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La Pavoni undressed

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  • #16
    Re: La Pavoni undressed


    I though that as well, but the thermal expansion can manage a great deal more than mains pressure, so there will still be some back flow. The coffee tek (who knows just about every machine ever made) reckons there was a check valve in that machine. He was actually quite adamant on this point. Anyway way problem diagnosed and it will be resolved.

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    • #17
      Re: La Pavoni undressed

      Mark,

      I just checked my drawings and mine doesnt have a check valve in the path either.... so it does rely on the mains pressure....

      You are right- some water will be forced back by the expansion in the heat exchanger.... but unlike yours where it will be boiling (no pressure in the heat exchanger) therefore a far greater volume will be expelled- mine is at 5 Bar and wont boil- so a relatively small volume will be expelled.

      Wont a check valve cause a build up of pressure in the heat exchanger as the water heats up - expands- and has no where to go? And it will start at 9 Bar pressure at a temp of about 93C and heat up to 125C - quite a bit of expansion = extra pressure in the sealed system possibly causing strain on the brew solenoid :-/ :-/

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      • #18
        Re: La Pavoni undressed

        Originally posted by JavaB link=1170566881/15#16 date=1173229091
        Wont a check valve cause a build up of pressure in the heat exchanger as the water heats up - expands- and has no where to go? And it will start at 9 Bar pressure at a temp of about 93C and heat up to 125C - quite a bit of expansion = extra pressure in the sealed system possibly causing strain on the brew solenoid :-/ :-/
        Unless the machine is a very different design than most any water left in the heat exchanger will do the same thing it does on your Cimbali JavaB, exit via the grouphead and (assumming the machine has one) the 3-way pressure relief valve.

        The brew solenoid regulates the water flowing into the HX, not out of it. The HX is not a sealed system.


        Java "Hhhmmm....Hydraulics" phile
        Toys! I must have new toys!!!

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        • #19
          Re: La Pavoni undressed

          Java....

          Now youve got me more confused than normal!

          Ive just checked my hydrologics drawing...... and water goes from the pump, appears to loop through the group head, into the heat exchanger, back to the group, through the 3 way and if the three way is on- out via the group head.... if the three way is off..... the water passage from the HX is blocked i.e. a sealed system.... the group is however vented via the three way - but not the HX. :-/ :-/

          What have I got wrong?

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          • #20
            Re: La Pavoni undressed


            JavaB, the 3-way will usually hold back about 12 bar before being forced open. This acts as an expansion valve and does the job. Other machines have dedicated expansion valves (like the Pav as the E61 group wont work as an expansion valve).

            Its true that with mains pressure, the initial expansion will occure until the pressure is equalized and then the flow will stop. Thats probably good enough to stop excessive hot water reaching the pump.

            Ive seen a few Bezzeras and they all include a separate exapnsion valve and check valve to prevent any flow back to the pump. This is common for most commercial and prosumer machines according to the tek whose brain I pick on occasion.

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            • #21
              Re: La Pavoni undressed

              Ah....

              Done some more checking....

              There is a pressure relief valve (11.5 Bar) on the run to the groups.... just after the pump with a vent line into the drain collector....

              And on the output side of the pump (shown on the pump drawing!!) is a non return valve....

              So all is again good in La Cimbali land!

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              • #22
                Re: La Pavoni undressed

                Originally posted by JavaB link=1170566881/15#18 date=1173242893
                Java....

                Now youve got me more confused than normal!

                Ive just checked my hydrologics drawing......
                The drawings in the Cimbali manuals leave a lot to be desired. They bear only a passing resemblance to the physical item.

                On Cimbalis the water comes up in a copper pipe and passes through a hole in the bottom rear of the grouphead into another copper pipe which makes a loop and feeds the water through the top front of the grouphead into another copper pipe where it then goes into the 3-way valve. From the 3-way valve the water then goes into another copper pipe which feeds it through a hole in the upper rear of the grouphead, coming out a copper pipe on the other side which then loops around and injects the water into the HX at the top center of the rear of the grouphead. The water from there goes down a poly pipe to be dumped into the bottom of the HXs tube where it is flash heated to steam and exits the HX via a small hole in the grouphead. It travels the length of the grouphead and exits via the showerscreen. There is a 3rd line attached to the 3-way valve. That is the pressure relief line which vents into the drip tray.

                As you can see from that description with the brew switch turned off the HX is open directly to the atmosphere via the pressure relief line and the showerscreen on the grouphead. There is no way short of a faulty 3-way valve and a packed portafilter (or the channel that feeds the showerscreen) that any water heated in the HX can make its way back to the pump or the rest of the system.


                Java "A little less confuzzled?" phile
                Toys! I must have new toys!!!

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                • #23
                  Re: La Pavoni undressed

                  Java,

                  Got to agree with your comments re drawings..

                  But my reality is different to the drawings - and your description....

                  Copper pipe up to the top left side of each group (about the centre).... through a hole in the casting and out the other side (right) where it loops via a copper pipe back to the centre rear of the group assembly and then out through a copper pipe into the centre of the heat exchanger "cavity".... It then returns via a channel cast/drilled into the brass to the three way mounted on the right side (near the front)..... the other side of the three way goes via a machined channel into the group head above the showerscreen.... with the third leg going into the drip tray.....

                  TOTALLY different .... OK it is a dossatron but that shouldnt make any difference...

