The balance decided what leaves. These two ratios decide what circulates, and circulation is where the energy bill lives.

Reflux ratio: $R = \dfrac{L}{D}$ — R equals L over D. $L$ is the condensed liquid pumped back onto the top tray, $D$ is the product drawn off, both in kmol/h, and $R$ is bare. Read it in plain English: R moles come back down the column for every one sold.

Boilup ratio: $V_B = \dfrac{V}{B}$ — V-B equals V over B. $V$ is the vapour the reboiler raises back into the bottom tray and $B$ is the bottoms drawn off. Same shape, other end of the column, and it is the steam valve written as a number.

The multiplier is the part worth memorising. All the vapour reaching the condenser is the reflux plus the product, so $V = \left(R + 1\right)D$. Condenser duty, reboiler duty and column diameter all scale with that $V$, not with $D$. Raise the reflux ratio from 1 to 3 and the product flow has not changed at all, while every piece of hardware around it has doubled.

Two near-relations get confused with $R$ and both are real. The internal reflux ratio $L/V = R/(R+1)$ is always below 1 and is what a tray actually experiences; quote it into a correlation written in the external one and the design comes out short. And a column with a partial condenser defines its reflux against a vapour draw instead. Check which one a published figure means before comparing two columns.

The economics are a genuine optimum, not a limit. Below about 1.1 times the minimum reflux the tower grows without bound; above about 1.5 times it, the stage saving has flattened while the utilities keep climbing. Their sum has a shallow minimum near 1.2 to 1.3 — shallow enough that operability and future debottlenecking often pick the final number instead.