10 



PROCEEDINGS OF THE NATIONAL MUSEUM 



It is clear, therefore, that any effort to produce extremely low 

 condenser temperature is not economical since the cost per BTU 

 of refrigeration increases rapidly as temperature is reduced, but the 

 vapor-pressure differential increases only moderately. A tempera- 

 ture of —40° C. is satisfactory and is within the range of conventional 

 freon-12 or freon-22 refrigeration units. 



Another factor to be considered in designing a condenser is that 

 the rate of water loss from the specimen wUl be very low^ and, thus, 

 the heat load on the condenser's refrigeration unit will be slight. 



The heat leak through the insulation of the condenser chamber 

 should be calculated exactly, as for the specimen chamber. 



Temperature Control: Temperature may be controlled by a simple 

 mercury control switch with a 2- or 3-degree low-temperature differ- 

 ential. The control employed in the Smithsonian system is a sealed 

 mercury switch, triggered by a vapor-pressure-actuated Bourbon 

 tube. Exterior adjustments with visible settings over a calibrated 

 dial are employed. 



Figure 5. — Schematic of Smithsonian freeze-dry apparatus (^= specimen chamber; B= 

 condensing chamber; C= recompression valve; Z)= vacuum pump; E, /"= refrigeration 

 compressors; G= vacuum gauge). 



Vacuum pump. — The vacuum pump, very important to the freeze- 

 dry system, should be selected with care. There are many suitable 

 pumps available. 



When a refrigerated condenser is employed as a vapor trap, the 

 only function of the vacuum pump is to reduce air pressure mechani- 

 cally within the chamber and evacuate the noncondensable gases 

 released from the specimens during the drying process. 



The capacity of a pump is described in terms of displacement. 

 Displacement, expressed in liters per second or cubic feet per minute, 

 is a measure of a pump's capability to produce a vacuum in a given 

 time, as well as to keep the system evacuated as gases are released 



