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  • Pressure and flow

    Ok, all you gurus...

    I have a Silvia with a Synesso 18g 57mm ridgeless basket. I have adjusted the pump opv to 9 bar.

    I have a Pneu-presso with a 51mm, ridged, double basket.

    I grind (with my Sunbeam - yes Im going to replace it at some stage...) to get a 25sec pour on the Silvia.

    When I grind with the same settings, use the Pneu-presso, same quantity of grounds, and tamp with the same force (as near as I can), I get a 25 sec pour using only 4.5 bar. If I up the pressure to 5 bar it shoots out faster.

    Theres more crema on the Silvia, but both coffees taste equally strong (no tracking thru the puck, I assume).

    Why do I get the same flow with half the pressure? Is it that the much higher pressure from the Silvia is compressing the puck even further? Or maybe the higher temp?

    Gonzo

  • #2
    Re: Pressure and flow

    My thoughts are that there are possibly a number of factors:
    1. Different number/size of perforations between the baskets.
    2. Same tamp pressure on a deeper basket (presuming the smaller diameter basket is deeper) may result in a less compacted puck.
    3. 9bar at OPV may not be 9 bar at shower screen.
    4. Not sure how the pump functions exactly, but perhaps the flowrate is limited?

    My money is on point number 2.

    Comment


    • #3
      Re: Pressure and flow

      Lever machines tend towards having deep and narrow baskets as opposed to wide and shallow as this presents less of a flow restriction as the puck expands. Id hazard a guess that most levers are extracting at around 6 bar compared to the 8 or 9 bar of pump driven machines, and the principle is the same for your presso baskets.

      Id also agree that the VST basket perforations are more restrictive than the presso baskets by design.

      Comment


      • #4
        Re: Pressure and flow

        Hmm. Im running all these ideas through my mental model of the situation and this happens:

        Perforations: I cant see the perforations being the "bottleneck". If theres no coffee the water just goes straight through. The puck is the bottleneck.

        Tamp pressure: If I put the same force on each basket, the larger diameter basket gets the lower pressure. Thats the one that flows the slowest (Silvia). Cant see it....

        Pump Pressure: My pump pressure was measured at the portafilter, after the shower screen, with a Greg Pullman-style gauge. Its not that.

        Flowrate: Both machines are capable of pumping much more water than ends up in the cup. Not that.

        Less flow restriction as puck expands: I dont understand the mechanism here. If each puck expands by, say, 10% then whatever depth the puck was will increase by 10%. That should increase the resistance to flow by...10%?? It should be the same effect for both. Any more info on this?

        Gonzo

        Comment


        • #5
          Re: Pressure and flow

          Originally posted by 49414054414C2E0 link=1327651715/3#3 date=1327757633
          Less flow restriction as puck expands: I dont understand the mechanism here. If each puck expands by, say, 10% then whatever depth the puck was will increase by 10%. That should increase the resistance to flow by...10%?? It should be the same effect for both. Any more info on this?
          That applies only if there is room for expansion between the grounds and the shower screen. Once the grounds hit the shower screen, the expansion can only go into the empty spaces between the grounds--i.e. the path the water takes. Less path, higher pressure needed for same flow.

          Greg

          Comment


          • #6
            Re: Pressure and flow

            Originally posted by 2D25243025284A0 link=1327651715/3#3 date=1327757633
            Hmm. Im running all these ideas through my mental model of the situation and this happens:

            Perforations: I cant see the perforations being the "bottleneck". If theres no coffee the water just goes straight through. The puck is the bottleneck.

            Tamp pressure: If I put the same force on each basket, the larger diameter basket gets the lower pressure. Thats the one that flows the slowest (Silvia). Cant see it....

            Pump Pressure: My pump pressure was measured at the portafilter, after the shower screen, with a Greg Pullman-style gauge. Its not that.

            Flowrate: Both machines are capable of pumping much more water than ends up in the cup. Not that.

            Less flow restriction as puck expands: I dont understand the mechanism here. If each puck expands by, say, 10% then whatever depth the puck was will increase by 10%. That should increase the resistance to flow by...10%?? It should be the same effect for both. Any more info on this?

            Gonzo

            From a fluid flow perspective, EVERYTHING is a restriction. Any reduction in flow area will create a pressure drop.

            With regard to the deeper puck, this is how I see it. When you tamp, the grounds will be more compressed at the top, and less at the bottom. This is because the tamping force will be progressively absorbed by the grounds, and the walls of the basket. This is in part due to elasticity in the grounds.

            In a deeper (and narrower) puck, this is more pronounced. So at the base of the narrow basket the grinds will be further apart, and under less compression, than those in the larger diameter basket. This will partially offset the effect of a decrease in driving force (lower pressure).

            Comment


            • #7
              Re: Pressure and flow

              Yes, MrJack, you are right about the pressure drop. However, I suspect that the pressure loss across the perforations would account for 0.1 bar of the 9 bar available.

              And your comments about the tamping effects I accept also. However, were talking about a difference in diameter of 57 to 51 mm. Its not much, so I find it hard to believe that such a small diameter change would make such a large difference in flow/pressure characteristics.

              Try this: when Miss Silvia hits the puck with water, the first thing that happens is the top part of the puck gets hot and wet (steady...) and this slurry then plugs any further quick permeation of water, so that when, all of a sudden 9 bar arrives, the puck is put under a lot of compression. 9 bar gives around 300kg of force at that diameter. This is probably 20+ times the force I exert when I tamp.

