Propane heater shield and re-radiator???

[walt]

This was somewhat one of those Covid projects that got a fair amount of use last winter in Arizona because we could at least be a little bit social by always staying outside. I try lots of things that do not work but this one seemed a good thing so I spent the time to document what I measured. Might be interesting, might not be.. but it is December after all and the sailing subject is a bit limited for most. Anyhow.. I will take the risk that this ends up being a discussion about home insurance (yep.. I know). Black body radiation is part of the discussion and is interesting just by itself.

One discussion we have had so far is would there be a benefit if the shield in the link was thicker metal. I dont think so.

Link below

black body heat shield

Edit Jan 19.. I both modified the shield and did a much more accurate measurement of the radiated power. In this thread, I had over estimated the radiated power for the Propane heater on its lowest setting but it still ends up being about 733 watts. Link here black body heat shield If you have already looked at this, please do a browser "refresh".

 

[Rich Stidger]

I would check the regulations regarding clearances for wood/coal stoves to nearby combustible surfaces and use those regulations as a guide to determine if you have a safe installation.

What does the heater manufacturer have to say about clearances to a ceiling? There are also gas-fired radiant heaters that are designed to mount to a ceiling. What are their guidelines?

 

[Ralph Johnstone]

'Fraid I can't buy this one.

The problem is that the black body will absorb heat and then, as a hot body, CONVECT a lot of that heat upwards in directions which do not benefit the user. I think you're looking for a perfect reflector which will not heat up but will reflect the original radiant heat back to a useable target.

 

[walt]

I put one reference to an article on black body radiation in that link which is interesting.

Most Heat Loss Is Black-Body Radiation

In a previous post I used statistical mechanics to show how you’d calculate the thermal conductivity of any gas and showed why the insulating power of the best normal insulating materials was…

www.rebresearch.com

Of course a hot object will have both convection and radiation but I think the model I used for calculating the radiated power is correct and based on the measured temperature of the shield says the radiated power is in the range of 1400 to 1600 watts. This also "feels" about right in actual application. There is of course convection heat lost upwards and I dont know what this is. I did measure the ceiling temperature and it does get warm and that would likely be mostly from convection heat. FYI, the ceiling got a lot hotter without the shield (and I think the model in the that link is correct).

Also in that link is an interesting calculation of radiated and conducted heat from a human.

, the radiative heat loss is calculated from the equation above: 2 *1* 5.67×10−8 * (8.43×109 -7.57×109) = 97.5 W. This is 23.36 cal/second or 84.1 Cal/hr or 2020 Cal/day; this is nearly the expected basal calorie use of a person this size.

Regarding the conducted heat from a human..

we find a Wattage of heat loss of 2*.024*(30-22)/.02 = 19.2 W. This is 16.56 Cal/hr, or 397 Cal/day: about 20% of the radiative heat loss, suggesting that some 5/6 of a sedentary person’s heat transfer may be from black body radiation.

 

[walt]

I think you're looking for a perfect reflector which will not heat up but will reflect the original radiant heat back to a useable target.

Also interesting regarding IR reflectors What Materials Reflect Infrared Light?

The propane heater has a shiny aluminum cap and Im guessing.. but believe that is by choice and is intended as an IR energy reflector. Ie, the shiny al cap reflects (mirrors) some of the main burner IR energy back downwards.

However, with this type of propane burner, I dont think the heat going up is radiation. I think it is more likely conducted heat escaping upwards from under the cone and from conducted heat from the cone itself since the shiny AL is not a good radiator. Something which can capture conducted energy and get hotter because of the captured energy is best. The science of black body emissions says that as the temperature of an object goes up, the radiated power goes up. And it fact the radiated power goes up by the fourth power of temperature.

Black objects make the best IR absorbers and emitters. (woods stoves are typically black). https://physics.stackexchange.com/questions/159856/why-is-black-the-best-emitter

 

[kloudie1]

Walt, I think you've gotten the low hanging stuff.. Maybe the next step would be to put a 10-12 inch clear border across the top of the windows (vinyl dodger material?) so that a larger pocket of warm air would be near the ceilng, essentially insulating the ceiling so it wouldn't accept radiant heat from people and stuff on the deck. (and still allow great views and allow the combustion products to vent) .
Good job.

