Analysis of explosion accidents with heat recirculation dryers and their inherent safety improvement

- A feasible inherent safety improvement for heat recirculation dryer is put forward.
- Nitrogen application for the oxygen concentration control presents the basis of safety improvement.
- The necessity of adopting oxygen concentration control with nitrogen replacement and evacuation was studied.
- The double circuits of nitrogen inlet and exhaust were designed to avoid formation of explosive gas mixtures in the dryer.
- The drying time and energy consumption were substantially reduced in the improved safety process.

The explosion accidents occurred at the drying workshops in China were found mostly to be caused by explosive gas mixtures formed in drying chambers. During the heat recirculation drying of surface coatings, solvent vapors are released forming explosive mixtures when the lower explosion limit of their concentration in air is reached. The analysis of exploded dryers and the numerical modeling served their inherent safety improvement ideas presented in this research. The improvement in heat recirculation drying safety was achieved with evacuation and oxygen concentration control using nitrogen. The drying gas double circuits at inlet and outlet of the drying chamber were installed to maintain an inert gas minor excess pressure (≤0.1 MPa) in the drying chamber. Oxygen concentration thus did not exceed 7.0 vol %. The drying time was substantially shortened, and the energy consumption was thus reduced for 67%. The present study provides an explosion prevention strategy with regard to inherent safety of heat recirculation dryers in design and operation.