                  Go Figure??????? :-/ :-/

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                  • #24
                    Re: La Pavoni undressed

                    Heres a pic of the grouphead on my Cimbali. The green dot is the waterline from the pump. The white line is the route of the incoming room temp water with the arrows showing its direction of flow. The red arrows show the direction of flow of the water at the end of the shot when the 3-way valve closes off the inflow of cool water and opens the path to the pressure relief line. In this pic the relief line was already disconnected from the 3-way valve assembly.


                    Java "A picture is worth, well a few words anyways!" phile

                    Toys! I must have new toys!!!

                    Comment


                    • #25
                      Re: La Pavoni undressed

                      Originally posted by JavaB link=1170566881/15#20 date=1173245351
                      Ah....

                      Done some more checking....

                      There is a pressure relief valve (11.5 Bar) on the run to the groups.... just after the pump with a vent line into the drain collector....

                      And on the output side of the pump (shown on the pump drawing!!) is a non return valve....

                      So all is again good in La Cimbali land!
                      On the Grimac there is a massive valve, and one of the smallish 6 mm? pipes from it goes to the drain collector.

                      The valve obviously is operated electronically on demand for the boiler and heat exchangers. But I can also operate it manually by pushing in, and holding, a knob on the front of the machine.

                      --Robusto

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                      • #26
                        Re: La Pavoni undressed

                        Originally posted by JavaB link=1170566881/15#22 date=1173247436
                        Java,

                        Got to agree with your comments re drawings..

                        But my reality is different to the drawings - and your description....
                        Got a pic?


                        Java "Wants to see" phile
                        Toys! I must have new toys!!!

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                        • #27
                          Re: La Pavoni undressed

                          Originally posted by Javaphile link=1170566881/15#25 date=1173248081
                          Got a pic?


                          Java "Wants to see" phile
                          Mmmm -didnt photograph this area whilst doing restoration..... will have to take some....

                          Check your mail.... sent you some pages from the service manual.


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                          • #28
                            Re: La Pavoni undressed


                            Im pretty sure Javaphile has confused the brew path a bit. If I follow the description of the brew path, then when the 3-way opens at the end of the shot, the pressure from the group will force coffee crud through the HX first before it gets to the 3-way. Theres no manufacturer that would use such a design. The 3-way is usually as close to the group as possible to minimise the path that coffee crud will travel before it is ejected. The diagram shown on the coffeeparts web page is consistent with a 3-way close to the brew head.

                            JavaBs explanation is what Id expect. The the expansion valve and check valve close to the pump seems pretty common. The expansion valve often has some silicone hose feeding into the drain.

                            Theres not too many elements in a coffee machine, but most are there for a reason and its important to know what does what and why, so you can diagnose any problems and fix them. Its clear that none of us has the wealth of experience that a professional coffee tek has. Thats why my trip to the coffee tek this morning was invaluable.

                            Cheers,

                            Mark.

                            Comment


                            • #29
                              Re: La Pavoni undressed

                              Originally posted by Sparky link=1170566881/15#27 date=1173250363
                              If I follow the description of the brew path, then when the 3-way opens at the end of the shot, the pressure from the group will force coffee crud through the HX first before it gets to the 3-way. Theres no manufacturer that would use such a design. The 3-way is usually as close to the group as possible to minimise the path that coffee crud will travel before it is ejected. The diagram shown on the coffeeparts web page is consistent with a 3-way close to the brew head.

                              Mark.
                              Not to mention all that Caffetto in the HX And it would sure take several blind shots for any to appear from the relief pipe - which I know is not the case.

                              Theres not too many elements in a coffee machine, but most are there for a reason and its important to know what does what and why, so you can diagnose any problems and fix them. Its clear that none of us has the wealth of experience that a professional coffee tek has. Thats why my trip to the coffee tek this morning was invaluable.
                              And thats the great advantage of this site..... we can all share our knowledge - and between the lot if us - hopefully get it right!

                              Comment


                              • #30
                                Re: La Pavoni undressed

                                Originally posted by Sparky link=1170566881/15#27 date=1173250363
                                Im pretty sure Javaphile has confused the brew path a bit. If I follow the description of the brew path, then when the 3-way opens at the end of the shot, the pressure from the group will force coffee crud through the HX first before it gets to the 3-way. Theres no manufacturer that would use such a design. The 3-way is usually as close to the group as possible to minimise the path that coffee crud will travel before it is ejected.
                                Thinking about it (I know I shouldnt be doing that this late at night!) youre absolutely right. And as I study the pic more and think back to when I had it all apart I remember now being confused as to the exact path the water took through the grouphead. The 3-way valve actually sits in front of the pipe that in that pic appears to go through it and is mounted directly to the side of the grouphead with the only pipe connecting to it being the pressure relief drain line.

                                This would allow the water (and backflush chemicals) to go from the shower screen a short distance through the grouphead and then out via the 3-way valve, before it reaches the HX.

                                The question that then of course begs to be answered is what is the path of the water through the grouphead between all the different pipes and the 3-way valve?

                                Has anyone ever seen a cutaway view of one of these groupheads?

                                The diagram shown on the coffeeparts web page is consistent with a 3-way close to the brew head.
                                Got a link? I cant find any hydraulic diagrams there.


                                Java "Going to sleep before he confuzzles even more people" phile
                                Toys! I must have new toys!!!

                                Comment

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