Gaggia Classic- a solution to the aluminum boiler?

[The Cadillac Man]

Hi everyone,

This is my first post to alt.coffee!

I've been doing a little reading on the shortcomings of my Classic,
and I wanted to put this idea out to the group... Obviously, being
made of aluminum, the boiler doesn't hold heat like a brass boiler
would, so why not strap blocks of brass or copper or somesuch to the
sides of the boiler? It would take longer to heat up initially
because the boiler would have to heat the brass as well, but in theory
the heat retention of the metal should stablize the heat of the
aluminum boiler quite a bit during a shot. I pulled the top off my
machine and I think that there easily room to attach some heavier
metal on all four sides of the boiler.

Anyone have any thoughts on this or see any glaring errors in my
logic?

thanks,

Mark.

[D. Ross]

| I've been doing a little reading on the shortcomings of my Classic,
| and I wanted to put this idea out to the group... Obviously, being
| made of aluminum, the boiler doesn't hold heat like a brass boiler
| would, so why not strap blocks of brass or copper or somesuch to the
| sides of the boiler?

Heat retention isn't a problem with the Gaggia boiler; the integral elements
keep the boiler plenty hot.

Some have speculated that there might be a thermal instability problem
because of the boiler's small size when compared to the shot size. In
practice this is not a problem, as evidenced by the Norwegian specialty
coffee association's tests - Gaggias came out as stable as most machines
(more stable than the Silvia), though were somwhat downrated for some other
reasons.

The complaints about the Gaggia boilers are (a) an irrational fear of
aluminum, and (b) real concerns about electrochemical corrosion due to the
bimetallic construction.

- David R.
--
Less information than you ever thought possible:
http://www.demitasse.net

[Craig Andrews]

Post by D. Ross
| I've been doing a little reading on the shortcomings of my Classic,
| and I wanted to put this idea out to the group... Obviously, being
| made of aluminum, the boiler doesn't hold heat like a brass boiler
| would, so why not strap blocks of brass or copper or somesuch to the
| sides of the boiler?
Heat retention isn't a problem with the Gaggia boiler; the integral elements
keep the boiler plenty hot.
Some have speculated that there might be a thermal instability problem
because of the boiler's small size when compared to the shot size. In
practice this is not a problem, as evidenced by the Norwegian
specialty
coffee association's tests - Gaggias came out as stable as most machines
(more stable than the Silvia), though were somwhat downrated for some other
reasons.
The complaints about the Gaggia boilers are (a) an irrational fear of
aluminum, and (b) real concerns about electrochemical corrosion due to the
bimetallic construction.

Yes, galvanic corrosion.
Craig.

Post by D. Ross
- David R.
--
http://www.demitasse.net

[Jack Denver]

Adding a hunk of brass to the outside would increase bimetal corrosion.
Aluminum is more galvanically active than copper but less than zinc so I
don't know which would get sacrificed.

Gaggia's thermal stability may be related to the relatively large heating
element which must permit some of the incoming water to be heated "on the
fly" . Aluminum also has very good thermal conductivity so it can pass the
heat (both that retained in the aluminum and the fresh heat that the element
generates during the shot) to the water quickly - not as good as copper but
better than brass (which is why you don't see many brass cooking pots).

I get the feeling that thermal instability could be minimized thru
manipulation of the steam switch. By manually turning on the steam switch as
the shot begins (or perhaps even a few seconds before to account for lag)
you could give the boiler a better chance of keeping up than if you rely on
the click thermostat. Even the most sophisticated PID cannot know in advance
that a shot will be starting soon, so it's time to anticipate by cranking up
the element, but a human can.

