Got a small question about condensers(The one you would use for separation in fractional distillation)

[Sigmarus]

So basically,i have always wondered why the water outlet tube is on top of the condenser while the water inlet tube is on the bottom.Thanks!

[hypervalent_iodine]

If you put it at the top, you tend to get a persistent air pocket and the condenser never really fills with water. Not the case if you have it going in from the bottom.

 

Edit: the explanation for that is simply gravity, by the way. Going in from the top, the water just falls straight down to the outlet and you get the air gap / pocket at the top end. From the bottom the water has to fill up the condenser before it reaches the outlet, so no air bubble. This is also true when working with reflux condensers.

[Sigmarus]

Thanks alot!

[studiot]

Hypervalentiodine has offered a very practical reason but there is also a thermal one that is important in other situations as well.

 

The evaporate enters at the bottom and you want maximum cooling at this point.

Cooling depends upon temperature difference and this is maximised as the difference between the evaporate at its hottest and the cooling water at its coldest.

 

The principle is used in a different way in hot water cylinders.

Hot water rises over cold water and does not mix rapidly with it.

So if you put the replacement cold in at the bottom, the water in the top of the cylinder remains hot and you can continue to draw off hot water.

Doing it the other way round the cold would fall through the hot and mix somewhat, and you would be drawing of much colder water much sooner.

[hypervalent_iodine]

Hypervalentiodine has offered a very practical reason but there is also a thermal one that is important in other situations as well.

 

The evaporate enters at the bottom and you want maximum cooling at this point.

Cooling depends upon temperature difference and this is maximised as the difference between the evaporate at its hottest and the cooling water at its coldest.

 

The principle is used in a different way in hot water cylinders.

Hot water rises over cold water and does not mix rapidly with it.

So if you put the replacement cold in at the bottom, the water in the top of the cylinder remains hot and you can continue to draw off hot water.

Doing it the other way round the cold would fall through the hot and mix somewhat, and you would be drawing of much colder water much sooner.

This is true, but not so important if the water is constantly running, and even less so if it is chilled. When doing reactions under reflux, you still use the same approach, yet the warmest part in this set up would be the bottom where the water is coming in. The reason is really just because it's difficult to fill the reservoir up if you go from top to bottom.