So I was visiting Ninth Street Espresso, and Ken Nye was telling me that all the equipment for his newest location had been selected, EXCEPT FOR THE ESPRESSO GRINDERS. And the reason the grinders were unselected was because (from the point of view of heat buildup in a high-volume location) the available grinders SUCK: they get so hot that the beans are literally cooked as they sit in the hopper waiting to be ground. In the ensuing discussion I inadvertently went into LECTURE MODE, and I hope poor Ken, Bob and Nick didn't get their minds blown with my impromptu blabbering on the thermodynamics of grinding coffee.And now I'm thinking, there's probably four or five of you out there that are actually interested in this kind of stuff, so why not toss it out into cyberspace? And I'm hoping talking about this will hasten the day when grinders for high-volume locations DON'T suck.
Please note: I'm going to propose some numbers which are guesses or approximations. They may not be perfect, but they'll be close enough for our purposes. People who are allergic to math and to numbers can skip to the last paragraph (or more likely, skip this post entirely).
There are TWO sources of heat in an espresso grinder:
1. the electric motor, and
2. friction from the grinding process itself.
Electric motors are never perfectly efficient. Typically, for every unit of electrical energy you put in them, only 65-90% comes out as mechanical energy. The other 10-35% is wasted as heat. Particularly in shops that grind each shot to order, the constant start-stop-start-stop cycles make grinder motors particularly inefficient, probably hovering around 70%.
My Robur draws about 900 watts and grinds an 18 gram dose in about 3.5 seconds. Other grinders are similar in their energy consumption. For shops putting out one shot a minute during a morning rush, this corresponds to an average continuous current draw of about 52 watts.
That means the heat generated by the motor would be 30% x 52 watts = 16 watts, and the mechanical energy transferred to the grinding chamber would be 70% x 52 watts = 36 watts.
The kicker here is that the motor is only generating 30% of the heat. So various ingenious designs that cool only the motor are solving only 30% of the problem.
We can estimate that the 36 watts of average mechanical energy fed into the grinding chamber is used in several ways:
1. About 4 watts is absorbed by the coffee, heating it up 15-20 degrees F
2. About 3 watts is used to mechanically eject the coffee from the burrset
3. Perhaps 4 watts is used to overcome bearing, belt or gearbox friction.
This leaves an estimated 25 watts of mechanical energy that turns into heat through friction as the bean particles are dragged through the burrset and crushed against each other. This 25 watts heats up the burrs big time! It is the reason why, after a short time in a morning rush, the grinder adjustment dial on a grinder can be too hot to touch.
Here's an analogy for what's going on: have you ever burned yourself by grabbing an incandescent light bulb that had been on for a while? Well, if you somehow buried a 25 watt incandescent light bulb into your grinder's working parts, and left the bulb on for an hour or two, you can imagine that the grinder would end up getting REALLY HOT: 25 watts of continuous heat is a lot in an enclosed chamber. And this is pretty much what's going on inside the grinding chamber of any high-quality espresso grinder during a morning rush.
Of course grinder manufacturers know this, and try to arrange for convection or conduction to carry away some of the heat. But it's not enough. I hear that some folks have tried to use computer cooling systems to accomplish the same thing. Perhaps this approach will work out in the long run.
But the bottom line is, it's not enough to remove the electric motor's heat from your espresso grinder. In a high volume shop, you've got to find a way to remove heat directly from the grinding chamber. When we learn to do this, our grinder heating problem will finally be solved.
This may seem like an odd thought, but couldn't you use a cold gas jet to cool the chamber after grinding a dose?
ReplyDeleteExpanding gasses are cooler than their storage temperature, so they'd absorb heat out of the chamber. I believe their cooling effect would be significantly faster than simple air circulation systems (like fans) because of the adiabatic cooling created by expansion.
I envision a 2-step process, where an air "purge" pulse clears the grounds from the chamber, moving them into the doser. Then the some sort of valve would have to close blocking the grinding chamber off from the doser and the bean reservoir before a long pulse of air (perhaps tied to a thermostatic valve) was vented through the grinding chamber to cool it. The cooling air pathway could include channelling through heat-exchanging fins to maximize heat transfer and reduce the length of time needed for the pulse.
