Specific latent heat of vaporization – refers to the amount of heat energy required to convert one 1 kg of a substance from a liquid to a gas or conversely the amount of heat energy to be removed to go from a gas to a liquid. This conversion is called a phase change and it requires more energy to change phase than it does to merely change temperature.
If it is known how much condensate of what type of solvent is collected over what period of time and the required condenser temperature, the latent heat of condensation can be found on Google and the heat load can be calculated.
Heater size – When the requested data is not available then select a refrigerated circulator that matches or exceeds the heating element wattage of the Rotary evaporator bath at the lowest required condenser temperature specified by the customer.
Vapor Pressure Temperature – is the boiling temperature of the fluid. Lowering the air pressure with a vacuum pump lowers the boiling point.
This is why water boils at a slightly lower temperature at high altitudes than at sea level. Some Rotary Evaporators have vacuum pumps attached to lower the pressure in the rotary flask which also lowers the boiling point of the fluid in the flask. If the flask is operating under a vacuum, the vapor pressure temperature or the temperature required to condense needs to be known.
20 degree rule – when the desired condenser temperature (setpoint) is not known, the 20 degree rule can be used as a guideline. It states that the condenser should be 20°C lower than the vapor pressure temperature and the rotary evaporator water bath temperature should be 20°C higher than the vapor pressure temperature.