Water spray is widely used in fire prevention and heat radiation protection due to its high infrared (IR) attenuation. Recently, the calculation and theoretical analysis of water spray IR attenuation have been partially solved, where multi-scattering in the water spray, distribution of droplets, concentration of droplets and water optical constant in the infrared atmosphere windows are considered. However, is there a minimum for spectrum transmission of water spray in the IR atmosphere windows band? If yes, what are the water spray parameters? These questions are still to be solved. This article combines Lognormal Monte-Carlo Method (LNMCM) and Single Diameter Monte Carlo Method (SDMCM) to calculate water spray infrared transmission. The droplet mean diameter is studied with SDMCM when the water spray infrared transmission reaches the minimum. The geometric deviation of water spray is calculated with LNMCM at the same conditions. The infrared transmission is calculated and analyzed with changing droplet diameters, on the stipulation that total water content of air keeps unchanged. In this condition, droplet concentration increases with droplet diameter decrease. Calculation results show that there is a minimum IR transmission of water spray in the atmospheric windows band, which means there is a maximum IR attenuation. Infrared transmission of water spray reaches the minimum when the droplet mean diameter is 12 micron and geometric deviation is less than 1.5. According to these, it is possible to achieve maximum infrared attenuation with limited water supplied, as long as the spray nozzles parameters are in accordance with the results.