If compressed air isn't fast enough, a colder, inert gas like CO2 could be used for the cooling pulse. Though another plain air purge might be necessary after cooling if the CO2 changed the flavour of the coffee.
As an added bonus, the first pulse of gas would also blow the grounds out of the grinding chamber into the doser, so you'd have no stale grinds on the next run. (Or the waste associated with chasing them out of the doser with new grounds)
With some muffling, the noise would be quieter than the steam wand.
There are a few problems with the calculations used to support the author’s assertion that the grinder motor only contributes to 30% of the total heat to which coffee beans are exposed.
ReplyDelete1. It is assumed that 100% of the work produced to crush the beans (the author’s stated 25 watts) is converted into heat. If it was all converted into heat, then there would be no energy left to crush the beans. This assertion ignores the first law of thermodynamics dealing with the conservation of energy.
2. The statement that 4 watts is absorbed by the coffee is unsupportable without knowing the thermal conductivity of the coffee.
3. How was the 4 watt figure for bearing friction estimated?
I realize that the numbers are guesses, but the entire theory rests upon the validity of those numbers. There are too many “about” and “perhaps” statements for me.
Great discussion.
ReplyDeleteI was speaking with a design person from a grinder manufacturer earlier this year about grinder heat and their surprising R&D discovery was that most of the heat comes from the grinding itself as opposed to the motor.
This became readliy apparent when they had moved the motor off the burrs and still found similar high temperatures in the coffee, indicating that it was the grinding and not the motor that caused excessive heat in ground coffee.
Turn grinders sideways. Why not explore the same vertical grinding disc set-up as those supermarket grinders use? Many fully-autos use this arrangement too. Doing it that way you can off-set the hopper so that it is not sitting directly above the burrs (cooking the beans) and use a feed screw to load the beans down and into the burrs. Also gravity would prevent stray grounds caking up the grind chamber. 3PH motor and big stonking flat burrs. Heat-sink fins on the main body of the grinding chamber.
ReplyDeleteOtherwise for the time being busy shops must use multiple grinders.
Thank you everyone for taking the time to comment.
ReplyDeleteCrosius, that's an interesting concept. I know that someone on one of the amateur forum was using air to blow the grounds out of the chamber and chute and into the doser. No attempt at cooling, though.
Jeff, This isn't the forum to get too techy, but I'm pretty sure it's correct that 100% of the energy that goes into actually crushing the beans is converted into heat. This is entirely consistent with conservation of energy.
The 4 watts that heats the coffee comes from my observation that the grounds come out 15F hotter than they went in. It requires 0.24 BTU to raise 18 grams of coffee (with a specific heat of 0.4) 15F. 0.24 BTU per minute is the equivalent of 4.2 watts.
The 4 watts for bearing losses is just a very generous estimate (10%).
Anyway, I was simply making the point that it's not the motor, it's the grinding process. Also, the 15F is probably very low, the grounds probably heat a lot higher.
Jay, I agree completely.
Paul, I'm not competent to comment on all the aspects of your design suggestion. But one of the key advantages of a conical burrset running on a vertical axis is that the grounds can drop straight out of the burrs into your portafilter, and a minimum amount remains in the grinder to bake before the next shot.
By the way, yesterday I had a brief phone conversation with an engineer who's designing a new grinder for a prominent high end espresso company. He confirmed that, in my original post I got most of the facts right, some of them wrong, and that the actual ratio is probably closer to 10% of the heat generated is from the motor, 90% is from the actual grinding of the beans!
Hi Andy always great to read you,
ReplyDeleteSo if I do get that right not even high-end (anticipated) grinder like the prototype LaMarzocco at Long Beach won't solve the heating issue? Like Paul wrote :
What about grinder that have a burr set in a vertical or angled way instead of horizontal (like KitcheAid ProLine or Mythos by Nuova Simonelli), would those design help reduce heat?
The goal is to get the hopper far from the burr. Any cooling design would somehow end up with condensation problem coffee grounds sticking all over the burr area, specially in high humidity place like Pacific NorthWest ;-)
Thx
nic
Hi nic, thanks for commenting. Your question is a good one, but please see my followup post.
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