Are you smarter than kirbic? Physics heat question

Pentax DA 18-135mm F3.5-5.6ED AL [IF] WR DC

12/18/2010 09:02:53 PM · #1

I thought that might get some attention. :) Last summer I put a gas fire pit in my back yard. It has two rings and is raised about two feet off the ground. It's great in the summer for roasting marshmallows and sitting next to. However, despite being rated at 90,000 BTUs, it isn't particularly warm to sit next to in the fall/winter. Most of the heat goes up. This got me thinking.

A propane heater (those lampost types) puts off seemingly much more heat in a horizontal or even downward direction. I assume this is because it uses radiant heat (by heating the wire mesh with the jets of propane) rather than convection heat (which I assume my fire pit is doing by heating the air above it). I wondered if there was a way to boost the heating power of my fire pit by taking advantage of radiant heat?

Does anybody think this is possible? My first thought was to create a metal mesh ring that would sit over one of the rings. I'm goofing with that right now, but I'm not sure if different metals have different abilities to radiate heat or if the natural gas flames, which aren't directed as jets, are hot enough to cause the metal to radiate. My first attempt was not particularly successful. The ring warped quickly and did not sit right over the flames. Amazingly, although I got a few sections to glow, when I removed it from the fire it did not feel like it gave off much heat and was touchable within seconds.

Another possibility is to replace to fake logs I have (which I believe are some type of cement) with something else like ceramic. This is expensive though and I would want to guarantee it would work first.

Other ideas?

Message edited by author 2010-12-18 21:05:23.

tnun

Ricoh GR III

12/18/2010 09:13:50 PM · #2

What you need are the blades from a chopper, coated black (absorbant) and reflecting on each side, suspended over your fire pit. (There are better demonstrations of superior intelligence, but this is a start).

GeneralE

Canon PowerShot S3 IS

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

12/18/2010 09:41:03 PM · #3

Heating something like stones which have low conductivity but high capacity (specific heat?) probabl gives you the best long-term way of storing/redistributing radiant heat, but you have to sit pretty close.

I think concrete may have too high a water content to work well -- something like a pizza stone or kiln bricks would be best. Be very careful of heating rocks found in riverbeds or on the seashore -- they can contain enough internal water to explode when heated, like popcorn only deadly ...

You could also fashion a metal hyperboloid reflector mounted above the flame ring to try and direct the hot air outwards.

You could also do what (I'm told) the Russians do, and use the flame to heat water and drink lots of hot tea -- puting a half-liter of hot water inside you, where it has to pass through all of your tissues to "escape" will heat your body far more efficiently than trying to raise the ambient tempterature in order for heat to diffuse through your tissues inwards, especially since you're probably wearing insulating clothing. Maybe the heater will feel more effective if you try standing next to it naked ... or build a small shed around it (i.e. a sauna) rather than trying to heat up the whole State of Oregon with your dinky 90,000 BTU burner ... ;-)

And no, I don't think I'm smarter than

kirbic at physics ...

sfalice

Canon PowerShot SX60 HS

12/18/2010 09:50:49 PM · #4

Are you smarter than kirbic? Physics heat question

In a word: No.

(Ask Kirbic)
:))

Mike

Tamron SP 24-70mm f/2.8 Di VC USD Canon Mount

12/18/2010 09:54:02 PM · #5

this is why gas fireplaces have ceramic logs. The ceramic logs get heated by the gas flames and then radiate the heat.

try adding those, they are pricey but they would work great.

edit: on second thought, its seems most contain lava rock that glows pretty but does nothing for heat...

Message edited by author 2010-12-18 22:37:12.

Morgan

Nikon D800

Nikon AF-S Zoom-Nikkor 28-70mm f/2.8D IF-ED

12/18/2010 09:56:46 PM · #6

Physics, my favourite topic. I recommend a location displacement strategy followed by a core heat induction - go indoors and have a hot chocolate.

Cory

Canon EOS-1D Mark IV

Canon EF 85mm f/1.2L II USM

12/18/2010 09:57:41 PM · #7

I see no reason that a carefully crafted 3-d grid of 1/4-1/2 iron rods (think re-bar) wouldn't work really well, especially if you put a 3/4 round reflector over it..

I can tell you from personal experience with outdoor heating pits/chimineas/burners, etc, that a reflector and something to add thermal mass really do make a huge difference..

