That is simply a function of "thermostat" setting, and heater size / flow rate.
Yes. You could have the heated delivery tube be at ~120 oC at the beginning, and 90 oC at the end.
Yes. Just imagine your "vessel" stretched long and thin (into a tube), with heater wire wrapped around.
The pump and heater are triggered on when water is requested.
It is your product...
On all the time, on only when the pump is running... you choose.
David A. Smith
David, This is also an attractive option. Let's assume we have a vertical flash-heating tube and beneath it the piston chamber, with the two tubes separated by a one-way valve; the valve could be as simple as a flexible washer screwed into the ceiling of the piston chamber so that it seals a ring of eight 3cm portholes which connect to the boiling tube; that's exactly how one of these vintage machines works, only with a kettle above the piston-chamber instead of a flash-heating tube:
| H20| | H20| | H20| heat | H20| | | | |
------ valve |....| |PSTN| |PSTN| piston |PSTN| chamber
Downward motion of the piston pulls the edges of the flexible washer down into the piston chamber, "unplugging" the port holes, and water is drawn down from the unpressurized vessel above, by suction and gravity, into the piston chamber.
The draw is never more than ~2.5 ounces at a time. That's the maximum volume of the piston chamber available for water when the piston retracts completely.
The draw from the retracting piston is instantaneous, so the water closest to the valve would not have a chance to heat up before it gets drawn down into the piston chamber. The water must therefore be at temperature when it reaches the valve. And the water must be kept at temperature once it reaches temperature. The flash-heater must have two states, super-heat to bring cold water to temperature quickly and low-heat to keep hot water hot.
How hot would the heating element have to get to bring 2.5 ounces of water in copper tube to 98c in ~10 seconds?
If 2.5 ounces could be brought to and kept at 98C by this heat-tube device, the device would solve the problem of how to get water from the reservoir into the kettle and from the kettle into the piston chamber. The fill pump would no longer be necessary. The fill sensors would no longer be necessary. It would be a continuous draw system with the water being heated while in transit, and all we would need is a temperature sensor and a feedback mechanism to control the two basic ON-states of the heating-element.
Regards Liam
Why so big? Did you mean 3mm?
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Why not a full "shot"? 250 ml is closer to 8 ounces...
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Significantly hotter than 98 oC. This would be hard on the piston seals also, going from unheated tube, to heated tube, to a tube that is heated by the water (and expanding).
It wouldn't have to be required, even with a classical kettle.
Chew it around a while and see if you like the flavor. Lots of different ways to skin a cat (so to speak).
David A. Smith
Yes, 3cm was a typo. The diameter of the circular arrangement of holes is itself only 35mm, and the individual holes are ~3mm.
A single espresso is only about 1 to 1.5 oz. A double 2+ oz.
"A double espresso is a 47-62.5 mL (1.5-2 ounce) extract that is prepared from 14-17 grams of coffee through which purified water of
88-95°C has been forced at 9-10 atmospheres of pressure for a brew time of 22-28 seconds. The espresso should drip out of the porta-filter like warm honey, have a deep reddish-brown color, and a crema that makes up 10-30% of the beverage."I was talking 250ml when considering the heating kettle approach. With the kettle, we'd need to have water enough on hand to make several doubles in a row without a downtime for recovery, plus some extra for a preliminary flush to bring the brass group up to temperature. You don't want to brew when the group is cold, otherwise the temperature of the water drops precipitously and the coffee is underextracted and tastes really sour. The brew water starts at 98C and loses a few degrees as it enters the piston chamber and flows to the brass group and moves down to the coffee. Temperature of the water as it reaches the ground coffee should be 88-95C. I prefer to the higher end of that range, 92-94C, though perhaps the user could be allowed to set the target temperature, if it wouldn't complicate the machine too much.
We could have this (rotated horizontally):
-------------------..........::O:::::::::::::::::: H20 H20 ---> H20 | H20 PISTON SPRING
-------------------..........:::::::::::::::::::::
Key:
----- is the flash-heat tube ..... is unheated copper tubing | is the seal ::::: is the piston chamber O is the porthole to the group
The seal between the piston chamber and the unheated copper tube would be some distance away from the heating element. Shouldn't an EPDM seal be able to withstand the ambient temperatures if the seal is some centimeters distance away from the heating element? Can the flash device be insulated?
I don't understand how we'd get water from the large (e.g. 2 liter) reservoir into the 250ml heating kettle without some kind of pump. Please elaborate.
Regards Liam
Dear Liam:
What is the water source? Isn't it some sort of pressurized, filtered water supply? Or are you doing a free-standing counter-top model, filled by jugs?
Keep in mind that a standard percolating coffee pot does exactly what I describe, moving a whole lot of water from a low location to a higher location at high temperature, by "flashing" small quantity of water to steam.
The water reservoir (not the kettle) is an *unpressurized* pour-over. The kettle (or flash-tube) is also unpressurized and sits higher than the piston-chamber so that the water can be *gravity-fed* to the piston chamber.
Yes, but we're not builing a coffee-scorching Krups-style steam-powered machine. We're building a very different beast indeed!
Temperature-stability is the overarching goal and the margin of error for temperature variation is very slim, a few degrees.
We want to pass ~1.5oz of water at ~93C +/- 1C through ~7g of coffee which has been ground to a fineness and compacted to such a density that
9 bars of pressure cause the water to take approximately ~22 seconds to be forced through the coffee :-)Take a gander:
Regards Liam
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