Post by D. Ross
| I've been doing a little reading on the shortcomings of my Classic,
| and I wanted to put this idea out to the group... Obviously, being
| made of aluminum, the boiler doesn't hold heat like a brass boiler
| would, so why not strap blocks of brass or copper or somesuch to the
| sides of the boiler?
Heat retention isn't a problem with the Gaggia boiler; the integral elements
keep the boiler plenty hot.
Some have speculated that there might be a thermal instability problem
because of the boiler's small size when compared to the shot size. In
practice this is not a problem, as evidenced by the Norwegian specialty
coffee association's tests - Gaggias came out as stable as most machines
(more stable than the Silvia), though were somwhat downrated for some other
reasons.
The complaints about the Gaggia boilers are (a) an irrational fear of
aluminum, and (b) real concerns about electrochemical corrosion due to the
bimetallic construction.
- David R.
--
http://www.demitasse.net

[Francis Vaughan]

Brass has a specific heat of 0.377 kJ/kg.K
Aluminium has a specific heat capacity of 0.91 kJ/kg.K

So per unit weight aluminium actually has 2.4 times the heat capacity.

However we must then factor in the density.

Aluminium has a density of about 2600 kg/m^3
Brass has a density of about 8500 kg/m^3

Brass is 2.6 times as dense as Aluminium.

So, for the same volume of metal, brass has only 8% greater heat
capacity.

Brass is in general stronger, so we probably end up using less metal in
a boiler.
Overall there is really no difference. On the other hand, insulating
the boiler could make a huge difference, at least to the losses.
Stability during the shot is mostly dependant upon mixing effects with
the cold water entering. Here a bigger boiler seems to make the most
difference. Although adding a lot of mass may help a bit - you still
need to get the energy from the metal into the water quickly. The
group's mass probably makes the biggest difference here.

Also, as a rough approximation water has the same heat capacity per
unit volume as these metals. So making the boiler bigger would have
the same effect as adding metal. Again, bigger boilers win.

[D. Ross]

| Also, as a rough approximation water has the same heat capacity per
| unit volume as these metals. So making the boiler bigger would have
| the same effect as adding metal. Again, bigger boilers win.

Except they don't. In the tests done by the Norwegian specialty coffee
association, the within-shot temp stability of the Gaggia Classic was as
good or better than that of other machines with much larger boilers,
including the Sylvia and the Zaffiro.

- David R.
--
Less information than you ever thought possible:
http://www.demitasse.net

[Cordovero]

Post by D. Ross
Except they don't. In the tests done by the Norwegian specialty coffee
association, the within-shot temp stability of the Gaggia Classic was as
good or better than that of other machines with much larger boilers,
including the Sylvia and the Zaffiro.

It really is a superb design. The boiler is also very capable of quickly
getting to steam temp.

C

[Jack Denver]

I've never done any such testing myself, but I seem to recall that others
have reported that within shot stability of Silvia is excellent, within a
couple of degrees for the 1st couple of ounces. I'd be curious to see for
example what kind of shot volumes or flow rates the Norwegians were working
with.

In a single boiler, at some point the incoming cold water is going to mix
with the stored hot water and the temp is going to drop, but if the design
is good, this will happen at a point that is beyond the normal shot size.

Also keep in mind that some authorities (Illy) think that it is actually
beneficial for the water temp to drop as extraction goes on, so that
absolute stability may not be the correct goal (Schomer notwithstanding).

Post by D. Ross
| Also, as a rough approximation water has the same heat capacity per
| unit volume as these metals. So making the boiler bigger would have
| the same effect as adding metal. Again, bigger boilers win.
Except they don't. In the tests done by the Norwegian specialty coffee
association, the within-shot temp stability of the Gaggia Classic was as
good or better than that of other machines with much larger boilers,
including the Sylvia and the Zaffiro.
- David R.
--
http://www.demitasse.net

[D. Ross]

"Jack Denver" <***@netscape.net> wrote:

| I've never done any such testing myself, but I seem to recall that others
| have reported that within shot stability of Silvia is excellent, within a
| couple of degrees for the 1st couple of ounces. I'd be curious to see for
| example what kind of shot volumes or flow rates the Norwegians were working
| with.

Well, to be honest some of the Norwegian results struck me as somewhat
strange. However, as you know for years and years people on this group have
made comments questioning the Gaggia thermal stability, based on no actual
evidence but rather on a gedanken-conviction that the small boiler must be
problematic. Usually the comments were made by Silvia owners engaged in
hypercompetitive machine advocacy. In the absence of actual evidence, such
comments are purely speculative, and if nothing else the Norwegian results
put the burden of proof on those who are denigrating the Gaggias.
Unfortunately, these comments live forever in the Google archives and the
group mythology, as witness the original poster's assumptions about his own
machine.

