Reply With QuoteYou mean thermal mass ?
Sitting in my desk chair, my hands happened to wrap around the chrome armrest support. I noticed it seemed very cold. Rather than pulling my hands away, I figured I would leave them there and eventually my body warmth would heat the chrome sufficiently to be comfortable. However, if the mass of the chrome was actually far greater than I surmised, perhaps the 'cold' would resist (and maybe overcome) my body heat to where it would never feel comfortable.
Acknowledging that "cold" is actually just the absence of heat to some degree, is there a way to quantify the amount or potential of the 'cold' to withstand a fixed influence of heat (my body)?
I suppose it's a question of "newtons" and such, but the relationship escapes me.
You mean thermal mass ?
There would be 2 factors at play here.
1. The ‘resistance’ of the metal when transferring and then distributing the heat your body gives off
2. The perception of warmth which will be dictated but the difference in temperature between you and the metal.
Any metal will be able to transfer the warmth but it’s mass and structure will influence how fast. The source intensity and locality of the heat source will also factor. In short your temperature and the mass of the metal will be the determining factors, knowing these will allow you to quantify the resistance the metal your chair offers.
With regard 2. You feel the chair is cold but what is cold? It’s likely room temperature or close to it, you are around 37 degrees c, the temperature of the metal is likely less than this meaning it will feel cold to touch, how cold is dependant on the difference in temp.
Now if you think I sound like I know what I am talking about, you would be wrong.
Short answer is yes but not really.
Your body produces BTU's of body heat so if you can't produce enough to sustain you're body temperature, you will cool down.
In practice it would be VERY difficult in the situation you describe because of the small contact area so it would have to cool your blood from your hand to the point it brings your entire body temperature down.
Increase the surface area though and it's easy to cause hypothermia, like in the cold sea.
Ha ha ha you sir, to an ignorant laymen like myself have done a very good job of convincing me you do indeed know what you are talking about.Originally Posted by Sinnlover
Here goes...cold is a feeling. It is an expression of the difference in temperature that you experience.
To lessen the feeling you need to reduce the difference in temperatures. The chair needs to get warmer or your hand needs to get cooler. Both happen and reduce the feeling that the chair is 'cold'...(but your hand may start to feel cold).
To warm the chair up takes energy...from your contact and this cools your hand. Your hand gets more heat (energy) from the blood stream, but if it's cooled enough the blood vessels constrict to restrict the blood supply (and heat loss). That reduces the energy available for transfer and your skin turns blue. Hint...you have a cold hand.
Were this a cold piece of wood you wouldn't have so much of a problem. Wood is a poor conductor so the heat doesn't move away from the point of contact. With only a little heat transfer, you'd get a cold bit of wood with a warm surface where you grasped it.
Metals, including chrome, steel, aluminium, etc, are good conductors so you're trying to heat up the whole thing. The heat you supplied in one part rapidly moves throughout the mass of metal. Until you heat it up enough that the energy flowing away from the contact point is minimal, it will still feel cold to the touch with a warm hand, but not with your cold hand as the temperature difference (surface to surface) has lessened.
Sorry if that's as clear a mud...I'll try to answer a question or two if you have any.
Yup, that's it. The limiting factor here is your body's ability to supply heat to the small proportion of your skin that is in contact with the metallic chair.
Your body is meant to conserve its resources, so it reduces the flow, but the metal will grab all the heat it can, and although it will warm up, it does so very slowly as a lot of the supplied heat will conduct itslef off round the rest of the chair.
And as said before, your body is an excellent sensor of temperature, but in a ludicously small range.
A shower (or hot tob) at 40 Deg C is too hot to bear, but something feels really cold at about 5 Deg C (and if it were -10 Deg C, it wouldn't feel much colder).
But in the range of 5-40 Deg C, it is very good at sensing difference
Sweets and Pickle have described the science behind what you have observed, the physics term for it is thermal conductivity which is a relative measurement of a material's heat transfer characteristics. Styrofoam has a very low TC while most metals are high.
There's also the heat capacity of the material to consider.
Substances with a high heat capacity (like water) will heat up less than something with a lower heat capacity (like metals) for a given energy input.
You asked for a nerdy answer so her it it.
