How long to heat up mild steel in an oven

[Maynut]

Hi,

I need to calculate how long it would take a 1m x 0.5m x 0.003 mild steel sheet to reach to 200^c from ambient 20^c when placed in an oven at 200^C.

The CP of mild steel is 0.490kj/kgK and density is 7860kg/m^3
I can get to the energy required, but im struggling finding a method for time.

If anyone could give me a nudge with a keyword i may be missing, or principle I could use I would very much appreciate it.

With Kind Regards
Mark

 

[LittleInch]

Try convective heat transfer coefficient

https://www.engineeringtoolbox.com/overall-heat-transfer-coefficients-d_284.html

https://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html

The difference in whether the oven is forced air draft or natural makes a massive difference

The issue is that as the differential temp gets lower the heat transfer gets lower and hence temp vs time takes on a curve.

A 3mm steel sheet of that size with huge surface are to volume ratio should be somewhere in the region of less than a minute? ( I estimate about 15 seconds to get to about 190)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[chicopee]

With a steel bar 3mm thick, it will be quick. Also at 200dC, heat transfer should most likely include radiation.

 

[LittleInch]

Most equations etc though are for a hot steel surface being cooled or in contact with a lower temperature liquid. This is the other way around hence without knowing any details about the oven, I assumed only hot air at circa 200C. Also don't know the colour of the thin plate. Black versus shiny surface might make a difference. Probably of less than 10 seconds...

Of course if the oven heating elements are exposed like a grill element you're going to get some radiation, but if it's only hot air then there won't be any?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[racookpe1978]

1. Is the oven preheated? (already ABOVE the required 200 degrees C - you need that EXTRA temperature above 200 degrees C to get the steel up to 200 degrees!
2. Is the oven evenly preheated to that 210-220-240 degrees, or is only the middle of the oven that hot - if the steel is fed through on rollers or a chain under the heating elements then back out again, then the dynamics are very different.
3. Most important: Radiant heat down from heating elements, or is the steel plate passing over the top of heating elements, or is the heat from exposed burners or external burners directly impacting the steel?

Remember: Homework assignments - such as this simplistic question - are not permitted.

Our assignment to you: What is the Cp of this alloy steel? What is its emmisivity (as-heated color or shininess?)

Example: A cookie sheet (slightly thicker than than your 0.003 (inch or mm ???) thick sheetmetal plate) begins cooling down from 393 degree F (400 degree nominal) towards that room air temperature within seconds of coming out of the oven. The over may take 15-20 minutes to heat up. To get a 400 degree cookie sheet, the oven needs time to REHEAT back to its 424-450 degrees while it is heating the cookie sheet because you opened the door to put the cookie sheet inside. But, since the oven is still hotter than 392 deg F even as it recovers its lost energy and as it heats the room air introduced with the cold cookie sheet, the cookie sheet itself is heating up towards its goal.

The real world is more complex than homework assignments. Real ovens are not found in textbook questions.

 

[Maynut]

Hi, Thanks for the responses,

The oven is forced convection, and should have a consistent temperature of 200^c throughout the envelope.
The reason for the question is we are having a 37m oven installed for powder coating. I need to make sure that the initial temperature can be reached, and that the powder coating curing process can be carried out in this distance at a given feedrate (around 1.5m/min). All the units are SI.
The material that will be processed will be hot rolled or cold rolled steel, and the surface will be pickled and oiled, although the pickling acid would have been removed in the pre-treatment section.

The heaters are typically 600w running power.

Thanks again for your feedback, I will look into your suggestions

With Kindest Regards
Maynut

 

[racookpe1978]

In my test of a preheated 425 F oven with still air and the heating elements below the flat thin plate, the dry dark sheetmetal did not reach 400 deg F (200 nominal C) until 8-12 minutes after insertion, depending on how long the oven door was held open putting the plate inside. Initial temperature was 70 degree F (25 deg C) for the steel plate. Temperature by contact thermometer.

The cool liquid on the actual sheet metal will require more energy to heat up and begin boiling off.

Strongly suggest moving the air around within the oven, as long as you do not "blow" the expensive heated air out of either end. The metal is so thin you will need to be very careful that hot spots do not occur in the oven - those will melt off/burn the powder, where the cold spots will not melt the powder properly.

 

[EdStainless]

How much power will it take to heat the steel?
How many 600W heaters do you have?
If you are doing cut pieces 1m x 0.5m x what thickness??? (0.003mm?) What do they weigh each? What about the coating, or is this pre-heat?

In forced convection they will heat quickly, the issue is do you have enough heater power to keep the air at 200C when loading continuously.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[IRstuff]

"until 8-12 minutes after insertion"
Interestingly, the spec time for Costco's pizza ovens is around 7 minutes.

THIS has some curves on page 2. Pay particular attention to the flattish parts of the curves; this is what most of the previous posters have alluded to, that the temperature behavior is asymptotic , and if you need to hit at least 200°C for at least 10 minutes, then you need an oven temperature on the order of at least 220°C, to ensure that the curing starts as soon as possible.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[Maynut]

Hi All,

Thanks ever so much for all your responses.

Very much appreciated.

Kind regards
Maynut

 

[LittleInch]

I just did a basic calc which got to a power requirement of 26 kW for heating without any losses, based on a continuos feed. If it's just the single plate which is slowly going through the oven that's a different story, but at 1.5m/minute you're going to get a lot of differential temperature effects, bending, warping etc. If not that's a lot of 600W heaters. ....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Maynut]

Hi All,

Quick update.

In the end i used Lumped thermal analysis. This method seemed to give meaningfull results.

The heating output from the oven was 600kW, not sure where i got 600W from.

Thanks again for all your help!

Kind regards
Mark

 

[MacGyverS2000]

The heating output from the oven was 600kW, not sure where i got 600W from.

<chuckle> Forgot to carry the one!

That certainly has a major effect on how fast the piece is heated...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

Any math is going to get botched up by one of a bunch of variables being guessed wrong.

You should do an empirical test. Find an oven around town, tape a small cheap thermocouple to the middle of a clean plate no one will mind running thru their oven and use your multimeter's T/C mode to watch and log the result. Use that aluminum tape.

Or have the company you're buying the oven do the test. They won't bat-an-eye at that as a lot of customers will need that test done before signing up for an oven. I have laser companies prove their lasers before buying. I have machine tool companies machine parts before buying. Marking companies demonstrate their products on our production materials etc etc. Those are all way harder than running a piece of metal thru an oven. I suspect the oven company has all the ride-thru sensor stuff and loggers already.

Keith Cress
kcress -

http://www.flaminsystems.com