- David R.


|
| In a single boiler, at some point the incoming cold water is going to mix
| with the stored hot water and the temp is going to drop, but if the design
| is good, this will happen at a point that is beyond the normal shot size.
|
| Also keep in mind that some authorities (Illy) think that it is actually
| beneficial for the water temp to drop as extraction goes on, so that
| absolute stability may not be the correct goal (Schomer notwithstanding).
|
|
| "D. Ross" <***@math.hawaii.NOSPAM.edu> wrote in message
| news:***@localhost...
| >| Also, as a rough approximation water has the same heat capacity per
| > | unit volume as these metals. So making the boiler bigger would have
| > | the same effect as adding metal. Again, bigger boilers win.
| >
| > Except they don't. In the tests done by the Norwegian specialty coffee
| > association, the within-shot temp stability of the Gaggia Classic was as
| > good or better than that of other machines with much larger boilers,
| > including the Sylvia and the Zaffiro.
| >
| > - David R.
| > --
| > Less information than you ever thought possible:
| > http://www.demitasse.net
|
|

--
Less information than you ever thought possible:
http://www.demitasse.net

[Mark Stewart]

Let me start by saying I'm the original poster. Sorry for the change in
names... I was using google and for some reason it won't let me reply. It
got me to finally set up my usenet on Outlook, if nothing else. Anyways-


Caught in my own trap. Yes, I'm going by the hearsay which I'm certain has a
lot of Silvia oneupsmanship driving it. I was approaching this as a
theoretical question rather than any personal problem I've had with my
machine.

I'm of the viewpoint that consistancy is the key to a learning to make a
good shot. Along those lines having a consistant temperature throughout the
pull would seem sensible, but Jack Denver brought up the point that (quoted)
"some authorities (Illy) think that it is actually
beneficial for the water temp to drop as extraction goes on, so that
absolute stability may not be the correct goal "

So, taking that viewpoint into consideration, as long as the drop in temp is
consistant shot to shot, it shouldn't make a difference.

Again... all this is at theoretical level. Brass or aluminum boilers, temp
drop or no, wil be of negligible difference compared to good beans, freshly
roasted, properly ground, properly measured and tamped.

Back to theory.

Francis Vaughan got all periodical on me (very good post, Francis, btw) so
I did some reseach and decided copper rather than brass would be the proper
material. It has both higher density than brass and higher specific heat
than aluminum. The only negative thing I see is that having a higher
specific heat might mean it could cool the boiler by leaching heat away from
it.

I think what I need to do is a couple of tests to see whether there is a
noticable difference in temperature between the start and finish of the
shot. If I can determine that I'll strap some copper material to the boiler
and see whether there is a change in that temperature differential. Finally
I'll see whether I can notice a difference in the taste or in the
consistancy of the shots I pull.

This will take awhile, as I'm currently waiting for a thermal fuse for my
Classic, which was the reason I started lurking around alt.coffee to begin
with. Once I'm up and running again and accustomed to my new used grinder
I'll post my findings.

Thanks to everyone who posted a reply thus far.

Mark.

Post by D. Ross
| I've never done any such testing myself, but I seem to recall that others
| have reported that within shot stability of Silvia is excellent, within a
| couple of degrees for the 1st couple of ounces. I'd be curious to see for
| example what kind of shot volumes or flow rates the Norwegians were working
| with.
Well, to be honest some of the Norwegian results struck me as somewhat
strange. However, as you know for years and years people on this group have
made comments questioning the Gaggia thermal stability, based on no actual
evidence but rather on a gedanken-conviction that the small boiler must be
problematic. Usually the comments were made by Silvia owners engaged in
hypercompetitive machine advocacy. In the absence of actual evidence, such
comments are purely speculative, and if nothing else the Norwegian results
put the burden of proof on those who are denigrating the Gaggias.
Unfortunately, these comments live forever in the Google archives and the
group mythology, as witness the original poster's assumptions about his own
machine.
- David R.
|
| In a single boiler, at some point the incoming cold water is going to mix
| with the stored hot water and the temp is going to drop, but if the design
| is good, this will happen at a point that is beyond the normal shot size.
|
| Also keep in mind that some authorities (Illy) think that it is actually
| beneficial for the water temp to drop as extraction goes on, so that
| absolute stability may not be the correct goal (Schomer
notwithstanding).
|
|
| >| Also, as a rough approximation water has the same heat capacity per
| > | unit volume as these metals. So making the boiler bigger would have
| > | the same effect as adding metal. Again, bigger boilers win.
| >
| > Except they don't. In the tests done by the Norwegian specialty coffee
| > association, the within-shot temp stability of the Gaggia Classic was as
| > good or better than that of other machines with much larger boilers,
| > including the Sylvia and the Zaffiro.
| >
| > - David R.
| > --
| > http://www.demitasse.net
|
|
--
http://www.demitasse.net

