Infiltration: The Hidden Enemy
Proper ventilation performance, especially during cold-weather periods, is affected by many factors including the performance of the planned primary inlet system and the hidden, unplanned leakage inlets from cracks and construction gaps (i.e., infiltration). This article will describe research conducted at Iowa State University to determine the extent and influence of infiltration on ventilation performance and some on-farm practices to help reduce the unwanted consequences from excessive infiltration.
The specific objectives of this fact sheet are to describe,
1. The role of static pressure control during minimum ventilation periods,
2. The influence of unplanned leakage areas, or infiltration, on static pressure control, and,
3. On-farm practices to reduce the negative consequences of infiltration
Static Pressure Control and Minimum Ventilation
In exhaust fan ventilated buildings, representing the vast majority of mechanically ventilated pig housing systems, the fans develop the necessary energy to pull fresh outside air into the building.
The suction level, or static pressure, that is developed forces fresh-air into the building through any opening between outside and inside the building. The hope is that this fresh-air is drawn in through the primary inlet system, generally in the form of a planned ceiling inlet system. However, cracks and construction gaps in the building shell represent added openings, albeit unplanned, and if not minimized can negatively affect fresh-air distribution resulting in poor air quality and uneven thermal conditions in the building. The influence of these unplanned openings can be significant, especially during minimum ventilation periods where a limited amount of fresh-air is expected to originate from planned ceiling inlets where it can be properly distributed throughout the building. Figure 1 represents the minimum ventilation (i.e., Stage 1) fan system design for a typical 1,000-hd W-F building in the Midwest to satisfy moisture and heater combustion gas control.
Proper fresh-air distribution requires that the inlet system be sized, by ventilation stage, resulting in an operating “balance point” static pressure between about 0.05 and 0.10 inches water column (in wc). For weaned pigs, at minimum ventilation for this 1,000-hd building, implies a total inlet area (outside to inside of building) ranging from 3.5 ft2 to 2.5 ft2, to maintain an operating suction static pressure of 0.05 and 0.10 in wc, respectively. For market pigs, at minimum ventilation for this 1,000-hd building, requires a total inlet area ranging from 17 ft2 to 11.5 ft2, to maintain an operating suction static pressure of 0.05 and 0.10 in wc, respectively. A summary of this is given in Figure 2. The end result is that the minimum ventilation fan system is designed to generate the airflow required for ventilation needs (in this case moisture and combustion gas control), and the inlet system is sized that allows the operating static pressure to be within desired ranges for proper fresh-air distribution. The fan system does not care where the required inlet area originates from.
The Influence of Infiltration on Static Pressure Control
Research was conducted on 19 pig finishing barns in Iowa for the purpose of quantifying the extent of infiltration (Jadhav et al., 2018a, 2018b, 2019). Curtains were closed to their as-installed limit with the resulting as-installed curtain overlap, all fan back-draft shutters were allowed to close naturally, all external doors were closed, and all primary ceiling inlets were closed and sealed with duct tape. An existing ventilation fan was used to provide a series of suction levels on the building shell and a FANS unit (Gates et al., 2004) was used to measure the ventilation rate. In this manner, all fresh-air intake originated through leakage areas around the building shell. The “as-installed” infiltration level, averaged across all measured buildings, is shown in Figure 3 along with the fan curve for wean age pigs. Figure 3 indicates that the amount of infiltration, on average, provided significantly more wean-age inlet area than would be designed for with the primary inlet system itself, resulting in a lower-than-desired operating
static pressure of about 0.02 in wc. This result implies that the inlet area from leakage locations amounted to about 9.5 ft2, or 2.7 times the required inlet area that would be designed with a primary inlet system itself. In fact, the infiltration area that was measured (on average across all barns) provided the required inlet area (for a desired minimum 0.05 in wc static pressure) up to about 5,500 cfm or 5.5 cfm/pig; the equivalent minimum ventilation needs up to a 100 lb pig. Ultimately, this implies that fresh-air distribution will suffer due to sporadic and unplanned inlet areas throughout the building and a significant lowering of the desired operating static pressure.
Reduce Unplanned Barn Leakage Areas
Clearly, the extent of infiltration in pig finishing can represent the vast majority of inlet area required for typical desired operating static pressures, especially for immature pigs. The research that was conducted (Jadhav et al., 2018a) aimed to highlight practices to minimize the unwanted influence of infiltration. A strategic procedure of sealing curtains (with duct tape), sealing all external doors (with duct tape), and sealing all fans and pump-outs (duct tape + tarpaulin) was conducted to pin-point areas of infiltration and strategies for minimizing. The results indicated that on average, about 20% of infiltration originated from curtains, 20% originated from improperly sealed back-draft shutters and pump-outs (which predominated), with the remaining 60% from “other” unquantifiable building shell sources such as ceiling panel joints and the interface between walls and the ceiling (results at 0.05 in wc building shell static pressure difference). Table 1 summarizes the findings of this research.
Table 1. Minimum ventilation inlet area requirements for a 1,000-hd W-F facility required to maintain a desired operating static pressure of 0.05 in wc.
As shown in Table 1, significant improvement in minimum ventilation performance can be achieved by sealing unused curtains, unused fans, and all pump-out joints. Sealing of curtains (beyond a simple 2-3 inch overlap) and unused fans can be achieved with interior plastic sheeting with an example for unused fans shown in Figure 4a. Pump-outs were a major source of fan-related infiltration implying that the cover lid-to-concrete bump-out and the interface between the pump-out lid and fan need special attention (Figure 4b).
The remaining 60% of infiltration originated predominantly from wall-to-ceiling joints best controlled with foam insulation in the attic but this is an expensive option. Controlling curtain,
pump-out, and unused fan infiltration is clearly worth the extra effort heading into minimum ventilation periods.
The unplanned inlet area provided by infiltration reduces the amount of fresh-air entering through the planned inlet ceiling. This in turn affects the desired operating static pressure at minimum ventilation which negatively affects fresh-air distribution in the building. Winter sealing of all pump-outs, unused fans, and curtains are cost effective measures that can significantly reduce the negative impact from excessive infiltration.
Gates, R. S., Casey, K. D., Xin, H., Wheeler, E. F., & Simmons, J. D. (2004). Fan assessment numeration system (FANS) design and calibration specifications. Transactions of the ASAE, 47(5), 1709–1715. http://doi.org/10.13031/2013.17613
Jadhav, H, S.J. Hoff, J.D. Harmon, and D.S. Andersen. (2018a). Swine finishing room infiltration: Part 1. Quantification and prediction. Applied Engineering in Agriculture 34(2): 413-424.
Jadhav, H, S.J. Hoff, J.D. Harmon, and D.S. Andersen. (2018b). Swine finishing room infiltration: Part 2. Infiltration as affected by room characteristics. Applied Engineering in Agriculture 34(4): 735-745.
Jadhav, H and S.J. Hoff. (2019). Use of air infiltration in swine housing ventilation design. Applied Engineering in Agriculture 35(3): 325-338. (doi: 10.13031/aea.12965).
MWPS-32. (1990). Mechanical ventilating systems for livestock housing. ISBN 0-89373-075-0.