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Air driven shot to increase thermal stability of small boiler?
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Discussions > Espresso > Espresso Mods > Air driven shot...  
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jroach
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Posted Sun Jun 30, 2013, 4:07pm
Subject: Re: Air driven shot to increase thermal stability of small boiler?
 

I'm claiming low caffeine levels...  

Started to sketch out the system, the air over hydraulic has a 'piston' that transmits the force from the larger area to the smaller area.  so the force on either side is equal with differing areas.  Been a foggy day.  

Good luck,

Ciao
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jonr
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Posted Tue Jul 2, 2013, 9:14am
Subject: Re: Air driven shot to increase thermal stability of small boiler?
 

The Myprressi Twist certainly has some fresh ideas.  It looks like it has a 4F intra shot temp drop and that you can control starting temperature with the number of times you preheat by filling/dumping the bowl.  That sounds a little cumbersome and inaccurate.  http://www.youtube.com/watch?v=If1GbNc2hoI

A system that is continuously bringing new hot water into the head should not suffer from the heat loss/drop that it has.  And PID control would provide an accurate and convenient starting temp.

As an aside, I often see claims along the lines of "the heavy brass ... maintains accurate temperature".  Water stores more heat than brass.  So if you want to achieve steady temperatures through thermal mass, add more heated water to a system and don't be concerned with how much brass it has.  Insulation is also beneficial for achieving consistent temperatures throughout a system.  IMO, there is nothing wrong thermally with aluminum, stainless steel and plastic parts.
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billc
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Posted Thu Jul 4, 2013, 9:36am
Subject: Re: Air driven shot to increase thermal stability of small boiler?
 

To clarify a bit about pressure/force.  Pressure is a measure of force per unit area (in the USA it is lbs per square inch - we use the units of Bar for espresso machines).  So if you know what the pressure is you can figure out what the force is on an item in the system just by knowing the square area.    There are "pressure loses" in the system due to restrictions and change in orifice size so the pressure may vary slightly in the overall system.  Electric valve take advantage of this by using a small orifice in the valve.  The valve piston is pressed against the orifice by a spring (spring provides a force).  The valve manufacturer needs to make sure the valve will hold the 9 bar pressure (they usually hold about 12 bar).  To be able to use a small spring in a small valve package they need to have a pretty small force acting on the spring.  This is accomplished by reducing the valve orifice size to an open area that when you calculate the force (pressure X area) it is less than the force of the spring.  This is why the orifice on the electric valves for espresso machines (holding back the 9 bar pressure)are about 1.2mm or less in diameter.  

THe measure of pressure in espresso machine is important so the force acting on the coffee is the same for all machines.  Portafilters may change in diameter and the overall force on the portafilter changes with the diameter of the basket but the force on the coffee remains the same for a given pressure.  You can prove this by looking 1 square inch of coffee in any machine.



BillC
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billc
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Posted Thu Jul 4, 2013, 9:41am
Subject: Re: Air driven shot to increase thermal stability of small boiler?
 

JonR,
Great idea.  If you need any parts let me know.   There have been a couple of ideas similar over the years.  Make sure you do not expose the heating element!  The only issue I heard what the the air reservoir had to be pretty large to maintain constant pressure.   I look forward to hearing your results!!!  There are a few capsule machine that use similar methods.


BillC
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CoffeeRon
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Posted Thu Jul 4, 2013, 10:50am
Subject: Re: Air driven shot to increase thermal stability of small boiler?
 

billc Said:

 The only issue I heard what the the air reservoir had to be pretty large to maintain constant pressure.

Posted July 4, 2013 link

Using a cartridge/regulator as in the Mypressi may be a good alternative. True there would be the ongoing cost of the cartridges, but cost per shot is really not much from what I've read on the Mypressi and the space savings of the nice small package may be worth it.
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jonr
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Posted Thu Jul 4, 2013, 7:44pm
Subject: Re: Air driven shot to increase thermal stability of small boiler?
 

billc Said:

JonR,
I heard what the the air reservoir had to be pretty large to maintain constant pressure.