[Francis Vaughan]

The only negative thing I see is that having a higher specific heat

might mean it could cool the boiler by leaching heat away from it.

This shouldn't actually happen. For two reasons. One: specific heat
isn't the same thing as thermal conductivity. So there is no actual
correlation between the two. That said, copper does of course have
quite a good thermal conductivity - but that is coincidence. So,
reason two: The drawing of heat away can only happen when the object
drawing the heat away is cooler. The whole point of this exercise is
to get it as hot, or perhaps even hotter. The only source of heat loss
is the environs of the boiler. The rate of heat loss to that will be
pretty much the same no matter what the boiler material. It is
dominated by convection effects, and a to a lesser effect radiation is
important. Convection is essentially dictated by the physical layout,
and radiation by the emissivity of both the boiler and the inside of
the case. Mirror polised aluminium is unbeatable for emissivity (for
about the first day of use :-| ) Insulating the boiler remains the
proverbial good idea.

Having a good conductor of heat will be good since it can get the
energy to the cooler water faster. Sadly water itself is a rotten
conductor (unless it is physical movement of the water you use to move
the heat energy about.)

[Jack Denver]

You are starting out with a mass that is hotter than either the incoming
water or the air so it will lose heat to both sides and it is a sort of a
race. At some point if the air side is better cooled, the mass will become
cooler than the water and it will start drawing heat from the water instead
of heating it. This has a lot to do with the shape of what you are attaching
to the boiler as well as the temperatures on both side. If you were to
attach something that had a lot of surface area in the open air , it would
lose heat to the atmosphere more quickly. Insulating the air side would
tend to prevent this. .

What we call "radiators" or "heat sinks" are really heat exchangers - the
heat will travel in either direction, depending on which is the hotter side.
The material does make a difference- this is why heat sinks on computers and
radiators on cars are made of some very conductive metal (copper or alu).
OTOH, here using something with poor conductivity (wood) the heat would
transfer poorly not only to the air side but also poorly back to the water
side so you'd get little effect in either direction. The degree of contact
between the added mass and the boiler also has an effect on the heat
transfer back to the water - given the convoluted shape of the boiler it
will be hard to have much contact unless you create a mold. Using thermal
grease between the copper and the aluminum would also help thermal tranfer
as even a small air gap will interfere. As an aside, materials that are
good electrical conductors tend to be good thermal conductors and vice
versa.

Given all the problems above, you'll get more bang for your buck just from
insulation (not to mention that copper will cause aluminum to become the
sacrificial metal).

Post by Francis Vaughan

The only negative thing I see is that having a higher specific heat

might mean it could cool the boiler by leaching heat away from it.
This shouldn't actually happen. For two reasons. One: specific heat
isn't the same thing as thermal conductivity. So there is no actual
correlation between the two. That said, copper does of course have
quite a good thermal conductivity - but that is coincidence. So,
reason two: The drawing of heat away can only happen when the object
drawing the heat away is cooler. The whole point of this exercise is
to get it as hot, or perhaps even hotter. The only source of heat loss
is the environs of the boiler. The rate of heat loss to that will be
pretty much the same no matter what the boiler material. It is
dominated by convection effects, and a to a lesser effect radiation is
important. Convection is essentially dictated by the physical layout,
and radiation by the emissivity of both the boiler and the inside of
the case. Mirror polised aluminium is unbeatable for emissivity (for
about the first day of use :-| ) Insulating the boiler remains the
proverbial good idea.
Having a good conductor of heat will be good since it can get the
energy to the cooler water faster. Sadly water itself is a rotten
conductor (unless it is physical movement of the water you use to move
the heat energy about.)

