Robert,
There needs to be air to reflect heat with shiny stuff. Shiny stuff won’t work when covered up by other stuff, unless the other stuff is transparent to IR. Shiny stuff does not block conduction, and it is terrible as insulation.
Insulation works in the dark.
IR works in the dark, too.
IR works in the dark, too.
Depends on how one defines ‘dark’.
Normal anthropocentric definition of dark is outside the visible spectrum. We all emit photons in the non visible infrared spectrum and plenty of phenomena emit photons in the Non visible above UV spectrum.
Anhyhoo, check out the official answer on the Reflectix website, under “What if there is no air space present on either side of the product?” (3 questions above the Radiant Barrier heading):
https://www.reflectixinc.com/a…equently-asked-questions/
“No Air Space = No Reflective Insulation Benefit (An R-1.1 is provided from the product itself for the Reflective/Double Bubble material.)”
Also:
“Research has shown that the insulation value of reflective bubble foil insulations and radiant barriers can vary from RSI 0 (R-0) to RSI 0.62 (R-3.5) per thickness of material. The effective insulating value depends on the number of adjacent dead air spaces, layers of foil and where they are installed.
If the foil is laminated to rigid foam insulation, the total insulating value is obtained by adding the RSI of the foam insulation to the RSI of the dead air space and the foil. If there is no air space or clear bubble layer, the RSI value of the film is zero."
- Office of Energy Efficiency, Natural Resources Canada, “Keeping the Heat In”, 2012, pg 27
And:
"An Radiant Barrier System (RBS) with a 3.5" gap has a performance that varies significantly between operating at a temperature of 140 °F (i.e., a solar heated roof sheathing) with an R-value of about 10 and a temperature of 0°F (a cold winter night) with an R-value of only 2.5. Add additional variables such as dirt accumulation on the barrier and a wide range of performance values can be quoted. In most cases, the annual benefit of an RBS relative to an inch of insulation is small or non-existent:"
- U.S. Department of Energy, Radiant Barriers, 2011
The more hands on experience I have with insulation for avoiding heat loss was with a thermic oil circuit for heating our fish oil hydrolysis reactors, we used rock wool (three inches thick) wrapped in aluminum foil (0,2 mm thick). Without insulation you could not get more than 3 meters near the pipes if you wanted to stay alive, and with insulation you could actually touch the aluminum foil and feel it warm with the naked hand.
Display MorePerhaps foil plus 3 inches of rockwool could be compared with foil alone.. lenr-forum.com/attachment/10181/
on the acrylic box..when that is built.
or alternatively theorize on the separate contributions of convection and radiation.
https://www.engineeringtoolbox…-heat-transfer-d_431.html
in my garage the foil alone seems to keep the emitted radiation bearable ..
in our endless blue skies summer when the metal skin temperature is 50 C.
the foil reduces 24 kilowatts
to less than one kilowatt..
I won’t wonder why your shed is 323 C...
But keep in mind, if the foil is touching the hot side surface, the benefit of reduced emissivity could be offset by increased convective heat dissipation on the foil surface because that surface will be hotter than a lower emissivity surface.
Thanks.. the problem is the calculator..er!
The foil isn't touching the roof... there is an airspace..but that gets up to 50C or more..above the foil
the temperature of the roof on top is up to 60 ...perhaps I need to paint it white..
I also need to get rid of the dead possum that ripped the foil off when if fought the rats..
The increase in radiation is proportional to the emissivity
In the garage .The correct figures approximately are 128 W for 0.03 foil versus 3.6 KW without foil. (e=0.85).
adding rockwool would make little difference.. where radiation is the primary heat transfer mode
3.6KW means running to aircon on full during the day...
In the R20 airbox the radiation losses start to get much more significant when the acrylic wall is 40C.
For 40C/30C the radiation losses are 184W /60Wfor emissivity of clear acrylic =0.95
For 40C/30C .. much less 5W and 1.8W for foil of emissivity 0.03..
This was the main benefit of using the foil.. reducing radiation loss.
Check the calculator,,er..
The lid of the box, with bubble foil insulation installed, is 33 C, with a 200 W heater inside, with 15 C ambient air. So even though it is not radiating heat, it is still getting hot and presumably convecting that heat. The sides also heat up, with a gradient from bottom to top.
Last night I opened up an air gap all around, between the foil and the box, so it should be interesting to see what effect that ultimately has.
Considering the cost of the acrylic box, I don’t recommend anyone else bother using one unless they will be fine with about 80-85 % recovery. (It is nice to be able see in.) However, a sealed box made of a layer of 1 inch thick aluminum-faced polyiso insulation inside (R6) and a layer of 2 inch thick XPS (usually pink or blue stuff) outside (R10) would be far more effective at trapping heat and a lot cheaper.
with bubble foil insulation installed, is 33 C,...presumably convecting that heat
the radiative heat loss assuming 30 average for the whole 2 sq meterbox is about 2W.
however passive convective heat loss may be larger... btw 45W and 100W.. assuming a 30--15 delta T,,
1) assuming an HTC of 1.5..heated side down horizontal
Heat loss = HTC x delta T x Area
= 1.5 x 15 x 2 = 45W
2) assuming a HTC coefficient of 3.4 vertical?