 

[walt]

I probably wont do any more since I think it solved my original problem of possibly not being safe. And I think it is safe as I have measured a bunch of temperatures and this did significantly reduce the ceiling temp. Ceiling still gets up in the 100F range near the shield and this also will radiate. In post #4 example from the link, even a human body radiates 97.5 watts. So I think the ceiling is still capturing some of the heat energy and re-radiating as is but how much.. dont know.

Regarding the question of spec, as mentioned in the page I did, I bought the propane heater and in reading the manual (after I bought the heater) discovered that it specified either 30 inches or 3 feet gap between the top of the heater and the ceiling. I did look at a couple other heater options and pretty sure these will all have some distance to the ceiling spec. My heater is actually about 14 inches below the ceiling and I chose (took the risk) to try an experiment shield.. and measured a bunch of stuff to convince myself of the safety.

This topic actually does have something interesting regarding at least trailer sailboats using propane heaters. I have a Mr Buddy heater in my 1990 Mac 26S and could not operate it on the floor because with limited room, it almost started my dogs tail on fire a couple times. I moved the Mr Buddy heater to a higher spot that was Im guesing half a foot (maybe a little more) under the top liner. The heat from this is radiant so I never thought to check specs (and I still dont know if that heater has a ceiling spec) but one day I just happened to check the temperature of the fiberglass directly above the heater. Meter only goes to 100C and the temp was over the limit. The heat actually slightly distorted the FG. I didnt have a better spot to put the Mr Buddy so added the fan and this brought the liner temp way down to something acceptable. In this case the fan air also ran heat back to the cabin so was a good thing. Fan always is on if the heater is running.

 

[JamesG161]

Very interesting.

I think you're looking for a perfect reflector which will not heat up but will reflect the original radiant heat back to a useable target.

You like to experiment, Walt, test this on your current set up. Spray this over the Black painted Metal.

Or perhaps a combination of insulation and reflection installed in front of your black metal.

Master Flow 20 in. x 25 in. Duct-board Return Air Box-DRAB20X25 - The Home Depot

You might consider this Ducting Board Design...

Black on the inside and Silver on the outside. I wonder why they did that?


Jim...

 

[walt]

Thanks for the reply and ideas and interesting and good for discussion.. Yep.. shiny silver coatings are better at REFECTING radiated energy. However, I think what I did is best and my argument again below.

Quick note.. radiated energy is like visible light, only a much longer wavelength. Radiated energy travels at the speed of light. Black body radiation science and equations are for radiated energy. Convection energy is due to a fluid flow transferring energy (air in this case). So convection energy travels anywhere the fluid (air) is able to flow.

Radiated energy in the spectrum we are considering here does not pass though a sheet of Aluminum of the thickness used in the heater. Radiated energy that is directed at an aluminum sheet is either reflected or absorbed. Radiated energy generally needs line of sight. The Aluminum cap at the top of the heater completely BLOCKS all the radiated energy from the heater element from going upwards. Because it is shiny AL it actually reflects the IR energy downwards. Glass will pass IR radiated energy but we dont have glass, we have sheet Aluminum.

So, I do not believe any radiated energy gets past the top heater cap shield since radiated energy needs line of sight. Its only air carried convection heat that comes up past the cone on the heater. The shiny top cone of the heater itself will get hot and then create both convection and radiated energy. But shiny AL is not a good radiator. The heat escaping the heater itself going upwards is mainly from convection air flow because the heater top cone blocks (absorbs or reflects)100% of the radiation from the main heater element.

Since we dont have radiated energy leaking upwards, something that reflects energy that is not even present is not a good solution. The best thing is to collect the convection heat carried by the upwards hot air flow (the corrugated steel is probably good for this). The captured energy will then cause the temperature of the shield to heat up. And the well known science of black body radiation says that the increase in heat will result in an increase in radiated energy.

So.. Im not going to try any suggesting for a shiny shield as I think the main heat flow upwards is from convection and not radiation. Once the shield heats up, we want it to be the best radiator as possible and black objects make better radiation emitters (and collectors).

FYI, I do have some insulation above the corrugated metal shield and the silver coating of the insulation shown likely would be a good thing to reflect radiated heat from the TOP of the shield back to the shield. This is what you want as it causes the shield to run hotter and hence increase the black body radiation from the bottom side which effectively "recycles" the heat energy back to folks under the metal shield.

But.. I could not find that type of insulation that had a comfortably high temperature rating. The stuff I used has a 1200F temperature rating.

