The Slim Jim is equipped with multiple boilers: one large stainless-steel steam boiler to produce hot water and steam, one stainless steel boiler per group to brew coffee. A heat-exchanger with balanced cross-flow and an electronically controlled cooling circuit ensures
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well-conditioned supply of pre-heated water for the coffee boilers to ensure enormous capacity in a relatively compact machine. Each boiler has its own electronic temperature control system, heating element and safety devices, offering extremely precise temperature stability at high volume.
Incorporated in the steam boiler is a very large heat exchanger (HX) that is part of a thermo-siphon loop. At the top of the thermo-siphon loop water is fed from the HX manifold block towards the individual groups. Cross-flow at the feed-side of the HX conditions the temperature of the water flowing to the manifold block. If for any reason the HX manifold block gets too hot the necessary amount of cold water is added in the flow towards the individual coffee boilers.
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The coffee boilers are made from a 4mm stainless steel tube and a 10mm thick flange. The group head is made in 5mm stainless steel and is welded to the boiler. All welding is done both on the outside for looks as well as on the inside to prevent possible crevices in which corrosion is known to start. All stainless is 316L. Boiler and group head together contain approximately 0.8 litre of water.
The boiler slopes slightly to allow for a proper constant thermo-siphon heat flow inside, reaching the full length and height of the boiler and group head. Both the top and the bottom of the group head are removable, sealed by O-rings.
Cold water entering the machine is first directed through a heat-exchanger located inside the steam boiler. Exiting the heat-exchanger its temperature is higher than needed because the steam boiler runs at 125 °C. A restrictor in the circuit makes that the water in the distribution block is very close to the temperature of the coffee boilers. This way the distribution block is kept warm during idle of the machine.
Temperatureinside the distribution block is monitored with a sensor. If by any chance the water in the distribution block is too hot some cold water will be added in the flow to the individual coffee boiler when a group is activated. The water thus conditioned will have only a minor deviance from the (pre-set) temperature of the group.
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Inside the coffee boiler, the temperature of the water is measured and controlled by a highly sensitive probe connected to a PID (proportional-integral-derivative) controller. This is a control loop feed-back mechanism. A nearly intelligent system, learning from the past, responding ahead, steering the 450 Watt heating element by pulses through a solid-state relay. There is no moving part in the entire heating and controlling system. This set-up ensures an extremely fast reacting system as well as a very reliable one. The water temperature can easily be changed in the Barista menu of the interface.
The controller offers an ECO mode, lowering the temperature of an individual coffee-boiler to 50 °C during longer periods of idle.
At first installation it is important all air is bled from the coffee boiler. Air can be compressed, water cannot. When the group lever activates the group solenoid valve and pump for the first time, all air is automatically expelled by the inrush of water because the pick-up tube is located directly under the group`s top which is the highest point in the system. When the heating system reaches and maintains the perfect desired temperature, the challenge this system poses is to not suffer any variation in temperature until the water hits the coffee grinds.
Law of nature demands that thermo-syphon flow will commence when changes in water temperature are present. Water heated by the heating element will go up. In the coffee system the warmer water will flow up to the very end of the group head where it will cool slightly and travel downwards again to the bottom of the boiler. The boiler and group are specifically designed to offer ample space allowing the warmer and cooler flows to pass each other. The “sweet spot”, the most stable temperature in this system, will develop at the highest point, right underneath the group top. The pick-up tube is positioned right in that sweet spot, running to the 3-way group solenoid valve. This valve is installed at the lower end of the boiler. A position as hot as possible. The valve itself is made from stainless steel.
When brewing, the water never leaves the coffee water boiler/group on its path towards the coffee. It runs via a tube inside the group, starting from directly under the group top, to the stainless steel three-way solenoid valve. Stainless transfers heat ten times less than brass, so is far more isolating. From the solenoid valve it is routed through another tube towards the bottom of the group. Both these tubes run parallel in the higher part of the boiler. This path makes for an extra temperature stabilizing effect.
The stainless-steel bottom plate of the group is at a low position. This causes its temperature to be below the temperature of the water. Water passing the bottom on its way to the coffee grounds, should not lose any of its heat to the metal bottom.
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Therefore, the dispersion block reaches up through the bottom plate, so the water enters the dispersion block directly without contact with the group bottom. This block is made from polymer plastic, a nearly temperature-neutral material. As this block remains cool, the fats and oils in the coffee do not bake on, effectively keeping it clean for a long time. This block consists of two pieces offering a nicely divided dispersion of the water.
The group screen is hold in place by the surrounding group gasket. To pull out the gasket with screen for controlling, cleaning or replacing, a special tool is provided with each machine in the Barista Kit. The gasket ring sits against the group bottom that has a small outward ridge. This ridge is pushed into the gasket each time the filter holder is locked into the group, thus providing excellent sealing. The slight moving up of the gasket ring, because of the flexibility of the material, provides an ergonomically correct, soft comfortable feel for the barista. Full sealing is achieved without having to pull the filter holder firmly. The group gasket is made in a heat resistant material that does not harden and go brittle but remains flexible.
The group screen’s circumference is pushed against the bottom of the group making the screen robust. Together with the gasket surrounding the screen, it is impossible for coffee grounds to ever reach the insides of the group. The supporting horizontal base of the screen is thick to resist deformation. This base is perforated and covered with a stainless-steel membrane, not a woven mesh. This membrane provides a very even dispersion of the water during extraction. It also strongly filters the water sucked up when the pressure is discharged at the end of the shot. This again ensures a cleaner inside of the group.
The importance of pre-infusion still seems to remain under-appreciated. On machines offering a very confined brew temperature a well-executed pre-infusion becomes imperative: without they will not be able to extract the full range of complexities hidden in the coffee. Properly infusing the grounds makes for uniform wetting and swelling of the grounds which in turn reduces the danger of channelling while also locking in fines, thus preventing them from travelling downwards and clogging the filter.
Even more importantly however, pre-infusion allows a tranquil warming and softening of the fats and oils in the coffee grounds before extracting these with high pressure. This clearly results in a higher yield of solids in the cup. These fats and oils are the actual carriers of the delicate aromas and flavours. A good infusion system allows for a finer grind with larger surface area making it easier to wash out the oils and fats.
When the brew process is started by moving the lever down, the group solenoid valve is activated and the pump starts to run. A 0.6 mm restrictor is located at the supply side of this 3-way valve reducing the maximum flow-rate through the valve. After passing the group-valve the tubing splits in two: one line feeds water onto the coffee-bed, another line feeds water into the pre-infusion cylinder.
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Movement of the piston of the pre-infusion cylinder is restrained by a double spring gradually increasing load while being compressed. At the same time pressure onto the coffee bed gradually increases. At about 4.5 bar the piston has reached it total travel and from that moment on brew pressure rises quickly until pump-pressure is reached. The pressure gauge is connected to the pre-infusion cylinder which means that there is an open connection all the way to the coffee puck. The gauge displays the actual extraction pressure, not just the pump pressure. The barista can monitor exactly how infusion and extraction pressure develops.
When the group-valve is de-activated at the end of a brew, pressure on the coffee bed is immediately released as flow in the valve reverses and water in the pre-infusion cylinder is pushed through the group-valve into the machine’s drain thus flushing possible very fine coffee grinds out of the valve after each brew.
