Screw-less dispersion screens

[James Hoffmann]

This is something I've been thinking about a bit since talking on the phone to Klaus a few weeks ago. What prompted the post was Kees annoucing his new Speedster production and part of it being a screw-less but apparently easily removable dispersion screen.

http://www.keesvanderwesten.com/at-the-workshop_new-speedster.html

If this is possible, and easy then surely a good thing? I have seen it on other machines, but usually it was a pain to remove the screen as the piece went up behind the gasket.

Will this become more widespread? If maintenance isn't a problem then is there any other downside?

[phaelon56]

The La Marzocco diffusion screen screw distributes the water horizontally out two small openings - correct? I'm curious what difference there is in distribution when the water from the gicleur/jet comes into the back side of the screen.

My home machine has the E-61 style grouphead and uses a screw-less screen. When it's freshly and thoroughly cleaned - which I can really do only when I change gaskets - the apparent distribution of water into the puck is excellent. But once it gets some blockage here and there - which occurs several weeks after the gasket change - distribution is not good and no amount f regualr water backflush and periodic puro-caffing resolves it.

A screw-less screen thats' easy to change sounds like a great idea - IF it remains easy to change.

[Alistair Durie]

If there is a little headroom (space) between the coffee and the screen, there shouldn't really be any difference without the screw. however the question is still in my mind: does the coffee expand up to touch the screen during brewing, or not until after the pressure is cut?

[phaelon56]

If there is a little headroom (space) between the coffee and the screen, there shouldn't really be any difference without the screw. however the question is still in my mind: does the coffee expand up to touch the screen during brewing, or not until after the pressure is cut?

I suspect that depends on how the basket was dosed and tamped. There have been folks - who've adopted or at least tested minimal tamping and dosing the comes just about to the top of the basket. In that case the coffee always touches the screen.

Are you thinking that without the screw there's a greater chance of expansion during the brew process? Is that what you're getting at?

[Alistair Durie]

A common theory is that with a little headspace that the puck is not touching the screen during the pressure of brewing, it expands after the pump turns off.

[paul_pratt]

The older E61 groups had notches cut into them which I can only assume is to enable easier screen and gasket removal without breaking the rubber. I wonder why they stopped with this system? I have another older machine in which the screen pops on and off, the outer edges of the screen have a roll which then grips the dispersian block. So you can remove it by prizing it off with a flat screwdriver and you don't touch the gasket.

[Klaus]

I would think that screw-less would provide a more even distribution of the water - no matter if there's space above the coffee or not. The way I dose (around 18,5 - 19 g. /double) there really isn't much room, and the coffee expands to touch the screw after just a few seconds of wetting. From that point on no water is pouring down directly in the middle.

So I can see the benefits of screw-less dispersion screens, but I'm not sure if the ones Kees are using are easily removable. I do think you have to take out the whole gasket as well, making it troublesome to do on a daily basis.

What I would like to see is a threading on the side of the dispersion screen, so you simply screw it on and off the dispersion block/group.

[James Hoffmann]

As for the expansion of coffee - I believe if there is a lower pressure wetting phase of extraction then the coffee is likely to briefly touch the screen but is then pushed away by the full force of the pump until the sudden expansion at the end of the shot when the solenoid dumps the pressure.

[JaanusSavisto]

What I would like to see is a threading on the side of the dispersion screen, so you simply screw it on and off the dispersion block/group.

I was thinking along similar lines, only You beat me to it, Klaus
that would definitely be the easiest way around using a screw in the middle.

[Henry Buchtel]

As for the expansion of coffee - I believe if there is a lower pressure wetting phase of extraction then the coffee is likely to briefly touch the screen but is then pushed away by the full force of the pump until the sudden expansion at the end of the shot when the solenoid dumps the pressure.

I dunno about the second part, but if you drip some hot water on the puck you can see it immediately expand and puff up... like watching cookies baking in fast motion. This doesn't happen with cool water though, it just gets slowly absorbed.

