Ventilation

Elite Agri Solutions strives to provide background information on topics which are hard to research. In cases where no reputable print resources were available for us to reference, we interviewed industry experts, so it is inevitable that the contents of this document will contain inaccuracies and bias. Use this as a resource to help you ask the right questions, not as a source of definitive answers. Elite Agri Solutions and its employee will not be responsible for the consequences of any decision made based on this guide. Where text or data has been copied directly, the sources have been noted, otherwise it can be assumed that all the information in this guide has only been curated by Elite Agri Solutions and is not our original property.

 

The primary functions of ventilation are to regulate temperature and to remove moisture and undesirable gases. The priority of each of these purposes must be weighed and ventilation rates adjusted according to conditions.

 

Each species has its own requirements, so the ventilation for a cattle barn will tailor to a much different set of needs than say a broiler barn. Despite the large differences across livestock, the fluid mechanics and heat transfer principles remain the same throughout.

 

If you are looking for plug and play calculations for ventilation, refer to OMAFRA’s publication 833 “Ventilation for Livestock and Poultry Facilities” we have collected the important work sheets from that document into “OMAFRA Ventilation Heating and Cooling Work Sheets”.

 

This document provides a more technical approach to determining ventilation needs. It will take a full day’s work, and perhaps some research outside of this document to fully understand the concept of psychometrics.

 

Very useful resources for this endeavor include the free and available online, but slightly outdated “Canadian Farm Buildings Handbook” and the up to date, but very technical “ASHRAE Fundamentals Handbook” available in print and online for a fee.

 

Even if you are planning on using the plug and play worksheets provided by OMAFRA a quick readthrough of this document will greatly help your understanding of ventilation fundamentals.

Terminology

Specific Heat Capacity: The quantity of heat required to raise a one-kilogram mass of a material by on degree Celsius

 

Sensible Heat: The quantity of heat energy associated with a change in temperature (kJ). It can also be described as – (mass of material) x (specific heat capacity) x (change in temperature) Sensible heat is released by livestock via conduction radiation and convection.

 

Latent Heat: The energy released when water changes phases from vapour to liquid or conversely the energy absorbed during the change from liquid into vapour. It is usually taken to be 2428 . Latent heat is released from livestock in the form of respired water vapour or evaporative cooling.

Humidity Ratio: The weight ratio of water in the air

 

Relative Humidity: For ventilation purposes it can be considered to be  both are measured at the same temperature and the value is described as a percent.

 

Dry Bulb Temperature: The temperature measured with an ordinary thermometer that is kept dry and shielded from radiation.

 

Wet Bulb Temperature: The temperature measured with the same thermometer except that it is wrapped in a wet wick and has air passed over it until the temperature stabilizes. The bulb is cooled by the evaporation of water from the wick and provided the air is not saturated the thermometer will show a lower reading than the dry bulb. Relative humidity can be found on a psychometric chart if both the wet and dry bulb temperatures are known.

 

Dew Point Temperature: The temperature when the air can no longer hold a constant volume of water and some must precipitate out. Under constant pressure and humidity but changing temperature, this is the point when condensation will form.

 

Enthalpy: The heat energy contained in a mixture of water vapour and air. This includes both sensible heat, indicated by dry bulb temperature, and latent heat contained in the water vapour.

Heat Balances

Inside of a livestock building there is a balance between heat sources (i.e. animals and supplementary heaters) and heat removal (i.e. ventilation and building). To maintain a constant condition, the total heat gained must be equal to the heat lost. To maintain this balance one or more of the factors must be adjusted.

 

Animal Total Heat

This factor varies directly with the number, size, feed intake and activity level of animals. Total heat is a combination of both the latent heat and sensible heat and is removed from bodies by radiation, convection and conduction to the environment around them. Sensible heat is transferred from an animal’s body to its surrounding via convection, radiation and conduction. Latent heat comes from evaporated moisture, usually a product of respiration, for some animals it is product of sweating. Latent heat in a barn can also come from sensible heat in the air passing over moist surfaces of the barn and evaporating moisture. Sprinkler systems artificially increase the amount of heat that is transferred from livestock as latent heat. The proportion of sensible and latent heat given off by livestock varies with temperature. It is described by a latent to total ratio and increases with ambient temperature.

 

Supplemental Heat

Includes all heat other than that produced by the animals. Some supplemental heat sources are not easily controllable such as the heat produced by motors and lights. In cold weather situations heaters are often needed to correct heat balances. Some fuel burning heaters exhaust directly into the building and contribute latent heat as well as sensible heat.

Building Heat Loss

All heat that is transferred through the building envelope, except for heat lost by air exchange is considered building heat loss. R value is a measure of thermal resistance of materials. It can be described in units of .