              So Im thinking now that all the tamping does is make sure there are no crevices for water to by-pass the puck. The high pressure from the Silvia simply compresses the puck so much that you can get bugger-all water through it. I suspect that if I reduced the setting of the OPV to, say 4.5 bar, Id end up with about the same amount of flow. Its counter-intuitive but it makes some sense.

              Thoughts?

              Gonzo

              Comment


              • #8
                Re: Pressure and flow

                Remember, when there is coffee in the basket, youre not talking about open perforations; grinds will protrude into the perforations, and alter the flow path. If you could somehow remove the basket altogether and check, I think youd find it accounts for more than 0.1 bar dP.

                I think the way you described the pressure rise at the puck is a little simplistic, and giving a false impression of what really happens. I doubt the puck gets hit suddenly by a stream of water at 9 bar. If it did, tamping would be pointless as the puck would be heavily disturbed.

                Rather, I think the channels leading to the shower head will progressively fill with water, and the pressure at the top of the puck will rise quickly, but not instantaneously. The water will likely have penetrated a fair way into the puck before full flow occurs.

                Also, I doubt water would just hit the puck and compress it as though it was impervious. Its like the difference between putting a bucket under a high flow tap, in comparison to a sponge.

                That said, most of the systems I look at dont involve solid particles (beyond fixed beds), and most of my experience is with high pressure gas; so in this case Im really just guessing based on my understanding of fluid flow.

                Comment


                • #9
                  Re: Pressure and flow

                  Gonzo - interesting thoughts. I tried the following experiment with the two machines I have here - the Expobar Minore III and the Nuova Simonelli Oscar.

                  Both set to 9bar pressure at the puck with the right flow through.

                  Both give identical (as near as I can time it) with the same VST basket. Both give different flows with their own respective baskets.

                  So the basket design does influence the flow in a measureable way. The other thing that might be happening is the brew temperature is likely to be different. Different temperatures will alter the rate at which the system extracts.

                  As you extract, the extracted stuff (ie coffee ;D ) leaves the puck and flow restriction drops. I did measure this once - the flow increases significantly. I have always put this down to the equivalent of the really fine material being flushed from the filter bed so the filter bed is coarser.

                  I think the actual dynamics of the extraction process are actually quite complicated. Id hate to have to model it!

                  /Kevin

                  Comment


                  • #10
                    Re: Pressure and flow

                    Good point Kevin (and interesting results)! Probably also that the viscosity of the fluid (which significantly influences frictional pressure losses) decreases as the oil content falls.

                    Comment


                    • #11
                      Re: Pressure and flow

                      Kevin, thanks for the info about the baskets - my two machines are so different theres no way I could do that.

                      MrJacks earlier point about the loss of tamp effect as you go down the puck may be counteracted by basket shape. My Synesso basket has a nice rounded corner on the bottom, but the presso one is square. It may mean that the bottom layers of grounds are squeezed even tighter by the radiused corner and thus the flow is slowed.

                      Yes, MrJack, the flow does increase in speed with time. I thought it was just the water finally getting to the bottom of the puck, but it may well be the viscous effects of the oils.

                      Brew temperature: yes, I thought that might be an issue - again, with my equipment I can only do rough comparisons. Certainly with higher temps youd expect to get more oils.

                      Good discussion!

                      Gonzo

                      Comment


                      • #12
                        Re: Pressure and flow

                        Originally posted by 69566E45474F240 link=1327651715/9#9 date=1327879258
                        Probably also that the viscosity of the fluid (which significantly influences frictional pressure losses) decreases as the oil content falls.
                        Unless youre using a Yemeni Theyre just thick-to-the-end! I hadnt actually considered the viscosity to be an issue, but of course it is.. Cripes - Id really not like to model this stuff!

                        The other thing I noted is that the gauge on the Expobar does drop slightly as the shot progresses. Thereby lending further credence to the nett flow increase.

                        /Kevin

                        Comment


                        • #13
                          Re: Pressure and flow

                          Originally posted by 49484F020 link=1327651715/11#11 date=1327893909
                          Cripes - Id really not like to model this stuff!

                          /Kevin

                          Oh, I hear you there. CFD on a batch liquid/solid extraction process...*shudder*

                          Comment


                          • #14
                            Re: Pressure and flow

                            Originally posted by 043B03282A22490 link=1327651715/12#12 date=1327899472
                            Oh, I hear you there. CFD on a batch liquid/solid extraction process...*shudder*
                            Unless you have PhD slaves students to do it ;D

                            But on a more serious note - I tried to have a bit of a play using the Wega in the Tea room today. The machine has flow sensors and tries to deliver a precise volume (but fails!). I put my VST basket and the synesso basket head-to-head. The synesso shot ran for 25Secs (the grinder is set for this basket) and the VST ran through in 15. Same grind. So further proof that the actual basket does affect this stuff.

                            It is nice and easy to do these experiments with a 2-group machine!!

                            Anyway..

                            /Kevin

                            Comment


                            • #15
                              Re: Pressure and flow

                              Well, I guess that explains it (well my initial query anyway).

                              Thats really good info, Kevin - I assume you put the same amount in each basket.

                              That is a really large difference in time! When you hold the VST and Synesso up to the light, do the perforations in the Synesso look more restrictive? I know its not a good test, but I thought Id ask anyway.

                              Gonzo

                              Comment

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