Regards, Henry

[phaelon56]

I dunno about the second part, but if you drip some hot water on the puck you can see it immediately expand and puff up... like watching cookies baking in fast motion. This doesn't happen with cool water though, it just gets slowly absorbed.

Regards, Henry

True - but there's a big difference between dripping hot water on a puck and forcing it through at 9 bars of pressure in an enclosed environment. But on occasion when I have grossly underdosed just to see what resulted... my leftover pucks tended to be soupy and not firm.

[Jason Haeger]

I dunno about the second part, but if you drip some hot water on the puck you can see it immediately expand and puff up... like watching cookies baking in fast motion. This doesn't happen with cool water though, it just gets slowly absorbed.

Regards, Henry

True - but there's a big difference between dripping hot water on a puck and forcing it through at 9 bars of pressure in an enclosed environment. But on occasion when I have grossly underdosed just to see what resulted... my leftover pucks tended to be soupy and not firm.

When you stop the shot, there is still a lot of water in the basket taking up the head space.

When the pressure is released, the force on the puck is removed, and the puck is free to expand/mix with whatever water is present.

No head space means very little water, and thus, "dry" pucks.

[gscace]

I dunno about the second part, but if you drip some hot water on the puck you can see it immediately expand and puff up... like watching cookies baking in fast motion. This doesn't happen with cool water though, it just gets slowly absorbed.

Regards, Henry

True - but there's a big difference between dripping hot water on a puck and forcing it through at 9 bars of pressure in an enclosed environment. But on occasion when I have grossly underdosed just to see what resulted... my leftover pucks tended to be soupy and not firm.

Yeah, but most of the puck doesn't experience much pressure drop across the particles if one subscribes to the idea that fines migration to the bottom of the cake is what produces most of the resistance to water flow. I'd be interested to see videos of extractions using Illy's
transparent portafilter. Then you'd be able to see for sure. I bet Bill Crossland knows as well.
I suspect that the coffee easily expands enough to push against the screen while brewing but i bet Bill knows and Illy surely does.

-Greg

-Greg

[Henry Buchtel]

True - but there's a big difference between dripping hot water on a puck and forcing it through at 9 bars of pressure in an enclosed environment.

I hear you, but the puffing up is instantaneous (to my eyes), while it might take several seconds to build up pressure and start a downward flow. You can try stopping a shot at 1 or 2 seconds to see the reaction of the puck. How significant this is is another question

re: the Kees group- is there a part which takes over the dispersing function of the LM dispersing-water + holding-shower-screen screw?

Regards, Henry

[Alistair Durie]

As far as whats happening in the portafilter, we're all talking theory as I'm not sure anyone can provide any real data on this. I vaguely recall that the Illy clear grouphead ran at well below normal brewing pressures.

[gscace]

As far as whats happening in the portafilter, we're all talking theory as I'm not sure anyone can provide any real data on this. I vaguely recall that the Illy clear grouphead ran at well below normal brewing pressures.

I bet it's not too hard to measure and I bet I could do it pretty easily if I'd just get my ass off of my skates and down to the secret basement lab of Espresso Research HQ.

Regarding the other business of dispersion screen design - Ya gotta get the water to flow through the dispersion screen more or less symmetrically. It's easier to do this if the water enters the region above the screen in the middle, and if the flow is not directed downward, which concentrates the water flow in the center. Marzocco's design gets the water introduced in the center with no downward flow, figuring that even though the flow moves sideways from just a few holes drilled into the special cool Marzocco screw, the distribution of water above the screen is sufficiently uniform to do the job. Knowing Bill I bet they might have done some flow modeling to prove it. Their models might also show that it's pretty easy to saturate the center even though there's a screw there that blanks off part of the dispersion screen. It wouldn't surprise me too much if this were so, even though it doesn't look as uniform and nice as the absence of any retaining screw. If the water doesn't enter the region above the screen on center, then you have to account for the assymetry somehow. One method is to have a secondary brass block that is counterbored on the top with holes drilled in it from top to bottom. When the block is fitted into the group the counterbore forms a cylindrical (realy short cylinder) chamber that gets filled by water when the pump is activated. The water then flows more or less evenly through all of the small holes and then through the dispersion screen. Note that the position of the small holes doesn't make for perfectly uniform water flow onto the cake. The number and position of holes makes for "good enough", and it's perfectly fine as long as the flow through the holes isn't too fast. There's a geometric relationship between the dimensions of the chamber above the small hole and the flow velocity that is important in that once the chamber is big enough, the flow through the holes becomes uniform even if the entry point of the water above the holes is off center. But it's got to be a pretty big chamber and I'm not sure if the chambers in groups I'm familiar with are large enough, particularly early in extractions when the coffee absorbs lots of water and flow rates are high.