 

[walt]

FYI, here is a picture of the heater and shield above the heater with some illustration that I added. You can see that the shiny Aluminum cap of the heater assembly blocks all line of sight radiation energy (in red) from the heater element to the shield above. The convection heat shown in yellow carried by air flow easily moves from the heater element upwards past the heater cap to the top black shield.

And once the top black shield heats up from capturing convection energy from heat flow, it will radiate heat downwards according to the black body equation.

Since the heat energy going up past the heater assembly is convection, there is little sense in trying to reflect radiation. You really want to best collect the energy from the convection so that the shield gets as hot as possible. Then let the black body radiator do its work and radiate heat back downwards to people sitting under the heater and shield.

 

[JamesG161]

My last reply is to try my suggested experiments or read this...

Heat transfer - Wikipedia

Every thing radiates energy.

Jim...

 

[walt]

Thanks for the link. Im not sure what the point is but interesting to read.

 

[Ralph Johnstone]

You had the answer back in your post #7.

Install a big, plantation style, slow turning fan above the propane heater and send those lazy, hot, molecules of air back down where they can do some good.

 

[walt]

You are correct if the goal is to keep the ceiling from getting too hot. The air flow of the fan would definitely work. When I tried the fan in the boat, I think it reduced the temperature of the liner by a huge 60C (going by memory).

FYI, I actually do have a fan like what was suggested in the last post next to the shield. The initial idea was that it would pull convection air from the panel and direct it downwards. However, when I actually tried the fan, it didnt seem to do anything. I did not try just a fan alone and if somehow placed perfectly, would in fact recycle some of the convection heat going up. Maybe the fan right above the heater blowing downwards probably would do good and might be a good idea. But.. I did not try this based on the fan experiment I did do which I abandoned and then pursued the black body radiation idea (adding insulation above the shield to increase its temperature/ radiation). FYI, the fan now is used for keeping cool on hot days and increasing air circulation.

But, if the goal is to capture that energy and re-direct it downwards making the heater more efficient, I have an actual data point of producing 1400 to 1600 watts of radiated power from the upper shield (unless you want to argue the black body science). I think that is fairly good actual results.

 

[walt]

One more thing to point out.. all of these ideas to increase heat from the IR heater somewhat go out the window if its a windy day. Really not much of a safety issue of the ceiling overheating on a windy day. Ambient wind quickly removes any convection heat coming off the heater. But.. even in wind, the heater still works nicely as the output downwards is mostly radiation. On calm days, you get a nice distributed bath of radiated heat from both the heater itself and the upper shield.

 

[Benny17441]

Seems to me that a fan blowing horizontally in between the Roof and the black body would displace and dissipate the rising hot air among a much larger section of the roof. Obviously the size of the fan and area coverage would determine the effectiveness. How hot does that roof get in the Arizona summers?

 

[walt]

Cation.. a bit technical.. Last night I looked at the heat flow off the top of the Propane heater trying to determine how much power was due to radiation and how much was due to convection. Convection is from both the combustion gas and air heated by the assembly. Radiation is line of sight depending on temperature, area and emissivity.

I tried to measure the temperature of the propane heater top cap but my meter only goes to 100C and the cap was hotter than this. The top cap is commercial sheet aluminum and from this link Emissivity Coefficients common Products has an "emissivity" of .09. This means that the cap is good at reflecting 91 percent of the IR from the burner downwards where you want the heat to go. But the .09 emissivity means that the top surface is a poor black body radiator, only emitting 9 percent of an ideal blackbody. Measured the area of the top cone and all I know about the temperature is that its over 100C. So I calculated the radiated power at 150C and 200C using the method previously linked.
Heater top cap AL radiation power at 150C is 42.9 watts.
Heater top cap AL radiation power at 200C is 89.69 watts

This is overall is a good thing (good design) as you want the cap to reflect heat downwards but not radiate a lot of heat upwards.

I also took a burning incense stick and looked at the air flow. No numbers but there is considerable air flow up past the burner section that bends around the cap and then flows a little inwards above the cap before going up.

Based on measuring temperature of the top painted black shield, I know its radiating in the 1400 to 1600 watts of power. Even if the cap on the heater got up to 200C, the radiated power of 89.69 watts means that likely most of the heating of the top shield is due to hot air convection.

