242 BELL SYSTEM TECHNICAL JOURNAL 



were vented to the recovery system, with a fire screen installed in the 

 vent line. Attached to the mixing tanks is a carbon dioxide fire ex- 

 tinguishing system. One opening is within the tank and another one 

 is above the explosion relief. This carbon dioxide system will be re- 

 leased automatically if a quick rise in temperature occurs within the 

 mixer and it can also be operated through a hand pull box on the out- 

 side of the building. Operation of the carbon dioxide system in the 

 mixers or in any other place in the cellulose acetate plant causes all 

 electric power in both buildings to be turned off instantly, sounds a 

 siren and calls the fire department. 



To avoid the possibility of static sparks while loading the flake cellu- 

 lose acetate into the mixer, a humid atmosphere is obtained by injecting 

 steam. The danger is further lessened by the avoidance of metal in 

 the container used for carrying the flake acetate. 



In the coating machines themselves, precautions have been taken 

 both to prevent fires and to render them harmless if they should occur. 

 A temperature well above 1,000° F. is required to ignite acetone and 

 the electric heating elements are operated at about half this tempera- 

 ture. The temperature of the drying chamber is controlled by a 

 pyrometer which turns on or shuts off the electric heating current as 

 required. As a further safeguard, a second temperature control, set 

 at a slightly higher temperature, is arranged to act if the first one fails, 

 shutting off the current and sounding a gong. Here again a carbon 

 dioxide fire extinguishing system is permanently attached with an 

 outlet in each drying chamber. Automatic discharge would be diffi- 

 cult to control because of the temperature conditions in the chamber 

 and the carbon dioxide is controlled entirely from hand pull boxes. 

 Hand CO2 equipment is also available in convenient locations. Ex- 

 plosion vents covered with aluminum foil are located in the rear of 

 each unit. 



However, as previously mentioned, there is no danger of explosion 

 of the acetone-air mixture in the drying chamber if the percentage of 

 acetone is maintained at less than the amount that will propagate 

 flame, about 2.5 per cent. Acetone-air analysis instruments of a 

 standard make are permanently installed, taking readings from each of 

 the drying chambers and from various points in the exhaust lines. 

 These instruments operate on the basis of difference between the 

 thermal conductivities of air and of atmospheres containing various 

 amounts of acetone vapor. Two platinum coils, heated by a constant 

 electric current, form two arms of a Wheatstone Bridge. Around one 

 of these coils is room atmosphere, containing whatever moisture and 

 other impurities it may, while around the other coil are the gases drawn 



