Is There a Benefit to Installing Extra Pad on a House?

When designing a tunnel-ventilated house, it is important to install the proper amount of evaporative cooling pad. Too little pad will result in excessive static pressure, which will reduce air speed, and therefore bird cooling. Too much pad will result in increased initial cost, more pads to maintain/replace, and a larger area of reduced air movement near the tunnel inlet end wall.

Pad area is based not on the size of a house but rather on the air-moving capacity of a house's tunnel fans. Traditionally, it has been recommended that a house should have one square foot of six-inch evaporative cooling pad (45° X 15° flute angles) for every 350 cfm of tunnel fan capacity at the design static pressure (pad design velocity = 350 ft/min). The design static pressure is the expected static pressure when all the tunnel fans are operating. Though there are a number of factors that determine the static pressure, in general, a house with a higher design airspeed will tend to have a higher static pressure than one with a lower design airspeed. For instance, the static pressure (measured at the fans) in a typical broiler house with an air speed of 600 ft/min will be around between 0.12 and 0.15". A broiler house with 800 ft/min, 0.17" - 0.20". Therefore, in a modern broiler house with air speeds between 600 and 800 ft/min, pad area should be determined by the total air moving capacity of a house's tunnel fan capacity at a static pressure of between 0.15" and 0.20".

Example A:
60' X 600' house
Design air speed = 600 ft/min (@0.15")
Tunnel fan capacity = 350,000 cfm (@0.15")
Pad height = 5 ft
Pad area = 350,000 cfm/350 cfm per square foot
Pad area = 1,000 square feet of pad.
Pad length = 1,000 square ft / 5 ft pad length = 200 ft or 100 ft per side.

Example B:
60' X 600' house
Design air speed = 800 ft/min (@0.20")
Tunnel fan capacity = 460,000 cfm (@0.20")
Pad height = 5 ft
Pad area = 460,000 cfm/350 cfm per square foot
Pad area = 1,300 square ft of pad.
Pad length = 1,300 square ft / 5 ft pad length = 260 ft or 130 ft per side.

Though a pad design velocity of 350 ft/min has been widely used for decades, it is important to realize that it is possible to use a slightly higher design velocity without significantly affecting our ability to keep our birds cool during hot weather. Thirty years ago, the pad area for a house with a six-inch evaporative cooling pad (45o X 15o flute angles) was determined using a design velocity of 400 ft/min (one square foot of pad for every 400 cfm of tunnel fan capacity). The decrease in design velocity through the pads over the years is due to the thought that increased pad area would result in lower static pressure, higher air speeds, and increased cooling of the incoming air. But is this true?

At a pad velocity of 400 ft/min, the static pressure or work required to pull air through a six-inch pad is approximately 0.065". It is essential to realize that this is only a portion of the static pressure the tunnel fans are working against. It also requires work/pressure to pull the air from the large pad area into the smaller cross-sectional area of the house and down the length of the house. So, though the "pad pressure" may be only 0.065" the total pressure the tunnel fans are working against in a modern broiler house (house air velocity between 600 and 800 ft/min) will tend to range between 0.15" and 0.20".

Reducing the pad velocity to 350 ft/min (increasing the pad area) decreases the pad static pressure by approximately 0.01" to 0.055". As a result, the total static pressure would theoretically decrease to between 0.14" and 0.19" (house air velocity between 600 and 800 ft/min). A change of pressure 0f 0.01" would have a negligible effect on house air speed.

How would the cooling of the incoming air compare at a velocity of 350 ft/min vs 400 ft/min? Interestingly, the cooling produced by a pad doesn't change much with the speed of the air through the pad. In fact, at 95° F, increasing the air speed through a pad from 350 ft/min to 400 ft/min would only result in a half-degree or less decrease in incoming air temperature (Figure 1). The truth is that decreasing the air speed through a pad from 400 ft/min to 350 ft/min (increasing pad area by approximately 15%) will generally have a negligible effect on a grower's ability to keep their birds cool during hot weather.

What does affect a grower's ability to keep their birds cool is pad and fan maintenance. For instance, though increasing pad area by 10-15% may lower the pressure by approximately 0.01" the fact is as dirt and minerals build up on pad surfaces pad static pressure can increase well over twice that amount (Figures 2 and 3).

Loose fan belts and worn pulleys can also have a far more dramatic effect on house air speed and, therefore, bird cooling than relatively small changes in house pad. In fact, at a static pressure of 0.15", a reduction in fan speed of just 10% due to a loose belt or worn pulley can result in the air moving capacity of a fan being reduced by as much as 20%. It is unlikely that all a house's tunnel fans would be spinning 10% slower, but if just half the fans had loose belts, it could result in a loss of house air velocity of between 50 - 100 ft/min.

In summary, the key to keeping birds cool during hot weather is not so much the precise pad area a house has but how well it and the fans pulling air through it are maintained.