Temp vs Pressure in Zinc heat up

........[where angels fear to tread].........

Assuming the heat source is the same and its input rate is the same before and after then the effect of pulling a vacuum above a solidified pool of zinc will be a marginal increase in the heating rate, as removing the atmosphere from above the vessel will eliminate heat loss by convection.

Now, if the zinc were fragmented rather than solid, one might be relying on the atmosphere between the fragments for a measure of heat transfer from the heat source to the fragments, in which case, removing the atmosphere would slow the heating rate down marginally.

How did the vessel become filled with solid zinc, one wonders? Could the zinc be kept molten in storage, so as to save operating time by eliminating the cooling event and the re-melting step?

The heat source will be the same BTU either way. The zinc is actually pumped into 55 gal drums before the kettle is changed, then packed back into the new kettle. The industry theory has always been that the tighter you pack the zinc the faster it melts. My ideas was to remove the air from between the zinc so that the heat will transfer faster. Thanks for the reply.

While pressure has a significant effect on boiling point, it has a nearly negligible effect on melting point.

In theory, pulling a vaccuum will decrease the melting point as well as the evap. point. Water can boil at 50 deg C if pressure drops to the right point. Pressure cookers work on this principle. If the pressure is increased, the boiling point is increased, and the food will cook faster because otherwise the cook temperature would never exceed 100 deg C. So the principal of reducing your melting time by reducing pressure is correct, but as others noted, it is probably not worth the logistical effort needed to create that environment around your metal.

Also, if you were to achieve this lower temperature melting of your metal, as soon as the material is exposed to atmosphere, it might flash freeze on the surface making it impossible to pour into your mold.

Friend--I cannot agree with your analysis. Although there is some variation in melting point as pressure changes (usually rising with increasing pressure because the liquid is less dense than the solid), I believe that any effect would be very small in the problem as given. In my handbook the melting point of zinc is given within 0.1 deg.C, with no reference to pressure. When you create a vacuum, you are only varying the pressure by a relatively small amount.

You also state that there could be the possibility of flash freezing. Your wording sounds like you expect the entire kettle to solidify. Did you leave out the heat of fusion (which would raise the temperature of all the zinc as a small portion solidifies), or leave out the possibility of supercooling?

I agree that the biggest effect mentioned in the discussion is the very modest effect of convection within the gas mixture surrounding the zinc, and its very modest increase in the speed of heat transfer during the time between the starting of heating and the creation of a liquid layer on the bottom.

-jmm

Let me re-iterate the things that I stated which you have misinterpreted:

You say: "Although there is some variation in melting point as pressure changes (usually rising with increasing pressure because the liquid is less dense than the solid), I believe that any effect would be very small in the problem as given"

My Statement: "So the principal of reducing your melting time by reducing pressure is correct, but as others noted, it is probably not worth the logistical effort needed to create that environment around your metal."

I am clearly saying that it won't work, but in theory there would be some change in melt temp with pressure. So in fact we agree. By the way, I'm sure there is a pressure reference in that look up table at the bottom in fine print.

Again, You say: "You also state that there could be the possibility of flash freezing. Your wording sounds like you expect the entire kettle to solidify"

My Statement: "Also, if you were to achieve this lower temperature melting of your metal, as soon as the material is exposed to atmosphere, IT MIGHT FLASH FREEZE ON THE SURFACE, making it impossible to pour into your mold."

This is pure speculation on my part as evidenced by the word MIGHT.

Now, since you have incorrectly represented my point (which is pretty rude), I would like to challenge one of your closing statements: "I agree that the biggest effect mentioned in the discussion is the very modest effect of convection within the gas mixture surrounding the zinc"

First of all, you are not agreeing with me because I never said that because it is completely bogus. It is not convection which is transferring heat through the air or gas between the chunks of metal, which would be the transfer of heat from a solid to a moving fluid (or vice versa). Rather, the heat transfer vehicle would be CONDUCTION which is the transfer of heat through a solid or stationary fluid (in this case a stationary fluid).

I do not appreciate being mis-quoted and having such a bogus attack on my post which was based completely on ignorance. Perhaps you would be prudent to look more carefully at someone's wording before making assumptions, because you appear rediculous.

Let's review: PWSlack (post #1) discussed two opposing effects of pressure on heat conduction--decreased energy loss above the surface of a solid mass when pressure is decreased, and decreased transfer in the interstices between particles in a fragmented state as pressure is decreased.

cmorg6 (post #3), who appears to be the original poster, clarified the conditions to be a kettle refilled from 55 gallon drums, so it does not appear to be a solid mass.

Combining these two posts yields the probable conclusion that: initially the best heat transfer to the solid zinc is without a vacuum, but over the entire melting period, a vacuum (or partial vacuum) would result in a lower heat loss from the kettle and a slightly lower heating time. This change over time is because of convection (or lack thereof in a vacuum).

Howetwo (post #2), noted a negligible effect of pressure on melting point, in contrast to a significant effect of pressure on boiling point.

You (post #4), agreed, stating "In theory, pulling a vaccuum will decrease the melting point as well as the evap. point.", then illustrated this with an example of the boiling point of water at lower and higher pressures.

We agree on the conclusion to that paragraph, but appear to disagree on the reasoning behind it.

Later in my post (#5) when I referred to "the biggest effect mentioned in the discussion" I was referring to all the posts on this thread, not just your own. If that caused confusion, my lack of clarity in the reference is at fault. I did misread your comment about flash-freezing--but did not intend rudeness thereby.

Convection is a non-negligible mode of heat transfer at different stages in the question as raised in the original post, although its overall contribution is probably minor. To imply motivation or personal character by saying: ". . . such a bogus attack on my post which was based completely on ignorance." does not help either of us.