Spray evaporative cooling to achieve ultra fast cooling in runout table

Spray evaporative cooling, in lieu of conventional laminar jet impingement cooling, has potential to achieve the anomalously high strip cooling rate of Ultra Fast Cooling – 300 °C/s for a 4 mm thick carbon steel strip – in Runout Table of Hot Strip Mill. In the present study, evaporation time of a single droplet impinging on a hot carbon steel strip surface has been analytically evaluated as a function of droplet diameter from fundamental heat transfer perspective based on the premise that a spray can be considered as a multi-droplet array of liquid at low spray flux density. Droplet evaporation time thus evaluated has been used to estimate strip cooling rate achievable in Runout Table of Hot Strip Mill by spray evaporative cooling. The proposed analytical model predicts that it is indeed possible to achieve the ultra-high cooling rate of Ultra Fast Cooling by spray evaporative cooling by suitable reduction of droplet size. A general analytical expression has also been developed to estimate critical droplet size to achieve Ultra Fast Cooling as a function of steel strip thickness. Predictions of the analytical model have been validated using CFD simulation with a modified Discrete Phase Model.