Why 15 is the magic number for your recirculating chiller

Recirculating chillers are essential equipment for cooling and condensation of solvent during rotary evaporation. A common misconception is that setting chillers to the lowest temperatures offers optimal results during distillations. I am here to give you more details about the importance of energy balance, how to calculate cooling capacities and what cooling temperature guarantees sufficient condensation.

I was chatting with my nephew the other day about nice activities that we’ve done in the past. He suddenly remembered that I took him to the circus one year and we rejoiced at the memories. One performance that stood out in my mind was the balancing act. The performer had climbed on top of a ladder and was juggling more and more objects in the air. His colleagues kept throwing more and more at him and the audience was extremely impressed. But after the 20th baton or so, he suddenly slipped, lost his balance and was lucky to be saved by his diligent assistants.

Well, we often try to push to the limit, yet finding the right balance is usually more important than achieving the best results. In the circus act, the audience was more than happy with a balancing feat of about 15 objects. There was simply no need to throw more in, risk the performer’s safety and affect the outcome of the performance.

The theme of balance is recurring not just in life but on the blog. I’d like to devote some more time to the subject here, this time when I talk about chillers in rotary evaporation with you.

The recirculation chiller is needed to support the rotary evaporator with sufficient cooling to condense the vaporized solvent. When setting up the chiller, you should always remember that the distillation should be in balance. What I mean here is that the energy for evaporation and condensation of the solvent should be equal. I’ve already brought up the topic of energy balance in my blog post about the delta 20 rule in rotary evaporation, in case you want to read up more on the subject.

Every solvent has its own heat of vaporization, also known as heat of evaporation or as enthalpy of vaporization. The term describes the energy needed to transform a given quantity of a substance into vapor at a given pressure. Or in simple terms, heat of vaporization is the energy required to evaporate a substance.

How can we calculate cooling capacity?

Let us try to find this energy for an example solvent, in this case water. The maximal distillation rate with a laboratory rotary evaporator is about 1.3 liters per hour. Let us use this value to see what this means in W. We use the following equation:

The heat of vaporization of water is 2428 J/g or Ws/g, so:

Hence, 877 W of energy are needed to evaporate 1.3 kg of water.

Now, I just explained that we need to maintain a balance between the energy of evaporation and energy of condensation. Therefore, 877 W cooling capacity is needed to condensate the evaporated solvent.

But bear in mind that to achieve the maximal distillation rate (1.3 L/hour), the rotary evaporation needs to be operated at a high bath temperature of above 80°C. However, the typical bath temperature is usually set at about 50°C. Of course, at different bath temperatures the distillation rate is also different (see table below).

Bath temperature	Boiling point	Distillation rate of water	Corresponding power
80°C	30°C	1.3 L/h	877 W
60°C	30°C	1.0 L/h	674 W
50°C	30°C	0.8 L/h	540 W
40°C	30°C	0.6 L/h	405 W

The table shows that at a typical bath temperature of 50°C and a boiling point of 30°C, 500 W is used for the evaporation of 0.8 liters of water per hour. To maintain the energy balance, 500 W cooling capacity is needed for this distillation. And with these conditions, our distillation rate of water is a respectable 0.8 L/h.

Now we’ve got this figured out, let us try to determine the optimal cooling temperature. Well, many people assume that setting the chiller to the lowest possible temperature is the best approach. But are they correct?

For the best evaporation performance, we need a significant temperature difference between heating bath and boiling point. But we still need to consider that the boiling point should be above the ambient temperature or the collected distillate starts to boil in the receiving flask. The vapor might then get lost in the vacuum pump.

Keeping this in mind, the recommended boiling point should be about 30°C in most cases. For an efficient condensation, a minimal temperature difference of 15°C is sufficient in most cases.

A sufficient condensation for most solvents can then already be guaranteed with +15 °C cooling temperature.

At this magic number, water as cooling medium is sufficient, there is less condensation of humidity on the cooling hoses and there is no ice forming on connections and hoses.

Still, are you more efficient with lower cooling temperatures?
Well the cooling capacity is related to cooling temperature. Take the table below for the BUCHI Re-circulating Chiller F-305:

Cooling temperature	Cooling capacity
20 °C	620 W
15 °C	550 W
10 °C	440 W
0 °C	250 W
-10 °C	80 W

As you can see, low cooling temperatures are inefficient, since at such low temperatures, the chillers lack the required cooling capacity for the condensation.

So without any clowning around, you can comfortably set your chiller at optimal, rather than lowest cooling temperatures. I hope you keep reading the blog for more magical tips on how to balance your lab life.

Till next time,