A vacuum pump has an important role in Freeze Drying. If the pump works and your chamber doesn't leak then you've overcome the 2 most common problems and Freeze Drying happens. If the pump doesn't work, then Freeze Drying doesn't happen. But what constitutes working? And if everything seems OK but your process isn't getting to the right vacuum level, where do you start troubleshooting?
We recommend starting at the vacuum pump for 2 reasons:
- It is the easiest thing to quantify and troubleshoot
- It is the source of your vacuum
Ever had a computer issue? The 1st thing you do is shut the system down to see if that fixes it. Well with a vacuum pump, the 1st thing you should always do is change the oil.
The appropriate frequency of oil changes can vary from very often (every time the freeze dryer is used), to seldom (every few months).
How often do you need to change the oil in your vacuum pump?
- Depends on the type of vacuum pump
- Rotary vane pumps: the high concentrations of corrosive vapors that are pulled into the pump quickly condense and combine with the pump oil to form a thick molasses type substance that significantly affect the performance of most rotary vane pumps. In order to keep these pumps operational, the end user must change the oil and clean the oil chamber on a very regular basis
- Diaphragm type vacuum pumps may require less frequent oil changes
- Depends on the use of the freeze dryer and the solvents that are being processed
- Depends on your firm’s standard operating procedures (S.O.P.)
So what constitutes a pump working? The S.O.P. ought to define how the system should run, and what acceptable parameters are. It should define what the base pressure of the pump should be, and what the chamber needs to achieve in order to have a successful process. Some may quip that if you aren't below 100millitorr you aren't freeze-drying, but the reality is that the recipe that the scientists came up with and documented in the S.O.P prevails.
In general, the vacuum pump needs to get to a higher vacuum / lower pressure than the chamber needs to be. How much lower does it need to be? That depends on a variety of factors, but in general you'd like to see a vacuum pump that can achieve about 50millitorr higher vacuum/lower pressure then the target pressure of the chamber. This means if your chamber needs to be at 100millitorr, then the vacuum pump should achieve at least 50 millitorr.
How do you test the base pressure of the pump? There are a few ways:
- Implement a foreline solution that allows in-situ testing of the vacuum pump
- Disconnect the vacuum pump and perform a baseline test to see the ultimate vacuum the pump alone can pull
- Document the result, and save it in a place that can be referred to later.
If the pump baseline yields a satisfactory result, the next step is to look at the chamber leak rate. The S.O.Ps should define what an acceptable chamber leak rate is. A common leak rate number is 15 millitorr per minute. To determine the leak rate:
- Pump down the chamber to its lowest pressure.
- Put a vacuum gauge on the chamber
- Valve off the chamber so the pump is disconnected and isolated from the chamber, and the chamber is sealed tight.
- Note the leak rate, either by reading it on a vacuum gauge that determines leak rate, or by creating a time vs. pressure graph
Comment below letting us know what your SOPs are in terms of base pressure and acceptable leak rate, and other vacuum related variables. If you do, we'll anonymously share all those answers with all those that participate, and enter you in to win a free Bluetooth Bullseye Precision Gauge that makes Freeze Dryer Troubleshooting much easier!