shot. Here's the scoop.

The VBM DD v2 has its E61 thermosyphon fed by large 10mm brass tubes that exit the brew boiler in the top and bottom through the boiler end caps. There is a flow restrictor in the top thermosyphon tube that is about 1.5mm in diameter. This means that the brew water feeding the shot comes mostly from the bottom of the boiler through the lower thermosyphon tube. The pumped water is injected from the bottom of the boiler through a tube that projects upward in the boiler but the water flows out of the injector sideways through two 3mm holes.

With the stock injector, the boiler water flow allowed the cold water a short path to the lower thermosyphon tube. This meant that no matter how stable the PID kept the boiler there was cooling during the shot. A preheat coil around the boiler helped but did not eliminate the decreasing temperature during the shot. The solution was to make an injector that puts out water at the top of the boiler. In addition, the new injector uses a single 1/16" hole oriented to create a faster flow. I oriented the hole to cause a whirlpool action around the boiler. This ensures that the path newly injected water takes is longer, must go past the heating element prior to leaving the boiler, and oriented away from the thermosyphon outputs. I was amazed at how much difference this made! The other mods each make a minor contribution to better temperature stability but this one made the biggest difference.

My brother has installed a PID on a Gaggia Coffee and has looked at the boiler internals. That classic Gaggia aluminum boiler injects water at the base of the boiler tangentially to the side of the boiler to introduce a whirlpool and it has a center tube for the group head feed that takes water from the top of the boiler. It's clear that Gaggia engineered the boiler so that the water flows near the external heating elements and must take a long path before it exits.

These two examples make it seem to me that water flow within the brew boiler is a crucial design component. I haven't heard this discussed here so I'm hoping these observations bring another perspective to what is important. I'm curious what the water flow looks like withing the small brew boilers in the crop of newly introduced double boilers.
Curtis
LMWDP #551
“Taste every shot before adding milk!”