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This is not a peer-reviewed article.
Tunnel Ventilation for Freestall Facilities Design, Environmental Conditions, Cow Behavior, and Economics
C. A. Gooch and R. R. Stowell
Pp. 227-234 in Fifth International Dairy Housing Proceedings of the 29-31 January 2003 Conference, (Fort Worth, Texas, USA), ed. K. A. Janni. ,Pub. date 29 January 2003 . ASAE Pub #701P0203
Abstract
Tunnel ventilation, the practice of installing large exhaust fans with high flow rates in one endwall of a barn to draw air longitudinally down the barns length from an inlet located in the opposite end wall, has recently received favorable usage by many dairy producers in the United States. The success of tunnel ventilation varies depending on several key variables. Several recently published research reports compare and contrast tunnel ventilation to natural ventilation. Tunnel barns were shown to provide a slight improvement in some aspects of the cows environment during potentially stressful conditions. Overall many barns that employ tunnel ventilation provide a better environment for housed cows than if they were naturally ventilated. Economically, an analysis of all associated cost for employing tunnel ventilation shows that payback, measured in sustained milk production, is achievable, especially for longer barns. This paper aggregates our research work and field observations to provide a single reference that can be used to obtain upto-date knowledge of tunnel ventilation for dairy freestall barns.
KEYWORDS. Airflow, Heat stress, Thermal environment.Introduction
The concept of tunnel ventilating animal housing facilities first originated with poultry structures but has recently received considerable interest by the dairy industry. Many dairy producers have found that tunnel ventilation improved the air quality in their barns during summertime conditions. The first dairy facilities targeted for tunnel ventilation were naturally ventilated barns that were poorly designed, sited, and/or oriented, and tie stall and stanchion barns that had inadequate mechanical ventilation systems. The widespread success of these initial applications has resulted in a number of dairy producers retrofitting tunnel ventilation to existing freestall barns. New freestall barns have been specifically designed and constructed to incorporate tunnel ventilation as well. All tunnel-ventilated barns have the two-fold goal of ensuring predictable summertime ventilation and air movement over cows bodies both of which are essential components to reduce heat stress.
Literature Review
Much of the recent ventilation research performed for dairy cow housing has focused on tunnel ventilation for freestall barns. Gooch and Timmons (2000) provided a detailed review of the application of tunnel ventilation to freestall barns. Gooch et al., (2000) performed a cash-flow analysis to investigate the sustained milk production required to pay for a tunnel ventilation system. Stowell et al. (2001a&b) reported on the performance and environmental conditions, respectively, of tunnel ventilated freestall barns. Cow activity and performance within tunnel ventilated barns was reported by Stowell et al. (2001c).
The purpose of this paper is to aggregate all of our research work and field observations to provide a single reference that can be used to obtain a thorough knowledge of tunnel ventilation for dairy freestall barns.
BACKGROUND
Description
Tunnel ventilation is a simple summertime ventilation system designed to provide air exchange and air movement concurrently in a barn. Fans (called tunnel fans) are usually placed in one endwall of a building. They are operated to create a negative pressure in the barn causing air to be drawn into the opposite endwall opening (Figure 1). Once in the barn, fresh inlet air travels longitudinally through the structure and is exhausted by the tunnel fans. In theory, for tunnel ventilation to function at its maximum potential, all sidewall, ceiling, and floor openings must be sealed to form the tunnel. However, field experience has shown that improved airflow at cow level can be achieved in barns that have freestall rows along the barns sidewalls by positioning the bottom curtains (bottom curtain opens from the bottom up) of a split curtain sidewall slightly open (5 to 10 cm; 2 to 4 in) during tunnel system operation.
Figure 1. Plan view of a typical tunnel-ventilated freestall barn.
