The picture on the left shows the basic reflux setup consisting of are heating mantle, a round bottom flask, a Liebig condenser used in reflux mode and a gas adapter on top. The picture on the right side shows the flask with a matching heating mantle and a stirring bar.
In choosing the size of your setup, one has to keep the following issues in mind:
1. The flask should not be more than half-filled to allow for enough room during expansion and/or boiling. If a gas is formed in the reaction, the flask size should be larger.
2. The entire setup is clamped at the neck of the flask (using a proper sized clamp). The clamp has to have rubber tips in order to prevent that the joint will be crushed during the clamping. The clamp also has to be stable (=no partically broken arms) because it carries the weight of the entire setup.
3. The inlet of the water is on the lower end of the condenser, the outlet at the upper end independent if the condenser is used for reflux or distillation. This way a low flowrate of the water will be sufficient for the cooling because the outer jacket of the condenser will fill up with water first before it reaches the outlet.
4. The heating mantle should fit the flask size. A heating mantle that is either too small or too big, has a poor contact with the flask, which damages the heating mantle. The lack of contact causes the heating mantle to overheat and burn out. A heating mantle that is slightly to big can be filled with sand to achieve a better heat transfer.
5. Most heating mantles have no temperature/power control. An external device has to be attached to control the power input. Never plug the heating mantle directly into a wall plug!!
6. If you heat a system, you need to have an 'overpressure valve' somewhere in your setup. It does not matter what you heat, the media always will expand (more or less) and will ultimatively build up a significant pressure, which can cause heavy explosions, followed by a fire if a flammable solvent or pyrophoric materials are part of the reaction mixture. In many cases, the setup will be connected to the inert gas part of your Schlenk line. It is imperative that the stopcock is opened before starting the heating. Afternatively, a bubbler with mineral oil can be used as pressure exchange valve that allows the overpressure to be released without air getting into the setup.
7. The joints have to be lightly lubricate.
8. Lastly, a spin bar has to be added to the flask to mix the solution during the reaction/reflux.