Estimating heat, mathematically

[Ed62]

Let's say I have a heat lamp 17" above a target. The temperature is 95 degrees. How can I figure how much to raise the lamp in order to get 90 degrees? Or is this even possible? I know you can figure light intensity mathematically, but not sure about heat.

Ed

[atgcpaul]

I don't know the answer but this should be interesting. I'd imagine ambient
temperature plays a role in it.

I'm also wondering if heat transfer has a "maximum velocity" where an object
can only attain a certain temperature if given an infinite amount of time to
absorb that heat. I think the answer to this and the answer to the above
would be closely related.

[BigguyZ]

As I understand it, Heat lamps work off of emitting light at infrared wavelengths. The intensity of light is an inverse square relationship to the distance of the source. So as you pull the lamp futher from the subject, the intensity goes down pretty quickly.

What I'd reccomend is setting the lamp up so it's adjustable. Then, get a termometer and make adjustments until the temp is dialed in.

[BigguyZ]

I don't know the answer but this should be interesting. I'd imagine ambient
temperature plays a role in it.

I'm also wondering if heat transfer has a "maximum velocity" where an object
can only attain a certain temperature if given an infinite amount of time to
absorb that heat. I think the answer to this and the answer to the above
would be closely related.

I failed thermodynamics, but I do know that heat transfer is based on a differential. SO the greater the difference in temparatures between the item and the enviroment, the faster the transfer of heat. So, there would be a point that the item would reach an equilibrium- where the heat was being transferred to the environment at the same rate it's absorbing energy from the lamp.

[Kristofor]

Your critical distance is going to change over time based on how much heat is needed to keep the object some changing number of degrees above the ambient temperature. The rate will change as the room temp changes, if there is precipitation, evaporation, wind, etc.

To me the easiest way to do it if it's not too sensitive of an object would be to have it at whatever the closest position needed is for the lowest temp/most evaporation/most windy conditions and then use a thermometer control to turn off the light when it hits the set point...

[pelligrini]

Do incandescent heat lamps put out the same amount of heat over the life of the bulb?

I know that fluorescent lamps will not put out the same amount of light over time. The output will diminish over time. I don't know if it's the same for incandescent fixtures though. I have a feeling that is not the case.

[LCHIEN]

when all the things are equal the temperature of the object is going to depend upon the temperature of the ambient; when the ambient goes up or down the object will rise or fall by the same amount.

The rise of the temperature of the object above ambient depends upon the heat input and the re-radiation of the some of the heat energy. When it readhes equilibrium the temperature rise will be constant and the amout of energy given off (by conduction and radiation) will be equal to the amount of energy received by radiation.

The heat energy is being radiated into an 3-D angle, unlike the sun which radiates more or less evenly into a 360 degree in all directions probably your heta lamp has a reflector. Changes in distance may put the heated object in a less intense part of the beam, but in general the energy radiated into a cone of a small angle is constant so as the distance grows the area being illuminated increases with the square of the distance so the energy per square area is 1/distance-squared.



There are so many variables for any long term control you would use a feedback loop to control temperature... the feedback would measure the temperature with a infrared sensor and use a electronic heater/motor control (e.g. a electrically controlled dimmer) to increase or decrease the brightlness of the heat lamp to raise or lower the temperature.

But just guessing, assume room temp of 70F, you want to go from Delta T of 25 to 20, then go from 17" to about 19".

you could use a light dimmer instead of moving the light...

[elderfrost]

Radiant heat transmission

This is a situation where most of the heat is transmited as radiant energy. The amount transmitted is generally not affected by the distance from the source and the recieving surface. It is more affected by the color of both the hot surface and the receiving surface; a black surface on both surfaces provides the most heat generation. The air between the surfaces also absorbes some of the energy. Think about the suns radiant energy. An object a long ways from the sun gets warm from the radiant energy. If you have experienced radiant heating, either in the floor or ceiling, you can feel comfortable even though the air temperature is cool.

In this case, distance will reduce the heat received by warming the air. Experimentation is the easiest way to determine the effect of distance. It would be difficult to calculate the effect of distance.

[Mr__Bill]

Some time back in a feed and grain store I saw a chart of how many brooder lamps were needed at what distance for a given ambient temperature for X sq feet and x number of chicks*. So..... it must be possible to figure it out, and if you have a local G&F that services real farmers they may have a chart or something. However a quick look with Google and it seems the chick crowd relies on trial and error and that may be the quickest way for you.


Bill
over here on the big ocean with a lovely overcast day.

* Now I expect you are wondering just what the heck I was doing looking at something like that. Well, my son had geckos and we needed to keep them warm and the heated rock was not enough and we thought perhaps they needed some artificial sunshine. We ended up with a heating pad.