Help with Newton's Law of Cooling

[Mark Nez]

Hi - I have been trying to figure out how long an object takes to cool and I am just banging my head on the desk. Info:

Metal object is placed into an oven set to 302F (150C).
The object is 3' long and 2.75" diameter, and is solid metal (steel).
The object is "baked" in the oven for 30 minutes at 302F and then the oven is turned off, door closed.
Oven is of industrial type, and is approx. 3ft by 3ft big (oven cavity).

Once the oven is turned off, how long would it take the object to cool to ambient (72F) temperature?

If anyone could help I wold be overwhelmed with happiness :)

Also - educated estimates are fine!

Thanks!
Mark

 

[berkeman]

Hi - I have been trying to figure out how long an object takes to cool and I am just banging my head on the desk. Info:

Metal object is placed into an oven set to 302F (150C).
The object is 3' long and 2.75" diameter, and is solid metal (steel).
The object is "baked" in the oven for 30 minutes at 302F and then the oven is turned off, door closed.
Oven is of industrial type, and is approx. 3ft by 3ft big (oven cavity).

Once the oven is turned off, how long would it take the object to cool to ambient (72F) temperature?

If anyone could help I wold be overwhelmed with happiness :)

Also - educated estimates are fine!

Thanks!
Mark

Welcome to the PF.

How well insulated is the oven? That will make a big difference.

Is there a window so you can see the object? If so, do you have an IR thermometer that you could use to take some data? That may be the quickest way to figure this out.

Why are you leaving the oven door closed if you want the object to cool?

 

[Mark Nez]

Thank you Sir! Great bike BTW!

The oven will be insulated very well, no window, and will have a thermometer in there eventually...I am in a meeting and was just trying to get a "ballpark" on timing for a prototype assembly we are working on.

 

[berkeman]

The oven will be insulated very well, no window, and will have a thermometer in there eventually.

So the intention it to maximize how long it takes to cool off? If I had that object in one of my ovens, it would probably take 12-24 hours to cool most of the way back to ambient. Can you say what the application is, and why you want to maximize the cooling time?

 

[Mister T]

Once the oven is turned off, how long would it take the object to cool to ambient (72F) temperature?

The oven and its contents will cool, but how fast depends on how well heat energy is transferred from the oven to the surroundings. Insulation inhibits the transfer of heat energy by conduction, and there are formulas that can be used to calculate the magnitude of the effect. They depend on things like the R-value of the insulation and the surface areas of the oven walls. But heat energy is also transferred via convection and radiation. Again, there are formulas that can be used to calculate these effects. But all you'll get from that is an estimate. You really need to measure the time if you want to get an accurate value.

 

[Mark Nez]

The oven and its contents will cool, but how fast depends on how well heat energy is transferred from the oven to the surroundings. Insulation inhibits the transfer of heat energy by conduction, and there are formulas that can be used to calculate the magnitude of the effect. They depend on things like the R-value of the insulation and the surface areas of the oven walls. But heat energy is also transferred via convection and radiation. Again, there are formulas that can be used to calculate these effects. But all you'll get from that is an estimate. You really need to measure the time if you want to get an accurate value.

Just a rough estimate would work. I am going to have test run and proper calculations done by the developing engineers, again, this is just a prototype design meeting and trying to set estimates on timing so we can get some ballpark figures for the prototype 1 build. Thank you for the info and help!

 

[Mark Nez]

So the intention it to maximize how long it takes to cool off? If I had that object in one of my ovens, it would probably take 12-24 hours to cool most of the way back to ambient. Can you say what the application is, and why you want to maximize the cooling time?

not really maximize, just get an idea of what we are looking at in terms of time for scheduling of personnel. I was in the ballpark of 6-8 hours based on past tools but wasn't sure if there was a calculator that I could "fudge" to get estimates. In this rare instance (at this point of time) just an educated guess is all I was hoping for. But, sounds like there are too many variables and I understand the hesitation of giving an answer based on the lack of hard numbers needed to calculate. Thank you for your fast replies!

I am actually thinking we are going to get cooling ovens for manufacturing at this point...

And no sorry, I can't discuss any of the project details...just a direction drilling tool for one of the top O&G companies :)

 

[Chestermiller]

Is the object taken out of the oven after the heating, or is it left in the oven?

 

[Mark Nez]

Is the object taken out of the oven after the heating, or is it left in the oven?

Left in

 

[Chestermiller]

I suggest you get an oven thermometer and first measure temperature vs time as the oven cools without the object present. This will give you the Newton's cooling coefficient exclusively from the oven to the surrounding room. We can then set up a model for the case with the object present within the oven.