OE 3rd Shot 49mm Lever Group Heat Sink
To introduce the concept of a group heat sink for the small group home lever machines...Olympia Cremina, La Pavoni EPC-8 and EPC-16, and Elektra MCAL we first must make some assumptions about the original design intent for these machines. We can only make certain assumptions of course, but these machines were, and are, produced by companies with a long history of commercial espresso machine design and the home machine designs are an extension of the commercial products, but in our opinion, are modified to meet consumer preferences for home use. The first obvious design point is the size of the machine. Second would be ease of use in a practical home environment, meaning that the machine should heat up quickly and be ready for use in the shortest amount of time possible. Other features such as steam capability and user friendly touches are considerations as well, but our heat sink idea stems from thinking about the first two design elements....size and efficiency for home use.
The size of the machine, and therefore the group, means that the groups on these home machines are fairly small, not in relation to the size of the boiler or case, but small in a thermodynamic sense. The massive group of a commercial lever machine makes heat management a fairly simple task...the mass of the group means that the temperature of the group is generally stable and constant once the machine is at steady state thermodynamically...it takes a lot of hot water to raise the temperature of the brass group and the group itself, once heated sheds a lot of heat to the air around it...the air acts as the ultimate heat sink in this case due to the large surface area of the group.
The smaller size of the home lever group means that the group is much more unstable and subject to rapid temperature increases when exposed to boiler heated water either on a plain water flush or when pulling a shot. The temperature of the group increases with each shot and the rate of cooling in the air around the group is slower, again due to the small size and mass of the group. This means that on successive shots the group tends to overheat...a relative term, but best put, it becomes too hot and the shots become bitter or burned.
So, the argument goes, that these machines have small boilers and are meant to only pull one or two shots before cooling the machine down to refill the boiler...a sort of internal control of the system...but a .8L boiler, such as on the earliest Olympia Cremina machines is capable of producing far more than just 2 shots...keep in mind that the home appliance nature of these machines may be interpreted that you get 2 shots of coffee before work and do not need any more, or you and your partner each get 1!! But we all know that the actual user of any appliance does not always fall into the demographic of the designer of that machine...some of us want THREE!!
Now consider the second point....fast warmup time for home use. These small machines have very small boilers and heating elements running at 800 to 1000 watts. The boiler size to wattage ratio is much higher on these machines than a commercial machine (in fact some commercial machines have 4 to 6 liter boilers and run at 1250 watts) so in a sense these small machines have what would almost be considered "flash boilers" for a very fast warmup time.
The new Elektra MCAL, the Cremina and the La Pavoni machines all have extensive metal to metal contact of the group to the boiler group flange, and are sealed by a simple O-ring. This means that the maximum amount of heat is transferred to the group to allow the fastest time for not only boiler warmup but group heating as well to be ready to pull a shot in the shortest amount of time. But again, the problem, first shot temperature good, second shot perhaps bordering on a bit hot, third shot, too hot...this goes back to group size once again.
We have experimented with different group to boiler seals to provide a thermal break at the group to boiler junction (teflon, silicone, rubber, compressed paper) and in each case found that a thermal break incorporated here caused the group warmup time to suffer dramatically and made demands that the user now, instead of turning on the machine, jumping in the shower, coming back and pulling a shot before zipping out the door to work, would have to make use of a number of warming flushes to get the group up to temperature...but how many flushes? Enough to deplete the water in the small boiler? Too many make the group again too hot even before that first shot....too much variation here and remember, this is a ANALOG machine and that is a central reason why so many people love the machine....it is not digital and has no monitoring features but a pilot light and possibly a manometer...the machine should be as the manufacturer designed it. Fast heat, fast warmup time, without a lot of fiddling.
The OE 3rd Shot
Our goal for the 3rd Shot Heat Sink is implied in the name...to enable us to pull that 3rd shot without experiencing overheating of the group and that dreaded burned shot. People have reported some success in group heat management by use of a wet cooling towel or cooling the group in a bowl of water, or locking in a cold portafilter. We wanted to design a device that:
- allows controlled group cooling that could fit into a user's general shot pulling routine without a lot of fuss.
- does not require modifying the machine from its original design (possibly creating a whole new set of issues).
- can be used without additional probes, wires, sensors, or clutter on the machine or counter top.
- is reliable and logical with no moving parts
The result is the OE 3rd Shot!! Very simple to use at any point of the machine heat cycle, just lock it in the group and it does all the work for you. The 3rd Shot will reduce the temperature of the group by about 12 degrees F overall from the first lock in.
Working example 1: Using a 1992 Stock Olympia Cremina with K themocouple attached to the group running at steady state temperature (about 1/2 hour of warmup time = group temperature of about 194F, room temperature = 70F pstat set for machine to run at 1 bar off / .8 bar on). 3rd shot at room temperature. Lock in the 3rd shot and temp reduced by 5 degrees F in first minute, 5 degrees more in second minute, rate of heat transfer slows (in accordance with Fourier's Law of Heat Conduction) at this point as the 3rd Shot gets warmer and the system stabilizes at about 182 F. Upon removing the 3rd Shot the group temperature remains stable at this temperature point before slowly rising back to it's steady state....lots of time to pull the shot.
Working Example 2: Same Cremina under same conditions but with 3rd Shot cooled in running cold tap water. 8 degree F heat drop in first minute followed by 4 degree drop in second, but again stabilizing at approximately this 12 degree drop in temperature from first lock in of the 3rd Shot.
To see the OE 3rd Shot in action click here: OE 3rd Shot
A general reading of Fourier's Law is that the rate of heat transfer from one body to another is in direct proportion to the heat differential between the two bodies, times the cross sectional area of contact. Since the cross sectional area is constant, this basically simplifies to the greater the temperature difference between the group & the 3rd Shot the faster the heat will move from the group to the 3rd Shot. There is less heat sink effect when the group is cooler, and the 3rd Shot is warmer (a smaller differential between the two bodies). A greater rate of heat transfer occurs when the group is hotter, and the 3rd Shot is colder (cooled in water, freezer, etc.) . Your mileage may vary but we are finding that the 3rd Shot gives a very predictable heat sink action...a timer may help fine tune this but is not essential.
From practical experience we find that using this simple device as part of our usual espresso prep routine allows good temperature management control and as the name implies, gives us that great 3rd (and 4th and 5th) shot!
|
