NO. 3549 FREEZE-DRY PRESERVATION — SHOWER 17 



evaporator inlet as possible. The valves in the Smithsonian system 

 are mounted inside the chambers, where maximum efficiency is gained. 



Bulb Location: For satisfactory expansion-valve control, good 

 thermal contact between the bulb and the suction line is essential. 

 The bidb, which controls the expansion valve, should be fastened 

 securely mth two metal straps to a clean section of the suction line 

 inside the chamber. The bulb should be located near the midpoint of 

 the line around the coU. It should not be near the bottom of the line 

 because a refrigerant-and-oil mixture is usually present there, which 

 woidd residt in mcorrect control of the expansion valve. 



Filter and Drier: These should be installed in the liquid line ahead 

 of the thermostatic expansion valve. 



Sight Glass: Further protection is easily and inexpensively provided 

 with a sight glass through which the refrigerant level can be deter- 

 mined by the presence or absence of bubbles in the liquid line. Bub- 

 bles indicate that the refrigerant level is low. 



Thermal insulation. — Insulation material is required to sub- 

 stantially reduce heat leak, which loads the refrigeration unit. This 

 material may be glass wool, wdth a K factor of 0.29, rock wool, wdth 

 a K factor of 0.26, compressed cork, with a K factor of 0.30, or other 

 similar material. The Smithsonian freeze-dry apparatus is insulated 

 with a foam-in-place plastic (polyuri thane). Foam-in-place plastics 

 range in density from 2 to 10 pounds per cubic foot; they offer K fac- 

 tors of from 0.02 to 0.24. These plastics can be used \\dthout great 

 difficulty and can be made to conform to any contour. Such proper- 

 ties make plastic foam an ideal material to use as insulation. 



Calculation for the determination of thermal impedence and heat 

 load is as follows: 



K= insulation factor (BTU/hr./ A = Ambient temperature (dif- 



sq. ft.) ference between the inside 



S= surface area of insulation and outside of chambers in 



(sq.ft.) ''F) 



IT= thickness of insulation (in.) 



<M2A = BTU/perhr. 



Temperature reading and recording. — Temperatures through- 

 out the drying process should be carefully watched and accurately 

 recorded. The record is indispensable for establishing drying times 

 and for determining ideal temperatures for various types of specimen 

 material. An automatic temperature-recording device is valuable 

 for indicating the temperature variations as the refrigeration com- 

 pressors are cycling, and it may also caU attention to problems that 

 would otherwise go unnoticed, such as a faulty temperature-control 

 switch. 



