There are several fundamental safety precautions which should be applied to all dryer designs.
These requirements call for a minimum below which the ventilation rate is never reduced. This predetermined, fixed value is calculated from solvent input and flammability characteristics. Fundamental safety does not require use of an analyzer but, if none is used, the LFL limit is usually decreased to about one half the value allowed when an analyzer is installed.
Without an analyzer present, there is little corrective action in case of a fault. Safety depends on:
- the accuracy of the calculations for flammability of the solvent mixture(s) to be used
- maintenance of the minimum ventilation rate
- limitation of the solvent load by mechanical means
If the solvent load increases to a high level, the hazard of fire or explosion is limited solely by the magnitude of solvent increase and presence or absence of an ignition source. If the increase exceeds the margin of safety as designed, a fire or explosion can occur. Failures can include:
- excessive coating weight
- increase in line speed
- reduction in ventilation
- change in solvent mixture
- increase in web width
The minimum ventilation calculations should be made for the “worst case.”
This includes the maximum weight or liquid volume of solvent to be applied, the fastest speed of the process, the maximum web width, the maximum flammability of the solvent types to be used, and also a consideration of likely failure modes such as the loss of tension, loss of solvent metering, or loss of ventilation. Without an analyzer, the highest margin of safety should be used, which usually means the solvent concentration limit is 20% or 25% LFL.