Posted July 4, 2013 link

In my case, I'd use an air compressor/tank that I happen to have.  It will pump up to 10 bars and then that gets regulated down to 9.   Of course most people wouldn't want to plumb to an air compressor in the basement.

I'm not sure that people would go for the 4 shots/cartridge that the Mypressi gets in a countertop machine. But I don't know of a small, inexpensive way to produce air pressures like that (~50 bars).

For a practical machine that minimized the effect of incoming cool water, I think an near neutrally buoyant insulated baffle that keeps the incoming water separate from the outgoing might work.  When the shot stops, the baffle could slowly sink back into position near the bottom of the boiler.  Or maybe just staggered fixed baffles would be enough to minimize mixing.

Then there are all of the ways to add heat on the fly as cooler water comes in.   Ie, automated temperature surfing and thermo blocks.
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jonr
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Posted Sat Jul 6, 2013, 7:36am
Subject: Re: Air driven shot to increase thermal stability of small boiler?
 

Thinking about Bill's comment, if one wanted to keep things small, then the air/water pre-pressurized tanks used in well water systems provide a model.  Ie, the boiler could contain an air bladder pre-pressurized to 9 bars.    Say you then pressurize the tank/bladder to 12 bars by adding water with the water pump.  Using Boyle's law and a 2.5 oz shot size, I get 8 oz for the needed bladder size (10.5 oz minimum total boiler size).  That seems quite reasonable - no external air compressor or air tank needed.  The water pump would run before the shot, but not run at all during the shot (remember, the idea is to avoid incoming cool water which disturbs the temperature).

I assume there are flexible plastics that could handle the temperatures involved.  An added plus is that the water pump could be downsized. It needs to pump up to 12 bars, but it doesn't need to do it quickly.  With a pressure regulator on the output, intra shot pressure would be a steady 9 bars and pulsations from a vibe pump would be gone.  And of course the boiler temperature should be steadier and with only a few oz of water in it, the boiler would heat up quickly.

A downside is that the Gaggia boiler is too small to try this design.
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jonr
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Posted Mon Jul 15, 2013, 11:37am
Subject: Re: Air driven shot to increase thermal stability of small boiler?
 

Continuing with the out of the box thinking, what if an air pump could reverse direction?   Pulling a vacuum on water causes it to immediately boil and cool down.  So the idea would be to have the entire system (boiler, group and tubing) slightly overheated (say ~210F) and then have the pump first drop the pressure to .8 bars (absolute) which would cause all of the water to boil and quickly drop to 200F.  Then reverse and push the uniform 200F water out at 9 bars.  The advantage is that it should create a more even temperature everywhere in the system.  Water maintained at 200F in a boiler isn't hot enough to maintain a group head at 200F.  But 210F (or whatever) would be.

I have no idea if this would be less expensive that something more traditional - say a large boiler maintained at 200F and an electrically heated group.
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CoffeeLoversMag
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Posted Tue Jul 16, 2013, 1:46am
Subject: Re: Air driven shot to increase thermal stability of small boiler?
 

If you try to change the original design, you may damage the machine or may change the operation procedure. Removing the copper tube inside may leak that portion because you change it with 9 bar air which is not fitted or may not fit.

If you happen to meet a problem on your espresso, the original designer is knowledgeable on how to improve its design... Never introduce anything that can give you more problems. Just change your espresso machine to a better one.

 
Did you know...? Dark roast coffees actually have less caffeine than lighter roasts due to the fact that the process of roasting burns off caffeine.
www.coffeeloversmag.com/theMagazine
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kolu
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kolu
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Posted Sat Jul 27, 2013, 5:49am
Subject: Re: Air driven shot to increase thermal stability of small boiler?
 

I was thinking about that a bit more - didn't such high air pressure end up in dissolving a rather big amount of air in the water? I mean it will be much like a making of sparkling water/soda.

Which isn't really the right substance to make your coffee from - just try making espresso from bottled sparkling water and you'll know what I'm talking about :)
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