| | Cooling | Summary
With high density broiler production, delivering air of acceptable quality to each bird space becomes very important. The temperature of the delivered air must be similar to the temperature requirements of the bird. The air speed must be relatively low when ambient temperatures are below the birds comfort zone to conserve bird body heat and must be relatively high when above the temperature comfort zone to encourage bird heat loss and reduce heat stress.
Air delivered to a bird space must remove bird heat (about 15 W for a 41-day broiler). If this doesn't occur ambient temperature will rise. Air speed in the vicinity of the bird plays a large role in accomplishing this. An air speed of 3 m/s (used in tunnel ventilation) is 3 times more effective than an air speed of 0.3 m/s (stagnant air) in removing heat from the bird. If the heat removal rate is less than that produced the bird will become heat stressed and reduce feed intake. The air entering the bird space must dilute the air in the bird space to acceptable air quality (ammonia <25 ppm, respirable dust (< 0.5 microns) less than 25 part /ml, and carbon dioxide less than 2500 ppm.
The design of the inlet and exhaust fan performance are the essential components in ensuring that the air enters the building uniformly and at an appropriate air speed that will encourage good mixing within the bird air space. Inlets must be planned and not merely a "hole in the wall". Planned inlets are designed so under a negative pressure of 0.12 inches, the air will project into the room at an airspeed of 5 m/s (1000 ft/min) and cause effective air mixing. If mixing is not adequate ceiling mounted fans may be used to encourage air mixing. Maximum available inlet area (m²) should be approximately equal to design ventilation rate (L/s) divided by 4000. This ensures that the inlet opening allows air to enter at 5 m/s or 1000 ft/min into the bird space at a static negative pressure of 0.12 in. This assumes that the inlet is 80% effective. Another rule of thumb is to design an inlet area that is 1.5 times that area of the exhaust fans.
A common problem with inlets is that the static pressure on the attic side of the inlet is not zero. This means that the air is constricted upstream from the inlet. This could be due to attic louvres or soffit openings being plugged or not designed properly. Upstream constrictions to air flow must be opened. Soffit or attic louvre areas should exceed the area required for inlets.
Inlets must respond to changes in ventilation rate. If there is not enough inlet area, the exhaust fan operation will be strained since they are working against a higher than necessary static pressure and performance will be decreased. This can result in fan capacity being reduced by 25 percent or more. As more fans come into operation, inlets must opened to maintain an incoming airspeed of 1000 ft/min or 5 m/s and yet maintain a constant static negative pressure of about 0.12 inches.
Producers must decide on a maximum ventilation rate (5 cfm bird or 2.4 L/s bird). Choose fan capacity for fans at 0.12 in of water gauge. Make sure fan capacity includes hoods, shutters, and inlet guard grills if they are included. If fans deliver as designed and inlets have sufficient area, change in temperature (T inside - T outside) should not exceed 3°C.
Self-designed systems can lead to performance failure. All the fans and inlets can be installed correctly, however they may not operate well as a system. Considering the importance of ventilating a high density broiler space correctly, ventilation systems should be installed by building contractors or equipment suppliers. Once a complete system has been installed the producer should receive a guarantee of performance for given environmental conditions and that the individual components will work in harmony with each other. A ventilation system that is guaranteed to operate satisfactorily over a typical range of outside temperature conditions will result in a better environment in the bird air space.
To maintain optimum performance, fans must be serviced on a regular basis. Fan shutters, housing and blades should be free of dust. Dirty fan blades (feathers wrapped around the cutting edge of blade) can decrease performance by 25%.
Cooling
When outside temperatures exceed 27°C, inside temperatures will exceed 30°C at ventilation rates of 5 cfm/bird. At these temperatures, the birds will have difficulty in dissipating their body heat at these high temperatures. Fogging systems become a feasible alternative to reducing the temperature of the bird space. Air temperature is reduced through the evaporation of water. Evaporation of 10 kg/h of water removes 25 000 kJ/h of heat or that heat produced by 600 broilers. To ensure maximum evaporation of water, droplet size and nozzle placement need to be considered. Currently, systems are being installed where nozzles operate under 1000 psi and deliver approximately 2 gal/h. A fine mist is produced that appears to evaporate entirely.
One 2 gal/h nozzle at 1000 psi appears to be adequate for 250 birds. This results in the inside air being 6°C less than outside, whereas no nozzles would result in a 3°C increase. This means that without birds, the temperature would drop 9°C with fog cooling.
This can be illustrated by the following calculation:
2 gal/h will remove 22,750 kJ/h or 6325 W of heat.
250 birds will produce 7500 kJ/h of sensible heat (8.5W/bird)
Ventilation rate is 600 L/s or 2500 kg/h of air
Change in temperature
= (7500 - 22750 ) kJ/h /2500 kg/h (cooling)
= 6°C less than outside
Change in temperature
= 7500 kJ/h /2500 kg/h (no cooling)
= 3°C greater than outside
As ventilation rates are increased, the effect of fogging to cool becomes reduced since the inside temperature approaches that of the outside. When fogging is required, the ventilation could be dropped to 4 cfm /bird from 5 cfm/bird and result in an inside temperature being reduced from 19°C to 17°C, however the relative humidity increase from 80 to 90%. A ventilation rate of 4 to 5 cfm/bird will provide optimal cooling.
In Summary
producers should consider the following:
| 1. | the ventilation equipment should operate well as a system, |
| 2. | consider that it be installed as a system, be commissioned by a supplier and its performance guaranteed, |
| 3. | maintain and clean the ventilation equipment (dirty fan blades, housing, grills, plugged inlets to the building will result in a temperature difference between inside and outside far in excess of 3°C), |
| 4. | Fog cooling is very effective in lowering the bird ambient temperatures (up to 6°C) when outside temperatures exceed 25°C, |
| 5. | Use planned inlets whose area(m²) is approximately equal to ventilation rate (L/s) / 4000, and |
| 6. | provide a maximum available ventilation rate of 5 cfm/bird. |
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Articles referred to:
| 1. | Extension articles written by Michael Czarick and Michael Lacy on Inlet Design and Fan Performance, Cooperative Extension Service, The University of Georgia, Athens, Georgia. |
| 2. | Barber, E.M., J.S. Strom, M.R.L. Bantle, S. Morsing and J.J.R. Feddes. 1994. Ventilation performance standards-Applying Danish experience in Canada. Appl. Eng in Agric. 10(3):395-401. |
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Dr. John Feddes
University of Alberta |
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