 

R (imperial) = RSI (Metric) X 5.678

R-values are additive so if you have a wall framing assembly with an R of 20 and you finish the wall with gypsum board with an R of 0.45, the finished wall will have an R of 20.45

 

(Theoretically)

The total building heat loss can be expressed in terms of the difference between inside and outside temperatures. This is done by finding the area of each building component excluding the floor (i.e. framed walls, doors, etc.)  and dividing each area by the RSI value for that material. The resulting ratios are added up and multiplied by the difference between the inside and outside temperatures to find the partial building heat loss in watts. You exclude the floor for this because the floor is in contact with the soil which will be a much different temperature than the air.

 

The old school of thought was that heat lost to the ground through a slab is rather insignificant to that which happens at the perimeter of the building. It is very common now for warm environment buildings to have insulation under the slab. Unlike the above ground building envelope which has a nearly uniform temperature on each side, the temperature in the ground is a gradient. To our knowledge there is no simple way to estimate heat loss through a slab.

 

If the floor is uninsulated except for at the foundation, heat loss is calculated by taking the perimeter of the building and multiplying it by a perimeter heat loss factor. This results in the perimeter heat loss in units of watts.

The total building heat loss is then the addition of the two.

 

Understanding Psychrometrics

A Psychrometric chart is an incredibly useful tool when evaluating environmental conditions in a barn, by simply knowing two of the following conditions any of the other conditions can be found on the chart.

  • Wet- bulb temperature
  • Enthalpy
  • Relative humidity
  • Humidity ratio
  • Dew point temperature
  • Volume of one Kg of dry air

This chart can be used to gauge minimum requirements before a barn is built and can also be used to adjust ventilation and heating conditions in existing barns.

To use the chart, find the intersection of the lines that represent two known values. This point is called the state point and all other values can be determined by reading the four additional lines that intersect there.

Moisture Control

Ventilation for moisture control is a concern for cold weather. It is necessary to remove moisture from the facility so that Relative humidity does not exceed healthy levels. If relative humidity exceeds 100% the dew point has been reached and condensation and fogging will occur. High moisture levels in the barn result in poor respiratory and bedding conditions.

 

Low temperatures have very little moisture carrying capacities, for example air at 80% relative humidity and 20  holds about 15g of moisture per kg of dry air, meanwhile air at 80% relative humidity and negative 30  holds about 0.2 g of moisture per kg of dry air. Minimum ventilation systems should be designed for worst case scenarios when conditions outside the barn are at low temperatures and nearly saturated. The information that is required to calculate the minimum ventilation for moisture control in cold temperatures is:

  • The minimum out-door design temperature and maximum outdoor humidity for your area this can be found in the “ASHRAE Fundamentals Handbook” we have scanned the tables containing “Climatic Conditions for Canada” and you can find them with our other resources.
  • The lowest critical temperature inside the barn for the specified livestock, this can be found in the “Lower Critical Temperature” table of OMAFRA’s Ventilation for Livestock and Poultry Facilities. Acceptable maximum humidity can be judged based on preference.
  • The latent heat production per animal, which can be found in the “Canadian Farm Building Handbook”

The steps required to calculate this is:

  1. Using the dry bulb temperature scale on the bottom of the ASHRAE psychometric chart No.2 locate the value for minimum outdoor temperature. (Step 1 on example chart)
  2. Find the relative humidity curve that is closest to the maximum outdoor humidity for your area. (Step 2 on example chart)
  3. Follow the vertical line up from the dry bulb scale to where it intersects with the relative humidity curve that you identified in step three. This point of intersection is the state point (Point A on example chart).
  4. Using the enthalpy scale find the value of the line that comes closest to intersecting with the state point. The enthalpy scale is the series of parallel line located above the 100% relative humidity curve. You will have to use a ruler or imagine that the lines are continuous. (Step 3 on example chart)
  5. Repeat steps 2,3 and 4 for the temperature and humidity that is desired inside of the barn.
  6. If a horizontal line is followed from the sate point of the outdoor conditions, over to the vertical line that represents the inside temperature the intersection of these two is the called the turning point. (Step 7 & point C on example chart)
  7. Find the line on the enthalpy scale that comes closest to intersecting with the turning point, this is know as the turning point enthalpy. It represents the divide between sensible enthalpy and latent enthalpy. (Step 8 on example chart). In the case where the desired barn temperature falls on a separate chart, draw the horizontal line on the chart that has the same humidity ratio as the line in step 8.
  8. Subtracting the turning point enthalpy from the enthalpy for desired indoor conditions gives the latent portion of enthalpy.
  9. The mass flow rate of air required to satisfy the desired conditions is then found by dividing the latent heat production per animal by the latent portion of enthalpy found in the step above.
  10. To convert from a mass flow rate to the ventilation rate multiple by the fraction 2/9.
  11. To convert from L/s per animal to ft3/min per animal, divide L/S by 2.11888.