[hudson]

Post by Jack Denver
Given all the problems above, you'll get more bang for your buck just from
insulation (not to mention that copper will cause aluminum to become the
sacrificial metal).

Won't that stop the cup warmer from working? I know that you can warm them
up with water first but it all seems to work very well at the moment without
altering anything. Could this also make the thermal fuse more prone to
failing?

Best Regards,

Stuart Hudson
(happy classic owner)

[Francis Vaughan]

I would imagine that this is due to poor mixing of the input water
dominating. If the inlet water happily jets across the boiler and sits
in the vicinity of the outlet, the stability will be really
compromised. Large boilers in general tend to stratify, and the inlet
water doesn't mix with the hotter water - which we hope is near the
outlet. However when I say larger, I'm talking about household hot
water systems. For an espresso machine we have a really small volume,
and the mixing effects are going to be pretty evil to model. Short of
a mixed mode computation fluid dynamics model it is going to be pretty
hard to predict what a particular small boiler is going to do. In that
respect long term empirical experience of the designer is probably the
best answer.

Drawing the conclusion that the stability effect was due to the size of
the boiler is the cited tests is drawing a very long bow. It could
just as easily have been due the the height, height to width ratio,
location of feed points, diameter of feed pipe, location of heating
element in the boiler - something which sets the Gaggia (with it's
external heating elements) apart from the others. So why did they not
conclude that an external heating element was the crucial difference?
Just deciding it was the volume of the boiler is totally bogus science.

There are quite a number of very broad assumptions in the argument I
made above - mostly about the rate of heat conduction (or equivalently
mixing of water.) I was addressing the issue of brass versus
aluminium. After that it gets really messy.

[j***@gmail.com]

I have a Gaggia Coffee, which I believe uses the same boiler.

I observe about a 20 degree drop throughout the shot, but I reasoned
that since I have PID control, I always get a reproducible starting
point temperature, and I am quite happy with my shots.

I have some experience with Norwegians, and I would conclude they had
too much caffeine before testing the Gaggia.

Jan

P.S. I calculated that the Gaggia heating element (1500-2000W) has
sufficcient power to replenish all the heat that is lost during a shot:

80degC*60g=4800cal (assumes heating from Room temperature to boiling,
and 2oz espresso volume)
4800cal/24sec=200cal/s=840J/s=840W

[Jack Denver]

PID's work best under steady conditions. In the case of the extreme drop
that is experienced when you pump cold water into the boiler, the finesse
of a PID works to your disadvantage because what you want to be doing is
running the element full blast to try to make up for the massive heat loss.
I'd suggest that you turn on the steam switch as soon as you start your shot
or even a few seconds before and see if you get less of a drop that way.

Post by j***@gmail.com
I have a Gaggia Coffee, which I believe uses the same boiler.
I observe about a 20 degree drop throughout the shot, but I reasoned
that since I have PID control, I always get a reproducible starting
point temperature, and I am quite happy with my shots.
I have some experience with Norwegians, and I would conclude they had
too much caffeine before testing the Gaggia.
Jan
P.S. I calculated that the Gaggia heating element (1500-2000W) has
80degC*60g=4800cal (assumes heating from Room temperature to boiling,
and 2oz espresso volume)
4800cal/24sec=200cal/s=840J/s=840W

[dadvocate]

Post by Jack Denver
PID's work best under steady conditions. In the case of the extreme drop
that is experienced when you pump cold water into the boiler, the finesse
of a PID works to your disadvantage because what you want to be doing is
running the element full blast to try to make up for the massive heat loss.
I'd suggest that you turn on the steam switch as soon as you start your shot
or even a few seconds before and see if you get less of a drop that way.

You cannot do that with a Gaggia solenoid machine, like the Classic, as
switching to steam mode cuts the solenoid over so the shot is stopped
and the machine pumps through the OPV instead.

