Fast air quench for Aluminum plates 1

[andrewjmorin]

I've got racks of aluminum plates-- 1/4 inch thick, about 1500 lbs total-- solution treated to 870F that are then water quenched to achieve T6. Works pretty well if you don't mind distortion effects from the thermal shock. B-(

I want to change this to an air quench, but there is a time factor in the process. We have something like 30 seconds to reduce back down to normal room temps.

Can anyone help me figure out what kind of volume of air would be required? Are we talking tanks of compressed air, or could a few good blowers do the job?

 

[IRstuff]

I don't think you can get there from here.

Your total joule heat divided by the time
= 500 J/gm-°F * 1500 lb * 800°F / 30 s = 9 gigawatts

Assuming 14 W/m^2-°F and 80 m^2 of surface area, you'd only be able to flow 444 kW

With water, I assume there's lots of sturm and drang, with plenty of evaporation, which works very well at sucking up joules.

TTFN

FAQ731-376

 

[Compositepro]

I imagine this is a very common problem for a lot of people. Distortion would be caused by uneven cooling and it is very difficult to quench cool a plate evenly. You might get some better advice in the Metaurgy forum. Polymer quench additives would probably help.

 

[IRstuff]

I would think that distortion is invariably a consequence of rapid thermal changes. Unless you could arrange to have the plates cooled uniformly, there will always be temperature differences across the plate during quenching, which would induce stress, and distortion.

TTFN

FAQ731-376

 

[andrewjmorin]

IRstuff thanks for taking the trouble to run those numbers for me up there. Is there a reason you stopped? Once you know the power involved, will the heat transfer to the air not submit to a similar solution?

 

[andrewjmorin]

Perhaps more information might help:

The water quench tank contains approximately 2000 cubic feet of water. What would be the volume of air that could provide equivalent cooling?

 

[IRstuff]

I misread a decimal point as a comma; that's what I get for not using reading glasses ;-)

It's only 9 MW, duh...

I did do the airflow, resulting in 444 kW of heat removal for a fairly high airflow.

To recap, for your 30 second timeline, you need to have a heat flow of 9 megawatts. For a reasonable air flow, you can only achieve about 400 kW, that's a factor of 25 difference.



TTFN

FAQ731-376

 

[andrewjmorin]

So, if you don't mind my asking, what's the airflow you used?

 

[IRstuff]

About 12 knots, but that's a diminishing return, at triple that air speed, you'd barely double the heat transfer coeffcient.

TTFN

FAQ731-376

 

[andrewjmorin]

Terrific, thank you.

So a (12 x 25 x 1.5 x 1.15 =~ ) 520 MPH airstream might be in the ballpark?

 

[IRstuff]

Yep, that might do it, but it might also be horrifically turbulent, which lowers the efficiency.

TTFN

FAQ731-376