Experimental Studies of Using Water Mist Cooling for the Tempering of Glass

Very high velocity air jet impingement has been applied during the cooling process of glass tempering. In order to reduce the usage of high-pressure air in the process, it is intended to demonstrate the feasibility of using water mist to enhance the air-cooling. The multiple jet experiments were performed using air jet velocities between 40 and 90 m/sec containing low mass flux of water mist that was varied from 0 up to 0.145 kg/m2sec for each jet. The experiments include different glass thickness. The optimal tempering conditions were explored. The mechanisms of mist cooling are revealed from the experiments of a single water mist jet impinging on hot stainless steel plates. Since the droplets are small, on the order of 20 micron, heat transfer distribution of the water mist has a similar form as the air jet cooling. The total heat transfer coefficient can be viewed as two separable effects: the summation of the heat transfer coefficient of the convective air and of the impinging water flux, respectively. The heat transfer of multiple water mist jets on larger glass is studied. When the liquid flux is too high or when the surface temperature falls below the Leidenfrost temperature, excessive local thermal stress will occur, which leads to cracking of glass. It is possible to reach the optimal tempering with a moderate amount of water applied on the glass surface for a short duration of time. The mist cooling demonstrates a definitive saving on the use of high-pressure air. When using mist cooling, the energy requirements of the system are significantly lowered. The mist cooling creates more refined fracturing in the punching tests of tempered glass. For glass thicker than 3 mm, the particle counts due to water mist improve about 121%. For 2 mm glass, the improvement is about 38%. The mist cooling of the thinnest glass is still not able to give the desired particle counts. But definitive improvement toward this objective is shown. Further studies of other alternatives may give a chance to achieve this goal.