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Tweaking Grow-Finish Ventilation

At first glance, everything in the relatively new 2,000-head, grow-finish barn seemed to be in order. But as Mike Brumm, an independent consultant from North Mankato, MN, and a team of four Extension specialists began taking measurements for a complete energy audit of the facility, it soon became apparent that their five senses didn't begin to capture what was really going on with the facility's ventilation

But as Mike Brumm, an independent consultant from North Mankato, MN, and a team of four Extension specialists began taking measurements for a complete energy audit of the facility, it soon became apparent that their five senses didn't begin to capture what was really going on with the facility's ventilation system that day.

The group's plan was to complete an energy audit to be used as a case study for ventilation workshops.

The 41-ft.-wide, grow-finish barn had two, 1,000-head rooms split by a work room at the center, where a controller moderated heat and ventilation.

When the group conducted a walk-through evaluation, the pigs had been in the facility for 13 weeks and marketing was scheduled to begin soon. Daily gains were averaging 1.60 to 1.85 lb. with feed conversion ranging from 2.65 to 2.95 lb. of feed per pound of gain.

“Once we began gathering data, we could see something wasn't right in the facility,” Brumm notes.

Each fully slotted room had four, 24-in. pit fans evenly spaced along the south side and a fifth fan on an end wall. Two pit fans were variable speed; the remaining three were single speed.

Fans were rated at 7,010 cfm at 0.05 in. static pressure. All fans had shutters.

Twelve ceiling inlets were evenly spaced over the center aisle of each 1,000-head room. The manufacturer's rating was 3,120 cfm per inlet. Each inlet had two baffles with 24 × 3.5-in. openings. Table 2 shows inlet settings for minimum ventilation rates.

The ventilation system was designed to provide about 35 cu. ft./min./pig (cfm/pig), which calculated: 7,010 cfm/fan × 5 fans/1,000 head = 35.0 cfm/pig fan capacity, and 12 × 3,120 cfm/inlet/1,000 hd = 37.4 cfm/pig inlet capacity. An Airstream controller averaged the readings from two temperature probes mounted in each room.

The controller temperature was set at 67°F. Minimum ventilation was provided by two, variable-speed pit fans with the motor curve 4 at 60% and 1°F bandwidth (Table 1).

At Stage 2 ventilation, two single-speed pit fans kicked in with 1°F differential. And, at Stage 3 ventilation, a single-speed wall fan came on at 1°F differential.

Problems Arose

The team soon discovered that incorrect settings on the weight-adjusted air inlet baffles were compromising the facility's ventilation system and causing a host of problems.

“That type of inlet is notorious for bouncing on windy days,” Brumm explains. “The producer had locked the inlets so they only opened so far because they were causing rapid changes in the static pressure of the facility. That's the correct thing to do in windy weather conditions, but when he forgot to unlock them, that's when the ventilation problems started.”

The building's normal, fan-assisted ventilation capacity was 35 cfm/pig. With the inlets restricted, the rate dropped to 20 cfm/pig.

“Because there wasn't enough air entering the facility, the fans were being starved. Under those conditions, static pressure increases, and that triggers an increase in fan speed and fan staging because they have to work harder to move the proper amount of air,” Brumm explains.

Despite the fans working harder, the temperature inside the facility began to rise, requiring more electricity. At that point, the controller turns on another fan.

“Now, you're using even more electricity, but the air still isn't coming in at a rate to meet the fans' needs. Static air pressure is still very high. If the temperature continues to rise, the next step is the controller drops the curtain,” he says.

The facility's curtain overlapped the header by 3 in. The curtain controller was set to move the curtains at a rate of 1 ft./min. With cycle settings at 15 seconds, the curtain opened 3 in. during each cycle and then stopped for two minutes to gain a temperature reading before moving again. The controller's dead-band, the variable difference between when the heater shuts off and fans began to increase speed, was 1.5°F.

Normally, fans shut off when the curtain opens just a few inches. This owner had modified controller settings so the fans stayed on until the curtain was open 10-12 in.

Because the facility was being starved for air flow, the static pressure was so high that the curtains were sucked tight against the walls and therefore couldn't open properly.

Each room contained two, 250,000 Btu unvented heaters, 1.5°F offset from set point and 1°F differential.

Eight stir fans per room, located along the south wall, were tilted downward approximately 15° and were set to blow across the room (south to north). These fans operated at 16°F above the room temperature set point. Summer cooling misters were set to come on at 89°F.

The producer's electric bills were averaging $130-$150/month for the past two years.

Trouble-Shooting the System

On the day of the walk-through, the outside temperature was 34°F and there was a northwest wind. Room temperature had increased 5°F in 5 minutes; static pressure was 0.062 in. and inlet velocity was 700 ft./min. (fpm). Humidity was 80%, carbon dioxide was 2,500 ppm, ammonia was 25 ppm and hydrogen sulfide was less than 1ppm.

