16 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 119 



Repairs should be made while the system is evacuated because the 

 vacuum helps a sealant penetrate a leak. Unless the pressure is 

 unusually high when the pump is operating, it may be assumed that 

 a leak is small and can be sealed with glyptol varnish or lacquer 

 applied by brush. The instant that either material reaches the 

 vacuum through a pinhole leak, it dries and plugs the leak. If the 

 pressure remains high, the leak is large and should be plugged with 

 soft solder. 



Refrigeration compressors. — The major consideration in refrig- 

 eration is that constant temperature be maintained. 



The hermetically sealed refrigeration unit is composed of a motor 

 and compressor shaft of 1-piece construction. The motor (cooled by 

 the flow of refrigerant gas) and compressor assembly are within a 

 gas-tight housmg that is welded shut. This method of construction 

 eliminates the need for certain parts (pulley, belt, compressor fly- 

 wheel, and compressor seal) found in an open unit and, of course, 

 avoids the servicing and replacing of those parts. 



One objection to this type of unit is that, under freeze-dry operating 

 conditions, there is some danger of the motor's burning out because 

 of the very small amount of refrigerant being cu"culated. 



The Smithsonian's compressors, however, have not overheated 

 despite continuous operation for several years in a room with an 

 average temperature of about 90° F. If overheating should occur, it 

 could be remedied by installing a water-cooled condenser in the 

 discharge side of the compressor. 



A second t3rpe, the air-cooled compressor, usually operates with a 

 belt and pulley; the motor is in the open, where it is cooled by air 

 circulating around it. 



Operating characteristics of both types of refrigeration compressors 

 are, otherwise, essentially the same. 



The selection of a fractional-horsepower refrigeration compressor 

 should be based upon calculation of heat load on thermal msulation 

 (see p. 17 and also table 1). 



Expansion valves.— For maxmium vaporization of the refrigerant, 

 it is important to select thermostatic expansion valves of correct 

 capacity. It is equally important that the valves be installed at the 

 proper locations, smce both factors can influence the success or failure 

 of the entire system. 



The thermostatic expansion valve selected for the condensing 

 chamber should be a type designed to control temperatures below 

 -40° C. 



Although the thermostatic expansion valves in either chamber may 

 be mounted in any position, they should be installed as near the 



