A load of tips on how to prevent condenser overload

Condenser overload can be damaging to your freeze-drying performance, the vacuum pump and the freeze dryer itself. To avoid the occurrence of this phenomenon, you should consider the importance of condenser ice capacity, as well as the freeze-drying rate. But a few other parameters also influence condenser overload, especially in real-life situations. Read this blog post to see how to avoid condenser overload to safeguard both your lyophilization process and your freeze-drying unit.

My daughter just moved to a new house and invited us over for dinner. We did the grand tour and she proudly showed us her kitchen, including her large new fridge. Her husband grumbled about the size and complained about the running costs of the fridge compared to the smaller unit they had before.

My daughter did not let that one slide. Instead she countered him with some pretty good arguments why a bigger fridge can be more cost-efficient. She mentioned that their previous fridge was often stuffed to the top with food. As a result, vents were being blocked off and food was spoiling faster. She often had to run the fridge at higher power, overworking the motor, increasing energy costs and negatively affecting the machine itself.

Indeed, overloading is a big no-no when it comes to fridges, washing machines, cars, and your back when carrying heavy grocery bags.

It is also a big no-no for your freeze dryer in the laboratory, especially when it comes to the condenser.

To avoid condenser overload, it is important to know how much moisture needs to be removed. If too much ice accumulates on the condenser coil, more isolation builds up and the collection will become less efficient. In the worst-case scenario, the condenser won’t be able to trap any more ice at all. Another point to consider is that using a freeze dryer with inefficient ice capacity can result in an increase in collector temperature. This higher temperature is caused by ice isolation and by poor vacuum in the system as vapor accumulates in the drying chamber and spoils the oil in the pump.

To prevent these unwanted effects, you should pay particularly close attention to two condenser specifications. The first parameter is the total capacity of the condenser, or how many kilograms of ice can be contained on the coil. The second is the condensing capacity or the freeze-drying rate. This parameter refers to how many kilograms of ice can be trapped onto the coil in a 24-hour period. The freeze-drying rates are usually measured on a manifold tree with shell-frozen water. Samples that are slant frozen or bulk frozen have slower sublimation rates.

These two are the most important parameters to consider because both the total amount of solvent in the batch, as well as the rate at which solvent sublimates can cause the condenser to overload.

Consider how these parameters affect condenser overload with the graph below. Here, you can see what happens to the condenser trapping capacity (blue) and the vapor load (red) in relation to the duration of the cycle versus moisture present.

condenser overload, condenser trapping capacity, vapor load, condenser load, freeze drying, lyophilization, freeze-dryer

If there is more vapor than the condenser can trap, the vapor will bypass the condenser and exit through the vacuum pump. This could happen if there is more solvent to sublimate than the total capacity of the condenser. Alternatively, condenser overload results when the drying occurs faster than the drying rate of the condenser.

As helpful as these graphs are, they are often obtained using standardized parameters and may not always be applicable to real-life scenarios.

Let me give you an example. Vapor, for one, will not escape the sample at an even rate throughout the cycle. In fact, drying is often faster at the start of primary drying when there is less resistance to vapor migration. It is possible that even when the performances of a condenser match, with process times between 24 to 48 hours on average, certain samples could generate too much vapor too quickly for the condenser to trap. This will then result in condenser overload.

Process temperature, container type and batch size are all factors that can influence the rate at which vapor is generated.

Applying too much energy at the start of the primary drying step can lead to vapor production that is too high. This in turn would cause an increase in pressure and a rise in condenser temperature as the condenser tries to cope with the excessive amount of vapor.

Hence, all these considerations should be kept in mind when designing a freeze-drying cycle in order to avoid condenser overload, protect your vacuum pump, your freeze dryer and your lyophilization performance.

I hope I have not overloaded you with information on condenser overload and freeze drying. If this is the case, be merry. I will talk about chromatography again in the next blog post.

Till next time,

The Signature of Bart Denoulet at Bart's Blog