 

Temperature Control

Minimum

Depending on the sort of livestock that is being kept, additional heat may be required to maintain the lowest critical temperature in the barn. Solving for the ventilation requirement to keep temperature inside the barn at desired levels requires knowing the following information:

  • The minimum out-door design temperature and maximum outdoor humidity for your area this can be found in the “ASHRAE Fundamentals Handbook”, we have scanned the tables containing “Climatic Conditions for Canada” and you can find them with our other resources.
  • The lowest critical temperature inside the barn for the specified livestock, this can be found in the “Lower Critical Temperature” table of OMAFRA’s Ventilation for Livestock and Poultry Facilities. Acceptable maximum humidity can be judged based on preference.
  • Sensible heat production per animal.
  • The portion of sensible heat per animal that is converted to latent heat (ie. Sweating and evaporative cooling, not respiration)
  • Building Heat Loss

The steps required to calculate this are:

  1. Using the dry bulb temperature scale on the bottom of the ASHRAE psychometric chart No.2 locate the value for minimum outdoor temperature. (Step 1 on example chart)
  2. Find the relative humidity curve that is closest to the maximum outdoor humidity for your area. (Step 2 on example chart)
  3. Follow the vertical line up from the dry bulb scale to where it intersects with the relative humidity curve that you identified in step three. This point of intersection is the state point (Point A on example chart).
  4. Using the enthalpy scale find the value of the line that comes closest to intersecting with the state point. The enthalpy scale is the series of parallel line located above the 100% relative humidity curve. You will have to use a ruler or imagine that the lines are continuous. (Step 3 on example chart)
  5. Repeat steps 2,3 and 4 for the temperature and humidity that is desired inside of the barn.
  6. If a horizontal line is followed from the sate point of the outdoor conditions, over to the vertical line that represents the inside temperature, the intersection of these two is the called the turning point. (Step 7 on example chart) It represents the divide between sensible enthalpy and latent enthalpy. (Step 8 on example chart). In the case where the desired barn temperature falls on a separate chart, draw the horizontal line on the chart that has the same humidity ratio as the line in step 8.
  7. The available sensible heat is equal to the sensible heat produce by each animal subtract the portion or sensible heat converted to latent heat per animal, subtract the building heat loss factor.
  8. Find the sensible heat portion of enthalpy by subtracting the minimum outdoor temperature from the turning point enthalpy.
  9. Moss flow air rate is the available sensible heat divided by the sensible heat portion of enthalpy.
  10. To convert from a mass flow rate to the ventilation rate multiple by the fraction 2/9.
  11. To convert from L/s per animal to ft3/min per animal, divide L/S by 2.11888.

Depending on the conditions it might be noted that the rate required to effectively remove moisture is higher than the rate to maintain minimum temperature. If this is the case, the barn is either going to be cold or wet. To adjust for this, sensible heat must be added to the system.

 

Maximum

Ventilation requirements for summer conditions are generally easier to calculate, under most warm weather conditions, ventilation requirements for moisture control are far superseded by ventilation requirements for controlling temperature and therefore the latter case is considered. To determine the ventilation rate required to keep temperatures within a certain limit relative to outdoor conditions, the following information is required:

  • Solar gain potential (if the barn has a white roof, or is relatively well insulated and the attic is adequately ventilated this is considered negligible)
  • The maximum out-door design temperature and humidity for your area, this can be found in the “ASHRAE Fundamentals Handbook”, we have scanned the tables containing “Climatic Conditions for Canada” and you can find them with our other resources.
  • Sensible heat production per animal
  • The portion of sensible heat that is converted to latent heat (i.e. sweating and evaporative cooling, not respiration)

 

The steps required to calculate this are:

  1. Calculate the sensible heat to be removed from the building by subtracting the portion of sensible heat that is converted to latent heat from the sensible heat produced per animal.
  2. The energy required to raise the temperature of dry air by is found by taking the difference between outdoor and indoor conditions and multiplying that value by the specific heat capacity of dry air, 1.006 kJ/(kg).
  3. Dividing the sensible heat to be removed from the building by the energy required to raise the temperature of dry air yields the mass flow rate of air.
  4. To convert from a mass flow rate to the ventilation rate, multiple by the fraction 2/9.
  5. To convert from L/s per animal to ft3/min per animal, divide L/S by 2.11888.

 

For tunnel ventilated buildings the most important factor for maximum ventilation is the air speed in the barn. The desired speed is typically between 200-500 ft/min or 1.0 – 2.5m/s[i]. Therefore, the required fan capacity (ft3/min) is attained by multiplying the barn’s net usable cross-sectional area by the desired air speed. For poultry, the bottom 1 ft of the barn is occupied by birds and equipment so is not usable for finding the net area. Likewise, other livestock will similarly reduce the net area inside the barn depending on livestock type and equipment arrangement.

