GAS DELIVERY SYSTEM continued


back to 5% (or the programmed concentration). Then that solenoid shuts off the gas flow until the concentration is reduced through consump- tion by the cell culture’s biological processes (relatively minimal over time), or until the door is opened again and the process begins anew. The actual time the CO2


is flowing for each door


opening is relatively short; it typically takes as little as 20 sec of gas flow to return the concen- tration to the required 5% level.


The key to comparing the flow of 6 lpm of gaseous CO2


per incubator must be weighed


against the duty cycle of how often the door is opened in an hour of daily use compared to how often the door remains closed over that 24-hr period. For very active operations, this may be as often as once every 5 min during operating hours; less active operations may be as little as once an hour. Determining the duty cycle is therefore critical to evaluating what each incubator’s average daily consumption of CO2


Table 3 – Evaluation of duty cycle vs total daily CO2 of CO2


required for supplying demand Duty cycle % of 6 lpm flow


Liters per minute at duty cycle Liters per day Lbs of CO2 Lbs of CO2


/day /week/incubator should be. In general, based on years


of experience, the author typically assigns a duty cycle of not more than 30% to active operations and not less than 10% for mini- mally active operations. Table 3 compares an evaluation of duty cycle to the total daily CO2 demand, based on 6 lpm of flow that can be used to then calculate demand in net pounds of CO2


required for supplying that demand.


For bioreactors, the demand, though required to be uninterrupted during operation of the reac- tor’s batch or cycle, is not required to be available 24/7, but rather only during duration of use. Typically this demand is expressed in pounds of CO2


becomes the obvious option, and your gas sup- plier should be consulted for sizing and locating the bulk installation. If demand is less than 1000 lb/day or if your usage points or location will not allow for a bulk installation, other forms of supply such as a liquid cylinder can be considered. The piping system and the supply manifold can then be sized such that a safe and efficiently green supply can be determined by making sure that the available supply supports your requirements while limiting or eliminating the vent loss posed by relief valves on these containers.


per cycle or pounds per hour of duration


of the cycle. Therefore, determining your daily demand is somewhat easier by simply totaling the pounds required per batch or cycle and how many of these the facility does during an average day. By using the information in Table 2, you can determine if your operation is of the size where cryogenic liquid or less likely a high-pressure cyl- inder is an option, or if the total demand requires a bulk installation.


Gas delivery system evaluation


and options Typically, if a building site allows for and can be piped for CO2


service during construction and the demand is greater than 1000 lb/day, then bulk


As with any bulk or sizable liquid cylinder instal- lation, area oxygen deficiency monitors should be installed in storage or use areas, and any pipeline-relief valves should be piped to an ap- propriate exterior vent line as per NFPA 50 and building safety code requirements. The relief valves on cryogenic liquid cylinders cannot be piped away and are exempt from this require- ment. For this reason it is critical that the gas delivery system be sized properly and incorpo- rates an “economizer” program to minimize the vent loss of any liquid cylinders that sit unused on the reserve side of the system.


To provide a continuous and uninterrupted supply of CO2


to either incubators or bio-


reactors from cryogenic liquid cylinders or high-pressure cylinders, an automatic switch- over manifold similar to that shown in Figure 2 is usually employed. These systems automatically switch to a reserve bank of cylinders once the primary side has been emptied. For installa- tions in which this supplies a single incubator or small number of incubators, the supply may be as few as one to two 50-lb high-pressure cylinders per side and operate on what is called a pressure-differential basis.


In these installations there is no risk of venting CO2


AMERICAN LABORATORY • 24 • SEPTEMBER 2013 since there are no relief valves, other than


demand to aid in calculating net pounds


50% 40% 30% 20% 10% 3.0


2.4 1.8 1.2 0.6


4320 3456 2592 1728 864 17.42 14.00 10.45 6.97 119


98 3.48 73.15 48.79 24.36


those on the pipeline or system that only actu- ate if the pressure control fails. If the demand for your operation is greater than 200–300 pounds of CO2


per week, then you should consider


using cryogenic liquid cylinders as the primary source of supply, and either high-pressure or liquid cylinders as the reserve supply. In this instance, a pressure-differential system should not be considered because it may result in false switching from a partially full liquid cylinder, since the pressure in these cylinders can vary with demand and can result in as much as 30% of their contents not being used. This will result in that volume of gas eventually being vented to the atmosphere as gas, either as it sits awaiting the next delivery or is vented by the supplier once it is returned to the supplier’s facility. Figure 3 shows a top-down view of the various valves, safety mechanisms, and component parts of a typical cryogenic liquid cylinder.


Failure to utilize 30% of the gas purchased is not what one would consider responsible “green operations” in terms of the environment and cost-efficiency. It is responsible to ensure that the system employs a means to accurately deter- mine when the liquid cylinder is empty by using


Figure 2 – A gas switchover system automati- cally switches to a reserve bank of cylinders once the primary side has been emptied for efficiency and cost-effectiveness.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60