Results and Discussion
Airspeed comparisons
Air velocities were consistently measured only within the Ohio barns. Morning and afternoon measurements from 5 to 6 visits to each farm provided the basis for this data.
The airflow within the two ventilation systems differed spatially as shown by average airspeeds at 18 interior locations within each barn (Figure 5). In the naturally ventilated barns, air moved the fastest along the exterior edges (X1 locations; X can be substituted for North, South, East, or West in any of the following references to location) and the feeding alley (X4 locations). At the exterior edges of these barns, the natural flow of air was just entering or about to leave the barns. Toward the center of a barn, the air at cow level slowed as the cross-sectional area increased and some airflow was pulled away from the floor (as observed by comparing X3 versus X2 airspeeds). The high airspeeds in the feeding alleys were likely due to the influence of the supplemental cooling fans and some channeling of airflow away from stall areas into the more-open alleys. Air flowed comparatively slowly in the center alleys (X5 locations) of these barns.
Air streams in the tunnel-ventilated barns were more stable than those in the naturally ventilated barns, but airspeeds were not uniform along the lengths and widths of the barns (Figure 5). Airspeeds in the center of these barns (X4 and X5 locations) and higher off the floor (X3 locations) were usually greater than in the corresponding freestall areas (X1 and X2 locations). Evidently, airflow was channeled toward the areas providing the least resistance to airflow and away from areas offering more resistance due to blockage (cows and freestalls) or confinement (shorter heights). Neither of the tunnel-ventilated barns in Ohio had ceilings installed. Rather, each had a shorter eave height and flatter roof (2.5:12 or 3:12) than that for a conventional naturally ventilated barn design. In Barn TV-OHC, large open doorways at the southern ends of the cow alleys provided almost the entire amount of inlet area (both large doors at the ends of the drive-through feed alley were kept closed when not in use). Consequently, airflow was preferentially directed to the freestall and feeding areas (X1-2 and X4 locations) in the south half of this barn. Limited data for the tunnel-ventilated barn in NY showed that channeling was more pronounced in a longer barn. In Barn TV-NY, ceiling baffles and a low ceiling were each employed within the barn to try to keep airflow nearer to the cows, and some air was allowed to enter along the bottom of the sidewall curtains to maintain air movement near the sides of the barn. Installation of a flat ceiling at eave height proved fairly effective in this regard. [Although not described here, added design for year-round ventilation is essential for barns with ceilings.]
The resultant cow-level airspeeds within the occupied regions of the barns were roughly a third those of the corresponding ambient winds for both ventilation systems, varying from about 20% to over 60% of ambient wind speed (Table 3). The two ventilation systems produced differing directional flows of air, as was expected. In the naturally ventilated barns, the direction of interior airflow was usually very similar to that of the ambient winds. There was little evidence that the supplemental-cooling fans altered the bulk movement of air through the barns, although lengthwise flow of air was enhanced in the vicinity of the fans. Within the tunnel-ventilated barns, airflow was much more unidirectional, flowing primarily along the long axis of each barn.
| Farm | Air velocity (mph) over the study period | |||||||||
| NW Quad. | NE Quad. | SW Quad. | SE Quad. | Outside | ||||||
| Mean | STD | Mean | STD | Mean | STD | Mean | STD | Mean | STD | |
| NV-OHC | 2.17 | 0.73 | 2.06 | 0.63 | 1.56 | 0.30 | 2.15 | 0.91 | 6.01 | 3.14 |
| TV-OHC | 1.48 | 0.44 | 1.97 | 0.81 | 2.51 | 1.01 | 3.40 | 1.45 | 6.47 | 3.74 |
| NV-OHW | 3.17 | 0.91 | 2.32 | 0.63 | 2.18 | 0.74 | 1.95 | 0.51 | 5.05 | 3.45 |
| TV-OHW | 1.85 | 1.23 | 2.99 | 1.45 | 1.49 | 0.46 | 2.32 | 0.71 | 7.17 | 3.90 |
| 1NV = Naturally ventilated, TV = tunnel ventilated, OHC = central Ohio, OHW = western Ohio, NW = Northwest, NE = Northeast, SW = southwest, Quad. = quadrant, and STD = standard deviation. | ||||||||||
The airflow rates through the tunnel-ventilated barns were controlled and, therefore, were more consistent over time than those through the naturally ventilated barns. However, airflow measurements did not show an advantage in the consistency of cow-level airspeed within the barns, as shown by similar ranges in standard deviations in resultant air velocity for the occupied spaces of the barns (Table 3). The values listed in Table 3 illustrate that tunnel ventilation did a similar job of maintaining air velocity in the cow spaces of the barns on a day-to-day basis as was achieved in the naturally ventilated barns with a base number of supplemental cooling fans (neither of the naturally ventilated barns in Ohio had enough fans to service both the stall and feeding areas at recommended levels).