So just because there's no screw in the middle of the dispersion screen doesn't mean it's automatically better. Ya gotta look at how the water gets introduced, and ya gotta consider the geometry of the upstream flow path.

-Greg

[Marshall]

A screw-less screen thats' easy to change sounds like a great idea - IF it remains easy to change.

Michael Teahan gave me a cardboard ring spacer to solve this problem. It prevents the gasket from adhering to the brewhead and lets you easily remove it with your fingertips. http://www.puly-usa.com/commerce/index.php?cPath=925_928

[Jason Haeger]

A screw-less screen thats' easy to change sounds like a great idea - IF it remains easy to change.

Michael Teahan gave me a cardboard ring spacer to solve this problem. It prevents the gasket from adhering to the brewhead and lets you easily remove it with your fingertips. http://www.puly-usa.com/commerce/index.php?cPath=925_928

While the shim works fine for home use, I've seen many situations in commercial applications where it hasn't made a lick of difference except in correcting the angle of the PF handle/correcting the head space between the puck and the shower screen.

It still took just as much effort to get the friggin' group gasket out. Every time.

[Marshall]

Regarding the other business of dispersion screen design - Ya gotta get the water to flow through the dispersion screen more or less symmetrically. It's easier to do this if the water enters the region above the screen in the middle, and if the flow is not directed downward, which concentrates the water flow in the center.

Would I be correct in assuming the Dalla Corte meets this description with its overhead boiler? I find my home E-61 requires some compensations for the "rear entry" design of its brewhead water inlet and am looking forward to giving that part of my ritual up (I currently load more coffee in the front of the basket).

[Klaus]

Greg, excellent answer. I was thinking about the LM screw this evening as I was closing the shop, and how is disperses the water over the screen.
Do you happen to know how the water is dispersed over the screen on the ones Kees use?

[gscace]

Greg, excellent answer. I was thinking about the LM screw this evening as I was closing the shop, and how is disperses the water over the screen.
Do you happen to know how the water is dispersed over the screen on the ones Kees use?

No, I'm not familiar with the Kees design.

Marshall, I don't have a Dalla Corte to look at, but I can quickly pull my La Spaz apart to see what they did. The DC group is a logical extension of the La Spaz group in many ways, so I might be able to give you the right answer that way. The way to get the right answer for sure is to email DC's US office, Holger Welz, or Paolo Dalla Corte at Dalla Corte in Italy. Holger and Paolo are at

Holger@Dallacorte.com
Paolo@dallacorte.com

-Greg

[James Hoffmann]

I have a La Spaz grouphead here - I will dismantle and photograph. The DC uses the same block I think but different screens. There is a little preinfuse thing they both do which is interesting....

Photos up in 10.

[Jason Haeger]

I have a La Spaz grouphead here - I will dismantle and photograph. The DC uses the same block I think but different screens. There is a little preinfuse thing they both do which is interesting....

Photos up in 10.

Photos below:





(underside of dispersion block)

[Jason Haeger]

It reminds me quite a bit of the domestic Gaggia group head design. Extremely similar.

[James Hoffmann]

Thanks Jason - I got distracted by a visitor to the roastery!

Will post more about this when I get home.