Calming the waters – Nucleate Boiling

Or, more academically: How nucleation sites affect the stability of boiling water and spitting 9Barista chimneys.

The problem

We’ve been getting sporadic reports from customers that their 9Barista machines have been spitting water out of the chimney during the brew process.

Our advice so far has been to reduce the amount of water put into the machine to lower the level of water in the upper boiler.

This worked for most, but isn’t a solution we like very much as we couldn’t work out why this was happening on some machines and not others. Much testing was done with different heating rates, different hardnesses of water, different grind settings – all to no avail. It’s a particularly sensitive topic for us as it was a continuous source of trouble during our early prototypes – and we thought we’d fixed it.

The theory

After (a lot) more reading about how boiling water behaves, we started thinking about this graph, which shows that there are actually several regimes of boiling water. The axis on the bottom is the temperature difference between the water and the heating surface. The vertical axis is the rate of energy transfer.

Our theory is that for some machines, the boiling water in the upper boiler enters the B-C regime, rather than A-B regime. In the B-C regime the boiling is intense, large bubbles are formed, which then rise to the surface, where they burst. The large amount of steam bubbles in the water effectively raises the level of the boiling water in the chamber, allowing some to escape through the chimney as the bubbles burst. The A-B regime by contrast is more of a gentle simmering, where small bubbles are formed, but many collapse within the water, and there is little disruption to the top surface of the water. We need to keep things in the A-B regime to prevent water from coming out of the chimney.

In order to do that, we need to increase the number of “nucleation sites” – places where bubbles can form. This will promote the creation of smaller bubbles, resulting in a more stable boiling process.

It turns out we can achieve this by simply roughening up the heat exchanger coil, and one of the surfaces in the upper boiler. Literally just rubbing the surface with sandpaper.

Using a strip of sandpaper to roughen the surface of the ‘upper chamber’ floor

We’ve been testing this solution on a few machines, and we’ve seen very consistent stabilisation of the boiling. It also sounds different – the boiling noise is much higher frequency – leading us to believe we are getting more and smaller bubbles.

We’ve also tried polishing the surface of the upper boiler to see if we can make the water-spitting worse. Pleasingly that worked too! So we have some confidence that we’re seeing the correct effect.

The solution

So, our formal advice for fixing a 9Barista that spits water out of its chimney is as follows:

  • Step 1: Unscrew the boiler and take off the portafilter from the machine.
  • Step 2: Remove the boiler O-ring (The red seal just below the heat exchanger coil).
  • Step 3: Roughen the brass surface below the heat-exchanger coil with 120 to 180 grit sandpaper. Try to scratch the surface as much as you can.
  • Step 4: Roughen the outer surfaces of the heat exchanger coil in the same way.
  • Step 5: Thoroughly rinse the group-head assembly under the tap to remove any residue from the sanding process.
  • Step 6: Reassemble – and give it a go!

Below is an image showing the two areas which will need roughening.

We have designed some “renucleationisation kits” and we’ll be offering them to any existing customer for free (free postage too). We’ll be sending out an email with a coupon to all 2nd batch customers – other customers please email [email protected] with your machine’s serial number, and we’ll send you one.

You’re also welcome to use your own sandpaper, if you prefer.

If your machine serial number is greater than 322, then your machine has already had this treatment applied during production – so you shouldn’t need the kit.

One reply on “Calming the waters – Nucleate Boiling

  • Chris Rowelle

    We live in a very hard water area and to prolong the life of anything that involves heating water, I regularly descale them. For a good few brews after I have descaled my 9Barista, the chimney ferociously spits water but once a layer of limescale has reformed it calms back down again. I have wondered why this might be happening and the nucleation issues would certainly explain it. I will roughen up the surfaces indicated the next time I descale my 9Barista and let you know how I get on.

    Apart from this minor issue, I love my 9Barista and during these lockdown times it is getting put through its paces numerous times a day and is a pleasure to use each and every time. The negative pressure release valve improvement has made a big difference to the user experience as my wife and I no longer have to wrestle the boiler apart each time we want another delicious espresso! Thank you for designing and manufacturing a wonderful machine.

    Reply

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