Heat loss = 102W.
passive HTCs referenced from
Display Morethe radiative heat loss assuming 30 average for the whole 2 sq meterbox is about 2W.
however passive convective heat loss may be larger... btw 45W and 100W.. assuming a 30--15 delta T,,
1) assuming an HTC of 1.5..heated side down horizontal
Heat loss = HTC x delta T x Area
= 1.5 x 15 x 2 = 45W
2) assuming a HTC coefficient of 3.4 vertical?
Heat loss = 102W.
passive HTCs referenced from
The whole exterior is not that hot, since ‘cold’ air is coming into the bottom on one side, however the upper 1/3 or so is fairly warm. The box sure isn’t losing 100 W, even with nothing (except acrylic) on the sides. With a layer of bubble foil on top (touching the box), and R10 plus bubble foil on the bottom, total losses (heat not accounted for at the air outlet) are about 35-40 W at 200 W input.
Acrylic, it seems, has quite high emissivity, so radiated heat from the heater will be readily absorbed by the box. Since we know how crappy those emissivity charts can be, I would have to look around for a while before I got a decent handle on what the appropriate emissivity might be. Consider that the radiated heat, absorbed by the box, will be exposed to forced convection on the inside of the box, putting some portion of that absorbed radiant heat back into the inside air.
With an air gap between the foil and the sides and top, heat recovery is just under 92 % currently.
Now there is a delta T of 15.9 C.
Edit: Average delta T of 16.2 and recovery of just under 91%. One outlier at 93.5%, which was only 2.5 W and 0.2 C ΔT more than the rest.
One outlier at 93.5%, which was only 2.5 W and 0.2 C ΔT more than the rest.
That recovery rate is much lower than Mizuno's at 2.5 W. It would be ~99%. Perhaps there is more heat leaking out of the bottom of the calorimeter chamber, or somewhere else.
In any case, the recovery rate is unimportant. As long as it is uniform, predictable and more or less linear, it does not matter. A stable ambient temperature is much more important.
That recovery rate is much lower than Mizuno's at 2.5 W. It would be ~99%. Perhaps there is more heat leaking out of the bottom of the calorimeter chamber, or somewhere else.
In any case, the recovery rate is unimportant. As long as it is uniform, predictable and more or less linear, it does not matter. A stable ambient temperature is much more important.
It was 2.5 W higher than the calculated power for others in a similar input range, due to a 0.2 C greater ΔT...
However, if bragging rights are on the line, for 3.76 W fan input, and no internal heater, I managed 5.6 and 8.7 W output a couple of times. (That cold air packs a punch.)
Sort of like pyramid power, but with a silvery cuboid instead.
My vane anemometer showed up today, and so I stuck it on the end of the outlet tube, and...
*sigh*...
it reads 6.38 m/s where I had 3.7 m/s averaged with the hot wire anemometer....
So I guess I found another rabbit hole to go down to figure out what that is about.
My vane anemometer showed up today, and so I stuck it on the end of the outlet tube, and...
*sigh*...
it reads 6.38 m/s where I had 3.7 m/s averaged with the hot wire anemometer....
So I guess I found another rabbit hole to go down to figure out what that is about.
Let the fun begin! Good to know that your instruments don't agree. That's a very usual problem and finding why is fun!
Let the fun begin! Good to know that your instruments don't agree. That's a very usual problem and finding why is fun!
Well, I am off to another adventure tomorrow, for about a month. I can’t take the calorimeter but I will pack the two anemometers and a tube and see if I get some time to work out what is going on.
My vane anemometer showed up today, and so I stuck it on the end of the outlet tube, and...
*sigh*...
it reads 6.38 m/s where I had 3.7 m/s averaged with the hot wire anemometer....
This is a good test.
In July a visitor brought a vane anemometer to Mizuno's lab. It showed results very close to the hot wire one. It was a little higher.
Someone told me that at this air speed, a hot wire anemometer is a better choice.
Well, here is a start: Hot wire anemometers report values in standardized units, while the actual flow rate is reported by vane anemometers. However, standards vary.
http://www.controlandinstrumentation.com/flow/standard.html
(TSI uses 21.1 C for its standard temperature)
My vane anemometer showed up today, and so I stuck it on the end of the outlet tube, and...
*sigh*...
it reads 6.38 m/s where I had 3.7 m/s averaged with the hot wire anemometer....
So I guess I found another rabbit hole to go down to figure out what that is about.
Have you tried putting a puff of airborne particles (e.g., smoke) through the system? I suppose it would only yield a minimum estimate of airspeed but it would be a good sanity check on your other measurements.