[Francis Vaughan]

Post by Jack Denver
PID's work best under steady conditions. In the case of the extreme

drop that is experienced when you pump cold water into the boiler, the
finesse of a PID works to your disadvantage

Sort of. For the manner in which a typical PID is tuned for a Sylvia
this is exactly true. But you could tune the PID to be very aggressive
at keeping the water at a more constant temperature during the brew.
Turn the D term way up. Downside is that you pretty much guarantee
that it will be much less stable under quiescent conditions. It would
be interesting for someone to auto-tune their Sylvia PID whilst making
shots, not as a steady state. I'm not actually sure it would manage to
auto-tune, but it would be interesting to see what happens.

This strikes to the heart of my criticism of PID control for small
boiler machines. In principle we have more knowledge than the standard
PID control allows us to use. The PID essentially tries to second
guess the onset of a destabilising event (through the D term.) But we
know a priori that said event is happening - we have hit the brew
switch. We know what the injection rate of water is (the golden rule)
and so knowing the temperature of the input water we know exactly what
the energy replacement required is. We could simply disable the PID
during brew and switch over to a static pulse width controlled powering
of the element to exactly replenish the energy.

There are two (or maybe 2.5) flaws in this. There is a lag time whilst
the heater gets up to temperature. This could be coped with by a delay
before the pump is started (more control electronics :-( ) and most
importantly, it assumes homogeneous water temperature in the boiler.
Which we know we don't have. This can be fixed with active stirring.
Which can be done (and I am about 1/4 the way through designing.) A
half a flaw is that the golden rule for flow rate assumes a single size
of shot - so we would need some extra input (say a switch) to change
from a single to a double basket.)

[j***@gmail.com]

I agree, the pulling of an espresso shot is a very reproducible event,
so it might be better to program the heater for the shot, than
controlling it.

If you look at Andy S.'s homepage, I believe he has a copper tube with
intake water wrapped around the boiler, that way, the heat loss of the
shot gets distributed over a longer time.

This is something which would be more helpful for a Gaggia than a
Silvia, but it is very tight in there.

[Francis Vaughan]

Post by j***@gmail.com
If you look at Andy S.'s homepage, I believe he has a copper tube with
intake water wrapped around the boiler, that way, the heat loss of the
shot gets distributed over a longer time.

The copper tube is an intersting idea, and one that should help a great
deal. One way of thinking about it is that it has increased the boiler
size, and enforced stratification of the input water and the remaining
boiler water. However there may be some issues. The copper tube to
brass boiler interface is very conductive. There is a large surface
area of water in the tube, so the rate of energy transport into the
cold water will likey be reasonably high - and will come, in part, from
the hot water touching the inside of the boiler. Interestingly,
properly tunned this might turn out to be a very stable way of creating
a drooping temperature profile. One could also tune the heat movement
with the addition of thin layers of a less conductive material between
the boiler and the tubing. This must be balanced with recovery time.

In general the idea of enforced stratification is likely very useful.
In a small boiler we don't have the scale that allows this to happen
naturally, as it might in a larger one. So the introduction of various
long tubes, baffles and the like may make for much more control. In
particular the removal of turbulent mixing, indeed possibly any sort of
direct mixing of hold and cold water, and relacing it with simple
conduction through layers of tube and water will yield a system that is
amenable to simple analysis. Rather than a mixed mode CFD problem it
can be modelled with tools as simple as SPICE. Once this is done we
can even include formal modelling of the PID parameters - or at least
simulation and itterative optimisation.

Curiously we end up with what might be considered as a glorified
thermoblock. But one with very exactly constrained and optimised
performance.

[Andy Schecter]

Post by Francis Vaughan
The copper tube is an intersting idea, and one that should help a great
deal. One way of thinking about it is that it has increased the boiler
size, and enforced stratification of the input water and the remaining
boiler water. However there may be some issues. The copper tube to
brass boiler interface is very conductive. There is a large surface
area of water in the tube, so the rate of energy transport into the
cold water will likey be reasonably high - and will come, in part, from
the hot water touching the inside of the boiler. Interestingly,
properly tunned this might turn out to be a very stable way of creating
a drooping temperature profile. One could also tune the heat movement
with the addition of thin layers of a less conductive material between
the boiler and the tubing. This must be balanced with recovery time.

