Evaporation and settling of semi-synthetic metalworking fluid droplets in industrial buildings

Abstract

Machining processes emit large amounts of semi-synthetic metalworking fluids (SSMWFs) droplets, which undergo both evaporation and settling in the industrial environment, presenting detrimental effects on both workers and equipment. This study experimentally and numerically investigates the evaporation and settling characteristics of oil in water (O/W) emulsion droplets formed by SSMWFs. Firstly, based on the separation characteristic of oil and water phases during the evaporation process, an evaporation model for SSMWFs droplets was developed, and the evaporation process can be divided into water evaporation and volatile oil evaporation. Next, the evaporation characteristics was analysed. It indicates that the SSMWFs droplets do not evaporate completely, but exist in a final state of equilibrium particle size, which is mainly affected by the percentage of non-volatile components and the initial particle size. The evaporation rate is mainly affected by initial particle size, relative humidity and ambient temperature, while the concentration of gaseous volatile oils in the environment has little effect on it. Increasing relative humidity of the local environment provides more available time for capturing droplet pollutants before the evaporation of volatile oil components, at 20 %, 80 %, and 95 % RH, it takes 3 s, 18 s, and 63 s for the volatile oil components to start evaporating, respectively. Finally, the settling characteristics show that the deca-micron-level SSMWFs droplets released in large quantities during machining can suspend for over 300 s, posing non-negligible inhalation exposure risks. The study provides guidance for assessing the impact of SSMWFs droplets on human health, and for designing effective contaminant control strategies.