How tunnel ventilated barns really work in hot weather

Many of the readers of this blog have tunnel ventilated wean-finish or grow-finish facilities. Others have seen these type of facilities and have questions about how they function in terms of meeting the heat removal needs of growing pigs. A common question I get asked – how much temperature variation is ‘normal’ for these facilities?

For readers unfamiliar with tunnel ventilated barns, these facilities use ceiling inlets for relatively uniform air distribution within the room of the facility for the lower stages of ventilation. Often times there are enough fans and ceiling inlets to accommodate 45-54 cfm/pig of ventilation capacity.

Once higher amounts of air exchange are required to remove heat produced by the growing pig, a curtain opens at one end of the relatively long and narrow room (often 50 ft x 196 ft or 40 x 240) as an additional air inlet to the room with exhaust fans at the opposite end of the room pulling the air across the pigs on its way to being exhausted by the fans. As more fans come on to increase the amount of air exchanged the curtain opens further and in most cases the ceiling inlets close so little if any solar heated air from the attic is pulled into the animal space. When the ventilation system reaches maximum capacity, 100% of the incoming air enters at one end of the room via the tunnel curtain opening with it being exhausted at the other end of the room.

Most ventilation experts size maximum ventilation rates with the expectation that during warm/hot weather the temperature of the air at the exhaust fans is no more than 2-3F warmer than the temperature of the incoming air (ambient). It is generally felt that it is not economical to install additional fan capacity to lower temperature variation below this target.

Keep in mind that it is impossible to have the exhaust air at the same temperature as ambient in warm weather. You’ve got a room full of growing pigs adding heat (upwards of 800 Btu/hr of air temperature per pig for market weight pigs) to the air as it passes through the pig zone so the exhaust has to be warmer than ambient.

To look at this temperature rise I reviewed data from a wean-finish site where the data logger recorded not only temperature but what stage the ventilation system was operating at the time of the temperature log. I chose to review conditions every 15 minutes over a 2 month period late last spring (May-June). The room held 1230 pigs that ended up weighing 273 lb in late June when marketing began.

In this facility, when all of the fans were operating with bigger pigs in the room the temperature rise averaged 1.9F. This was good news for me since I designed this particular ventilation system which includes turning off a majority of the pit fans at maximum ventilation.

As expected, the largest variation in temperature occurred when the room first entered tunnel mode. At this point a large share of the incoming air is from the ceiling inlets but some air is entering via the tunnel curtain and this cool air must travel the entire length of the facility to be exhausted by the end wall fans (with some of it being exhausted by pit fans along the way). At this point in the ventilation system staging the air temperature was 6.1F warmer at the fan end of the room than at the tunnel curtain end of the room on average, with peaks of 8F variation common.

As additional fan stages came on this variation decreased since the air was now moving faster through the pig zone resulting in higher exchange rates.

By nature of their design, tunnel ventilated barns will have variations in temperature within the animal space. Hopefully the above case study helps readers understand the cause of this variation and what the practical design limits are for this type of system. Next week I hope to write about the pigs need for temperature variation.

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