Barry, how did your preheat testing work out?

Maddie says "no chips, want more matto's"

cannonfodder wrote:Barry, how did your preheat testing work out?

i tested it briefly with the scace in march, and the initial results suggest it is pointless.

a few days ago, i hooked up the marzocco again (after about a month of disconnect), and i hope to re-test the pre-heat this coming week (if maddie will let me). i've got a 5lb bag of stale decaf that i can use for testing, and i'll do "real shot" temp measurements with the datalogger.

Interesting. I had added a short HX preheat to my little Isomac and it did appear to make a difference. I had to change my flush routine; it appears to work better with many short flushes vs. one long. But my little 1.5 liter HX pales in comparison to something like a LM. That is also a perceived cup change, I don't have a data logger, maybe one day, but for now my measurements are quite literally cheek and tongue.

DavidMLewis wrote:Hi,

Looking at these cutaways, it's not at all clear to me what causes boiler water to circulate in the group. I guess I can see that as water cools in the group, the cooler water will slide down back into the boiler, but by that point the damage has been done, thermally. Barry, I'm sure that you in particular have given this a lot of thought; what's the deal?

Best,
David

I'm sorry for kicking up this old topic but I think I've got the explanation for how the heat transfer works without water and without steam actively flowing through the head.

The system basically works like a standard heat pipe as they're used in today's CPU coolers. The water in the boiler is at brew temperature, and the water vapor above it too. But as you know from sitting in a cold car, the warm water vapor form your breath will condense at the windows, simply because they are the coldest part in the car once you're driving it.

The mechanism for these brew heads is that condensation occurs at the coolest point in the system, which will always be the brew head. The water vapor takes with it the heat of the boiler water, and with condensation transfers this heat into the metal. The condensed and cooled water then flows back into the boiler, where it will be heated again. As soon as the brew head is at the same temperature as the boiler water, condensation will stop.

Gabelstaplerfahrer wrote: The mechanism for these brew heads is that condensation occurs at the coolest point in the system, which will always be the brew head. The water vapor takes with it the heat of the boiler water, and with condensation transfers this heat into the metal. The condensed and cooled water then flows back into the boiler, where it will be heated again. As soon as the brew head is at the same temperature as the boiler water, condensation will stop.

I don't get what you're saying but please straighten me out if my understanding is erroneous. The saturated LM group head is attached to the boiler and the top of each group head is at the highest point in the system. After the machine is turned on, the brew boiler is filled with the autofill valve and has been allowed to reach stable temperature the installer then slowly loosens a hex nut/crush washer assembly on top of each group head. This is done until water begins to dribble out under pressure - around the edges of that nut - and then tightened to seal.

At that point the brew group / boiler assembly is fully "saturated" and there is no longer any air in the system. Where would condensation occur if there is no space for air? But I think that simple thermodynamics would explain why the water circulates. The grouphead has a lesser amount of mass in both metal and contained water than the brew boiler. The higher energy of hot water and thermal mass in the brew boiler would seek an equilibrium point with the group head. Hot water will move to the area of lower energy in the brew group thereby displacing the cooling water there and - in theory -this will be a continual process causing circulation in a saturated system - with condensation not being involved.

Can one of you kindly either confirm this as correct or instead explain what I may be failing to grasp?

saturated brew groups, as I understand it, are not heated to the point of water vaporization, so even if the boiler were not completely full (which I understand to be the case), you would not see very much efficiency in using the heat of vaporization/condensation method used in a heat pipe.

I believe that Owen's right--the temperature normalizes through what is basically a simple thermosiphon, which works because, indeed, the heat sink that is the grouphead is placed high relative to the boiler itself. Heat pipes are very good at resolving massive temperature differentials. I don't think they are as effective at maintaining equilibrium--certainly not as effective as filling up the entire brewhead with a huge amount of water, which strongly resists temperature changes and keeps the temperature pretty homogenous within the boiler. I believe that it's exactly that saturation of water in the boiler that is why the LM grouphead (and others) are called saturated.

I see, it didn't occur to me that water can actually fill the brew group. I thought it was sealed at the top and couldn't be filled with water. Of course, a filled brew group gives much more stability than just vapor. And I didn't know the boiler is always completely filled.

Gabelstaplerfahrer wrote:I see, it didn't occur to me that water can actually fill the brew group

Yeah, it actually needs to fill the brew group. On Synessos the brew water is actually drawn from the very top of the group so no water comes out until the group is completely full of water. On LM's that I've used you actually have to bleed the water out through a screw on top of the grouphead.

What I can't understand is why they didn't put the groups highest point lower than the highest point on the boiler? That way there wouldn't be any problems... Sure the machine would be a bit higher, but then again, these things aren't exactly small to begin with...

Also they could have solved all the "brew-path exiting the temp stable area" problems with just using a mechanical connection between the mechanical and electronic parts that can't be put inside the boiler. ergo, the impeller-part would be submerged inside the boiler and attached to the tube that carries water to the group, then connected with a shaft through the boiler wall to the rotation-sensor that tells the control box how much water is flowing. Only problem is that you would need to custom make these solenoids and impellers, or at least retrofit normal components with food-grade boiler-submersible water parts, but if you're all-ready making some of the worlds most expensive espresso-machines this can't be much of a problem, esp. when all your machines use the same parts...

Stuggi wrote:What I can't understand is why they didn't put the groups highest point lower than the highest point on the boiler? That way there wouldn't be any problems...

The group is the highest part of the boiler on purpose- that way there's a coherent convection circuit which includes the group head. Hot water flows up to the group & back down- this makes the group temperature most controllable. Its easier to predict the temperature at the top of this circuit & more difficult to predict the temperature at any other point in the circuit.

Also they could have solved all the "brew-path exiting the temp stable area" problems with just using a mechanical connection between the mechanical and electronic parts that can't be put inside the boiler. ergo, the impeller-part would be submerged inside the boiler and attached to the tube that carries water to the group, then connected with a shaft through the boiler wall to the rotation-sensor that tells the control box how much water is flowing. Only problem is that you would need to custom make these solenoids and impellers, or at least retrofit normal components with food-grade boiler-submersible water parts, but if you're all-ready making some of the worlds most expensive espresso-machines this can't be much of a problem, esp. when all your machines use the same parts...

That's what the Versalab machine does- the pump head is inside the boiler. (If you're interested just look up their patent application.) How does new water get into the boiler then? On the versalab you have to pull a few shots, then wait while the boiler refills & gets back up to temp. (10 minutes minimum.) That won't cut it in a busy cafe- you need to pull shots as fast as people walk through the door, all day long.

The way Synesso (and some other dual-boiler machines) do this is to pump cold water through a heat exchanger in the steam boiler into the brew boiler. The new water in the brew boiler is very close to the target boiler temp, so there's no thermal shock from pulling shots. It also means when your pump fails you just quickly replace a cheap part under the counter instead of cooling off the machine, disemboweling it, and messing with a bunch of gaskets. Again, downtime costs shops big-time.