Limitations
Tunnel ventilation is not generally an appropriate ventilation system for use in cool and cold periods. The air speeds associated with a well-designed tunnel system can result in cold, drafty conditions when operated at those times. A reduction in the number of tunnel fans operating or decreasing the capacity of each fan by slowing the electrical motor will result in reduced air exchange rates that may not provide fresh air throughout the length of the barn. If the same ventilation equipment is used during cool weather, then appropriate measures must be implemented to ensure adequate mixing and to prevent serious gradients in air quality along the barns length.
The maximum barn length that can be effectively tunnel-ventilated is another limitation. As the inlet air moves longitudinally through a barn, it becomes increasingly contaminated with air pollutants (i.e. dust, humidity, manure gases, and pathogens). At some point, the inlet air is no longer fresh and provides limited benefit to cows downwind from this point.
Design
The procedure to design a tunnel ventilation system consists of two straightforward steps. First, the required total fan capacity is determined. Then this value is used as an input variable to determine the size of the air inlet(s).
Determine Total Fan Capacity
Successful system design must achieve two goals: 1) air speed at cow level and 2) air exchange. Except for very long barns, air speed is usually the limiting criteria. The recommended air speed at cow level to promote cooling is 2.5 to 3 m/sec (500 to 600 fpm) (Shearer et al., 1991). To determine total fan capacity, a chosen air speed value in this range is multiplied by the cross sectional area of the barns air space calculated perpendicular to the direction of airflow. Field experience suggests a recommended air exchange rate during hot conditions of 0.5 m3/sec (1,000 cfm) per cow. Each of these goals should be considered individually during the design stage to determine which one will ultimately govern. Use the larger of the two values calculated above to determine the overall theoretical fan capacity required.
Determine Inlet Size
The size of an air inlet opening must be sufficient enough to allow adequate air volumes to enter the barn without having to overcome excessive resistance to flow from friction and turbulence. Higher resistance creates increased static pressure within a barn and decreases effective fan capacities. Inlets are best sized to provide a minimum of 930 cm2 (1 sq. ft.) of area for every 0.19 m3/sec (400 cfm) of fan capacity. With less than 930 cm2 (1 sq. ft.) of inlet area per 0.33 m3/sec (700 cfm) of fan capacity, increased static pressure reduces fan performance and provides less-than-adequate air exchange (Gooch and Timmons, 2000).
Environmental Conditions
In order to justify the extra capital and operating expense of tunnel ventilation, the environment in a tunnel-ventilated barn must be better than the environment in the barn if it were naturally ventilated. Cows that benefit from an improved environment will generate income to help pay for the extra costs. The main advantage of a tunnel-ventilated barn should be that cows have access to good airflow at all times during the summer regardless of natural wind conditions or where the cows are within the building. The main disadvantage should be the electricity needed to operate the ventilation fans to supply required levels of air exchange.
Stowell et al. (2001a) investigated the differences in performance with respect to inside vs. outside parameters (dry-bulb temperature, relative humidity, and temperature-humidity index) between three pairs of tunnel-ventilated and naturally ventilated barns with cooling fans, one pair located in the Finger Lakes region of New York State, one pair located in central Ohio, and one pair located in western Ohio. A synopsis of this work is presented below.
Overall temperature comparison
A well-ventilated barn will have an inside dry-bulb temperature that is equal to or slightly below that of the outdoors, unless evaporative cooling is employed. Barns with temperature differentials greater than a few degrees (warmer inside than outside) during a reoccurring duration of time can be considered to have ventilation shortcomings.
The average hourly dry-bulb temperatures for August 2000 are presented in Figure 2 for each barn monitored. The average temperatures shown for Hour 1 represent the means of the average temperatures from midnight to 1 a.m. throughout the month (representing a maximum of 31 days). Within each day, hourly temperatures represent the average of 10-minute readings from either a single sensor (outside temperature) or the four sensors in each barn (inside temperature).
