Understanding Energy Balance to Optimize Lab Efficiency: Rotary Evaporation

In this blog, I would like to discuss energy balance and its importance in laboratories. Using the example of a Rotary Evaporator, I shall explain the factors that influence and impact laboratory equipment and processes.

Every year, living in Switzerland, I find myself in a constant battle with my house in the mountains. The temperature outside fluctuates wildly throughout the year, from hot summers to snow-capped winters. My aim is always to keep the house – and myself – just warm enough. Cozy but not like a sauna. To do this, I must get the balance just right – the size of logs in winter, the airflow, and even the amount of ash in the fireplace can make a difference. Too little heat and I’m piling on sweaters; too much, and I’m sweating like I’ve run a marathon. Striking a balance is a bit of an art, guided by intuition, knowledge, and experience. The same balance must be considered in laboratories and is crucial to their design. A finely tuned laboratory can be the difference between an efficient process and a waste of resources.

In both my house and the laboratory, the same influencing factors are at play, namely a principle called energy balance. On a fundamental level, energy balance is best explained by the Laws of Thermodynamics that influence everything irrespective of the type of system or material in question. They hold true from the microscopic scale of atoms and molecules to the macroscopic scale of planets, stars, or entire galaxies. Understanding these principles is fundamental to the design of laboratory equipment, I recently talked with Peter, Buchi’s resident evaporation expert , about energy balance in a rotary evaporation system. The laws of thermodynamics ensure the glass can withstand the temperatures and pressures used during evaporation. They also dictate how energy will flow during the evaporation process. The laws are critical to laboratories that must understand energy balance for several reasons. The most important reason is to understand how much energy will enter the lab with each system, as this affects the type of ventilation system used, the air conditioning system, and even the fume hood variant.

How can you assess energy balance within a rotary evaporator?

As the First Law of Thermodynamics, also known as the Law of Energy Conversion, tells us, energy cannot be created or destroyed in an isolated system. The total amount of energy is constant, although it can change from one form to another, e.g., from kinetic to potential or thermal to mechanical energy.


• ΔU is the change in the internal energy of the system.
• Q is the heat added to the system (positive when heat is added, negative when removed).
• W is the work done by the system on its surroundings (positive when work is done by the system, negative when work is done on the system.

Therefore, the change in the system’s internal energy is equal to the heat added to the system minus the work done by the system on its surroundings.

When trying to understand how energy flows through a rotary evaporator, you need to know the basic energy balance.

The heat flux rate will equal the sum of the heat input, output, and reaction.

±𝑄𝑆𝑦𝑠𝑡𝑒𝑚 = 𝑄̇𝐼𝑛𝑝𝑢𝑡−𝑄̇𝑂𝑢𝑡𝑝𝑢𝑡±𝑄̇𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛

• 𝑄̇𝑆𝑦𝑠𝑡𝑒𝑚 is the rate of heat transferred into or out of the system.
• 𝑄̇𝐼𝑛𝑝𝑢𝑡 is the heat being added to the system from an external source.
• 𝑄̇𝑂𝑢𝑡𝑝𝑢𝑡 is the heat being removed from the system or lost to the surroundings.
• 𝑄̇𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛 is the heat generated or absorbed due to a chemical or physical reaction occurring within the system.

Therefore, the net heat transfer rate is the difference between the heat input and output, adjusted to account for any heat produced or consumed by reactions within the system. As no energy can be created or destroyed in an isolated system, all the energy will either be emitted as heat or go into the solvent’s phase transition.

A more detailed energy balance focusing on emissions shows that overall heat losses are a sum of the individual system components:

𝑄̇𝑆𝑦𝑠𝑡𝑒𝑚 𝑙𝑜𝑠𝑠𝑒𝑠 = 𝑄̇𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑙𝑜𝑠𝑠𝑒𝑠 + 𝑄̇𝐻𝑒𝑎𝑡𝑖𝑛𝑔 𝑙𝑜𝑠𝑠𝑒𝑠+

𝑄̇𝑃𝑢𝑚𝑝 𝑙𝑜𝑠𝑠𝑒𝑠 + 𝑄̇𝐼𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑒 𝑙𝑜𝑠𝑠𝑒𝑠 + 𝑄̇𝑅𝑜𝑡𝑎𝑣𝑎𝑝𝑜𝑟 𝑙𝑜𝑠𝑠𝑒𝑠

Picture 1: Pathway of energy in a Rotavapor® system

In the rotary evaporator, the chiller and the heating bath are the most significant contributors to total energy.

𝑄̇𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝑙𝑜𝑠𝑠𝑒𝑠 = 𝑄̇𝐶ℎ𝑖𝑙𝑙𝑒𝑟 𝑖𝑛𝑝𝑢𝑡 + 𝑄̇𝑉𝑎𝑝𝑜𝑟
𝑄̇𝐻𝑒𝑎𝑡𝑖𝑛𝑔 𝑙𝑜𝑠𝑠𝑒𝑠 = 𝑄̇𝐻𝑒𝑎𝑡𝑖𝑛𝑔 𝑖𝑛𝑝𝑢𝑡 − 𝑄̇𝑉𝑎𝑝𝑜𝑟

They divide their power input into two parts: the first is released into the surroundings as emissions, and the second goes into the solvent’s phase transition. Transitioning from a liquid into a gas requires energy, whereas transitioning from a gas into a liquid releases energy. It comes as no surprise that the heating bath and chiller also cause the most significant energy losses. Another important point to consider in regard to energy loss is the Delta 20 rule. When setting up the evaporation process, the ‘Delta 20 ’ rule refers to the temperature gradients between the heating bath, solvent vapor, and the condenser. The bath temperature should be 20 ºC higher than the boiling point of the substance you want to evaporate, and the coolant should be 20 ºC or more, lower than the vapor temperature. For example, you could set the bath temperature at 50 ºC to yield a solvent temperature of 30 ºC, which is subsequently condensed at 10 ºC. These 10/30/50 parameters are suitable for evaporation to bring and carry off accumulated energy efficiently. Neglecting the Delta 20 rule can lead to unnecessary energy losses in the system. Understanding these losses can be crucial for designing more energy-efficient systems and improving the overall efficiency of a system or process .

By analyzing the energy balance of laboratory equipment and processes, one can easily calculate how much energy will enter the lab as emissions. Understanding this can reduce the chance of over or under-dimensioning or choosing the wrong type of climatization. Under-dimensioning refers to components or equipment not being adequately sized or having the wrong specifications to handle the maximal expected requirements or loads. Correcting dimensioning is integral to preventing long-term issues, such as humidity, mold, and corrosion.

This understanding of energy balance in laboratory equipment drives chemists like Peter to design more efficient instruments and develop techniques to save energy by reducing energy output. One example of an energy-saving technique for rotary evaporators is “swimming/floating balls”. These are placed in the bath to minimize the loss of heat due to the evaporation of the bath fluid. The balls help with heat retention as they create a layer on the bath’s surface, reducing the exposed surface area of the liquid and the amount of evaporation. The latest rotary evaporators also have features such as eco mode designed due to an understanding of energy balance. I hope this article has illuminated your understanding of how the laws of thermodynamics can be applied to laboratory equipment and lead to more efficient practices going forward.

Till next time,

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