Jed sez a 150 watt hose wouldn't be very hot. GoatGuy says it would.
Well, let's do a little plausibility calculation, and see whose ballpark
we wind up in.
A 2 cm hose 3 meters long has surface area of about 2 * pi * 300 = 1900
cm^2.
In comparison, consider a 150 watt lightbulb. It's an approximate
sphere, and it might be 3 inches in diameter (that's a moderately fat
lightbulb). Then its radius is about 3.75 cm, and its surface area is
something like 4 * pi * (3.75^2) = 177 cm^2.
A 150 watt lightbulb gets hot as heck. On the other hand I've never
blown up a lit lightbulb by slobbering wet paint on it with a full
roller (which I've done a number of times in a former life) so "heck" is
not totally unbounded. Let's be generous, and say our 150 watt bulb
gets up to 200C above room temperature (that's going to be about 220 C,
which is stinkin' hot -- but, in fact, Wikipedia says the envelope temp
of a general service bulb can reach 200 to 260 C, so it's not outrageous).
Now, most of the energy of the bulb gets lost via convection to the air,
just as most of the energy of the hose gets lost that way. For
simplicity, let's assume the hose and the bulb both lose *all* their
energy that way. And let's assume the loss rate is linear in the
temperature difference with the air (which may even be true). Then the
temperature of the hose should be (*very* roughly)
(177 / 1900) * 200 = 19 degrees above room temperature
That's about 40 degrees C, or about 104 degrees F. Even if I'm off by a
factor of 1.5 in one of the assumptions it's still not going to be
especially uncomfortable to touch a rubber hose which is that hot.
I'd say Jed wins this one, hands down. Goat Guy must be using strange
hoses.
On 11-06-21 09:54 AM, Jed Rothwell wrote:
Daniel Rocha <[email protected]
<mailto:[email protected]>> quoted GoatGuy:
So, if a length of tubing is 3 or 4 meters long (per that longer video
with the blue bucket...) then it should be conducting away about (50 ×
3) = 150 W of heat. This will derate the system accordingly.
That is RIDICULOUS. This guy has no common sense. He has no intuitive
feeling for how much heat 150 W is! This estimate is off by a factor
of 10 at least.
The hose is obviously too hot to touch. Anyone who has ever used a
hose of this nature (including me) knows that a hose with steam in it
gets too hot to touch. If the entire 3 m length of the hose was
producing only 150 W it would only be a tiny bit warmer than the
surroundings. You could hold it anywhere. I expect that by sense of
touch alone you could not tell it is warmer than the surroundings.
That hose is radiating kilowatts of heat. Anything that size, that
hot, has to be.
- Jed