The chiminea is one of the most effective outdoor heaters I've seen, and the reason is the directionality of the heat, I've actually got an image somewhere that shows one burning in a snow storm and a perfect cone on the ground in front of it is clean of snow for 15 feet - the only real problem is that the durability of the clay ones from Mexico is pretty cruddy, I kill about one a year... (thinking about going cast iron..)

kirbic

Canon EOS R5

Canon RF 24-70mm f/2.8 L IS

12/18/2010 10:21:03 PM · #8

Drawn like a moth to a flame... you knew I couldn't stay away from this thread, LOL. Let's start from the beginning. You want to maximize the heat transferred outward from the burner, to the objects (people) surrounding it. There are three possible modes of heat transfer:
- Conduction
- Convection
- Radiation
Conduction doesn't sound like a safe idea, unless you use the burner to heat a tub of water and immerse your subjects in the water. That would be efficient!
Convection is working against you; cold air is drawn inward, heated, and exits upward. It actually cools your subjects as it moves inward!
We have left, radiation. In fact, radiation is the most efficient way to heat in this instance. Keep in mind that the amount of radiant heat transferred is proportional to the fourth power of the temperature difference between the radiating object and the absorbing object. It also depends on the "view factor." Think about the view factor as the angular size of the radiator as viewed from the subject.
So your objective is to create a radiating object that is as hot as possible, and relatively large, if possible. Higher temperature is more important than larger size.
Finally, you want an object with a high emissivity. Metals normally have low emissivity, but luckily their emissivities go up (way up) as they weather and oxidize. For all intents and purposes, don't worry too much about the emissivity, but try to stay away from polished, non-corroding metals. If you're using metal, use something that will oxidize.
I really like Cory's idea of a grid of rebar with a reflector. You could easily get the rebar red hot, and the reflector, which should be a cone or pyramid oriented point down, will help to re-radiate heat that is radiated upward. Anything you build should be made of thick enough materials that it doesn't buckle and warp.

RayEthier

Canon EF-S 18-55mm f/3.5-5.6 USM

12/18/2010 10:24:00 PM · #9

...what

Kirbic said or buy some ski-doo suits.

Ray

PGerst

Canon EOS-20D

Tamron SP AF 28-75mm f/2.8 XR Di for Canon

12/18/2010 10:46:24 PM · #10

The same problem exists with home fireplaces. The heat goes up...straight up the chimney. That's why I use The Grate Wall of Fire. Its a special grate that builds the fire vertically and against the back wall of the fireplace to radiate much more heat outwards.

It works like a charm. The heat is so intense that I can't sit directly in front of the fireplace.

Here are some photos of what it can do:

Message edited by author 2010-12-18 22:54:40.

Canon EF 85mm f/1.2L II USM

12/19/2010 12:08:01 AM · #11

Ok, I could almost follow that. :) I'm actually onto experiment #2 which is to take my old fire dish and place it upside down over the flames about two feet above them. I sat it on its old frame. I'm gonna let the fire go for ten more minutes and see how hot it gets. I was also hoping it would push the warm air outward since it can no longer go up, but initially it didn't seem to quite do that. I could even take the wire cover and put it right in the flames (removing the fake logs that are there) to see if that would add to it.

So would brick pavers be a good radiative heat source since fireplaces are made from them? I could make this thing all sorts of ugly in the winter as long as it gave off good heat. :)

Cory

Canon EOS-1D Mark IV

12/19/2010 12:18:11 AM · #12

Originally posted by DrAchoo:

Think reflector...

Just get a steel box of some sort and cut a side out of it.

ETA: I have a firepit that I built to use for larger fires, we just used the solid cinder-block type retaining wall blocks.. Not cheap, but get em hot and they do keep the heat coming, they keep the wind out, and the reflect a certain amount of heat...

But, in your case, it might be much easier to just use a prefabricated box of some sort..

Message edited by author 2010-12-19 00:20:54.

Mick

Canon EOS-5D

Canon EF 70-200mm f/2.8L IS USM

12/19/2010 12:20:38 AM · #13

Originally posted by kirbic:

In fact, radiation is the most efficient way to heat in this instance.

Yeah, it's called a big ass bonfire. It takes a real physics genius to figure that one out. :)

12/19/2010 12:38:37 AM · #14

Well, the dish did not work well. Interestingly it got quite hot to the touch, but you could hold your hand even an inch above it and not feel any radiation. Why doesn't it radiate? Do you have to hit a magic temperature? Is it the type of material?

PapaBob

12/19/2010 01:11:12 AM · #15

I saw an interesting fire pit near a hotel in the mountains, they used 4" pipes stack in a pile like logs with lines and holes burned through the pipes to make them somewhat resemble logs. The metal logs got hot and radiated heat outward. The logs were stacked several feet above the fire pit and it seemed to pretty well.

Art Roflmao

Canon EOS-6D Mark II

Canon RF 24-70mm f/2.8 L IS

12/19/2010 01:58:35 AM · #16

Here's my solution:

The trick is getting him to adjust the output properly.

kirbic

Canon EOS R5

12/19/2010 10:55:31 AM · #17

Originally posted by DrAchoo:

If you can even think about touching, it, it's not near hot enough. If it starts to glow, it's hot enough ;-)
Yes, you need to get whatever you intend to use as a re-radiator directly heated by the flames. FWIW, 90,000 BTUs sounds like a lot, but in this application it is not all that much.
Here's an interesting story related to radiation heat transfer. I used to play volleyball near a building with a west-facing block wall. After sunset, if you stood within a few feet of the wall, you could really feel the radiant heat coming off it. The temperature of the wall was not high, perhaps 120Â°F, but of course it was a *huge* radiator.

