WHAT DOES A “HEAT EXCHANGER” IN A COFFEE MACHINE MEAN?

The various machines (single boiler, HX, Lever groups, pressure profiling etc..), hints tips, problems issues
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Claudette
Posts: 21
Joined: Tue Aug 20, 2019 3:20 pm

The quick answer...... is basically its one boiler, with one element can produce the two different temperatures required for making a coffee. The lower of the temperatures is for the brewing, and the higher is for the steaming and hot water. The element is used to heat water in the boiler and the water in an inner pipe which supplies the water to the group head. It means that you can brew and steam at the same time.
The long answer is......“Heat Exchanger (HX)”, a very fancy term for a copper tube that passes through hot water. As the cold water flows through the copper tube, immersed in a container of hot water, it gets heated up. So the HX is really very simple and it is probably clear that the temperature of hot water exiting the heat exchanger depends on a number of things:-
  • How hot the water in the container (boiler) is
  • The surface area of the copper tube (or to put it simply, how long it is)
  • The temperature of the water when it enters the copper tube
  • How fast the water flows
The espresso machine designers will have to consider these factors in designing a machine that is able to produce enough hot water, without it being too hot and to minimise the requirements of a cooling flush. The Following refer to the diagram of the Heat Exchanger:-
Yellow Section
This section is what could be termed the cool side of the machine; it will contain the cold water tank (or possibly mains water inlet on a plumbed machine). Its task is to provide water at the correct pressure to the rest of the espresso machine. The pump can be either a Vibration pump, or a rotary pump. Its sole purpose is to pump water at high pressure (9 or 10 bar).
Note: Vibration pumps are unregulated and deliver water at a pressure of 15 bar (much too high for espresso). Rotary pumps have their own internal pressure regulation and can be regulated to deliver water directly at 9 bar.
The water is drawn from the cold water tank, into the vibration pump, and through a device called an OPV (pressure release valve) which lowers its pressure. You will notice that this OPV has a tube leading back to the cold water tank, because to regulate water pressure down to 9 or 10 bar, it has to allow some of the water back to the tank rather than into the machine.
The water now passes from the OPV to the rest of the machine; this water can now only exert a maximum static pressure of 9 or 10 bar. It next comes to something called an “Autofill Solenoid”. The “Autofill Solenoid” has two positions:
Water is directed to the E61 group (via the HX) for making coffee (normally in this position)
Water is directed into the main boiler to refill e.g. after steaming, or using hot water.
The function of this solenoid, as you may have guessed, is simply to ensure the main boiler can fill when needed, without pushing water through the E61 group.
Red Section
This can be termed the hot side of the machine, it’s where the water gets heated, steam is produced and everything is prepared for coffee making.
The main component of this section is the Boiler, as you can see it is full of hot water and steam. It can do this because the boiler is like a pressure cooker and inside it’s at a higher pressure than atmospheric. In fact the boiler is usually at around 1.2 bar. This means we have water at around 124C (yes hotter than boiling which is 100C) and an area of steam above the hot water. The boiler is maintained at this pressure by the little pressurestat which switches the heating element on/off as required to maintain 1.2 bar. These pressurestats are usually adjustable, via an adjuster on top (using a screwdriver.).
When we open the steam tap, steam comes out and the pressure in the boiler drops, this causes the water to instantly boil, producing more steam…very simple
When we open the hot water tap, steam pressure forces the water out
The autofill probe is just in the water, but when the water level drops as in steaming or drawing hot water, it senses this drop in level and tells the Control Board (brain). The brain then switches on the pump and sets the autofill solenoid to the fill the boiler position. Once the water level reaches the probe, the pump switches off and the autofill solenoid goes back to the making coffee position.
You can clearly see the HX tube within the boiler and this works no differently to the simple diagram given earlier.
Tan (or Coffee Coloured) Section
This is the business end of the machine, the part you normally see and interact with. All the previous wizardry was to deliver hot water at about 93-96C and 9 bar (140 psi) to this part of the machine for making coffee. Move lever up, pump starts, water passes through HX into group at 9 bar, into portafilter and through coffee….mmmm espresso!
The only slight complexity here is shown in the area marked by the green dashed line. The E61 thermosyphon (link). This is a simple passive circuit that allows convection currents to circulate through the E61 group to keep it hot (this increases temperature stability during the shot). You can read much more about it in the E61 thermosyphon article.
Lastly the poor old control board…well it’s box of electronics and controls the pump, water level, autofill solenoids and even has a protective circuit that prevents the heating element coming on if the boiler needs filling. It does a few other fancy functions at switch on, but it’s really a simple device. They are sometimes called level controllers and are usually made by Giemme or Gicar for prosumer machines. They are not meant to be user serviceable and if they go wrong are usually replaced. However often a fault is a simple transformer that runs one of the solenoids and these can sometimes be purchased cheaply for a few pounds and replaced using a soldering iron.
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