Ventilating for Air Quality

Another condition that must be monitored in a barn is the air quality level, it is often the factor that determines ventilation rate in the winter.

Time-weighted average exposure values (TWAEV), short-term exposure values (STEV) and ceiling exposure values (CEV) not to be exceeded for biological and chemical agents (Occupational Health and Safety Act, Ontario, 1986). Some USA values are also included.

Agent TWAEV STEV Agricultural operations with concentrations exceeding TWAEV
Carbon Dioxide
(ppm)
5000 30,000 Swine, poultry
Ammonia
(ppm)
25 35 Poultry, swine and dairy calves
Hydrogen Sulphide (ppm) 10 15 During manure agitation for swine, dairy and poultry
Carbon Monoxide (ppm) USA (86/87) 35

40

400

400

Poultry and swine facilities when unvented fuel fired heaters are maladjusted
Nitrogen Dioxide
(ppm)
3 5 Inside silos after filling
Grain Dust
(mg/m
3)
4 Livestock feed rooms and grain centres
Total Dust
(mg/m
3)
10 Most barns after animal feeding
Respirable Dust
(mg/m3) USA (86/87)
5 1

 

Weighing all Factors to Determine Requirements.

Efficiency

When considering the lifetime costs of choosing one fan over another it is important to calculate the operating cost for each fan. 3rd party testing information should be available from any reputable fan manufacturer.

 

The efficiency of fans is greatly affected by the type and condition of shutters, blades and diffuser cones. It is useful to compare the cost of operating one fan vs a competitor.  In order to do this calculation, you will need to find the air flow rate in L/s or CFM, the fan efficiency in L/s/W or CFM/W, the estimated annual operating hours hr./year and the hydro rate $/kWhr. This information can either be provided by the manufacturer or by a third party. The BESS Laboratory at the University of Illinois provides a comprehensive list of data for various models from major fan manufacturers.

 

 

For example, comparing information provided by BESS for two 48” fans (both tested at 0.05” static pressure), from different manufacturers a major difference can be seen. It is presumed that the fans run continuously for 1/2 of the year.

$/Year

 

This shows that the if the first fan with a higher efficiency is used under the test conditions and for ½ of the time, there is a potential cost savings of 326.30 $/Year. This is an important figure to weigh when considering fan models. Even if there is a larger upfront cost for the more efficient fan, the savings in operating costs within a few years might make it a better choice.

 

Generally larger fans operate more efficiently than smaller fans, so it may be beneficial to operate a few large fans rather than many small fans for maximum ventilation conditions.

The maximum volume of air that a fan can move occurs at 0” static pressure. The air flow ratio is an indicator of how well a fan will perform as static pressure increases because of dirty shutters, clogged cooling cells or other restrictions in air flow. It is generally calculated by dividing the amount of air moved at 0.2” static pressure by the amount of air moved at 0.05” static pressure. A higher ratio indicates a better fan, that will maintain more of its capacity at high static pressures. If considering evaporative cooling pads or light traps, operating pressures may be relatively high, so a fan with a good air flow ratio is important.

 

Accessories can greatly affect the performance of fans. Shutters and blade guards are standard on almost all fans: “You should expect a 10 to 15 percent reduction in air flow using inlet-side shutters and a 15 to 25 percent reduction using discharge-side shutters. Properly designed guards should disrupt air flow and efficiency by less than 5 percent.”[ii] Other accessories increase air flow: “A discharge cone and proper housing design can improve air flow by at least 15 percent.”[iii] As fans get dirty with dust and moisture, their efficiency can decrease by as much as 40 percent.[iv]

 

Understanding Closed Channel Flows

Flow volumes must be the same down the length of the barn, however minor losses due to friction occur, resulting in the highest static pressure near the fan and lowest static pressure nearest the inlet.

 

Jetting is a concept used for cold weather ventilation in most mechanical ventilation systems. The idea is that if the air inlet is positioned correctly, incoming air will increase velocity and form a jet of air passing over the surface of the barn ceiling. This improves the distance that air is thrown, as well as improving the consistency of air, mixing the warm air near the ceiling with incoming air.

Ventilation Types

Mechanical Ventilation

Axial (air moves parallel to fan’s shaft) fans are by far the most common type of fan used for livestock ventilation, the size of these fans is usually referred to by the diameter of the blade in inches. Fans are typically rated in a range of face velocities between 5m/s and 11m/s and a static pressure between 0.1 and 0.25 inches of water column. Two fans of the same dimension can have very different performances for moving air volumes and energy efficiencies. Capacities of specific fans should be provided by the equipment manufacturer and can often be cross referenced with independent test data from the BESS Laboratory at the University of Illinois. For preliminary reference, a list of some common sizes and capacities of axial fans is supplied.