When all five fans were on in a room, the inlet velocity climbed to 1,415 fpm and room static pressure to more than 0.125 in.

“The controller kept opening those curtains more and more until the facility started cooling off,” Brumm explains. “Once it began to cool, the fans and curtains returned to normal settings until the temperature rose again and the entire process started over.

“What we experienced in this facility really illustrates the negative effects of improper ventilation equipment settings,” he says.

When static air pressure settings are correct, air enters through inlets at approximately 800 fpm. As static air pressure increases, air speeds escalate.

Brumm's measurements showed air was entering the rooms at more than 1,500 ft./min., a clear indication that something wasn't working properly.

Variable-speed fans are designed to reduce the amount of air moved as temperatures fall, but may not function properly in windy conditions. “At less than full speed, a variable-speed fan's ability to function against high winds is reduced. If winds are high enough, variable-speed fans may even run backwards,” Brumm notes.

“Air flow brings fresh air to the furnace and the pigs, and removes moisture and contaminants from the building. When you're in a barn every day, you will have a sense of whether things are working properly,” he says.

Controllers can also malfunction and disrupt proper ventilation. “The ropes on that type of controller can get loose over time. If you don't perform maintenance on them and recalibrate the controller every so often, those inlets will either be opening too far or not far enough.”

In addition to higher energy costs, this type of scenario can damage equipment, as motors run while moving less air and curtain control cables are subjected to unusual stress. Temperature fluctuations can also affect the pigs' health.

Hog Slat, which designed and built the unit, recommended fan motor curve 4 and a minimum ventilation setting at 60% as a good starting point, never below 40%.

Other Recommendations

Additional post-tour recommendations from the group included:

  • Widen the variable bandwidth to 2°F and use motor curve 5 at 50%, minimum. That would allow for a slightly wider range in temperature for the variable-speed fans to respond, and could result in less on/off cycling of the single-speed fans.

  • Modify stage 2 and 3 fan settings so that stage 2 would activate one fan and stage 3 would provide for greater heat relief with two additional fans.

  • Reexamine the inlet stop settings, which the group felt were severely restricting mechanical ventilation. Consequently, the facility's curtain was opening prematurely because temperature increases weren't being controlled by the fans. Fans were working harder but not effectively affecting air flow. Therefore, the cost of operation was significantly higher.

  • When pig weights approach 200 lb., lower the set point to 62-64°F. Summer cooling settings were recommended to begin at 80°F.

Table 1. Fan Motor Curve Number
Fan Curve Number
1 2 3 4 5 6
Setting Voltage
100% 245 245 245 245 245 245
90% 141 166 182 186 220 192
80% 128 145 147 173 205 166
70% 113 130 136 156 189 156
60% 99 115 123 142 169 146
50% - - - 126 149 137
40% - - - - 130 -
Table 2. Inlet Openings at Minimum Ventilation Rate
Inlet from east end South slot opening (in.) North slot opening (in.)
1 1.25 1.25
2 0.63 0.63
3 1.50 1.00
4 0.75 1.00
5 0.75 0.75
6 0.50 1.25
7 0.75 .88
8 1.00 1.00
9 .88 .88
10 0.75 1.00
11 0.75 1.00
12 0.75 1.00
Average slot opening: 0.91
Table 3. Attic Insulation
Current insulation depth (in.) Adding 6 in. - savings/year Adding 6 in. - payback (years)
2 $4,800 0.3
4 $1,800 0.9
6 $1,000 2.1
8 $600 2.6
10 $400 3.8
Table 4. Perimeter Insulation
Current insulation depth (in.) Adding 1 in. - savings/year Adding 1 in. - payback/years
0 $1,300 0.7
½ $300 3.2
1 $100 7.0
Table 5. Even Slight Over-Ventilation
For given case:
10% over - $1,040 LP increase (27%)
20% over - $1,960 LP increase (51%)
30% over - $2,970 LP increase (77%)
40% over - $4,060 LP increase (105%)
50% over - $5,130 LP increase (132%)
59% over - $6,030 LP increase (156%)
virtually no investment — only management
Table 6. Lighting Example
Incandescent Compact Fluorescent
75 W 20 W
1,065 lumens 1,250 lumens
750 hour life 10,000 hour life
41¢ initial cost $2.69 initial cost
Operating 8 hours/day - 2.920 hours/year
219 Kwh or $21.90/year 58 Kwh or $5.80/year
Need 3.89 bulbs/year Need 0.29 bulbs/year
Total cost = $23.49/year Total cost = $6.58/year
$17 savings per year or payback in less than 4 months