By Brett C. Ramirez
Warm weather may be long gone and nowhere in the near future, but it’s always worthwhile to discuss the importance of maximum ventilation during summer.
For breeding and gestation (B/G) barns in particular, ventilation is critical to limiting the negative impacts of heat stress such as diminished reproductive functionality, decreased feed intake and the decreased chance at achievement of the genetic potential of resultant offspring.
As buildings are being remodeled to accommodate increased space requirements for sows, there are several factors which may impact required summer ventilation including: increased heat production due to activity, changes in stocking density, changes in barn size and if the ventilation system is also being replaced, changes to air exchange rate.
These substantial changes warrant a revisiting of the fundamentals of maximum ventilation to reduce the negative impacts of heat stress on sows.
Calculating Maximum Ventilation Rate The maximum ventilation rate is the greatest air exchange rate that the system can achieve and typically occurs around 8°F to 10°F above the set-point temperature. When calculating or evaluating the maximum ventilation rate for tunnel ventilated barns, there are several possible approaches:
Typical hot weather design ventilation rate for gestating sows is 250 cubic feet per minute (cfm) per sow to reduce heat accumulation and increase air movement. So, a room housing 1,128 sows would need a maximum ventilation rate of 282,000 cfm.
If the room is 62 feet wide by 400 feet long with an 8-feet-tall ceiling, the cross-sectional airspeed at this rate would be 569 fpm.
With an adjustment to heat production to account for modern sow genetics, at 250 cfm/sow, this barn may experience upwards of a 3°F increase in temperature.
If fans/shutters are dirty, belts loose, dirty evaporative cooling pads and added static pressure due to filters, etc., the once “designed” ventilation rate of 250 cfm/sow, may be closer to 212 cfm/sow, a conservative 15% decrease, which results in closer to 3.6°F increase in room temperature.
Impact of ventilation rate in a typical gestation building (1,128 sows; 62 × 400 feet)
200
52.8
452
+3.8
225,600
250
42.2
569
+3.0
282,000
300
35.2
683
+2.5
338,400
350
30.2
797
+2.2
394,800
Let’s consider the scenario of a remodel to change from stall B/G to open pens (i.e., Prop-12 compliant). More needs to be considered than just cfm/sow. Using our current 400 feet long example barn, assuming it was remodeled to pen B/G and now houses 904 sows with no changes to the building footprint.
If the total ventilation rate doesn’t change (i.e., continue to use existing system), there should not be a negative effect (increases from 250 cfm/sow to 312). However, if during the remodel ventilation equipment is upgraded there is now opportunity to ventilate less because there are fewer sows in the barn. The table below shows the implications of such an assumption.
Ventilation impact when designing for Prop-12 stocking density requirement (904 sows; 62 × 400 feet)
65.8
365
+4.3
180,800
52.7
456
+3.4
226,000
43.9
547
+2.8
271,200
37.6
638
+2.4
316,400
In this case, the target ventilation rate needs to increase from 250 cfm/sow to target a greater air exchange rate to decrease temperature gain inside the room. At least a 40 s air exchange would be a good target.
Therefore, the fully stocked barn required a little more 250 cfm/sow to achieve the 40 s air exchange while the reduced stocked barn requires a little more than 300 cfm/sow. The table to the right shows the impact of ventilation on different gestation barn conversion scenarios with italicized rows recognizing new stocking densities for Prop-12 compliance.
With any model or design calculation, the total static pressure the fans will experience (evaporative pad + filters + room + intakes + etc.) must be considered to ensure the fans can perform and deliver the desired rates.
In some cases, total static pressure can be high (>0.3 inch water column). When conducting ventilation design, it is important to calculate the different rates and consider tunnel airspeed, air exchange rate, and cfm/sow numbers to make informed decisions building and ventilation remodels.
Special thanks to Dr. Hyatt Frobose with JYGA Technologies/Gestal for helping supply example scenarios.
Ramirez is an assistant professor in agricultural and biosystems engineering at Iowa State University.
124.5 × 468 ft
2,550
73 s, +5.1°F
44 s, +3.0°F
31 s, +2.2°F
1,792
104 s, +5.7°F
62 s, +3.4°F
45 s, +2.5°F
51 × 372 ft
1,584
38 s, +5.0°F
23 s, +3.0°F
16 s, +2.1°F
624
97 s, +5.7°F
58 s, +3.4°F
42 s, +2.5°F
60 × 400 ft
2,623
29 s, +4.9°F
18 s, +3.0°F
12.5 s, +2.1°F
1,746
44 s, +5.5°F
26 s, +3.3°F
19 s, +2.4°F