On Thu, Feb 17, 2011 at 8:45 AM, Jed Rothwell <jedrothw...@gmail.com> wrote:
> Joshua Cude <joshua.c...@gmail.com> wrote: > > >> With 1 kW, you can raise the temperature of the water at 300 mL/min about >> 50C to give 65C or so, definitely too hot to touch. >> > > That is true, but the power was not 1 kW. It was 400 W. It was 1 kW at the > beginning of the experiment, but a flow calorimeter or hot water heater > cools down rapidly at these flow rates, so a few minutes after the power > falls to 400 W, the water would be lukewarm. > > In Japan, most kitchen and bathroom sink water heaters are the instant, > on-demand type that heat up the water as it flows through. Essentially, they > are flow calorimeters. A recalcitrant old gas fired one that I use often > goes off and stays off as the water is flowing. The water cools down > instantly. (Come to think of it, that's kind of dangerous. We should > probably get it replaced. > Well, the water did start to cool off. There's a dip in the temperature. But we don't know what the thermal mass of the inside of that device is. It is certainly more than the pipes themselves. What we do know is that it has enough thermal mass that it doesn't heat up in a few minutes; it takes about 30 minutes. And when it is shut off (as in test 2) it cools off rather slowly. The only place it cooled off quickly was at the end of test 1, when they upped the flow rate using tap water, by probably an order of magnitude. I suspect it is designed to have large thermal mass (maybe in hot oil, or even water under pressure), so that after the power is turned off, the thermal mass keeps the output at the bp for some time. That way, they can claim it is self-sustaining, even though it's just cooling off. Clever. > > - Jed > >