 

Common Axial Wall Fan Selections[v]

Blade Diameter in Inches Typical CFM Performance at 0.10” Water Colum
10 800
12 1200
14 1600
16 2400
18 3000
20 3500
22 4000
24 5000
30 7000
36 10500
42 14500
48 20000
55 22500
60 25000
72 38000

 

 

A generally accepted rule of thumb for sizing fan stages is that the first stage be 2/3 the rate calculated for winter moisture control with each successive stage slightly less than doubling the rate of the stage before it. It is common to install variable speed fans for the first two stages of ventilation so that the ventilation rate can perfectly match requirements.

 

Negative Pressure Ventilation Systems use exhaust fans to remove air from the buildings, creating negative pressure and drawing air into the barn through inlets. Positive pressure systems are not preferred because warm damp air would be forced out of the building leaving via the intended outlet, but also through any crack or leak in the building envelope. Warm damp air would be forced into building components and condensate, greatly speeding up the decay of building components.

 

The simplest cross ventilated barns draw air across the narrowest dimension of the barn, having a continuous inlet on one side of the barn and exhaust fans on the other. Variations can include chimney mounted fans, various inlet designs, or inlets on both sides of the barn if width becomes large enough. In barns for sensitive animals (calves, piglets, etc.) duct systems are used instead of stir fans to distribute and mix fresh air inside the building.

 

Tunnel ventilated barns draw air across the length of the barn. Exhaust fans produce negative pressure at one end and air is let in at the other end of the barn. Tunnel ventilation results in higher air velocities at animal level and enhanced evaporative cooling during hot conditions. In some cases, the end walls of the barn don’t provide enough area to accommodate inlets or fans. When this is the case fans or inlets are located around the corner on the end of the barn.

 

In situations where a below barn manure pit is in use, minimum ventilation is often achieved by having an exhaust fan pulling air from the barn via the manure pit. This system is used to reduce gas and odour levels inside the barn. It is rare that maximum ventilation be drawn through the slats, usually side wall fans are used to supplement during hot conditions.

 

Combination positive and negative pressure systems use fans to push air into a distribution network and use exhaust fans to draw it out of the building.

Weather hood should be installed on all stage 1 and 2 ventilation. Wind stop and weather hood on air inlets is necessary to prevent wind form influencing the static pressure of the barn.

 

Natural Ventilation

During summer conditions this system relies on large side openings to allow fresh air to flow across livestock buildings. It relies on the wind to provide air exchange. Cooling fans are installed inside the barn to locally provide airspeed at animal level when there is inadequate natural airspeed. Curtains or other paneling are used to reduce the wall opening area to reduce air flow when needed. Typically, the barn is positioned so that the narrowest dimension of the barn is perpendicular to the prevailing summer wind.

 

Under winter conditions, either chimneys or an open ridge allow buoyant warm air to rise and exit the barn, drawing fresh air in wall openings. Dampers in chimneys or an adjustable ridge opening control the rate at which air is exhausted.

 

Natural ventilation is relatively difficult to control and is not recommended for animals with narrow comfort zones or that are sensitive to drafts. However, it has the benefit of very low operating costs and requires no large backup generator in case of power outage. An average summer breeze can provide 3 or 4 air changes per minute all without using any electricity.

 

In Ontario a north-south orientation is most common, but a wind rose for a location near you can give a better indication of the dominant wind direction.

 

During cooler season ventilation, have curtains or rigid panels open from the top down to allow cold air to warm and loose its draft potential before reaching the livestock.

 

Extend chimneys a minimum of 1.5 ft above the highest point of the roof to have direct wind access.

 

Chimneys larger than 4ft x 4ft tend to have down drafts.

 

Open ridge 1-2in. of ridge opening for every 10ft of barn width. Rising the height of the open ridge 1.5-2.0 times the width of the opening increases air draw.

 

An alternative to an open ridge is the overshoot ridge, which has one slope of the roof extend past the peak over the other slope, the opening is then vertical gap between them. A curtain can be installed in the overshoot like a wall curtain.

 

The following figure and corresponding table are taken from OMAFRA’s Ventilation for Livestock and Poultry Facilities.

Suggested Dimensions for Custom Made Chimneys[vi]

Dimension (I) 24 in. x 24 in. 24 in. x 36 in. 36in. x 48in. 48in. x 48in.
Dimension (M) 600 x 600 mm 600x 900mm 900 x 1,200 mm 1,200 x 1,200mm
A 24 in. 600 mm 24 in. 600 mm 36 in. 900 mm 48 in. 1,200 mm
B 24 in. 600 mm 36 in. 900 mm 48 in. 1,200 mm 48 in. 1,200 mm
C 27 in. 676 mm 27 in. 676 mm 39 in. 976 mm 51 in. 1,276 mm
D 30 in. 750 mm 42 in. 1,052 mm 54 in. 1,352 mm 54 in. 1,352 mm
E 24 in. 600 mm 24 in. 600 mm 36 in. 900 mm 36 in. 900 mm
F 12 in. 300 mm 18 in. 450 mm 24 in. 600 mm 24 in. 600 mm
G 48 in. 1,200 mm 48 in. 1,200 mm 60 in. 1,500 mm 72 in. 1,800 mm
H 48 in. 1,200 mm 60 in. 1,500 mm 72 in. 1,800 mm 72 in. 1,800 mm
I 6 in. 150 mm 9 in. 225 mm 12 in. 300 mm 12 in. 300 mm
J 18 in. 450 mm 27 in. 675 mm 36 in. 900 mm 36 in. 900 mm

 

 

It should be noted that all chimneys should be insulated to prevent condensation.

