APPLICATION OF CO, 



Pressure 



During the initial purging of the storage facility, 

 pressure will build up because of the large initial volume 

 of gas applied. Some method must be used to relieve this 

 pressure. If the storage facility has an aeration duct at 

 the bottom, it can be left open during the initial purge 

 to relieve the pressure. If the storage facility is not so 

 equipped, a small opening can be provided at an access 

 door during the initial purge to relieve the pressure. The 

 area left open can be sealed after the initial purge. (See 

 "Starting the Purge.") 



Calculation of Theoretical Amount of Gas 

 to be Used in Initial Purge 



The following formula can be used to estimate the 

 free air space to be purged in a bin having a flat 

 top: Number of bushels of commodity x 1 .24 x A + B = 

 cubic feet of free air space 



A = approximate percentage of free interkernel 

 air space in the commodity, that is, soy- 

 beans, 35 percent; grain sorghum, 37 per- 

 cent; rice and shelled corn, 40 percent; 

 wheat, 46 percent; barley, 50 percent; and 

 oats, 58 percent. 

 B = cubic feet of free air space in the headspace 

 above the commodity determined by 3.14 

 r 2 h (r = radius and h = height) for cylin- 

 drical volumes. 

 Sample calculation: A concrete silo measuring 81 

 by 24 feet contains 27,500 bushels of wheat. Headspace 

 tfabout 10 feet. 



27,500 x 1.24 x0.40+ (3.14 x 12 2 x 10) = 18,161 

 cubic feet. One pound of liquid C0 2 produces 8.72 

 cubic feet of gas; therefore, 2,083 pounds of C0 2 theo- 

 retically should purge the atmosphere of the storage fa- 

 cility to 100-percent C0 2 . However, due to the mixing 

 of the C0 2 with the atmosphere in the storage facility 

 and due to the difficulty encountered in penetrating the 

 grain mass, this amount of C0 2 will not produce a 

 100-percent concentration. The necessity of relieving 

 pressure on the storage facility, especially when no pres- 

 sure relief, such as an aeration fan duct, is available at 

 the bottom of the storage facility also contributes to loss 

 of C0 2 . However, this formula will provide a good indi- 

 cation of how much C0 2 will be used in the initial purge. 



Starting the Purge 



After calculating and obtaining the theoretical 

 amount of C0 2 needed, the initial purge can be started. 

 The external vaporizer(s) should be preset to maintain 

 the gas at a temperature of 60° to 90° F— the faster the 

 flow rate, the more the vaporizer(s) will operate, and at 

 high flow rates some reduction in temperature may 

 occur. The regulating valve should be opened to start the 



gas flow. Initially, personnel should be stationed near 

 the areas where C0 2 is applied and where pressure is to 

 be released from the storage facility to ascertain if the 

 flow rate is too high. Some indicators that the flow rate 

 is too high are: 



1 . Grain in headspace displaced by gas flow. 



2. Duct tape loosened around seals. 



3. Excessive airflow around or through the 

 pressure relief area(s). (Some experience is 

 needed to be able to use this factor.) 



The initial flow rate for purging will be a function 

 of the capacity of the storage facility, the output of the 

 vaporizer(s), and the ability to relieve pressure. Our 

 experience has shown that in a concrete silo of 

 20,000-bushel capacity the initial purge rate should be at 

 300 to 400 pounds of C0 2 per hour, and one having a 

 30,000-bushel capacity should be purged at 500 to 600 

 pounds of C0 2 per hour. 



Gas Sampling During the Purge 



Analysis of the air should begin when about 25 

 percent of the calculated amount has been delivered into 

 the storage facility. In the simplest application, the gage 

 built into the supply tank can be used to measure the 

 amount applied. To determine its accuracy, this gage 

 should be checked against the one on the supply truck 

 during the filling operation. Uneven distribution of the 

 C0 2 can be expected during the early stages of the purge 

 because of different pressures arising as the air in the 

 storage facility is displaced and because of the difficulty in 

 penetrating foreign materials mixed in the grain. The C0 2 

 will distribute itself evenly throughout the grain mass 

 during the later phases of the purge. Once the desired con- 

 centration is reached, as indicated by analysis of the stor- 

 age atmosphere, the gas flow can be reduced or shut off. 



Gas Sampling and Application After the Purge 



After the desired concentration is reached, gas sam- 

 pling can be limited to the sample tube run just under 

 the surface of the grain. Because C0 2 is heavier than air, 

 the initial loss will occur in this area. When the concen- 

 tration falls below that desired, more C0 2 can be ap- 

 plied to bring it back to the desired level. An alternative 

 procedure is to allow the gas to flow in constantly at a 

 low rate to compensate for leaks and loss in the upper 

 areas of the headspace. If additional facilities are to be 

 purged while the atmosphere is maintained in others, a 

 "T" or "T's" and regulating valves should be installed 

 between the pressure regulator and the additional deli- 

 very hoses. Once the personnel become familiar with the 

 application of C0 2 into their particular storage facilities, 

 they should not have any difficulty maintaining the con- 

 centration after the initial purge. This can be accom- 

 plished with minimal labor and equipment. 



