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Hydraulics in the morning ben lee

A strong cup of coffee is sometimes needed when you are a hydraulics designer or engineer, whether it's to get you fired up for the day, to fuel a late night before a deadline or just as a break from a particularly difficult problem.  Probably the last thing on your mind is how hydraulics is at play making that coffee, but without clever hydraulic design those coffee machines at your favourite cafe would not function properly – much like many people don't function properly before their first caffeine hit!


To make the perfect coffee the water must be at 93°c, at a pressure of 9 bar, which is very achievable in the water boiler, but this must be maintained all the way to the brewing head, something that requires clever thermodynamic design.  Maintaining the thermal mass requires a large and insulating metal casing in the brewing head, as if the water cools down it will not brew properly, and water that is too hot makes the coffee taste bitter.  Bearing in mind that the same machine also needs to be able to heat water to boiling point to create the 100°c steam, that is used to froth the milk and it starts to get tricky.  Twin boilers are one way of achieving water heated to two different temperatures, the other being a heat exchanger that creates the two temperatures in the same machine.


When the 93°c water reaches the brewing head it must be passed through the puck of coffee grounds in the filter holder, but even here there is hydraulic engineering involved.  To make the perfect coffee the water must be first fed through at 3 bar to soak and pre-infuse the ground beans, but only for a few seconds.  Then the 9 bar pressured water can be flowed through the beans at a controlled rate.  Many commercial coffee machines automate this controlled pressure flow so anyone can use them, using a rotary pump or a reciprocating solenoid valve to achieve the perfect flow for each coffee type, whether it's a single or double shot.  Older machines, and those beloved by coffee purists are manual, meaning that the operator needs to know exactly how much to move a lever in order to get the water through at the right rate for each type of coffee.


Next time you stop for a coffee, pay attention to how the machine works and you'll appreciate that hot cup of coffee even more.

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