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Thread: Came here for the sweet diagram - Working on a La Cimbali M21 Plus rebuild

  1. #1
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    Came here for the sweet diagram - Working on a La Cimbali M21 Plus rebuild

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    Hello so I have been restoring a La Cimbali M21 Plus (I think its pretty similar to the Dosatron, no lcd and i think simpler steaming).

    Anyway I had been wondering for a while what the heck is going on with the group. A google Image search brought me to this sweet Diagram. Javaphile thanks for doing this it has helped a lot!

    I have a few questions if you don't mind:
    So i see how it works but... Why? The junior has the same group but without all of the passes through the group head (from what i can see in pictures).
    what advantage is their in not running the pup directly into the heat exchanger and the out the shower screen?

    Second how well does what is going on here work? Does it need a cooling flush? what kind of a temperature profile does it produce?

    second while the machine is idle between shots what happens with the water that is between the solenoid-the heat exchanger- and the pump(or whatever point it stops being able to flow backwards)? Is that section of pipe acting as a kind of boiler where water is heated in the exchanger and gradually heating up that whole section?

    Thirdly what improvements (If any) could be made to the design. For instance would a longer or shorter section of pipe between the pump and the group help to provide a better temperature profile?
    Would insulating any of the pipe or the group have a positive effect?

    Anyway anything you can offer by way of explanation on top of what you have already done in that picture would be invaluable.
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  2. #2
    Super Moderator Javaphile's Avatar
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    I have a few questions if you don't mind:
    So i see how it works but... Why? The junior has the same group but without all of the passes through the group head (from what i can see in pictures).
    what advantage is their in not running the pup directly into the heat exchanger and the out the shower screen?
    The mass of brass in my picture is from a 2 group La Cimbali M28, which is a commercial machine designed to continuously pull non-stop shots from both groups. I've not had any hands on experience with the Junior model and so I can not speak to whether the grouphead on it is the same size/mass as the truly massive one on the M28, also the interior channels may be different on the two models. Plus I suspect that the Junior is not designed to pull shots with no rest between them as the M28 is. My M28 is also an older design (20 years old?) so no doubt their design has changed over the years.

    Second how well does what is going on here work? Does it need a cooling flush? what kind of a temperature profile does it produce?
    It works very well.

    If you're not pulling continuous shots then it needs a cooling shot/shots. The volume, timing, and number of which is dependent on how low it has sat since the last shot.

    I've never hooked a Scace device to it so I couldn't say.

    while the machine is idle between shots what happens with the water that is between the solenoid-the heat exchanger- and the pump(or whatever point it stops being able to flow backwards)? Is that section of pipe acting as a kind of boiler where water is heated in the exchanger and gradually heating up that whole section?
    If you're talking about the solenoid on the 3-way valve the water going into the heat exchanger never passes through the 3-way valve. If you're talking about the solenoid that allows water from the pump to flow into the heat exchanger it is far enough away from the grouphead not to be affected by its heat. The water between the pump/solenoid and the heat exchanger is heated where it passes through the grouphead via the grouphead's latent heat so it is cooling the grouphead not heating it. No water is left in the heat exchanger as any injected into it is flash heated to steam upon entry.

    what improvements (If any) could be made to the design. For instance would a longer or shorter section of pipe between the pump and the group help to provide a better temperature profile?
    Would insulating any of the pipe or the group have a positive effect?
    Any change to the length of the piping between the pump and the grouphead would have no impact on anything as the water isn't heated until it gets to the grouphead/heat exchanger. Insulating the grouphead and/or any piping passing through it would dramatically change the thermodynamics of the grouphead, to the detriment of the coffee no doubt. The only possible change to look at that I can think of would be the size of the hole in the group jet (Part number 700230 on this page: Group head solenoid operated - Cimbali | Coffee Parts ). Some suppliers offer them with different size holes to change the profile of the shot.


    Java "Cimba!" phile
    Toys! I must have new toys!!!

  3. #3
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    Cimbali Injector?

    Thanks that is helpful. The "Junior group" is similar to your machine and mine except that it doesnt have the cooling pipes that go through the top. Thanks for pointing out their cooling function.

    So this is the bit that confuses me still:
    "No water is left in the heat exchanger as any injected into it is flash heated to steam upon entry."

    The heat exchanger. I have so far been able to work out from reading a few descriptions of how it all works only that it 'injects' water into the heat exchanger- which is different to designs that pass water through the boiler in a copper tube.

    What mechanism keeps water out of the heat exchanger or where does it go? If I am understanding the plumbing correctly the solenoid that stops the flow from the group is after the heat exchanger, which means that it should fill with water. Is it that the pressure from the heating of the HX empties it of water? if so does it also empty it from the hot side of the heat exchanger.



    I am having some trouble articulating what I mean. so the water flash boils at the bottom of the heat exchanger and rises to the brass of the group where it cools down. and I am guessing that the rate that this prcoess happens is directly related to the pressure of the water that you inject into the heat exchanger, so that lowering the pump pressure also lowers the water coming out of the group. So when the solenoid is closed some water would remain after the HX and in the HX right?

    thanks for taking the time to reply i hope that this line of questioning isn't too annoying
    Last edited by Javaphile; 18th October 2015 at 09:33 PM. Reason: Fix link

  4. #4
    Super Moderator Javaphile's Avatar
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    Ugh, I shouldn't post when I'm so tired. First off let me correct something from my previous post. This "If you're talking about the solenoid that allows water from the pump to flow into the heat exchanger it is far enough away from the grouphead not to be affected by its heat." should actually read like this "If you're talking about the solenoid over near the pump that allows water from the pump to flow into the boiler it is far enough away from the grouphead not to be affected by its heat.".

    Now on to your question.

    The heat exchanger. I have so far been able to work out from reading a few descriptions of how it all works only that it 'injects' water into the heat exchanger- which is different to designs that pass water through the boiler in a copper tube.
    A HX (Heat eXchanger) is a HX regardless of differences in designs. A cooper tube running through the boiler or a copper 'test tube' sticking into the boiler, they're both HX's and work on the same principle.

    What mechanism keeps water out of the heat exchanger or where does it go? If I am understanding the plumbing correctly the solenoid that stops the flow from the group is after the heat exchanger, which means that it should fill with water. Is it that the pressure from the heating of the HX empties it of water? if so does it also empty it from the hot side of the heat exchanger.
    The HX is at a temp well above the boiling point of water at atmospheric pressure. On most commercial HX machines boiler pressure typically runs around 1-1.2bars which equates to a temperature of roughly 122C plus or minus a bit depending on the exact pressure in the boiler. At the beginning of a shot the water entering the HX flashes into steam as the HX is at atmospheric pressure and then goes into the grouphead. As the pressure builds during the shot it is conceivable that water may exist in liquid form in the HX for a short period until the pump stops where upon the pressure drops and it flashes to steam. At the end of the shot any residual water on the HX side of the 3-way solenoid would exist outside of the HX proper as in it there would only be steam. Let a Cimbali sit long enough between shots and the water left in the grouphead and cooling pipes would also turn to steam as the grouphead overheats. Hence the need for a cooling shot prior to making the next espresso. Different machines have different designs so as always YMMV.

    Once again I'm posting this way late at night/early morning so hopefully it makes sense.


    Java "All posted out" phile
    Toys! I must have new toys!!!



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