Inside air temperature tracked outdoor temperature fairly closely in each of the study barns. The tunnel-ventilated barns displayed a slight advantage during the heat of the day for each pair of comparison barns. Mean inside air temperature was usually within 0.5 C (0.9 F) of mean outdoor temperature during mid-afternoon hours in each of the three tunnel-ventilated barns. In the naturally ventilated barns, the air temperature during these afternoon hours was usually 0.5-1.5 C (0.9 2.7F) higher, on average, than the outside air temperature.
Naturally Ventilated NY Barn Tunnel-Ventilated NY Barn
Naturally Ventilated Central OH Barn Tunnel-Ventilated Central OH Barn
Naturally Ventilated Western OH Barn Tunnel-Ventilated Western OH Barn
Figure 2. Average hourly dry-bulb air temperatures within each of the study barns for August 2000 (Stowell et al., 2001).
Temperature-humidity index
Air temperature and humidity combine together to cause a net effect on cows confined in a dairy barn. The net effect can be quantified by following the equation for temperature-humidity index (THI) as outlined in the ASAE Standards (1999). Contemporary high-producing dairy cows are thought to be in an environmentally stressful situation if the THI value is 70 or greater. A well-ventilated barn will have an inside THI value equal to or slightly less than the corresponding outside value.
The small advantage that the tunnel-ventilated barns exhibited in terms of temperature moderation was repeated for THI. Table 3 shows that each of the tunnel-ventilated barns provided a slightly cooler interior environment than its naturally ventilated counterpart when outdoor conditions were potentially stressful (THI 70 or above). The average advantages in terms of reduced differentials between indoor and outdoor environments were 0.4 C and 0.4 for air temperature and THI, respectively. These differences were statistically significant.
Table 1. Barn environments for conditions when ambient THI = 70 during summer 2000 (Stowell et al., 2001).
|
Applicable No. of hours |
Temperature differential, Ti - To , (C) |
THI differential, THIi THIo | |||
|
Farm ID |
n |
Meana |
Std. Dev. |
Meana |
Std. Dev. |
|
NV-NY |
265 |
0.89 |
0.58 |
0.98 |
0.67 |
|
TV-NY |
247 |
0.40 |
0.92 |
0.75 |
0.99 |
|
NV-OHC |
478 |
0.58 |
0.65 |
0.73 |
0.69 |
|
TV-OHC |
555 |
0.16 |
0.62 |
0.07 |
0.70 |
|
NV-OHW |
642 |
0.76 |
0.60 |
0.82 |
0.70 |
|
TV-OHW |
692 |
0.40 |
0.75 |
0.51 |
0.88 |
a The means for each barn pair differ at P < 0.01.
Spatial variation
As air moves longitudinally through a tunnel-ventilated barn the potential exists for it to gain temperature and humidity by picking up sensible heat and moisture. The longer the barn, the more potential for gains. Data collected from one of the tunnel-ventilated study barns that was comparatively long (268m) showed a 2-3 C increase in the time-averaged temperature during the day and 4-5 C during the night suggesting that this barn is potentially too long to tunnel ventilate effectively or additional fan capacity is needed to hasten the air exchange rate. Shorter barns (~50m) performed better with 1 C or less temperature gain in the day and no more than 2 C gain at night. The maximum time-averaged THI (compared to that outside) ranged from about 1 point for the shorter barns to nearly 4 points for the longest barn.
Air speed distribution
Providing sufficient air speed at cow level to enhance heat removal from cows during stressful environmental conditions is the second goal of operating a tunnel ventilation system. The theoretical average air speed is calculated by dividing the total in-place fan capacity by the cross-sectional area of the building perpendicular to the direction of airflow. As previously discussed, cows benefit from between 2.5 to 3 m/sec (500 and 600 fpm) of air speed over their bodies during stressful summertime conditions (Shearer et al., 1991).
