Ventilation: Get the Best From Your System
By Harry Huffman P. Eng
Get the Best From Your system
By Harry Huffman P. Eng
While we all recognize the need to supply fresh air for the birds and
that tem-perature sensors are used to operate the fans so that the
desired temperature can be maintained, we tend to forget about the
other functions of the ventilation system.
While we all recognize the need to supply fresh air for the birds and that tem-perature sensors are used to operate the fans so that the desired temperature can be maintained, we tend to forget about the other functions of the ventilation system.
All birds constantly expel water vapour and carbon dioxide into the air from respiration.
Since carbon dioxide is heavier than air, the carbon dioxide concentration at floor level can quickly build and limit the amount of oxygen available for floor reared birds. Adequate ventilation is necessary to limit this concentration to no more than 5,000 ppm to prevent negative effects on the birds. Some countries are actually recommending this upper concentration limit be lowered to 3,000 ppm.
Compare this to fresh outside air, which has a base carbon dioxide concentration of about 300 ppm and you can see just how contaminated the air in a poultry barn really is.
The birds are also expelling a large quantity of water vapour into the air space. If all of this moisture is not removed on a continual basis, it will be absorbed by the litter and increase the rate of manure decomposition. This results in the production of ammonia gas that can make the room very odourous and decrease the quality of air that the birds are inhaling. Excessively high ammonia gas levels will cause blindness and affect the birds’ ability to find the feed and water.
For most types of poultry the acceptable range of room humidity is 50 to 70 per cent. Higher relative humidity levels will quickly cause wet litter and odourous conditions. The maximum recommended level for ammonia gas in poultry buildings is 20 ppm. This is getting close to the 25 ppm time-weighted maximum average recommended for industrial workers based on an 8-hour work day and 40-hour work week.
The two basic properties of air that must be accommodated in every ventilation design are:
Cold air is heavier than warm air
• Therefore, it will fall or sink to floor level when introduced into a building
• It will not climb up sloping ceilings
Cold air does not hold much moisture
• For every 10 C degrees that air is warmed, its moisture holding capacity is doubled, Therefore, for the majority of the year when the outdoor air temperature is lower than the desired room temperature, this fresh air is introduced at the top of the sidewall and jetted horizontally across the ceiling surface such that a circulation pattern is established and the incoming air has sufficient time to mix with room air to warm prior to reaching the birds.
For this to happen, the room must be sealed tight enough that a small negative pressure or “vacuum” can be created by the exhaust fans. For fall, winter and spring conditions we are looking for a static pressure level of 0.06 to 0.08 inches water gauge. Every room should be equipped with one of these inexpensive measuring devices to quickly visualize whether too little or too much static pressure is present.
If you are running too much static pressure during cooler weather, then your facility is extremely well sealed and you simply need to open the air inlet a little more. However, this scenario is the exception rather than the norm. Most barns are too leaky and the fresh air can enter from too many undesirable air inlet locations. Of course, the remedy is to tighten the barn up.
The first and most obvious leakage is through the large summer fans that are no longer required. Inside mounted, insulated covers are commercially available for most wall fans and if not, custom-fitted units should be constructed. Be sure that all load-out doors and the large clean-out doors are weather stripped and clamped tight shut.
Unfortunately, in a good number of cases, these measures are still not sufficient and the producer is faced with running the air inlet totally closed to develop the correct static pressure. This strategy generally results in very spotty air introduction with large zones of little or no fresh air.
An excellent retro-fit is to install thin plywood blocking along the air inlet closure board with a short calculated gap or space left between each section of blocking such that the air inlet now closes against the new blocking and leaves these short thin slices of fresh air inlet as the preferred leakage locations for the minimum ventilation air.
Of course the other strategy that should be used in all rooms with floor reared birds (and some cage facilities) is internal air circulation. These fans (an example is shown on page 14) have a number of important functions that are often over looked. Firstly, they do help to ensure that all of the birds, regardless of their location relative to the fresh air inlet, receive some newer air. They help to mix the incoming air, which warms it more quickly, thus reducing its draft potential.
If supplemental heat is added, internal circulation fans aid its distribution and helps to eliminate both temperature differentials and stratification. This circulating air also helps move and dilute the heavier carbon dioxide layer near the floor and the birds. Lastly, since there are only a few points of exhaust operating during cold weather, the internal circulation fans help carry a well mixed sample of room air pass each operating fan for extraction.
Back to fresh air entering the building at the top of the wall: It does not take much of an obstruction to cause this air jet to lose its horizontal trajectory across the ceiling surface as well as lose its velocity energy. Any obstruction within eight feet of the air inlet will cause the incoming air jet to turn down into the room air space and fall to the floor before it has been adequately warmed, and possible drafts or even chilling of the birds can occur.
Therefore, all electrical conduits, water and gas lines should either be mounted on the wall below the air inlet or suspended off the ceiling to maintain a smooth ceiling surface in the direction of air travel. Note that a corrugated metal ceiling with the ribs running perpendicular to the direction of air travel is often the reason for wet litter on the air inlet side of the facility.
Most poultry facilities require some external source of heat (the exception being caged laying hens) to achieve good ventilation. Failure to provide this heat means that sufficient incoming air will not be adequately warmed and it can “sponge” up all of the water vapour being produced by the birds, causing the building to become damp and odorous. The two most common types of heaters are simple box type unit heaters designed to heat the room air and radiant tube heaters designed to concentrate the heat on the birds and floor area located beneath each unit.
In most cases, the typical heater installation simply allows the products of combustion to be dumped into the room air space. This additional water vapour and carbon dioxide must be vented from the room by some means and therefore more minimum exhaust fan capacity is required. More producers are now starting to vent their radiant tube heaters directly outside to improve air quality for the birds and reduce the need for extra minimum fan ventilation. Properly managed, this strategy should not impact your over-all heating energy costs.
Several types of alternative heating are now being used to reduce or even eliminate these extra air contaminants in the building. Hot water heating with a boiler and a combination of in-floor and air heating with distribution pipes produces a dry heat with all products of combustion exhausted from a separate boiler room. We are also seeing some use of passive solar heating along the air inlet and the use of heat exchangers on the minimum exhaust fans to help increase the minimum ventilation rate as well as reduce the over-all heating cost. These systems are improving winter air
quality for the birds but do have a significant capital cost associated with them.
Regardless of fan size, inlet type or heating equipment, the key to good ventilation in every building is having a control system in place that is user friendly and fully understood by the operator. Today’s modern ventilation system controller not only operates the exhaust fans, but opens and closes the air inlets, turns the heaters on and off and runs a summer sprinkler or high pressure cooling system. Typically, it will have at least 20 settings and often times 60 or more that must be programmed into the unit for proper operation.
It will pay to spend whatever time it takes to become totally comfortable with your ventilation system controller and able to make changes to the settings as necessary to get the best from your ventilation system. n
Harry Huffman operates an independent engineering practice in London, Ont., offering agricultural engineering expertise to the livestock and poultry industry. He specializes in ventilation consults on the farm. He has 35 years of experience and offers advice on: ventilation system planning (new construction or a system upgrade), in-barn ventilation troubleshooting and recommendations for improvement, detailed ventilation system analysis. He can be contacted via e-mail at email@example.com.