Posted Sat Jul 27, 2013, 5:28pm Subject: Re: Air driven shot to increase thermal stability of small boiler?
I think this idea of pulling espresso shots is very similar to something I did over a decade ago, with a project called Bar Monkey. Bar Monkey is a computer controlled mixed drink maker. The biggest issue I ran into was how to siphon liquid from about 20 different containers... on the cheap.
My solution involved 20 teflon-coated HPLV paint cans. The paint cans had an air input port and a paint output port. Obviously the output port from all the paint cans could conjoin into a mixing manifold and out goes comes the mixed drink. On the air input port, each can would have it's own air solenoid, which is controlled by the computer.
So how it worked was you have constant air pressure behind the solenoid. When the valve is opened, air pressure is applied to the liquid and the liquid is pushed out the siphon tube at the bottom of the can. In this case, the tube would be pointing upwards, so whatever level you cut the copper tube at, is your maximum shot volume.
To address the pressure issue, it doesn't matter if the many orifices within the system are small and different sizes (for all practical explanation purposes). What matters is the area of the top of the water level and the area of the coffee puck. These two figures will tell you how much PSI to introduce into your boiler. Read more here. The pressures within the smaller internal orifices will balance each other out. It's the two outer areas that are applying and performing the work. The small orifices only slow down flow, so that is about fluid speed, not pressure.
If I understand correctly, your aim is to "preload" your puck, soak, and infuse before pressure is applied? So, before pressure is applied, what is your mechanism of getting water to the puck and soaking? Will you still need the water pump to prep or will you be also using an air system... making it a 2-stage air process?
Slowing down the air pressure as it enters the boiler may be a good idea, since you could (not saying it will) blow up the boiler with a sudden impact of air (cold air, no less) into a hot and sealed chamber, and you might want to avoid water splashing. The splashing could affect your air pressure ratios. However, slowly introducing pressure may affect your shot, since the ideal espresso shot is pulled inside of 20-30 seconds. What is the time interval between 0psi and 130psi (if it were)? This interval should probably be factored into your preinfusion time, instead of part of shot time.
The last question I have is, you mentioned the heaters wouldn't have to be on, in your process, so the boiler would be roughly the same temp, or lower, than the water temp inside. After your shot is done, you'll have to refill your boiler with water to prep the next shot. You will still need a water pump, yes... or air pump? Then the heaters will kick in to prepare the water. Will recovery time be fast enough for the next shot? According to all the Gaggia advertising and consumer claims, the 1400 watts of power the Gaggia Classic has can recover in 17 seconds. Experience says it's longer than that, but I'd like to see how your implementation turns out.
jonr Senior Member Joined: 25 Jun 2013 Posts: 329 Location: Americas Expertise: I love coffee
Posted Wed Aug 14, 2013, 8:41am Subject: Re: Air driven shot to increase thermal stability of small boiler?
For the sake of completeness, here is an update. While I think that an air driven shot would provide a nice steady temperature and pressure, I no longer consider that a good goal. My opinion is that a slightly dropping temperature profile is beneficial and that some shape to the pressure profile may be beneficial. So my focus has shifted to computer control of these things (temp is working well, in the thinking stage for pressure).
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