Cooling effects and evaporation characteristics of fogging systems in an experimental piggery

Evaporation of water to the ambient air is generally a cost-effective solution to alleviate heat stress but is also critically discussed due to its increasing effect on indoor humidity. The objectives of this study were to investigate the impact on indoor temperature, humidity, and ventilation rate using a high-pressure fogging system inside a mechanically ventilated research facility for fattening pigs. Data were sampled quasi-continuously throughout four fattening periods, including information on water consumption and energy use of the fogging system, as well as on the average weight gain of the pigs. It was found that for the pigs, the positive effects of the reduction of sensible indoor temperature exceeded the negative effects of the increased humidity, mirrored e.g. by a reduction of the temperature–humidity index (alert situations were reduced from 15.5% to 0.8%) and an increased weight gain of the animals during hot summer conditions. Main effects were achieved on days with mean daily outside temperatures above 14 °C, during which water consumption of the fogging system averaged 4.9 l d−1 pig−1.The evaporation characteristics, such as the evaporative fraction as well as the time constant for reaching a steady state, were evaluated using a transfer-function model. Both were influenced largely by temperature and saturation deficit. While the evaporative fraction was 100% during steady state and 63% of steady state was reached within 65 s during warm and dry ambient conditions (28 °C; 53% relative humidity RH), the evaporative fraction dropped to 89% and 65% for moderate (21 °C; 69% RH) and cold/humid (13 °C; 83% RH) indoor conditions, respectively, and the time to reach steady state was nearly doubled for the latter. The information about such evaporation characteristics is crucial for an accurate control of fogging and ventilation.