We found out that airspeeds were not uniform along the lengths and widths of barns. Measured airspeeds within the lower portion of many tunnel-ventilated barns were noticeably lower than the design airspeed. Measured airspeeds in the central areas, like drive-through alleys, and higher off the floors were usually greater than in the corresponding freestall areas and other occupied cow spaces. This shows that airflow naturally channels towards those areas with least resistance to air movement and away from areas offering more resistance due to blockage (cows and freestalls). Longer barns appear to have a more pronounced airflow channeling effect resulting in little air movement at cow level.
Closing thoughts
It should be noted that the success of a particular ventilation system selected by a dairy producer for their barn depends on many independent variables, and the values for these variables differ from barn to barn and from farm to farm. Barn design, location, orientation, and position on the farmstead are some of the variables that affect the success of a particular system. Not all barns will naturally ventilate well nor will tunnel ventilation be successful in all barns. Given a good natural ventilation scenario and a tunnel ventilation comparison barn we did not find big differences in the performance of the two systems. However, a naturally ventilated barn that does not provide good air exchange will benefit substantially from the adoption of tunnel ventilation as the data showed for the New York tunnel-ventilated barn. The original construction of this barn featured natural ventilation though it was poorly designed. It had low building sidewall heights, a fully insulated ceiling (insulated along the bottom chord of the trusses), no ridge opening, and was positioned in a low lying area with higher topography located on the prevailing windward side. In this case, the addition of tunnel ventilation to this barn a few years prior to the study resulted in improvements of air quality that far exceeded what the data suggests when comparing it to its naturally ventilated pair.
Cow Behavior
Ventilation and cooling systems should be designed to promote and encourage the continuation of productive cow activity during environmentally stressful conditions. Productive activities include eating at the feed bunk and resting in stalls. Cows that are environmentally stressed will digress from productive activities as stressful conditions increase.
As part of the study previously discussed (Stowell et al. (2001a&b)), we also looked at the cows response to their environments. Data supported the notion that cows make behavioral modifications (move and adjust their posture) to take advantage of greater air movement. In two of the three tunnel-ventilated barns with two longitudinally oriented pens per barn, the tendency observed was that more cows were on their feet (standing at a bunk, in cow alleys, or in stalls) within upwind quadrants. Cows that were standing could be doing so to expose their bodies to higher levels of airflow. This could have an impact on cows in the long run as they may develop increased foot and leg problems.
As previously mentioned, the New York tunnel-ventilated barn was originally naturally ventilated, and it was also outfitted with 4 rows of cooling fans (which are still in the barn), 1 row over each head to head freestall row. The barn manager found that operation of the original cooling fans during environmentally stressful conditions helped to keep cows more comfortable and lying in their stalls than with the use of the tunnel ventilation system alone.
Economics
The capital, operational, maintenance, and repair costs associated with a tunnel ventilation system must be recovered from monies generated from selling additional milk as a result of sustained milk production of levels similar to those realized during moderate environmental conditions. Also contributing to system payback includes sustaining other herd health performance indices, such as breeding services per conception, fetal growth rate, and others. All cow heat stress related dependent variables contribute towards the rate of return on the tunnel ventilation system investment. The net return on investment is a function of how well cows respond in total to a heat stress condition in a tunnel-ventilated barn each summer beyond how they would respond if it was naturally ventilated.
A complete economic analysis of a ventilation system is complex and difficult to fully quantify. While an analysis of the fixed and operating costs associated with the tunnel system can be quantified, an accurate analysis of the cow is much more challenging. The complexity of measuring a cows complete biological response to heat stress combined with the lack of performance data to develop economic loss predictions make this difficult.
