Calculating air exchange rates from broiler livestock houses

Abstract

Listen

Translate article

Air exchange rates (AERs) were calculated from 2 full sized broiler houses. The first broiler house was a Louisiana stall (natural ventilation) and the second a mechanically ventilated stall. The following methods were tested in the naturally ventilated livestock house. Under hot summer conditions the CO2, heat and moisture mass balance models were assessed. Because of the hot temperatures and high AERs, mass balance model results were corrected, reasonable AER estimates were obtained. In the autumn, a tracer gas method was set up with only 6 sampling locations, mass balance models were also tested. The measured ventilation rates were low, but recorded significant correlation coefficients with outside temperatures (r2=0,73), wind speeds (r2=0,47) and were above the minimum recommended winter ventilation rates, except on 1 day when the wind speeds were very low and wind directions random. The tracer gas method results were correlated with the heat and moisture mass balance model results (r2=0,73 and 0,71, respectively) and the calculated AERs were within range of each other. The following methods were tested in a mechanically ventilated broiler house; pressure flow sensor, constant tracer gas injection and the mass balance models. Air volume discharges were accurately quantified with the pressure flow sensor, a high correlation (r2=0,95) between AERs and outside temperatures was recorded. These measured results were validated and considered to be the true AERs. The CO2, heat and moisture mass balance models were validated against the true AERs. The mass balance results were unpredictable, sometimes very accurate and other times out of range. The CO2 mass balance results at times correlated well with the heat and moisture balance results (r2=0,91 and 0,64, respectively). At best, the average 24h CO2 mass balance results were +54% above the true AERs (r2=0,78), the heat balance results -14% below (r2=0,66) and moisture balance +5% above (r2=0,61). To compare the accuracy of the mass balance models on a finer scale, day and night time averages from the true AERs were compared with the mass balance model results, no trends were observed. Methods of improving the mass balance results by incorporating bird activity into the model to account for diurnal variation, and using 2h intervals instead of 1h were tested. The mass balance results improved only sometimes with these amendments. Due to poor tracer gas/air mixing the tracer gas method did not obtain reasonable results. A SF6 sink was identified, located directly beneath the exhaust chimneys this depressed the air exchange rate results.