Grain Mill Roller Speed

[DGuggenb]

Small rollers don't work well. The reason large rollers are used is at least two-fold: the periphery speed is lower for equal throughput, AND the nip angle on the machine is much less. This is essential for good crush. From A TEXTBOOK OF BREWING, DeClerk; "The diameter of the rolls should be at least 250mm otherwise the feed will fall on to the rolls at an obtuse angle, and will not be snatched up by the rolls. The crushing time would also be too short with rolls of any smaller than 250mm." My experience bears this out.

There is also a very good discussion of roller radius, roll gap, forces & energy required for milling in MALTING AND BREWING SCIENCE, Briggs, Hough, Stevens, Young. There is a derivation of optimum roll nip angle of 16 degrees. Also backs up sound brewing science.

The periphery speed can be set to the same speed, the difference then would just be the length of the roller which is what would give the higher throughput. The peripheral speed is just the RPMs times the circumference, so that's not hard to match.

We're on the same page about the contact length being gentler with the larger roller, like I said, although it's not as much as some might think. Going from a 2" to a 10" roller, you're only doubling the distance that the grain is in contact with the roller. For this size, the grain is in contact with the mill for about 8mm until it hits the smallest gap and maintains contact with the mill for about .0026 seconds to hit that point. The 2 inch mill would be about 4mm and .0013 seconds at the same peripheral speed. To put a visualization to this, it takes a person more than 100 times longer to blink than the amount of time the grain spends getting crushed, even with a 10" roller.

I'm also just keeping this in context of the scale that was being asked in this thread. He's only crushing 120-200 lbs of grain, so he probably doesn't need to buy a grain mill that can do 5,000 lbs per hour and is more than double the cost of the rest of his brewery equipment combined, lol.

Don't get me wrong, if money wasn't an object definitely go for the larger mill.

[gitchegumee]

But the grain DOES know the difference

Small rollers don't work well. The reason large rollers are used is at least two-fold: the periphery speed is lower for equal throughput, AND the nip angle on the machine is much less. This is essential for good crush. From A TEXTBOOK OF BREWING, DeClerk; "The diameter of the rolls should be at least 250mm otherwise the feed will fall on to the rolls at an obtuse angle, and will not be snatched up by the rolls. The crushing time would also be too short with rolls of any smaller than 250mm." My experience bears this out.

There is also a very good discussion of roller radius, roll gap, forces & energy required for milling in MALTING AND BREWING SCIENCE, Briggs, Hough, Stevens, Young. There is a derivation of optimum roll nip angle of 16 degrees. Also backs up sound brewing science.

[DGuggenb]

Roller Speed

I am assuming you mean 2.5m/s is the tangential velocity on the roller. For a smaller mill with 2" rollers like many micros have you would need an RPM of 6000 to achieve that at the roller not at the motor.

v = 2.5m/s = rw = 0.0254(m) x w(rev/s)

w = 98.4 rev/s = 5905 rpm

For a 6 in roller...

2.5m/s = rw = 0.0762(m) x w (rev/s)

w = 32.8 rev/s = 1968.5 rpm

I guarantee no brewery is running their mill at 6000rpm or 2000rpm. You would shred the grains. Unless I am making a bad assumption, that rule doesn't work. RPM would be a better data point to look at or those speeds are wrong. Are you sure it wasn't 2.5 - 4 in/s? That works better.

I believe the intent is the speed at which the grain is moving through the gap. RPM just relates to the actual number of times the roller rotates per minute, not the speed at which the grain is getting crushed. 200 RPMs on a 12" roller would mean grain is moving through the gap at 3.2 meters/second. The equivalent grain speed on a 2" roller would require 1200 rpms. The grain doesn't know any difference. Big breweries just wouldn't use a 2" roller because you're spreading out the wear over such a small area and would be replacing the rollers many times each year. I imagine the lead-in to the crush is a little gentler since you'll have slightly more contact time with the roller, but since the grain is getting crushed at the same speed it's hard to say.

[BFCbc]

Gabe, your answer is in the link that BigMuddy sent you. RPM is NOT the deciding factor, periphery speed is.
"Rolls should run at 2.5 - 4.0 m/s on the periphery. "
I'd be wary of tiny rolls, too. They produce an inferior crush. A few percentage points on yeild will add up over a few years. Good luck!

I am assuming you mean 2.5m/s is the tangential velocity on the roller. For a smaller mill with 2" rollers like many micros have you would need an RPM of 6000 to achieve that at the roller not at the motor.

v = 2.5m/s = rw = 0.0254(m) x w(rev/s)

w = 98.4 rev/s = 5905 rpm

For a 6 in roller...

2.5m/s = rw = 0.0762(m) x w (rev/s)

w = 32.8 rev/s = 1968.5 rpm

I guarantee no brewery is running their mill at 6000rpm or 2000rpm. You would shred the grains. Unless I am making a bad assumption, that rule doesn't work. RPM would be a better data point to look at or those speeds are wrong. Are you sure it wasn't 2.5 - 4 in/s? That works better.

[gitchegumee]

Gabe, your answer is in the link that BigMuddy sent you. RPM is NOT the deciding factor, periphery speed is.
"Rolls should run at 2.5 - 4.0 m/s on the periphery. "
I'd be wary of tiny rolls, too. They produce an inferior crush. A few percentage points on yeild will add up over a few years. Good luck!

[ClaudiusB]

The BC Micro Brew series crushes 20 lb/min but requires two motors as per BC specs.
Each roller is driven by one motor.


Cheers,
ClaudiusB

[Gabe]

I believe the one your talking about is the home brew version. I have that one from times past. We are looking at the Micro series that BC makes. I was hoping for the neighborhood of 10-20# a min. I think a smaller complete mill from Apollo or Robix might be a better choice for us.

[ClaudiusB]

we are looking at sheaves tying to figure out our ratio.

Get a gear motor and you can build a nice looking mill.
For my toy mill I used a Bodine gear motor and run the mill at 94 RPM, 3 lbs a minute with 1-1/2" rollers.
The Barley Crusher mill design might limit your speed to below 500 RPM.
Call BC and ask for specs.

Cheers,
ClaudiusB

[Gabe]

Thanks

That helps for when we get it going, but right now we are looking at sheaves tying to figure out our ratio. This is for the Micro Brewery Mill that BC Entr. makes. Anyone else with an idea of an acceptable speed? We are a small brewery looking at 120-200# of grain.

[BigMuddyBrewing]

http://probrewer.com/vbulletin/showthread.php?t=13818&highlight=mill+crush

http://probrewer.com/vbulletin/showthread.php?t=13818&highlight=mill+crush

[Gabe]

I don't know if it matters much, as the manufactures of every mill spec their own out individually, but what should the typical speed of the rollers be on an average mill? We were thinking in the 250 RPM range? Is this way off or in the realm?