Rotary Evaporators Explained: How to Choose One

A rotary evaporator, or rotovap, removes a solvent from a solution gently and quickly, and recovers that solvent so it can be used again. It does this by combining three things: rotation, reduced pressure and a warm water bath. The result is fast evaporation at temperatures low enough to protect heat-sensitive compounds.

This guide explains how a rotary evaporator works, what the size of a system actually tells you, and how to put together a rotovap, chiller and vacuum pump that suit your work.

The parts of a rotary evaporator

Part	What it does
Evaporating flask	Holds the solution and rotates continuously
Heating bath	Warms the rotating flask gently and evenly
Vacuum pump	Lowers the pressure so the solvent boils at a lower temperature
Condenser	Cools the solvent vapour back into a liquid
Chiller	Circulates cold fluid through the condenser
Receiving flask	Collects the recovered solvent
Lift	Raises and lowers the flask in and out of the bath

How rotary evaporation works

The solution goes into the evaporating flask, which is fitted to the rotor and starts turning.
The vacuum pump reduces the pressure inside the system, lowering the solvent's boiling point.
The flask is lowered into the heating bath. Rotation spreads the solution in a thin film over the inside of the flask, which greatly increases the surface area for evaporation.
Solvent vapour travels into the condenser, where chilled coils turn it back into liquid.
The recovered solvent drips into the receiving flask. What stays behind in the evaporating flask is your concentrated product.

The 20 / 40 / 60 rule of thumb

A common starting point for setting up a rotovap is to keep about 20°C between each stage:

Heating bath: around 60°C
Solvent vapour: around 40°C, set by adjusting the vacuum
Condenser: around 20°C or colder

The gap between the vapour and the condenser makes sure the vapour condenses efficiently instead of being drawn into the pump. Many users run cooler (for example 50 / 30 / 10) to protect delicate compounds. Treat these as a starting point and adjust for your solvent.

What the size of a rotovap tells you

Rotary evaporators are named by the volume of the evaporating flask: 5L, 10L, 20L, 50L and so on. That is not how much you can process at once. As a rule, fill the flask no more than about half full, so a 20L flask runs roughly 10L of solution per load. You can keep topping it up during a run.

How fast you can work depends on the whole system, not just the flask: the heating bath's power, the depth of vacuum and, above all, how much vapour the condenser and chiller can handle. A large flask on an undersized chiller will not run any faster than a smaller system.

How to choose a rotary evaporator

1. Batch volume. Start with how much solution you need to process per day, then choose a flask size that handles it in a sensible number of loads.

2. Condenser and chiller capacity. Match the chiller to the condenser and the solvent you use. This is the most common bottleneck.

3. Vacuum pump. Rotary evaporation needs a steady, adjustable vacuum rather than the deepest possible one. A chemically resistant diaphragm pump, ideally with a vacuum controller, is the usual choice. See our vacuum pump guide.

4. Manual or motorised lift. A manual lift is simple and reliable. A motorised lift makes it easier to raise heavy flasks smoothly, which matters more as flasks get bigger.

5. Materials and safety. Check that seals and glassware suit your solvent. PTFE seals and good-quality borosilicate glass are standard. If you use flammable solvents, work in a well-ventilated area that is suitable for them, and follow the manufacturer's instructions.

If you would rather not match parts yourself, a turnkey kit pairs the rotovap with a suitable chiller and pump.

Getting the best from a rotary evaporator

Start rotation first, then bring the vacuum down gradually before lowering the flask into the bath. This reduces bumping and foaming.
Don't overfill. Keep the evaporating flask about half full or less.
Use a cold trap between the condenser and the pump to catch any vapour that gets past, protecting the pump. See our cold traps.
Empty the receiving flask before it gets close to full.
Look after joints and seals. Clean and lightly lubricate ground-glass joints, and replace worn seals before they leak.

Rotary evaporator FAQs

What size rotary evaporator do I need? Work back from your daily volume. Remember to fill the flask only about half full, and that the chiller often limits speed more than the flask does.

Do I need a chiller? Yes for any serious use. Tap water cooling cannot hold a steady, low condenser temperature, and poor condensing sends solvent vapour to the pump.

Which vacuum pump suits a rotovap? A chemically resistant diaphragm pump with adjustable control. You need a controllable, moderate vacuum rather than the deepest one.

Why is my flask bumping or foaming? Usually the vacuum was pulled too fast or the bath is too hot. Bring the pressure down gradually and lower the bath temperature.

Shop rotary evaporators

We stock Helios rotary evaporators from 5L to 50L with manual or automatic lift, along with rotary kits, turnkey solutions, chillers and heaters, vacuum pumps and glassware and adaptors. For availability or help putting a system together, call the Barnsley showroom on 0114 229 1807.