Re: Input Water Temperature on HX Machines

On Sat, 29 Oct 2005 17:25:58 -0500, ferret
<nobody@xxxxxxxxxxxxxxxxxxx> wrote:

>We have water towers in this area and the water temperature varies from
>tepid in the Summer to just above freezing in the Winter. I'm planning
>to do a machine upgrade this year and if the input water temperature is
>going to add yet another variable I'll have to adjust for, I'm not
>likely going to want an HX machine.


If the HX were an "ideal" heat exchanger, then at a constant flow rate
it would raise the temperature so the proprtional reduction between
the source (tap water temp) and sink temperature (the boiler) would
stay the same.

For instance, say the machine is designed to raise 15C water to 95C
with a 120C boiler temperature. In that case, the heat exchanger is
reducing the difference by 76%. If the water drops to 5C, the output
should drop to 88C. That would be a significant difference and require
a boiler pressure adjustment.

However, in reality, espresso machine HXs are far from ideal, since
Italian engineers are presumably aware of the potential problem.
Typically the amount of water in a plain heat exchanger, or in a
heatexchnger-thermosyphom-head combination for that type of machines,
is considerable. So is the stored heat in the brass parts. All this
acts as a big thermal capacitor, evening out the fluctuation of the
direct heating by the boiler. As long as this mass is at shot making
temperature, 5C to 10C fluctuations are simply damped out to around 1C
(this is my experience with my E61 Tea, and it presumably applies to
all the other machines that do as well).

If the inflow water temperature changes dramatically, the only
consequence, as Barry states, is that the maximum shot making pace
will change. If it gets very cold, it may mean one can only make 50
cups an hour rather than 60 from a commercial one grouper. This will
usually not be an issue in US cafes (where steam cpacity is the big
killer) and never in a home environment.

--
jim schulman
<jim_schulman@xxxxxxxxxxxxx>
.

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