Francis, I think your observations are spot on. I'm not all that happy with
the tube around the boiler thing. The tiny Silvia boiler still doesn't provide
very good stability. If the copper tube hadn't been such a hassle to install,
I probably would have taken it off by now! :-)

[Francis Vaughan]

I'm not all that happy with the tube around the boiler thing.

Hmmm, I'm a bit sad to hear that. I had been toying with something
similar as an idea, but was beginning to wonder how it was performing
in practice.

The tiny Silvia boiler still doesn't provide very good stability.

Any chance of some numbers? Before and after coil would be brilliant.

If the copper tube hadn't been such a hassle to install, I probably

would have taken it off by now! :-)

Don't you hate things like that? I have a few of those sorts of
efforts about. Mostly from my messing with HiFi days.

A couple of thoughts that might mitigate some of the effort. You could
leave it in place, fill the tube with water and seal it up. Would act
as an additional thermal mass. But taking up where I left off, if we
had a good idea of the thermal conductivity from the boiler water into
the tubing you could look at the idea of inserting some thin layer of
less conductive (I hesitate to say insulating - just more resistive)
material between the coil and the boiler. With a bit of luck it might
be possible to get the best of both worlds. Or go the whole hog and
put a sub reseviour inside the boiler that keeps the brew water away
from the rest of the water mass. Inside the little Sylvia this might
be a be bit squeezy.

[Andy Schecter]

Post by j***@gmail.com
P.S. I calculated that the Gaggia heating element (1500-2000W) has
80degC*60g=4800cal (assumes heating from Room temperature to boiling,
and 2oz espresso volume)
4800cal/24sec=200cal/s=840J/s=840W

Don't forget that the puck absorbs almost as much water as it weighs
initially. In other words, a puck containing 18 grams of coffee will absorb
about 16 grams of water. This water, of course, counts in your heating
calculations.

[j***@gmail.com]

Thanks, I did not think about that amount of water.
The other factor which I thought about, but didn't include in the
calculation is the amount of heat which gets absorbed by the boiler,
but not delivered to the water during the 25 seconds of the shot.

[Randy G.]

Great stuff that, Francis,

What about the fatigue factor of aluminum vs. brass from the repeated
heat/cooling cycles, as well as their comparative
expansion/contraction factors? Probably not a factor with espresso
machines becasue the heating element is likely to fail or the machne
will end up in the closet before the boiler cracks (in home use,
anyway).


Randy "trying to sound smart" G.
http://www.quiknet.com/~frcn/Coffee/Coffee.html

Post by Francis Vaughan
Brass has a specific heat of 0.377 kJ/kg.K
Aluminium has a specific heat capacity of 0.91 kJ/kg.K
So per unit weight aluminium actually has 2.4 times the heat capacity.
However we must then factor in the density.
Aluminium has a density of about 2600 kg/m^3
Brass has a density of about 8500 kg/m^3
Brass is 2.6 times as dense as Aluminium.
So, for the same volume of metal, brass has only 8% greater heat
capacity.
Brass is in general stronger, so we probably end up using less metal in
a boiler.
Overall there is really no difference. On the other hand, insulating
the boiler could make a huge difference, at least to the losses.
Stability during the shot is mostly dependant upon mixing effects with
the cold water entering. Here a bigger boiler seems to make the most
difference. Although adding a lot of mass may help a bit - you still
need to get the energy from the metal into the water quickly. The
group's mass probably makes the biggest difference here.
Also, as a rough approximation water has the same heat capacity per
unit volume as these metals. So making the boiler bigger would have
the same effect as adding metal. Again, bigger boilers win.

[Cordovero]

The only shortcoming of the aluminum boiler in real practice for a home
machine is the possibility of corrosion. If you're willing to sacrifice
some of the flavor of minerals, purified water does extremely well.

C

[DAVE4830]

I echo the (scientific) thoughts of the group. with my own experiences,
having owned 3 Gaggias over the years, the only minor concern is for
corrosion. That's why brass not aluminum hardware on boats! Use soft
water and be gentle with the descaling process. and -- Don't leave any
water in the boiler if you need to store the unit for an extended
period.

cheers

dave
PID guy in the South
910 616 0980