Cory

Canon EOS-1D Mark IV

Canon EF 85mm f/1.2L II USM

12/19/2010 11:07:02 AM · #18

Originally posted by kirbic:

Originally posted by DrAchoo:
Well, the dish did not work well. Interestingly it got quite hot to the touch, but you could hold your hand even an inch above it and not feel any radiation. Why doesn't it radiate? Do you have to hit a magic temperature? Is it the type of material?

If you can even think about touching, it, it's not near hot enough. If it starts to glow, it's hot enough ;-)
Yes, you need to get whatever you intend to use as a re-radiator directly heated by the flames. FWIW, 90,000 BTUs sounds like a lot, but in this application it is not all that much.
Here's an interesting story related to radiation heat transfer. I used to play volleyball near a building with a west-facing block wall. After sunset, if you stood within a few feet of the wall, you could really feel the radiant heat coming off it. The temperature of the wall was not high, perhaps 120Â°F, but of course it was a *huge* radiator.

Same thing happens out here in the desert.. Due to the incredibly rapid temperature drops that occur at night, it's really easy to perceive the radiant heat coming from any dark colored rock formation.....

Mike

Tamron SP 24-70mm f/2.8 Di VC USD Canon Mount

12/19/2010 11:42:07 AM · #19

This of it like a grill. you need a material to heat up and radiate, the flames are good at heating not radiating heat, this is why we cook with charcoal and gas grills have flavorizor bars.

the flames are great at heating an object but terrible at cooking food without scorching it.

you need a material to get very hot, you then use a metal shell or dish to bounce the heat radiating from that object, just like a grill hood.

Message edited by author 2010-12-19 11:46:10.

bergiekat

Canon EOS-5D Mark IV

Canon EF 24-70mm f/2.8L II

12/19/2010 11:53:06 AM · #20

Maybe try Fire and Ice? Just a thought.

There are many companies that sell this stuff and it is much warmer than fake logs or real wood. We use it indoors and love it! They often use it in outdoor pits, but it might require a different burner which would defeat the purpose, so I'll shut up now! :P

...AND, I"m a damn site smarter than Fritz...if you ask me the right question, muahahaha! :P {{hugs}}

Message edited by author 2010-12-19 11:54:57.

David Ey

Nikon D80

Nikon AF Micro-Nikkor 60mm f2.8D

12/19/2010 12:14:39 PM · #21

It seems you have already figured out that heat rises. Place your seating ABOVE the flame and you will be plenty toasty with 90k btu. Your BBQ cooker likely has the ability to adjust how high the meat is above the flame. Do this for your seating and you will be fine, if you have plenty of adjustment. You might want to test this by placing a meat thermometer at the lowest point your body will be while seated, probably your feet. This will likely be quite high so you might want to motorize the seat platform, hop aboard while it is low, make adjustment and then fire it up. I bet you can get all the stuff you need at Walmart and Home Depot. Forget that Radiation BS. You don't need it, and, you can paint it any color you wish, even white. You will find 90k btu much more than adequate which will make this setup more green which will make your neighbors happy. You might even be able to sell some carbon credits too.

Sometimes kirbic, and other scientists make things harder than they are. To summarize, heat rises. Get yourself where the heat is going and quit trying to redirect it. It's only natural and more efficient. I'm surprised the big K didn't think of this.

I'm glad I could help Dr Achoo. You are one of my favorite members. Mr. kirbic is too. He has always been willing to help me and while he is smarter than this old guy, he missed the simplicity of this one.

Good day.

docpjv

Fujifilm X-E1

Fujifilm XF 18-55mm f/2.8-4 OIS

12/19/2010 12:18:11 PM · #22

Build yourself a thin diameter stainless steel beach umbrella and plant it right in the middle of your fireplace. wait for the fire to have died down completely before trying to remove it by grabbing the 'umbrella-handle' LOL.

David Ey

Nikon D80

Nikon AF Micro-Nikkor 60mm f2.8D

12/19/2010 12:40:36 PM · #23

Hi Peter. I hope you are feeling better.

RayEthier

Canon EF-S 18-55mm f/3.5-5.6 USM

12/19/2010 12:58:24 PM · #24

Has anyone mentined a fan system. It has been mentioned many times that heat rises, but most gas fired furnaces and fireplaces have a fan system to send the hot air to other locations that straight up the chimney.

If you are using this outside, it is almost a waste of time and effort since you might be nice and toasty in the front, (assuming you are facing the fire) and will be freezing your hind site since all the cool (cold) air is rushing towards the source of the heat.

For what it's worth, I have a propane fired heater I use in my garage in the winter time and it works just fine... mind you my garage is insulated.

You best bet would be to throw a temporary tent over it to retain the heat and enjoy.

Ray