NO. 3549 FREEZE-DRY PRESERVATION — HOWER 19 



The main requirement for the deep-freeze chest is that it maintain 

 a constant temperature of — 15° to —20° C. The only alteration needed 

 is removal of its hinges and latch and the insertion of a wood spacer 

 between the chest and its lid to permit passage of a vapor line between 

 them. The vapor line must pass through the wood spacer. 



The hinges and latch should be relocated for the raised top; the 

 thickness of the wood spacer is determined by the diameter of the 

 vapor line that will pass through it. 



A small specimen chamber may be made from a 5-gaUon paint-spray 

 pressure tank roughly 18 inches in diameter and 21 inches high. A 

 hole must be cut in the side of the chamber for a vacuum line, which 

 can be brazed into place. 



The major consideration in selecting a gas-ballasted vacuum pump 

 is the volume occupied by the water vapor at the temperature and 

 pressure being used. Assuming that the chamber temperture is 

 —20° C, the vapor pressm'e is 0.8 mm. Hg. At a vapor pressure of 0.8 

 mm., one gram of water occupies a volume of 1200 litters. If the 

 pressure gradient is approximately 0.6 mm., we find one gram of water 

 will occupy approximately 1400 liters. A gas-ballasted vacuum pump 

 with a 25-liter-per-minute capacity will take approximately one hour 

 to pump one gram of water. A pump with a capacity of 79 liters per 

 minute wUl remove approximately one gram of water in 18 minutes. 



If the vapor line is 150 cm. (5 ft.) long, the diameter is calculated 

 to be approximately 2.5 cm. or 1 inch. Tubing IK inches in diameter 

 will allow a sufficient operating margin when using the larger capacity 

 pump. 



The chamber must not be loaded beyond pumping capacity. In a 

 system of this size, it is suggested that a single specimen be placed in 

 the chamber and a second one added two or three days later, followed 

 by a thu'd specimen on the completion of the first, and so on, in this 

 pattern. Observations of drying times during experimental drying 

 cycles will determine the capacity of each individual system. The 

 drying cycle can be interrupted, provided specimens are not permitted 

 to thaw. 



The apparatus described above is limited by two basic characteris- 

 tics of its construction: the vapor-handling capacity and the size of 

 the specimen chamber (fig. 8). As stated, the most efficient device 

 for removing water vapor from the specimen chamber is a refrigerated 

 condenser. If an external refrigerated condenser of sufficient size is 

 used, the only limitation to the size of the specimen chamber is the 

 size of the deep-freeze unit that holds the specimen chamber. 



A refrigerated condenser can be constructed within a paint-spray 

 pressure tank similar to the one used for the specimen chamber. The 

 condensing surface may be a coil of ^s-inch copper refrigeration tubing 



