Minimum ventilation challenges

With a light frost once again, I’m reminded this morning that colder weather is coming. With colder weather comes a reduction in the ventilation rates of our facilities as we need to exchange less air to remove the same amount of heat.

Winter minimum ventilation rates are the lowest rates we use and are based on the amount of air exchange necessary to control moisture in our production facilities. Often times, especially in farrowing and weaned pig facilities these rates remove more heat than that produced by the pigs so supplemental heat is necessary to maintain correct temperatures.

In some facilities we’re already approaching or are at minimum ventilation rates on these cool mornings. One of the most common problems I see with minimum ventilation is the lack of correct inlet adjustment which usually means air ‘dumps’ into the pig space without adequate mixing. In farrowing rooms this is often chilled piglets in the creep area while in nurseries this is often wet floors and dirty pigs.

When we operate our ventilation systems at 0.05” w.g. static pressure (which is the pressure we size fans at), inlet velocity is in the range of 800 feet per minute. If pressure is less because the openings into the room are too big or too many the inlet velocity declines and air doesn’t mix correctly and ‘dumps’ from the inlet.

The question becomes, what is the correct inlet opening to get 800 fpm velocity? The answer – not very big!

Let’s start with a 24 crate farrowing room. The Midwest Plan Service recommends 20 cfm/crate for the minimum ventilation rate to control moisture and I’m happy on most sites if I can get the system to operate in the range of 25 cfm/crate. For the 24 crate farrowing room, this is 600 cfm. If we want the air to move at 800 fpm from the inlets, this means the total inlet area to the room from all holes (broken shutter blades, missing door sweeps, inlets, etc) must be 0.75 ft2 (600 cfm / 800 fpm). This is just larger than a sheet of 8.5 x 11 paper which is 0.67 ft2 in area. Said another way, the sum of all the holes into this farrowing room must be about the size of a single sheet of paper – not very big!

Let’s look at a 1200 head nursery or single-stocked wean-finish room with the intention of 2 cfm/pig or 2400 cfm total ventilation at minimum. Doing the math, the total opening to the room must be 3 ft2 (2400 cfm / 800 fpm) or 4.5 sheets of paper! This isn’t much when you consider than an uncovered 36” fan shutter can ‘leak’ 250+ cfm and the big 50+” fans in tunnel barns can ‘leak’ 5-600 cfm each.

This is why I am an advocate of using clips or other closure mechanisms on inlets when trying to operate ventilation systems at minimum. Closing 50% of the inlets reduces the number of holes into the animal space, meaning the unclipped inlets must open further to get the correct inlet total area necessary for 800 fpm inlet velocity.

Many of you have heard me talk about the ‘Brumm Bald Forehead’ rule when talking about inlet adjustment. When ceiling inlets are adjusted correctly with a 7.5-8 ft ceiling, you should ‘feel’ the air coming from the inlet in your face/forehead when standing approximately 15 ft back from the inlet if you are my height (5 ft 7 in.). This draft should be at the same location at all stages of ventilation – that is the purpose of adjustable inlets. The winter challenge – accounting for all the other sources of air infiltration into a facility that add to the inlet capacity and adjusting the inlets accordingly to maintain velocity.

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