So is the top shield best as a good IR reflector (polished metal or silver) or a good IR radiator black. If the top shield is a good reflector, it would reflect the relatively low radiation from the heater top cap. However the convection heat that results in higher temperature of the shield would NOT be well utilized because a better reflector makes a poor emitter.

The top shield in black paint makes it better for both radiation collection and radiation emission. So the radiation of the top cap gets best absorbed by the top shield which increases its temperature. Either way the convection power gets absorbed. But the top shield in black is going to be best for re-radiating IR power back downwards so best utilized the convection heat (which likely is most of the heat power) but also completely utilized the radiation heat.

My conclusion again, the top shield is better in black for best IR power absorption and radiation.

The foil covered insulation posted a few back inspired the next experiment Im going to try to minimize radiation on the top of the shield (keeps the ceiling cooler) while possibly enhancing radiation on the bottom side where we want it (retired guy has time for unimportant things like this)

 

[walt]

How hot does that roof get in the Arizona summers?

a fan blowing horizontally in between the Roof and the black body would displace and dissipate the rising hot air among a much larger section of the roof.

The fan idea would certainly be interesting. I looked at my case and just dont have the clearance to even try the fan right over the heater. Horizonal fan idea.. in the link for emissivity, either pine or plaster board have fairly good emissivity (pine is .84, plaster board is .91) meaning they would make fairly good black body radiators. Seems like a lot of the ceiling could be turned into a radiator. You would want the patio ceiling insulated to retain the heat so that it radiated downward rather than dissipating upwards. Interesting good idea!

Ambient temps at this location.. I have seen 124F but I think the record is something like 128F. But.. its nice in the winter. Went sailing yesterday (Lake Havasu) wearing open shoes, shorts and a light jacket.

 

[walt]

One last post.. (hopefully) on this since its still December with the results on the Aluminum foil experiment.

I was curious which way the foil backed insulation that JamesG161 posted (thank you) would be installed for example in a home. The answer gets a little complicated by needing a vapor barrier but in general the foil faces in the direction you want warmer as the foil reflects IR. From this link Emissivity Coefficients common Products Aluminum foil has a very low emissivity of .04 meaning that it is very good at reflecting IR energy.

In my case, I am trying to capture either radiation or convection energy from the Propane heater which will cause the top shield to heat up. When the shield metal heats up, the IR radiation goes up with the fourth power of temperature. I want the radiation to all go downwards towards people sitting under the heater and shield. I dont want radiation going upwards as the energy would not only be wasted but also possibly heat the ceiling up to levels you dont want.

So.. I did try aluminum foil on TOP of the metal shield and under the high temperature insulation. The idea is that the aluminum foil would be good at reflecting radiation from the top of the shield and work with the top insulation to reduce any heat loss from the top of the shield. The links for the foil backed insulation say you need an air gap for the foil to work and I sort of get this because the foil lays over corrugated steel.

Did another experiment before and after the foil was added by marking a spot on the shield and measuring the temperature on that spot before foil and after foil. I did the two measurements about an hour apart on a day with no wind and let the shield heat up the same amount of time. No wind for either test. Im measuring this from under shield as this is where I would like to see an increase in temperature and hence an increase in black body radiation.

Well.. I did see an increase in temperature of 1.3C which is not much.. Using the blackbody equations, the calculated radiated power increase from the top corrugated shield was 38 watts. Hmm.. interesting but its not a lot of improvement. Picture below shows the aluminum foil on the top side of the shield and under the high temperature insulation.

Also somewhat interesting regarding the color of the shield. I looked up IR home wall heaters. These use AC home electricity to heat up a panel and then the panel radiates heat. Turns out that you can buy these in either white or black. https://www.warmlyyours.com/en-US/radiant-pan What to make of this LOL.. I would guess that you can make a good radiator with either a black or white surface. It also might mean that if the emissivity is slightly less, the temperature just gets a little hotter so that the radiated energy is the same either way. Anyhow.. I thought interesting.

Edit Jan 19.. I both modified the shield and did a much more accurate measurement of the radiated power. In this thread, I had over estimated the radiated power for the Propane heater on its lowest setting but it still ends up being about 733 watts. Link here black body heat shield If you have already looked at this, please do a browser "refresh" after clicking on the link..

 

[jssailem]

It also might mean that if the emissivity is slightly less, the temperature just gets a little hotter so that the radiated energy is the same either way

It also may mean that you give your customers white or black as a color choice. Just because marketing says so.