 

Wind Break

 

A wind break is an essential for a barn that has permanently open sides or where livestock are housed on an outdoor yard. Boards in a windbreak fence should be spaced according to the following table to slow the velocity of the wind through the fence instead of creating a current over the fence.

Windbreak Fence Board Spacing[vii]

Board Size ft Slot Width
Rough Cut Lumber in. Dressed lumber in.
1 x 4 7/8 ¾
1 x 6 1 3/8 1 1/8
1 x 8 1 ¾ 1 5/8
1 x 10 2 2
1 x 12 2 2

 

 

If planting trees for a wind break plant trees a minimum 60-65 ft upwind of the building or yard to be protected. Trees generally protect an area downwind equivalent to ten times their height. It is advised to plant two or more rows of trees with different species in each row. Cedar and spruce tend to work well together, as the spruce quickly gains height and the cedar fills out well closer to the ground.

 

Hybrid Ventilation

This system utilizes a mainly natural ventilation approach to summer ventilation with large open side wall inlets. Fans are generally not needed in the summer and curtain height is the primary control of temperature saving on the high energy cost of operating fans.

 

In the winter, the system can be more precisely adjusted than a natural ventilation system. The wall inlets are closed, and the barn operates as a negative pressure, mechanically ventilated building with either wall fans or chimney fans providing the necessary airflow. Ceiling inlets are typically used, so that the air can be passed through the relatively warm attic space before entering the building

 

Circulation Fans

Air mixing Ducts

Typical capacity of ducted circulation fans is 0.5-1.5 CFM/ft2 of floor area. Internal air flow speed should be less than 1200ft/min in the duct. The target for air speed coming out of each hole is 1,000ft/min. Ducts up to 50ft in length can be parallel wall tubes with equally spaced holes. However, tubes longer than 50ft should either be tapered or have variable hole spacing in order to evenly distribute air. Air flow capacity should be equivalent to 3-4 room air changes per hour.

 

Table 1

Hole Sizing For Circulation Ducts[viii]

Required Jet Distance (ft) Recommended Hole Diameter Size (in) Air Flow Per Hole (CFM)
6 1.8 3
8 2.4 5
10 3.0 8
12 3.7 10
14 4.3 15
16 4.9 20
20 6.1 30

Stir fans

1-3 CFM/ft2 of floor area.

Spaced no further than 100 ft apart for race-track style (recommended for barns narrower than 50ft).

Spaced into the barn 20-30% of barn width and no further than 50 ft apart for across the room or criss-cross arrangement (recommended for barns wider than 50ft)

 

Inlet

Location

Air can be inlet over the top of the wall framing (between the heels of the trusses), through the wall framing, or from the attic air space through the ceiling. All inlets should be placed so that air can be jetted across the ceiling during cold weather, allowing cold and warm air to mix before it reaches the level of the animals. Poorly designed systems don’t have a high enough intake velocity to jet the air, so instead it is dumped to the floor as a draft. Design entrance velocities so air is jetted across the ceiling at rate of 800-1000ft/min. To achieve this design velocity, static pressure in the barn is typically kept between 0.6 and 0.8 inches of water column. During the summer the static pressure should be slightly lower to maximize air flow. In warm weather inlet velocity is not as critical because the small indoor-outdoor temperature difference will not cause cold air to sink and cause drafts. Summer inlet velocities should be in the range of 500-600ft/min, unless using tunnel ventilation.

Tunnel ventilated barns should have all inlets located on the end wall or as close to the end wall as possible. Net opening should be 2ft2/1000CFM and the desired air speed inside the room interior should be 200-500 ft/min.

Design

The predominant styles of inlets for mechanically ventilated barns are modular and continuous.

 

Modular inlets can either be wall mounted or ceiling mounted. They are typically short enough to fit between either the roof trusses or the wall framing with limited modification. They have the benefit of being much easier to accurately adjust and have improved jetting ability during minimum ventilation. The baffle is either constructed from high-density polystyrene foam board or molded plastic depending on the case. It is important that the baffle be insulated to minimize condensation and potential freezing at the inlet. Typically, they are actuated in unison, linked by a cable or rod system. Plastic inlet baffles that are molded with aerodynamics in mind, have the benefit of being able to provide improved jetting if installed properly. If modular inlets are installed more than 10ft apart along a wall, pockets of dead air will occur between them.

