"Cydrome Leader" wrote in message news:nflqsr$cp$ snipped-for-privacy@reader1.panix.com...
Can you explain how vapor phase drying (as in making power transformers) and degreasing works? I've never come across a written explanation that makes any sense or explains the difference between dipping something in clean solvent and then shaking it off and drying it. ====================================================================
Not sure what vapor phase drying is, unless it is the final stages of vapor phase degreasing, but here goes:
If you heat a pot of clean solvent and then dip something in it for cleaning, that item gets pure solvent at the start but any residues that dissolve in the solvent leave it less than pristine so the first item isn't perfectly clean when you take it out, and the next item you want to clean will be even dirtier, so if you want clean solvent for the next item you have to throw out the first pot of solvent and start over, which uses up a bunch of solvent. If you use a tall pot with a cooling coil around the top and only put a small amount of solvent in the pot, when you heat it you will fill the pot up to the cooling coil with solvent vapor. Any vapor that reaches the cooling coil condenses and runs down the sides of the pot back to the liquid in the bottom. Basically this is a refluxing still. So long as any impurities in the liquid solvent have a vapor pressure much higher than the solvent vapor pressure, the vapor in the pot will be essentially pure solvent vapor no matter how much dirt has accumulated in the bottom of the pot. Now dip an item to be cleaned down into the vapor, but not all the way down into the liquid. Since the item is at room temperature (well below the boiling point of the solvent) vapor will begin condensing on the item, dissolving residues, and rinsing them into the liquid in the pot as the solvent drips off the item. However, the item is warmed up by the heat of condensation of the solvent and eventually reaches the same temperature as the solvent vapor. At that point condensation stops, along with the cleaning action. The item may appear moist or almost dry. Now when you lift it up to the top of the pot any liquid solvent on the item will evaporate as the item rises above the solvent vapor in the pot. The cooling coil is made tall enough so that this last flash of evaporation is captured and condensed back into the pot, to minimize any loss of solvent by "drag out". I assume that this last flash of evaporation is what you are referring to as vapor phase drying. The advantages of vapor phase degreasing is that each item gets very pure solvent and you aren't constantly throwing out solvent as it gets dirty, but the limitation is that once the item warms up you have to take it out and let it cool if you need more rinsing. Now, if you make the still a bit more complicated, by collecting the pure solvent that is condensing at the cooling coil and storing it in a second tank (returning any excess to the first pot so it doesn't run dry), this tank will always have very pure solvent so you can do lots of rinsing - this is the fancy system that John has. You can do vapor phase degreasing in the headspace of the first pot, and rinse with as much solvent as you have clean in the second tank by holding the part in the vapor of the first pot so all the liquid goes to the bottom of the first pot to be redistilled. In the old days chlorinated solvents like chloroform or trichloroethylene were used since they were great degreasers and had dense, heavy vapors that stayed down in the pot instead of filling the room. Not sure what is used now, but John said his was $100 per gallon so it must be good :-). Hope that wasn't too long and actually answered your question.
----- Regards, Carl Ijames