An explosion hazard results v\rhen propane or natural gas passes 

 through the generator uncombusted due to a malfunction and accumulates 

 to an explosive ratio with oxygen in the storage room. What are the 

 facts about explosive levels of propane and natural gas? For pro- 

 pane the lower explosive limit (LEL) is 1.1% in air and the upper 

 explosive limit (UEL) is 9.5^. This means that at any concentration 

 between 2.2 and 9.5% propane in air an explosion can occur if touched 

 off by a spark or flame. At less than 2.21 there is insufficient 

 propane to cause an explosion and at more than 9.5% propane there 

 is insufficient oxygen in the air to cause an explosion. Air con- 

 tains 1\% oxygen. A minimum of \\% oxygen is required to cause 

 an explosion with propane. In the case of Peabody CA storage explo- 

 sion this means that at least 1.1% propane was present while the 

 oxygen was greater than 11°. For methane, the primary component of 

 natural gas, the LEL and UEL are 5.31 and 14.0%, respectively, and 

 a minimum of 12.1% oxygen is required for an explosion. Incomplete 

 combustion of propane also produces carbon monoxide and ethylene 

 which are also combustible. Moreover, carbon monoxide may accumu- 

 late in work areas and this poses a health hazard as a poison. 

 There will be more about this later but let's return to some begin- 

 ning questions about CA generators. What are they? Why are they 

 used and how do they work? 



The term CA generator means a mechanical device that lowers 

 the oxygen concentration of the atmosphere by causing a net increase 

 in the percentage of nitrogen in a storage room. CA generators were 

 introduced in the early 1960 's and currently most of the controlled 

 atmosphere facilities in North America are equipped with some type 

 of generator. They are used to rapidly establish the desired low 

 oxygen level of 2 to 3% within 2 to 5 days after hermetically seal- 

 ing the refrigerated storage room of fruit. Without using such a 

 device it generally takes several weeks to lower the oxygen content 

 of the storage room to the same level as the fruit consume oxygen 

 in the process of respiration. There are numerous reasons and 

 beneficial effects of rapidly establishing the CA condition that 

 need not be elaborated here. 



Some CA generators adjust the level of both carbon dioxide 

 and oxygen while others adjust only oxygen and a separate device 

 is used for scrubbing carbon dioxide. Since carbon dioxide is 

 produced by the fruit by respiration some means is necessary to 

 limit carbon dioxide accumulation in both conventional or generated 

 CA storages. The CA generator provides a means to adjust the 

 oxygen level in the room independently of the fruit. 



There are several types of CA generators used commercially and 

 differ widely in basic principles. They include: nitrogen purge 

 systems using liquid or gaseous nitrogen; inert gas generating 

 systems which produce the low oxygen atmosphere external to the 



