The golden rules of powered ventilation

Some 40 years ago, after writing a few press articles based on my farm experience of attending to piggery ventilation, I got a tactful letter from the ventilationists at the then NIAgrEng (why do the British close down such valuable research and trial organisations) that made me sit up a bit.

It said: “John, while agreeing with what you advise, some of it is not based on the laws of physics. Why not come up for a couple of days and we will put you straight.”

Swallowing my youthful pride, I did just that and it turned out to be one of the best things I have ever done. They dismantled and then rebuilt my approach to piggery ventilation. One of the things Jeff Owen and his colleagues taught me – demonstrated so effectively from working airflow models – were the following critical rules of fan ventilation.

Get control of air movement. The fans and the internal shape of the room are the primary dynamic forces. Getting control can only be achieved by sealing the building. I still go into piggeries where the cracks and crevices around doors and windows must add up to 1.5 square metres. This is like trying to fill a bucket with a hole in it the size of a penny!

Ventilate the dunging area first and the pigs last, in any climate. Failure leads to wrong-mucking and vices.

In cold conditions, have a long air travel. This ensures good mixing of incoming colder air with the warm currents rising from the pigs, and slows it down to under 0.2m/sec over the pigs’ backs, keeping them inside their thermal comfort zone. The process is reversed and speeded-up in hot conditions so as to cool the pigs.

Fans do not direct the air. They are solely the source of power. If the building is adequately sealed they can be placed anywhere. Easy to access is good. 

Inlets direct the air pattern. Because of this, unlike fans, inlet size, placing and control of aperture are very important.

Inlet closure and fan speed must be interlinked by a controller. Programmable controller science has advanced tremendously in the past 15 years. Make sure your system is updated as it can now handle a wide range of variables that caused problems in the past. It may look expensive, but in my experience the payback on capital cost for growing pigs is still from only a few batches put through.

If there’s a choice, preferably exhaust the air. Mainly because exhausting the air makes it easier to stabilise incoming air should outside wind speed, temperature and direction vary frequently. Pressure systems also work, but are trickier and must be installed by an expert.

Under most climatic conditions, air should enter the inlet at not less than 5m/sec. This level is needed to drive the correct air patterns. If it’s very cold, then supplementary heating may be needed.

Fix both maximum and minimum ventilation rates. These depend on an often confusingly wide variety of influencing factors. A major advantage of modern control systems is that when these are set up correctly at installation, and checked annually, then all of them are taken care of within agreed forecasts of likelihood (that is, minimum/maximum outside temperatures and wind speeds).

In cold and exposed, windy areas, the power needed to meet target ventilation rates are best served by several small high-speed fans cutting in and out rather than a few large ones speeding up or down.

When altering sub-optimal ventilation causing the considerable penalties outlined in my last set of notes, or if completely upgrading premises to one of the new integrated systems, the service of a professional agricultural ventilationist is essential so as to get the physics right. Why these vital people are in such short supply across the world is lamentable.

Meantime, every pig farmer should have the basic kit of an anemometer, a set of smoke phials and an accurate, portable temperature sensor. These will teach him so much about his own piggery ventilation – and make the job more interesting too.  

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About The Author

John Gadd, who has spent 60 years' involvement in pig production, has had more than 2,800 articles about pigs published and has written three best-selling pig textbooks. With hands-on experience that includes managing a grow-out herd at 1,800ft in Banffshire, Scotland, and 20 years in the allied industries with Boots' Farm Department, RHM Agriculture and Taymix, he set up his own international pig management consultancy in the mid 1980s and has now visited more than 3,000 pig units in 33 countries as a pig management adviser. (Photo courtesy Bournemouth Daily Echo)