Continuous inlets are typically made of high-density polystyrene foam board that is joined by plastic H channel to form a continuous panel running the length of the wall. A cable system actuates entire inlet. The baffle is usually set so that in the closed position it forms a 45-degree angle between the wall and ceiling.

A less common type of inlet is called a porous ceiling inlet, this is where air is drawn into the room through the ceiling material. Traditionally this was the case with air being drawn through the mow floor. Punched plastic, spaced boards and steel strips are the most common types of material used to achieve this air intake. Ensure that the room is always under static pressure, else moisture will infiltrate the attic and insulation because there is no vapour barrier.

 

Wind Stop and Weather Hood

If an inlet is directly exposed to wind, the ventilation system may perform extremely different then it was designed to. To avoid this, it is essential that the inlet is properly hooded and has wind stops installed. If it is the case that the inlet is in the soffit of the building, the roof and facia may be slightly extended to act as the weather hood and the wind stop may be a board that extends horizontally a few inches under the eave, out from the wall of the barn. This is effective because the board restricts air that is being funneled up the wall of the building, and the facia blocks the direct wind pressure.

Another option for a wind stop is to have a wide baffle mounted to the wall of the barn 12-15” below the intake. Similarly, it blocks wind that is being forced up the wall of the barn but doesn’t restrict the size of the opening through the weather hood.

Tall weather hoods can have black perforated siding mounted as passive solar collectors on the wall of the hood with closure boards on the bottom of the intake hood. During summer months the board is open and air intake proceeds as a typical system would. Under cold conditions the closure board is shut and air passes through the perforations in the solar collector on the wall.

If black out conditions are needed, ensure that accommodations are made so that the weather hood is of adequate size to house dark out panels and allow for the increased intake area needed to deal with the air restriction.

Table 2

Sizing Air Inlets[ix]

Inlet Conditions Net Opening Area
Unrestricted 2.0 ft2/1,000CFM
Cooling Pad 2.2 ft2/1,000CFM
Black Out 2.5 ft2/1,000CFM

 

Calculations

In lieu of specified values, winter entrance air velocity can be recommended to be 1,000 ft/min with summer values at 600 ft/min.

For continuous inlets this is used to solve for the width of the inlet needed for maximum ventilation, this then determines the size of the baffle. The length is typically the full length of the barn minus a few feet at each end and a few feet for the actuator assembly.

For modular inlets the area of each inlet is fixed, so this is used to find the number of inlets required for winter ventilation.

If you have already calculated the air flow rate for each stage of your ventilation, you can determine the rough opening required at each stage of ventilation. This can be programmed into the ventilation controller and fine tuned as needed.

Controllers

Electronic controls provide excellent management of room temperature, humidity and air quality. These controllers can manage a system of multiple fan stages, heating and cooling systems. Advanced controllers can record performance data assisting producers in management decisions. Temperature set points can be programed to adjust day by day as a group of animals matures. Controllers should be matched to the level of functionality needed for the application. A mechanical thermostat control system should be hard-wired to override the electronic controller so that it will engage at least the first 3 or 4 stages of ventilation if the electronic system fails.

Terminology

Proportional Controller: Operates on the principle that there is always error, except when the controller is exactly at the set point. Most barn controllers are of this this type.

PID (Proportional-Integral-Derivative) Controller: Attempts to minimize error and keeps the barn as close to set-point as possible. Can lead to excessive fan operation and increased drafts in spring and fall. More extensive knowledge is needed to properly tune these controllers.

Main Set Point: The base temperature chosen for a room. Usually chosen to be at the lower end of the acceptable temperature for the livestock.

Relative Set Point: The temperature above or below the set point at which fans or heaters kick in. eg, a stage of ventilation is activated at +2.

Band Width or Differential: The difference in temperature allowed between a variable fan running at minimum speed and it is running at full speed.

Dead band: Minimum temperature change where nothing happens. For example, 1 increase in room temperature is allowed before the next stage of ventilation is engaged. Controlling dead band is one of the biggest advantages of electronic controllers over having a mechanical thermostat for each stage of ventilation. With older systems, thermostat accuracy limited how small the dead band of a system could be

Hysteresis: This is the offset value used to keep fans or heaters from switching on and off too quickly. For example, a fan stage will engage at 1 above set point but won’t disengage until the temperature has fallen to 0.5 above set point. If there is no hysteresis, fans would cycle on and off erratically whenever temperature was fluctuating close to that stage’s relative set point.

Temperature Sensor or Probe: Sends a signal to the controller indicating the temperature felt by the livestock. Placement is crucial, if this sensor is placed by a heater or in front of a drafty inlet the reading may be a very poor reflection of the majority of the barn. It is best practice to have multiple sensors and take the average of them.

