I have a 20L across international rotovap, and the manual says for coolant to flow into the top and out from the bottom. See picture. Typically coolant flow is the opposite in my experience - into the bottom, out from the top. At least that’s the way my SPD condenser is configured as well as my 2L rotovap, and the way it should be done with reflux condensers. This directionality should minimize air bubbles.
The 20L rotovap works just fine set up as per manual with coolant inlet from the top, outlet from the bottom. But I am wondering did AI make a mistake in their manual? Does it not matter? What is the directionality on your rotovap?
its good practice to always have the coldest coolant to interact with the hottest vapor, but i could see they having a frame of thought like; because the coldest coolant would then be at the vapor exit ensuring anthing that didnt condense below would condense at the top? maybe idk
If I remember correctly it’s sort of the same concept buchi used but they have the inlet outlet next to eachother in the lower back side of their coil. Should give a max level of volume in the condenser while under load.
But I can see them thinking of heat properties rising to the top of the condenser faster so as to cool the top more so then which in turn doesn’t allow for the vapor to have a chance to by pass to the vacuum port if it is on the top end.( seen some units that have bottom location vacuum ports as well like buchi & others.)
Coolest coolant contacting hottest vapor would be COcurrent heat exchange - which is not the more common practice for heat exchangers. The most common configuration for a heat exchanger is countercurrent (which is what AI’s document is showing) - cocurrent heat exchange is generally used for very specific modes of operation since its decidedly the less efficient configuration for heat exchange. That is to say, there is no reason to configure the less efficient mode of operation (co-current) unless there is a purpose to doing so (like if you’re trying to minimize condensate subcooling, for instance). I’ve attached a diagram and heat profile:
Heat transfer is actually going to be proportional to the delta T at any given point along that length of heat exchanger. In co-current you see the two streams pinch together and the delta T becomes smaller and smaller over the length of exchanger, right? Whereas with countercurrent the delta T remains much more constant over the full length of the exchanger. The implication here is that along that path length, heat exchange begins to drop off in co-current as you approach that pinch point. There is actually a technique used to size heat exchangers in industry called pinch-point analysis that relies heavily to this effect. Countercurrent maximizes the usefulness of your heat exchange surface area by maximizing the productive heat exchange along the full path length of the exchanger.
Yes, visually it may be appealing because the bubbles are purged from the lines faster, but a chiller with adequate flow rate should overcome this. Consider this visual - you set the rotovap up co-current (coldest fluid at bottom of coil, hottest vapor at bottom of coil, both travel upwards) then along that length of coil the glassware is getting warmer and warmer the higher up you go. That is to say, the glass at the top of the coil where fluid is exiting is now warmer than the glass at the bottom of the cool where fluid is entering. Now picture an excited lil’ superheated solvent vapor molecule under vacuum traveling upwards that has somehow not been condensed in that first few inches of contact with the coldest available heat exchange surface…if all the glassware that molecule is going to contact along its pathway up is warmer and warmer…what chance does it actually have to condense on warmer glass (with ever diminishing delta T) before it escapes the condenser and enters the vacuum system? Will that lil’ molecule exchange enough heat to condense before the heat exchange drops off? Maybe, but maybe not. With all else constant, you have a better chance of condensing that molecule if the coldest fluid is the very last thing that excited lil’ molecule sees before exiting the condensing section.
Do I think this particularly matters on a rotovap of all things? Barely, I think if your chiller is big enough you’ll hardly notice a difference. But again it is a definitive truism that countercurrent is the more efficient and therefore common heat exchange configuration than cocurrent - and countercurrent here is the configuration where the inlets are on opposite sides of the heat exchanger (hot vapor enters bottom, cold fluid enters top).
I ran into the exact same thing this week when emailed they told us to enter from the bottom of bottom condenser and feed up which is what I thought and why i emailed cause thats not what manual says, so thats how i plumbed it, i will know monday, and report back. I got everything vac’d down to 24"hg need to check some leak points and try again.
The reason for going bottom to top is the coolant will fill the entire bore of the coil and thus provide maximum surface area contact of the coolant with the vapors for maximum condensing power. Unless the coil bore (diameter) is so small that entering at the top still fills the coil completely, you will want to enter from the bottom
yeah i still havent figured out when you would ever feed from the top unless maybe if your pump is awesome and can overcome it just coming in and falling down instead of filling.
Maybe its simply cuz the circulation pump has to work less with gravity helping the flow instead of the pump always fighting gravity?
Maybe it has nothing to do with the cocurrent and countercurrent flows cuz honestly it wouldnt matter to much in this application imo.
A pump struggling will push less fluid thru per minute.
A pump using gravity will push max flow.
The only performance difference between counter current and cocurrent flows would be that one can flow faster than the other, meaning it pushes more coolant per minute than the other
If the hottest vapor comes in contact with the coldest part of the coil and doesn’t condense, why would it condense on the coil further up if it’s warmer?
Always have the fluid flow in the opposite direction of the vapor for best heat transfer
