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How should the water flow in your condenser? From the flask to the top of the condenser, or from the top of the condenser back to the flask? I could imagine both: flask to top will fill the entire condenser with water, because of the resistance of gravity, enlarging cooling area, or top to flask, because this will generate counter current exchange increasing heat flow, but then it is almost impossible to get a decent cooling surface without extreme water pressure.

This picture suggests the first.

Refluxing

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In general the cooling water is attached according to the countercurrent exchange mechanism to maximize the efficiency. Of course this doesn't work well for the Liebig condenser you pictured, but you generally wouldn't use this kind of condenser for reflux anyway. The most common condenser for reflux is the Dimroth condenser which can be used for countercurrent exchange without any problems as the entry and exit of the water are both at the top of the condenser.

Here is an image from Arbeitsmethoden in der organischen Chemie (page 43) that shows where to attach the cooling water for different condenser types.

enter image description here

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Water should always enter from the bottom of a condenser (the end closest to the flask) and exit from the top of the condenser. Doing it this way always ensures that your condenser will be full of cooling water. If you set it up the opposite way, and if for some reason the rate of water flow into the condenser decreased and became slower than the rate of water exiting the condenser, then the condenser would not remain filled with water. If the reaction were unattended, this could cause problems. BTW, here is a link you might enjoy. It contains a lot of lab tips, including "how to hook up the water tubing to a condenser".

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    $\begingroup$ Yes, I thought that would be the case, but then I remembered counter current exchange and started to doubt. $\endgroup$ – Jori Jun 25 '14 at 17:58
  • $\begingroup$ How should an empty space suddenly appear in the water? Bubbles, OK, one of several reasons why a Liebig cooler is totally unsuitable as a reflux condensor. $\endgroup$ – Karl Nov 29 '15 at 22:40
  • $\begingroup$ @Karl Gravity. If the flow come from the bottom, any flow will fill the condenser; if it flows from the top, the water will tend to flow under gravity to the bottom and low flow will mean the coils/free space isn't full as the flow will be determined by the rate water flows out under gravity not the inherent flow rate of the water. $\endgroup$ – matt_black Sep 24 '16 at 10:41
  • $\begingroup$ @matt_black 1000hPa is equivalent to a 10m column of water. Once the condensor is full of water, there is no way some vacuum space will appear in there. It can indeed be difficult to fill the condenser if you hook it up the wrong way. $\endgroup$ – Karl Sep 24 '16 at 18:28
  • $\begingroup$ @Karl "How should an empty space suddenly appear in the water? " If the top condenser connector is used as a water inlet, and if the inlet tubing falls off or otherwise fails, or if the water flow from the main is interrupted, then water will drain out the bottom condenser connector leaving the condenser jacket filled with only air. $\endgroup$ – ron Sep 24 '16 at 21:29
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Hook from the bottom flowing upward in case the total cocondensation doesn't happen before the vapor reached the totally liquid state. it will get the cooling first before the exit giving a positive condensation with easy cooling water adjustments.

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