Time to reach Melting point....

[NickE]

This is a pretty easy calculation, however I've been out of school too long and my last two jobs had very little thermo, or even math required.

If I know my mass, Cp, Mp and Ambient I can calculate the number joules required to make the mass of metal reach the Mp. How do I figure time?

Joules are convertable to watthour, I know the wattage of the heater, so then do I just divide the two?

IE: 20Kg, 0.167 J/gm*C, 138C, ambient 20C
Res: 394120J

Converting gives: 109.5 watthour

The heat required to get over the heat of fusion (Hf) is given by:

mass*Hf = 982000J

and conversion gives: 272.78 watthour

So the total required heat to melt 20kg of metal is 382.28 W*hrs

Do I just divide by the wattage? (I know that this is assuming 100% effective transfer of heat, but some assumption is always necessary.)

So a 1000watt heater will take 0.3823hrs? My empirical evidence doesn't support this result.

Even at 20% transfer I get 1.91hrs, still nowhere near the empirical result.

Or is my 1000watt melting pot just really that bad at transfering the heat to the metal?

thanks

Nick
I love materials science!

 

[IRstuff]

Nick,

The time has a lot to do with the material itself. You wouldn't expect a Shuttle tile to heat up in the same time as a chunk of copper. Therefore, the thermal conductivity of the material comes into play.

As a gross check, you can place all the thermal mass at the end of the thermal resistance and calculate it like an RC-time constant problem. Ostensibly, the time to heat should be no more than about 3 times the product of the J/K thermal mass times the K/W thermal resistance.

TTFN

FAQ731-376

 

[metengr]

Nick;

http://books.google.com/books?id=6z...&hl=en&sa=X&oi=book_result&resnum=8&ct=result

 

[btrueblood]

Nick,

1 Watt = 1 Joule/second

Taking your two figures for the heat input required = 1.376 MJ, divide by 1000 Watts, to get 1.376x10^3 seconds, or 22.9 minutes, or 0.38 hours - so your math checks ok.

Yes, the transfer is just that poor. You would probably find, if you hooked up a thermocouple and plotted T vs. time that the temp. climbs quickly, then slows as the pot gets hotter. Put another way, the heat transfer was quite efficient at low temperatures, but as the pot got hotter, more and more heat was lost to the surroundings (both as radiant losses and convection losses). Even when the wattage is being dumped directly inside the part (e.g. inductive or resistive heating), the radiant loss at 1900 F is pretty high, and the efficiency can be low.

 

[NickE]

Thanks guys, just wanted to make sure that I was doing the figures correctly.

The current melting pot I am using was picked by my boss, it is for melting wax, the energy density is not what I would chose for melting the alloy i'm working with. (52Bi-48Sn -- Eutectic, Melting point 138C)

There is a heaterband on the pot that is halfway up the side wall, this is not in contact with the alloy whatsoever when melting, the alloy is too dense to completely fill the pot.

I am not given the option to re-work the pot, nor can I buy a new one. I also cant use alternate methods to put heat into the system.

Ughhh... I think my boss is nuts.

nick

 

[insult2injury]

If you have empirical results, why do you need to calculate the time required?

I2I

 

[IRstuff]

What's the pot made from? You can get a crude estimate by figuring the thermal resistance of the pot from the heater band to the alloy, and throw in some heat losses along the way, and cranking with the thermal mass of the alloy.

Most crucible materials tend to have poor thermal conductivity, and you probably have some pretty massive heat losses.

TTFN

FAQ731-376

 

[btrueblood]

Could you put some dunnage (filler) in the bottom of the pot, so that the liquid level is up closer to the heater band?

I was thinking ball bearings...but if you are then dipping parts in the solder, you have a lot of void space in the bottom that never gets used.

Have a filler block of some alloy machined to fit the bottom of the pot?

 

[EdStainless]

How about filling the bottom of the pot with a castable ceramic? I assume that the outside of the pot is insulated. If not will it tolerate it?

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[btrueblood]

Another thought that gets around your boss: preheat the alloy slugs in an oven to say 50-100 C before putting them in the pot. Tell the boss you are doing so for safety reasons - "drying" the slugs before putting them into the pots, as this helps prevent steam explosions from splatting hot molten metal all over the operator.

 

[NickE]

Thanks for the help guys, Looks like the system bottle neck (melting time) can be overcome through a couple of methods.

1) keep a full charge molten in the pot at all times.
2) adjust the controller to over the desired pouring temp.
3) add solid metal as needed to maintain the pot temp at the desired pouring temp.

thanks
Nick