If you reduce the amount of cooling water fed to a reflux condenser, at some point it can't condense all the vapor, and some will begin to flow past the condenser. If you do this by design, then the vapor getting past the reflux condenser can be condensed by a second condenser and collected as product.
Consider the operation of the unit in diagram 4-14. Initially, tap A is wide open and tap B fully closed. All the cooling water flows through the reflux condenser, and 100% reflux occurs. After the column has achieved equilibrium, tap B is gradually opened. As more water flows through tap B, less water flows through the reflux condenser, and vapor begins to pass by without being condensed and returned to the column as reflux. This vapor reaches the product condenser, where it is condensed as product (at whatever temperature you choose!)
We've found that turning just tap B offers more than enough control to reduce the reflux ratio to around 90%, but different stills may have different characteristics. The advantage of having two taps is that control of the water flowing through condenser 1 can be reduced still further after tap B is wide open. Closing tap A fully when tap B is wide open results in zero reflux, converting the system to a fractionating still or, if the heat input is increased, to a pot still.
This option requires constant pressure cooling water if the reflux ratio is to be kept constant for any tap setting. You can get this by using a small water pressure regulator or by making a small header tank. Also, the piping for the cooling water has to be laid out carefully to prevent airlocks from forming.
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