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I am working on a project where 1L of cold water (20C) has to be heated (80C) very quickly (30 seconds).

There were a few designs I was considering

  1. Heater cartridge (similar to what you would find in a 3D printer) - I can get up to 800W
  2. Peltier - 10 peltiers would add up to about 700W, but would require a power supply that could handle 60A, which would be difficult to find for a reasonable price
  3. Nichrome wire - The challenge with nichrome is how to transfer the heat to water. I was thinking of getting insulated nichrome and rapping that around a metal container that would hold the water.
  4. Blow torch or other flame - If this is the best method, I would need help calculating the 'wattage'.

I don't think any of these methods would work because the wattage is too low in any manner. I know water heating is typically measured in kW, but on 120V AC, the circuit would break at 3.6kW (30A circuit breaker).

So my question is what is the best method of heating the water with the given parameters (not limited to the ideas I had)?

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  • $\begingroup$ What about microwaves? You probably will need multiple, since I do not believe there one that can deliver 8.4 kW (the minimum power that Dave Tweed calculated). The advantage of a microwave is that it applies heat somewhat evenly distributed over the entire volume of the water, so you do not need to rely on heat transport through the water itself. $\endgroup$ – fibonatic Sep 4 '16 at 14:52
  • $\begingroup$ My induction hob is 7.2kW (240V on 30A circuit), between the four burners. I have a 2kW kettle as well. Add them together and you get 9.2kW... but you'd have to get the right distribution of the 1L water between the four burners and the kettle. $\endgroup$ – AndyT Sep 5 '16 at 13:30
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The specific heat of water is 4.186 J/g°C, so heating 1000 grams of water by 60 °C requires about 250 kJ of energy. Applied over a 30-s period, this represents almost 8.4 kW of power — assuming that none of the power is lost elsewhere along the way.

One approach would be to pre-store the heat that you need for the water in some sort of thermal reservoir. For example, you could have a large block of metal (aluminum is cheap and has a high specific heat of 0.900 J/g°C) that you keep at 80 °C (or somewhat higher) using a heater and a thermostat. The total thermal capacity of the reservoir needs to be significantly higher than that of the water by about 10:1, so you'd need about 50 kg of aluminum. Pass the water through channels in or over the surface of the aluminum, and it will heat up in short order.

You could also use a reservoir of hot water (10 l or so) and pass the water you want to heat through a heat exchanger (e.g., coiled tubing) that's submerged in the reservoir.

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