The cooler is an important part of many rotary evaporator units, as it is responsible for cooling the coolant flowing through the condenser. Buying a chiller can be avoided, for example, by using a dry ice condenser or reviewing your own chiller, but in most cases a suitable chiller will be required.
Important factors to consider when shopping for the right cooler for your unit are:
Minimum temperature and cooling capacity
pump pressure
pump flow
In this article, we explain these factors in detail to help you find the right chiller for the job.
1. Minimum temperature and cooling capacity
The lowest temperature the chiller can sustain needs to be at or below the temperature at which the coolant is maintained for condensation to occur at a good rate. This temperature depends on the boiling point of the solvent. The "Law of 20" is a good guide when determining what temperature to set your chiller at. That said, your cooler should be able to provide a coolant flow that is at least 20°C below the boiling point of the solvent.
You'll probably want to set the temperature much lower than that to increase the rate of condensation. However, this can be counterproductive, as cooling capacity is reduced at lower temperatures. For example, a chiller with a cooling capacity of 500 W at 20°C will have a lower cooling capacity at 10°C.
A good rule of thumb when shopping for a chiller is to check the heating capacity of your rotovap heating bath. If your cooler can't match its ability to cool at the desired set temperature, it won't be able to keep up. When looking at a manufacturer's specifications, it is common to find multiple cooling capacities listed for a range of temperatures.
Setting the temperature too low could result in less cooling than the bath can heat. If the cooler is underpowered at the set temperature, the coolant will not reach the desired temperature and there will not be enough heat transfer to condense the evaporated solvent in the evaporating flask.
Instead, steam will flow into the Vacuum Pump, increasing wear and shortening the life of the pump components. It may even flood the pump, causing irreparable damage. Also, if you have an overheat alarm on your chiller, setting the temperature too low could cause the alarm to sound and the chiller to shut off completely.
2. Pump pressure
Another important factor to consider is the maximum pressure of the cooling pump. If it is subjected to too high a pressure (usually in the range of 10-15 psi), it increases the risk of the condenser bursting.
Some manufacturers list a maximum pressure in their specs, but most do not. As a general guide, most centrifugal pumps have a maximum pressure of 10 psi, making them suitable for use with regular glassware. Positive displacement and turbo pumps, on the other hand, tend to have higher output pressures, so using them may increase the risk of glassware breakage.
3. Pump flow
The flow rate of the cooling pump affects the residence time of the coolant in the condenser. The lower the flow rate, the longer the coolant has to warm up in the condenser. As the temperature increases, the efficiency of heat transfer from the evaporated solvent to the coolant decreases. In this case, there is a risk that not all of the solvent vapor will be recondensed.
This is not a common problem, as most chillers have sufficient flow relative to their cooling capacity, but it is something to be aware of.
