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Freeze Drying Principles 


Freeze-drying is a process in which samples are first frozen and then subjected to a high vacuum (10 -2 torr). Under these conditions ice will turn from a solid state to a gas state and be removed from the sample by the vacuum pump. This leaves everything but the water behind in a dry state.  Typical uses where this process is employed is where the user wants to preserve the qualities (benefits) of a sample and remove the water.  Examples in large scale real world applications include dried flowers and cooking spice production.  The main objective is to preserve the sample and removing oxidative components that could degrade the sample.

Typical System Set-up

The system has two parts:

  1. The freeze-dryer, which consists of the sample chamber where the samples sit and the condenser chamber (trap) where the samples are frozen.
  2. A vacuum pump capable of pressures to 10-2 Torr or better which provides the vacuum to the freeze-dryer.  These are typically HyVac two stage vacuum pumps either belt or direct drive.


Most samples are freeze-dried over night. Frozen samples are placed in the sample chamber with the condenser on. The vacuum is turned on. A vacuum gauge can be used to monitor the pressure and when it reads 10-2 Torr the sample should be sufficiently dry.  The pump will have to remove all the frozen water in order to move to this pressure it might take some time as the gas evolves from the ice.  This process is called sublimation; turning ice into a gas phase and bypassing the liquid phase, and time for processing can be calculated based on expected volume of water present in sample. The system is then vented to atmosphere and the samples removed.  

Care should be taken to check the vacuum pump oil on a regular basis as contamination from the water removed will be a constant threat to the health of the pump.  High contamination levels of water in the pump will get in the way of drying your samples.  A quick check for water contamination is the old "spatter" test is to take a drop of oil and find the nearest hot plate (coffee pot) and put the oil drop on the hot surface if water is present in the oil the drop will spatter.  Use of the gas ballast valve may help thru-put water vapor that reaches the pump from the trap.


Vacuum will not establish:

  • This means an air leak somewhere. Check that the seal between the sample chamber and the machine is clean. Check the lid seal.
  • If the most recently handled seals appear good, then the problem may be in the tubing connections. Use a screwdriver or pliers to tighten all the hose clamps. If there are still problems, get help. The acid trap cylinder may need changing or the connections re-greasing.

Water gets into the vacuum pump:

  • Turn everything off immediately. Drain pump of oil and flush with HyVac Flushing Oil. Let the pump run for 20 minutes and actuate the gas ballast valve knob on top of the pump. Finally new HyVac pump oil is put in. 

Changing the Pumps Oil

Used oil is toxic and hydrocarbons can contaminate the lab. Waste oil goes into plastic bottles. Paper towels, cardboard boxes, and wipes should be bagged separately from the regular wastebasket contents. Work as cleanly as possible and constantly clean up so that oil is not transported by hands or feet around the lab.

  1. Turn off pump. Disconnect and remove any traps or filters and oil mist filter.   Inspect for need of replacement.
  2. On pump, remove drain plug. Drain contaminated oil into pan. Tilt pump to remove as much oil and water as possible. Allow to drain for 5 to 10 minutes.
  3. Replace drain plug and fill pump with HyVac vacuum pump oil until visible in clear gauge. Replace oil mist filter and inlet trap connections.
  4. Turn on the pump to determine that base pressure of system is still correct.  Allow to run for several hours.  pressure should attain better than 10-2 Torr on the gauge.  If not check for leaks.

More from Wikipedia on Freeze Drying


11/27/2005 ęCopyright HyVac Products, Inc. All Right Reserved
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