Fouriers Law Q= k A Delta T
Where Q is the heat flow , K is the thermal conductivity of the steel AND your finger , A is area , delta T is the hot ( finger) cold chair.
Also as Kingstepper says there is the matter of heat storage, eg when you pick up a piece of polystyrene it feels warm since it has very low heat storage and its k value is very low.
Sweets also mentioned your own reflex, so when your fingers sense cold you blood vessels retract from the surface reducing your own “ K “ value, so its harder for you to lose precious heat.
As others have said , when you touch the steel chair handle it sucks heat along itself quickly as all the steel wants to be at body heat not just the bit you touch. PLUS there is another element in play , radiation, the chrome has a high reflectivity so its also taking your heat and radiating it to the room.
A number of years spent designing furnace linings as a yoof , has educated me thus.
And just as a leaving gesture, we all know if you double or triple your loft insulation you get a decrease in heat loss. BUT when it comes to lagging pipework more is not always better, as Fourier shows above say you have a hot pipe with 1/2” of foam pipe wrap and you decide to double the thickness, you increase the surface area on the cold side. That causes a bigger A and you lose money!
Last edited by higham5; 11th November 2022 at 15:29.
This site disagrees...link: "As a simple rule of thumb, the thicker the lagging, the better the protection; especially for thinner pipes."
That's because thicker lagging will not only increase the area, it will also increase the delta T, ie the outer surface will stay cooler than with the thinner lagging, thus decreasing the overall heat loss.
Yes seemed strange result to me too.
Yes, you are exchanging air which will remove heat by convection and forced flow - by closed cell insulation where the entrained air is not mobile.
Does hot travel to cold, or does cold travel to hot? When you open a fridge door, does the heat flow in and the cold fall out? I have always assumed that it is the hot air that moves into the fridge and forces the cold air out (heat = energy?).
Heat flows from hot to cold.
What you see when you open the fridge door - is the cold, denser air falling out the bottom, to be replaced by the warmer less dense room air - in to the top. It is done by the movement of air.
Pickle you really have to run the calcs. Insulation suppliers will promote more use of their products. The formula is K X A X Delta t . So its the relationship between Delta T and A if they balance I agree. If A increases more than delta t reduces heat flows quicker.
We were lagging hot eg 800 deg C pipes and the results were like a U shape. As thickness increases heat transfer reduces to a point, then as further thickness is added heat loss goes back up again. We used hp programmable calculators to run the sums, as heat storage came into play as well.
Clients never understood this as they were schooled in loft type insulation where clearly A never changes so the more you lag the more you save.
Anecdotally however if I put four jumpers on and three coats I get pretty hot! :):)
Steve
^^^
Well if 1/2” is better than 1”, isn’t 1/4” better than 1/2” insulation and, ultimately no insulation is best?
I'm sorry but, not having seen your calculations, I'm not convinced. Especially having seen this link. See also this Google Search, where I found both that link and a Heat Loss from Insulated Pipe spreadsheet...link. If you download the spreadsheet you can alter E15, the insulation thickness, and see the commensurate change in heat losss at J14.
With apologies to the OP for having diverted his thread...
If the insulation chosen has higher insulation properties than free air- then keep adding insulation for maximum effect.
Otherwise - how can bringing the ambient air closer to the heat source ever be desireable.
Pickle I defer to the wisdom, we must have been getting it wrong all those years. In mitigation they are using Calcium Silicate which is monolithic we were using ceramic fibre, so k values different.I'm sorry but, not having seen your calculations, I'm not convinced. Especially having seen this link. See also this Google Search, where I found both that link and a Heat Loss from Insulated Pipe spreadsheet...link. If you download the spreadsheet you can alter E15, the insulation thickness, and see the commensurate change in heat losss at J14.
With apologies to the OP for having diverted his thread...
Apologies for the thread error every day is a school day.
Of course this isn't a good example of heat flowing from hot cold, the cold air is still cold but somewhere else.
I was thinking that when I read it, but it was the scenario postulated, so I went with it..............
Yes, I'm sure you know this but an explanation is (for conduction anyway):-I was thinking that when I read it, but it was the scenario postulated, so I went with it..............