Temperature Curve: A programmed set of temperature values that correspond to the minimum temperature requirement as the livestock matures. The set point is adjusted daily to reflect the changing needs of the growing animals.

Minimum Ventilation Curve: A programed set of speed values for the stage 1 fan to ensure that humidity is kept at an acceptable level as the livestock matures. The amount of ventilation to control moisture increases daily to reflect the increased moisture production of growing animals.

Motor curve: When using a variable speed motor, the performance of the fan over its bandwidth is inputted into the controller so that it can ensure that it speeds up relatively linearly. The motor curve is essential to ensure that the controller is supplying the correct amount of ventilation.

Malfunction Alarms & Back Up

If the main electronic controller malfunctions, it is wise to have a simple electro-mechanical thermostat wired to the second stage of ventilation. The thermostat should be set slightly above the desired set point and wired so that it overrides the digital controller in the event of a malfunction. This won’t provide the ideal environment for livestock but will hopefully prevent high mortality that could accompany complete failure of the ventilation system.

Increasingly popular are alarm systems connected to cellular networks. In the case of a malfunction multiple phone numbers are alerted. These systems should have outdoor temperature compensation features to avoid false alarms. A battery power supply should be included as well so that if power is interrupted and the standby power system does not engage, an alert can be sent. These alarm systems should operate totally independent of the main controller and have their own temperature probes.

Attic Ventilation

When using the attic as the fresh air plenum, during hot weather ensure that two complete air changes per minute in the attic space.[x]

Choose a white roof steel and install R-5 insulation under the roof steel to reduce the solar load, if the attic is going to be used for summer ventilation.

Table 3

Minimum Attic ventilation Openings for Natural Air Movement[xi]

Building Width ft Minimum Opening Per Eave Per Foot of Building Length Maximum Building Length Per Ridge Ventilator
Continuous Slot Width in. Number of Circular Holes required per foot of Eave Circular Ventilator (1 ft2 area) ft 4ft Long Ridge Vent (2.5 ft2 area) ft Vented Ridge Cap (1.5ft2/10ft) ft
½ in. diameter 3/4 in. diameter 1 in. diameter
20 0.20 12 6 3 30 75 45
25 0.25 15 7 4 24 60 36
30 0.30 18 8 5 20 50 30
36 0.36 22 10 6 16 40 25
40 0.40 24 11 7 15 37 22
48 0.48 13 8 12 31 18
50 0.50 14 8 12 30 18
60 0.60 17 10 10 25 15
70 0.70 19 11 21 12
80 0.80 22 13 18 11
90 0.90 14 16 10
100 1.00 15 15
110 1.10 17 13
120 1.20 19 12
130 1.30 20 11
140 1.40 22 10
150 1.50 23 10

 

The National Farm Building Code of Canada 1995 sections 3.1.3.3. and 3.1.3.4. states that unoccupied ceiling spaces are to be separate by fire stops into compartments no larger than 30m (98.4 ft) in any dimension, acceptable materials for building fire stops are:

  1. 36mm (29 gauge 0.014in) sheet steel
  2. 6mm (1/4 in.) asbestos board
  3. 12,7mm (1/2 in.) gypsum board
  4. 5mm (1/2 in.) plywood, oriented strand board or wafer board with joints backed with similar material
  5. 38mm (1.5 in.) lumber

These fire stops should be nearly airtight and so each needs to be properly and independently ventilated.

Moisture laden air from side wall fans can be accidentally brought into the attic and cause condensation. To prevent this, use vertical style fan hoods or install solid soffit 4ft on either side of each exhaust fan.

If incoming air to ventilate the housing area is not coming through the ceiling, blocking must be installed between trusses. An appropriately sized gap (approx. 1in.) should be left to ventilate the attic, while protecting loose insulation from displacement by air currents.

If using the attic as the fresh air plenum, make sure that the insulation stop does not restrict the intake opening size. A raised heel truss will likely have to be used in this case. If the attic plenum is to be used in a minimum ventilation system where the minimum ventilation will operate intermittently, ensure that modular inlets close and seal or have backflow curtains installed so that moisture laden air does not backflow into the attic where it can condensate, wetting insulation and corroding the truss system.

[i] (Ward, 2013)

[ii] (Tabler & Wells, 2016)

[iii] (Tabler & Wells, 2016)

[iv] (Wheeler, 2002)

[v] (Amec Foster Wheeler, 2016)

[vi] (Ministry of Agriculture Food and Rural Affairs)

[vii] (Midwest plan Service, 1987)

[viii] (Ministry of Agriculture Food and Rural Affairs)

[ix] (Ministry of Agriculture Food and Rural Affairs)

[x] (Ministry of Agriculture Food and Rural Affairs)

[xi] (Ministry of Agriculture Food and Rural Affairs)