Variable ventilation rate control below the heat-deficit temperature in cold-climate livestock bulidings

Steady-state and transient analysis of three control strategies, either-or temperature (EOT) control, either-or-neither temperature (EONT) control, and temperature-humidity control (THC) are reported. Variable ventilation rates, temperature and relative humidity responses, and supplemental heat consumption were simulated using a dynamic thermal environment model for livestock buildings. Variable ventilation rates following the moisture balance curve can be approximated by using an EPT control strategy and properly sized fans and heaters. The first-stage fan should have a flow rate equal to that required for moisture balance control at the design outside temperature. The second-stage fan, when added to the first-stage fan, should provide a flow rate to meet the ventilation requirement at the heat-deficit temperature. The heater capacity must closely match the supplemental heat requirement at the design outside temperature. Practical difficulties in accurately sizing heaters and fans will limit the application of EOT control. The EONT control strategy does not automatically adjust the ventilation rate during the heating season but it is a better alternative than EOT control for short heating season barns. The THC involved sensing both temperature and humidity and was shown to be superior to temperature-only control strategies in terms of the ventilation rate modulation to simultaneously achieve the design indoor temperature and relative humidity. (Authors)