FREEZE-DRY PRESERVATION — HOWER 



13 



indicates that a vacuum-pump limitation can limit the number of 

 specimens that can be produced in a reasonable span of time. 



A 2-stage gas-ballast vacuum pump is recommended for all freeze- 

 dry use. Gas ballast on the pump enables it to remove condensable 

 vapors such as water vapor from a vacuum chamber with a minimum 

 of internal condensation. 



In the gas ballast, a valve provides a means for controlling the entry 

 of air into the exhaust stage of the pump's operation. The ballast of 

 air acts as a transporting medium carrying diluted vapors through the 

 exhaust part to the atmosphere. 



Such pumps are generally rated with ultimate pressure ranges from 

 1 to 10 microns; however, pumping speed usually begins to decrease 

 at about 100 microns, which is considered a satisfactory pressure for 

 freeze-drying. 



Oil should be changed frequently in any pump used in a freeze-dry 

 system. Even though the pump is gas-ballasted, a certain amount of 

 water vapor mil contaminate the oil and damage the pump. 



Table 3. — Vacuum table 



The possibility of future expansion of the freeze-dry system should 

 also be considered when selecting a vacuum pump. Under certain 

 conditions the addition of a second chamber may require doubled 

 piunping capacity (see tables 3-5). 



Vacuum-line dimensions. — The diameter of vacuum lines affects 

 the efficiency of a freeze-dry system smce a pump is effective only 

 if the vacuum lines are large enough to handle the vapor transfer. 



Conductance (the volume of flow of vapor through a line) is a 

 well-investigated function, and there are a variety of ways to calcu- 



