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.