Gooch et al. (2000) used a cash-flow analysis to determine how much sustained milk production was required to achieve a break-even investment. The analysis showed that relatively little sustained production is required to pay for tunnel ventilation in many cases based on a five-year payback period including geographic areas where tunnel ventilation would be used as little as 50 fan days per year (1,200 hrs.). A summary of the analysis is shown in Table 2. Variables analyzed included: barn configuration and size, fan performance, electrical cost, animal stocking density, days of use per year, number of days of continued benefit, purchase, installation, operation and maintenance cost, gross milk price, milk marketing cost, feed cost, and other variable expenses. For the purpose of the analysis, it was assumed that a positive cash flow in a relatively short period of time meant that the investment had a positive rate of return.
Table 2. Pounds of milk per cow per day required to break even using an energy-efficient fan. Analysis based on a five-year pay back period, interest rate of 8 percent, and no changes in operating costs due to inflation. Gross milk price = $12 per Cwt. and energy cost = $0.10/kW-hr.
Conclusion
Tunnel ventilation can be considered a good way to ventilate a dairy barn. The design of the system is straightforward and many fan companies offer an array of fans worthy of consideration. Applied field research has shown that tunnel-ventilated barns can provide somewhat of an advantage of reducing environmental stress during potentially stressful conditions. Even greater improvements may be seen with increased air speed in tunnel-ventilated barns. This may also help with providing sustained air movement at cow level one of the greatest shortcomings of most tunnel ventilation systems. Research has showed that cows will adjust their position in order to help promote cooling of their bodies, many times to a degree that they reduce other productive activities (eating and lying in stalls). A cash-flow economic analysis revealed that little sustained milk production is required to pay for the installation, operation, and maintenance of a tunnel ventilation system in many cases. Those considering tunnel ventilation for their barns are advised to compare the likelihood of achieving good ventilation by natural means with their given situation and contrast that to the high probability (but not without a cost) of achieving consistent air exchange with tunnel ventilation. Risk management approaches should be used to evaluate these systems.
On-line design assistance is available for producers and their advisors for use in designing tunnel ventilation systems and evaluating the economics of the design. The web address is: www.prodairyfacilites.cornell.edu. Click on interactive tunnel ventilation.
REFERENCES
ASAE. 1998. Design of ventilation systems for poultry and livestock shelters, EP270.5. ASAE Standards 1998 , p.590-607 . ASAE, St. Joseph, MI.
Gooch, C. A. and M. B. Timmons. 2000. Tunnel Ventilation for Freestall Barns. Dairy Housing and Equipment Systems: Managing and Planning for Profitability. NRAES-129. Natural Resource, Agriculture, and Engineering Service, Ithaca, NY. pp. 186 201.
Gooch, C. A., M. B. Timmons, and J. Karszes. 2000. Economics of tunnel ventilation for freestall barns. Presented at 2000 ASAE Annual International Meeting, Paper # 004101. ASAE, St. Joseph, MI.
Shearer, J.K., D.K. Beede, R.A. Bucklin, and D.R. Bray. 1991. Environmental Modifications to Reduce Heat Stress in Dairy Cattle. Agri-Practice , Volume 12, No. 4, July/August, 1991.
Stowell, R. R., C. A. Gooch, S. F. Inglis. 2001a. Performance of Tunnel Ventilation for Freestall Dairy Facilities as Compared to Natural Ventilation with Supplemental Cooling Fans. Livestock Environment VI. Proceedings from the Sixth International Symposium. ASAE. St. Joseph, MI. pp. 29 40.
Stowell, R. R., C. A. Gooch, S. F. Inglis, N. R. St. Pierre, and E. Joseph Beiler. 2001b. Cow Activity and Performance within Tunnel-Ventilated and Naturally Ventilated Dairy Freestall Facilities. Presented at 2001 ASAE Annual International Meeting, Paper # 01-4101. ASAE, St. Joseph, MI.
Stowell, R. R., C. A. Gooch, S. F. Inglis. 2001c. Environmental Conditions within Tunnel-Ventilated and Naturally Ventilated Dairy Freestall Facilities. Presented at 2001 ASAE Annual International Meeting, Paper # 01-4099. ASAE, St. Joseph, MI.
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