Something is hot because its particles (atoms/molecules) have more energy and are jiggling about more than something cold. On contact the hotter material will transfer some of its energy to the colder, its particles will move more slowly and the colder material particles will now have more energy and be moving faster. Thus heat energy has moved from hot to cold.
Explaining heat transfer and basic thermodynamics is not easy on an forum thread (hence the links to things found on google) - sit down with a whiteboard and a box of props - in front of people, and it becomes simple/easier.............. for all parties.
I will let a lot of the stuff in this thread stand, but not that! It's making my (semi-)retired Physics teacher glands twitch!
A body that is reflective will stay cooler in the sun because (broadly) it reflects the IR radiation. But a shiny silver surface is a poor emitter of heat, as anyone who's ever played with a Leslie Cube will remember.
The second law of thermodynamics gives us the idea that if two bodies are in contact then heat will flow from the hot body to the cold body.
Imagine walking into your bathroom in the middle of the night. You walk from the tiles (ooh, cold) to the mat (nice and warm!) but they are at the same temperature. They have to be - they are in the same room - they share room temperature. But the tiles feel cold because they are a better conductor of heat - they allow the energy from your foot (at 37C) to flow to the tiles (at 20C) faster - the energy leaves your foot faster, and your foot feels colder.
The matt feels warmer as the material is a poor conductor of heat, and the heat leaves your foot more slowly.
As you say - They ARE at the same temperature but the coeficient of heat transfer of the tiles/steel is greater than the rug - so will conduct the heat from the foot/tile surface away better.
I think Higham mentiond "suck it away" (or similar). Not quite accurate from a heat transfer perspective (not accurate at all in reality) - but illustrates it non-the-less.
(Leslie Cube was in my class all through high school)
That's all quite right, but I would suggest that the surface or the mat warms up very quickly (for the reasons you have given). Thus it is no longer at the same temperature as the tiles very soon after your foot makes contact. I imagine that a thermal imaging camera would detect warmer footprints on the mat than on the tiles as you walk across.
At the risk of being argumentative, no.
Stepping on the tiles feels colder as more heat has left your foot.
The mat is not going to heat up because it's an insulator, and resists the flow of heat - your foot "perceives" the mat as being warmer because less heat leaves your foot.
And
Generally, there is an inverse relationship between density and specific heat capacity. The tiles would show the footprints more clearly, with an IR camera, as
a) the tiles are a better conductor, and
b) they tiles have a lowered SHC, and therefore would display a greater rise in temperature.
The IR camera detects temperature. In the case of the mat, a) it's absorbed less heat energy, and b) it's greater SHC means a lower increase in temperature.
I humbly suggest. :)
I see where you're coming from, but beg to differ:
- Quite right
- If you put a hot surface (foot) in contact with a cooler surface (mat) I think that the laws of thermodynamics tell us that heat is going to flow. The mat is going to heat up in the contact patch but the heat won't go any further very fast
- Agreed
- Agreed
- I see what you infer, but I contend that it will take longer for the tiles to heat up as the surface temperature is immediately lowered by conduction away from the contact patch, whereas the mat's surface will heat up quickly because of its lower coefficient of conduction
- OK
- True
- Not true, IMO, see point 5
If you stand there long enough and image the site very quickly as you get off, both may have reached the temperature of the foot's surface. The tile surface will cool much more quickly as heat is conducted away than with the mat (all other things being equal, but ambient temperature and its effect on surface cooling is a bit beyond me at this time of night...alcoholic intake being what it is).
PS Happy to be proven wrong...it wouldn't be the first time.
I've been giving it some thought!
And I think...
I think that the greater heat loss to the tiles would be more visible to an IR camera than the smaller heat loss (and kept at the surface) on the matt.
But I've no evidence to back that up! I found something that had the potential to be an answer to the Q, but the video is awful!
https://www.youtube.com/watch?v=n1e8j56ijpQ
"More visible"...yes the footprint will be larger as the heated area will include the surroundings to which heat has been quickly conducted. But brighter (ie hotter), IMO, no as, althought the footprint may be smaller, the mat will retain the surface heat where it was imparted, even though it may take less heat transfer to raise its surface temperature.
I would love to agree with you, but then we would both be wrong :)
Thanks guys for the great variety of explanations...much appreciated.
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