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Help understanding Heat Exchanger Thermodynamics

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  • Help understanding Heat Exchanger Thermodynamics

    I am hoping someone with more experience can confirm / explain the Cimbali Jnr heat exchanger thermodynamics.
    My understanding from the diagram below is that water is pumped into the heat exchanger tube and then flows via solenoid through the group.
    The group has a very large thermal mass and will remain at a constant temperature.
    The heat exchanger tube is a fairly large reservoir which holds approximately (I haven't measured yet) 200-300ml of water.
    At steady state this water will be at 100 degrees (boiler temperature).
    When cold water enters at the far end away from the group it will force the hot water through the group.
    I can't imagine much mixing of the cold and hot before flowing through the group.
    So in effect the group has to cool the water down to brew temperature.
    Does this sound correct?



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  • #2
    You have the path correct, but the properties wrong. The HX tube is not full full of water nor is it at 100C. It is empty and it is at the temperature of the boiler which is typically at ~130C. But while the inside of the boiler is typically at 1.2-1.4Bar the inside of the HX tube is at atmospheric (1Bar) of pressure.

    When the shot is started the 3-way valve, Ec in the diagram, closes the path to the exhaust tube while leaving the path onto the grouphead open and the pump starts. When the cold tap water/tank water is pumped into the HX tube it instantly flashes into steam and expands into the grouphead where it cools and condenses back into water before reaching the portafilter. When the shot is stopped the pump is turned off and the same 3-way valve opens up the pathway to the exhaust tube. This opens the pathway through the HX and grouphead to the atmosphere allowing the remaining steam and water in the pathway to vent into the water catch tray leaving it empty and ready for the next shot which repeats the same process.

    I've moved this thread to the non-machine specific brewing machine forum as it deals with the operation of a generic HX system and as such is applicable to many different machines in addition to yours.


    Java "Thermo what?" phile
    Toys! I must have new toys!!!

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    • #3
      That makes sense. Much more thermally consistent than I was imagining. I wonder how much ambient temperature effects the groups passive regulation? Your explanation may also explain my water flow problem - the HX tube or seal is probably damaged as the tube had water in it when I removed it this morning.

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      • #4
        Originally posted by saeco_user View Post
        That makes sense. Much more thermally consistent than I was imagining. I wonder how much ambient temperature effects the groups passive regulation?
        With the massive brass grouphead the La Cimbali's have unless it's freezing in the room it won't make much difference.

        Your explanation may also explain my water flow problem - the HX tube or seal is probably damaged as the tube had water in it when I removed it this morning.
        Water in the HX can be caused by trying to pull a shot when the machine is not fully up to temp. Some of these machines can take 45 or even a full hour to come up to temp.

        With the generic HX questions answered, questions dealing with your specific machine should be dealt with in a new post in the Brewing Equipment - Extreme Machines ($3000+) forum. You'll find a number of threads in there dealing with several variations of the Junior that would be worth your having a read through before doing anything else first though.


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

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        • #5
          Will do. Am just starting on a rebuild. Was really just curious about how the HX worked. Thanks for explaining.

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