Tim
The thermal energy required to remove ab/adsorbed water molecules from a
heterogeneous material is about the same, irrespective of the temperature
(within reasonable limits and provided you do not have a phase change, such as
exceeding the Tg, which, incidentally, drops by about 20 °C when the board is
humid). If you use a vacuum oven, you have extra energy required to turn the
pump and operate the traps. The pump runs hot, after a time, and this heat is
dissipated into the atmosphere and not into the oven. If you use a cryogenic
trap to prevent oil vapour from entering the oven or moisture from emulsifying
the lubricating oil, this also requires energy to produce the removal of heat.
If you use absorber/adsorber traps, they require energy to regenerate. Overall,
vacuum drying requires 50-100% more energy than atmospheric pressure drying,
albeit the temperature may be considerably lower.
Brian
Timothy Reeves wrote:
> When you say that the energy is actually greater, you mean the energy
> imparted to the escaping water molecules, right? But what does that matter?
> What really matters is energy imparted to the board. The board itself may
> actually decrease in temperature if a low enough pressure is obtained. In
> that sense, Doug was right (but he should have said "less heat [transferred
> to the polymer, glass, and copper]")
> I know... you were just trying to point out the misconception most people
> have regarding temperature vs. heat.
> I love these technical arguments... :-)
> Tim Reeves
>
> > ----------
> > From: Brian Ellis
> > Sent: Thursday, June 1, 2000 7:54
> > Subject: Re: Baking of boards
> >
> > Sorry, Doug, the heat is more or less the same. The energy required is
> > actually
> > greater. It is the temperature that is lower.
> >
> > Brian
> >
> > Douglas Pauls wrote:
> >
> > > If you are really concerned about the effects of heat on the polymer
> > > qualities or the oxidation of the surfaces, consider using a vacuum or
> > > vacuum-make operation. Gets the water out with less heat.
> > >
> > > Doug Pauls
> > >
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