Problem: To reduce operational costs in today’s unstable economy, a lab or facility manager with a new ICP/Mass Spec system must cost-effectively use cryogenic liquid argon dewars to supply gaseous argon.
When it comes to specific critical gases requirements, one attractive option is the use of cryogenic liquid cylinders, commonly called dewars, to supply gaseous argon to ICP and ICP/Mass Specs. Argon’s unit cost per cubic foot/gaseous liter is much lower when delivered to your facility in cryogenic liquid form in dewars. However, since these containers store the gas in a cryogenic form until it is dispensed as a gas, there are two issues that can decrease the cost-effectiveness of using this mode of supply.
Since the argon is stored in self-pressurizing containers with relief valves and the cryogenic liquid is boiling inside, reserve containers waiting for the primary container to empty on a system will, over time, build internal pressure until the relief valve setting is reached and then begin to vent excess gas at a rate of 2 to 3 percent per day. This is the NER or Normal Evaporation Rate, referred to in the industry as the “use it or lose it” phenomenon. At this rate, if the reserve cylinder of argon contains 4,500 cubic feet of gas, it equals losing 135 cubic feet every day it sits in reserve. It wouldn’t take long to eat up whatever savings resulted from the lower unit cost.
Since most of the gas is in cryogenic form, the pressure in the container does not indicate if the unit is full, half full or empty. The small liquid level indicators on top of the dewars are hard to read and not very accurate. Since most gas distribution systems rely solely on pressure to change from primary to reserve cylinders, false low-pressure readings can cause lab personnel to remove containers with as much as 25 percent of the contents still inside.
Solution: CONCOA’s Intelligent Gas Distribution System IntelliSwitch II continuously monitors your reserve container’s pressure, using programmed software logic to determine if a container is truly empty or contains residual product. IntelliSwitch II 538 Series is the latest in the line of computer-controlled gas distribution switchover systems using very accurate transducers to monitor both primary and reserve inlet pressures. Proprietary software logic uses this information to drastically reduce evaporation loss by switching to the reserve dewar to supply the gas to the instrument before the relief valve opens, drawing down pressure in the container to a programmed point, and then returning to the primary dewar. This “economizer function” can virtually eliminate losses in the reserve dewar.
Additionally, when primary dewar pressure drops below the point at which the system switches to the reserve, IntelliSwitch II does not immediately conclude that the container is empty. It waits a programmable amount of time and while supplying the system from the reserve, watches the pressure reading on the primary. If the pressure rises above the switchover point in that time period, the computer concludes residual liquid was left in the container and switches back to it to supply the instrument. It does this repeatedly until the dewar fails to build pressure above the programmable switchover point, at which time it switches to the reserve, and sounds an alarm to indicate the primary is now truly empty. This “look-back feature” reduces the residual discarded when using dewars to less than 2 to 3 percent.
By eliminating evaporation loss and reducing the residual left in empty containers, CONCOA’s IntelliSwitch II optimizes the cost savings of using cryogenic argon to supply gas to ICP or ICP/Mass Specs.
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