Evaporative pad cooling model validation in a closed dairy cattle building

Abstract

A mathematical model of an evaporative pad cooling system to predict the dry-bulb temperature and relative humidity of air exiting the pad, its cooling efficiency and the water requirements for agricultural buildings was derived from first principles using mass and energy balances. The model uses the geometry of the evaporative pad as characteristic parameters and the water temperature, dry-bulb temperature and relative humidity of the outside air as primary boundary conditions. The model was validated with data from wind tunnel tests and field experiments on a closed dairy cattle building using an evaporative pad cooling system. These data also enabled new expressions for heat and mass transfer coefficients of a cellulose cooling pad to be developed and validated. The resulting model accurately predicted the main output variables in the closed dairy cattle building, with the prediction error falling inside the accuracy of the sensors used to measure the environmental variables (±0.5 °C and ±3% for dry-bulb temperature and relative humidity, respectively). The effect of pad water temperature on the output variables was obtained with the developed model, allowing to propose the pad water temperature as an effective manipulated variable for designing an evaporative pad cooling control system. It was found that, for each Celsius degree that the water temperature decreased below the input wet-bulb temperature, the output dry-bulb temperature decreased by 0.7 °C and the output relative humidity increased by 1%.