Flat burr grinders and controlling the exit chute? (e37s and spraying)

Grinders are one of the keys to exceptional espresso. Discuss them here.
nuketopia
Posts: 1305
Joined: 8 years ago

#1: Post by nuketopia »

So in the typical implementation of an electric powered flat burr coffee grinder, we'll have a non-rotating burr, usually the upper, and a rotating burr and some means to adjust the distance between them and thus, set the fineness of grind. Usually, the burrs are horizontal in a grinding chamber with the lower burr being driven by a motor, either directly or with some pulley arrangement. The lower burr carrier is in most cases the rotating one. The lower burr carrier is usually equipped with wipers which collect the grinds that emerge around the circumference of the burrs, and sweep them around the grinding chamber until the reach the exit chute. As the rotation speed is generally pretty high, the exit chute is in most cases, equipped with some kind of flap, screen or other device intended to discharge static electricity in the grinds and to slow their exit so they fall gracefully into the portafilter or into the doser.

Here is an example of the grind chamber, of a particular model of grinder I'm working with. Shown is the grind chamber, the lower burr and the 3 wipers located near each burr retaining screw. The exit chute is at the 6-o'clock position, along with an adjusting device for the exit flap. The burr rotates in a clockwise fashion, as viewed from this photo. This particular grinder runs at 1450 rpm and has an 83MM burr. That's about 24 revolutions per second, * 83mm * pi = 6.25 meters/second linear velocity.




The second photo is looking up the chute of the same grinder. The exit chute is covered by a thin metal flap, hanging on its upper hinge and free to swing as the collected grinds push it open. The adjusting mechanism is a flexible finger, which limits the travel of the flap. The finger is user adjustable and ranges from just short of touching the metal flap in the full closed position, to allow full and free travel in the fully open position. The finger is to the upper left in the shoot and the metal flap with a crease in it, just behind it. This is in a partial open position, where the finger would limit the flap to about half its available travel.



Most similarly designed grinders, such as the Baratza Vario and the Compak, etc, have some sort of exit control device in the grind chamber exit. In many cases, it is a silicone rubber flap. Some vendors offer these flaps in various thickness, which allows for differing degrees of resistance to the exiting grinds.

This particular design, allows the user to adjust the range of flap opening, for the best trade off of "static" and "clumping" due to grands being compressed against the flap when it is held nearly closed, to "static" when the grinds are allowed to flow freely.

--continued in next post--

nuketopia (original poster)
Posts: 1305
Joined: 8 years ago

#2: Post by nuketopia (original poster) »

As the grinder operates, the ground coffee is driven by the wipers into the grind chamber exit, which is a machined notch in the side the chamber wall.

The hi-lighted area is where the notch is, and the curved arrow shows how the grinds are propelled.



To exit, the grinds have to make a near 90-degree turn, press against the flapper and fall down the shoot.

And this is the genesis of the E37S "spraying" problem.

nuketopia (original poster)
Posts: 1305
Joined: 8 years ago

#3: Post by nuketopia (original poster) »

So how does this cause the spraying issue?

The grinds are driven into the exit notch and impact the inside wall of it on the left side. When conditions are right, the grounds compress into a lump and build up.



As this lump builds up and grows, it pushes against the flapper door.



When conditions are correct, the lump holds the flapper open. But there's no build up behind the right side of the door, leaving coffee grinds to exit freely at high speed from the grinding chamber from the right side of the flapper.

This results in a rooster tail of ground coffee that flies beyond the portafilter handle and out and to the left of the exit chute.

Since the chute on this model is open, this leaves an uncontained exit path for the grinds to fly out.

SAB
Posts: 364
Joined: 10 years ago

#4: Post by SAB »

There's nothing that'll shut down a discussion like a simple, accurate, thoughtful explanation. :D

I'm glad you've gotten to the bottom of the problem...first step in achieving a consistent solution.

Bear007
Posts: 17
Joined: 8 years ago

#5: Post by Bear007 »

I've had mine for about a week now, its only got 3lbs of beans though it but I promised myself I would fix the problem if it did happen. So it sprayed for the fist time yesterday, I added a small 1-1/2" outside, 1-1/4 inside, piece of aluminum tube to it. The tubing has a 45 degree angle cut at the end and I epoxied a small deflector to the top of the 45, after applying a small bead of silicone to very end of the new chute to isolate it from the grinder body, I used an industrial hot melt glue gun to attach it to the grinder itself. So far its working great.




OldNuc
Posts: 2973
Joined: 10 years ago

#6: Post by OldNuc »

It is a design defect in the motorized flat burr grinder. The manual flat burr grinder had the shaft oriented horizontally and the ground coffee fell out around the periphery of the burr at low velocity so the design worked. The EK43 is a motorized version of the early hand crank grinders. The large antique grocery store motorized grinders also are built similar to the hand powered versions. Motorized grinder performance would be markedly improved with a tangential exit chute with an exponentially increasing cross sectional area.

nuketopia (original poster)
Posts: 1305
Joined: 8 years ago

#7: Post by nuketopia (original poster) »

OldNuc wrote:It is a design defect in the motorized flat burr grinder. The manual flat burr grinder had the shaft oriented horizontally and the ground coffee fell out around the periphery of the burr at low velocity so the design worked. The EK43 is a motorized version of the early hand crank grinders. The large antique grocery store motorized grinders also are built similar to the hand powered versions. Motorized grinder performance would be markedly improved with a tangential exit chute with an exponentially increasing cross sectional area.

Exactly what I was thinking too. This would allow the grinds to decelerate without employing things like spring-loaded metal flap or silicone rubber curtain. Those impeding flaps, along with ramming the grinds into the side wall of the exit chute are what cause the clumping. This would result i the nice evenly distributed fluffy grounds we like so well.

OldNuc
Posts: 2973
Joined: 10 years ago

#8: Post by OldNuc »

That tangential design works well with pumps for fluids and slurries so it should work with coffee.

nuketopia (original poster)
Posts: 1305
Joined: 8 years ago

#9: Post by nuketopia (original poster) »

Especially if we look at the discharge as a particle-laden flow. The design of the exit could include a phase separator so that the air and coffee particles can be separated and the grinds directed by gravity into the portafilter at a controlled rate.

OldNuc
Posts: 2973
Joined: 10 years ago

#10: Post by OldNuc »

Replace that useless doser thing with a cyclone separator and actually have something useful. That would also get the outlet back into the front center of the vertical motor grinder. Portafilter or catch cup would have to seal to the bottom of the cyclone for it to function correctly.

Post Reply