The minimum ventilation calculations should be made for the “worst case.” A direct measurement of the ventilation rate - using properly situated low-pressure switches - should be connected to the control logic so that the process shuts down immediately if ventilation falls below the minimum required.
In such cases the following shut down logic should be applied:
Places where pockets of high concentration might occur through uneven ventilation flow or obstruction by the product being coated should be sought out and eliminated.
In general, the concentration immediately at the product being coated is very high, as the liquid solvent evaporates and only just begins to mix with air.
Variable ventilation controls allow the maximum reduction in ventilation, and thus maximum economy. It also allows some extra corrective action but also one important safety concern: Variable ventilation is based upon the measurement of the L.F.L. concentration and adjustment to ventilation rates through damper or blower controls.
There are two main types:
The control of variable ventilation rates can undergo an additional improvement that can result in the detection of certain system faults with a greater margin of safety than the previously discussed methods.
Recipe controls use modeling or prototyping or historical records of process variables to determine limits for detection of unacceptable deviations in the LFL control system.
There are four main detector types used for combustible gas analyzers:
