HOUSING PIH-60
PURDUE UNIVERSITY. COOPERATIVE EXTENSION SERVICE.
WEST LAFAYETTE, INDIANA
Mechanical Ventilation of Swine Buildings
Authors
James P. Murphy, Kansas State University
Don D. Jones, Purdue University
Leslie L. Christianson, University of Illinois
Reviewers
Wallace and Denise Bradway, Salem, New Jersey
Daryl Haerther, Strasburg, Colorado
Harvey Hirning, North Dakota State University
Leon and Pat Reiner, Tripp, South Dakota
Dennis Stombaugh, The Ohio State University
A well-designed and well-managed ventilation system is an
essential part of any pork production building. Mechanical venti-
lation involves many factors, which must be understood if it is
to function properly. It's a complete system of matched
components-insulation, tightly constructed building, supplemental
heat, fans, inlets, and controls-tailored to the environmental
needs of the animal. Omitting any of the components can result in
unsatisfactory performance. Simply adding fans without providing
adequate insulation and supplemental heat will not provide the
desired results.
Proper ventilation should:
o Remove moisture, gases, odors, dust and air-borne
disease organisms.
o Provide fresh air and distribute it uniformly without
creating drafts.
o Control air flow and supplemental heat to modulate
temperatures in winter and summer.
Building Construction
Insulation in the walls, ceiling, and along the foundation
and floor is necessary to keep the building warm and prevent con-
densation. Using adequate amounts of insulation reduces heat
losses and prevents condensation in cold weather. Adding or con-
serving heat in winter acts to increase the temperature of incom-
ing air, enabling it to absorb and remove moisture as it moves
through the building. Check the Midwest Plan Service Handbooks
and Pork Industry Handbook publications listed at the end of this
factsheet for more information on this topic.
Unplanned openings such as cracks around doors and windows
can nullify the benefits of well-planned ventilation inlets by
changing the distribution and velocity patterns of air entering
the building. For this reason, mechanically ventilated swine
buildings must be tightly constructed. Many operators eliminate
windows to reduce air leakage, condensation and heat loss. How-
ever, emergency ventilation openings should be provided if
standby power is not available.
Ventilation Principles
During cold weather, the air flow requirements of the build-
ing are based on the moisture produced in the building and on the
expected moisture content of the air entering and leaving the
building. The ventilation system is expected to remove this mois-
ture and to maintain a relative humidity between 50% and 70%.
Higher humidities contribute to condensation and respiratory
problems while humidities below 50% may also increase respiratory
problems due to dryness and dust.
During cold weather, the outside air entering the building
contains very little moisture. When cold air is brought into a
warm building, the air temperature increases which, in turn,
increases its moisture-holding capacity. This warmed air absorbs
moisture before being expelled by the ventilation system. Figure
1 illustrates how inside and outside air conditions determine the
ventilation rates for moisture removal. Under the conditions
shown in Figure 1, every cubic foot of exhaust air removes
(0.00831- 0.0003) / 13.3 = 0.0006 lb. of water. Since a 120-lb.
hog produces 0.19 lb. of water per hour, the ventilation rate for
moisture removal for these conditions would be (0.19 lb./hr./pig)
/ (0.0006 lb./ft.3) = 320 ft.3/hr./pig or 320 / 60 min./hr. = 5.3
cu. ft. per min. for each 120 lb. hog.
Cold weather is defined here as periods when supplemental
heat is needed, either continuously or intermittently, to main-
tain a constant room temperature. If a ventilation rate higher
than this calculated value is used, an increased heating bill
should be expected. Table 1 gives the recommended ventilation
rates for cold, mild, and hot weather ventilation.
Table 1. Recommended fan capacities (at 1/8 in. static pressure)
per pig or per sow and litter
____________________________________________________________________________
Ventilation rates, cfm
Cold Mild Hot*
Life Stage weather weather weather
____________________________________________________________________________
Unit- - - - - - cfm/hd (or sow + litter) - - - - - -
Sow and litter 400 lb. 20 80 500**
Prenursery pig 12-30 lb. 2 10 25
Nursery pig 30-75 lb. 3 15 35
Growing pig 75-150 lb. 7 24 75
Finishing pig 150-220 lb. 10 35 120
Gestating sow 325 lb. 12 40 150
Boar/Breeding sow 400 lb. 14 50 300
____________________________________________________________________________
* These rates may be reduced when supplemental cooling is available
in hot weather; and may be increased when air velocities on pigs
are low in summer.
** 500 cfm is the generally recommended hot weather rate in farrow-
ing, however local recommendations range from 250 cfm in northern
areas of the United States to 1000 cfm or more in the southeast and
southwest.
The rate for each season is the total capacity needed. For sow and
litter: 20 cfm/unit (cold weather) + 60 cfm/unit = 80 cfm unit
(mild); add 420 cfm/unit for 500 cfm/unit total hot weather rate.
Cold weather rate: In some cases, this airflow needs to be adju-
stable, due to a change in the number of animals in the room or due
to their growth. Ideally, at least one fan should operate at all
times when the inside temperature is above 35o F. Set a thermostat
to shut the fan off when the inside temperature drops below 35o F
and activate an alarm to notify the operator. This fan should sup-
ply the cfm rate listed in Table 1 under "Cold weather rate". The
fan should exhaust the air from above any stored liquid manure.
Mild weather rate: Provide additional airflow, thermostatically set
to start in 3-5 degree steps, from lowest desired temperature to
prevent sudden drops in temperature. These fans, together with the
cold weather fans, provide the capacity for outdoor temperatures up
to about 55o F.
Hot weather rate: Provide additional fans to supply the cfm rates
listed under "Hot weather rate". Some or all of these fans should
be operated when the inside building temperature is above 75o F. Hot
weather rate airflow capacity of sows and litters and breeding
animals can be reduced somewhat by utilizing drip cooling or zone
cooling (water evaporation or mechanical air conditioning) of sows
and boars. See MWPS-33 for detailed design information.
Types of Ventilation Systems
There are three general types of mechanical ventilation sys-
tems: positive, negative and neutral pressure systems. Classifi-
cation of the system is based on the air pressure of the enclo-
sure relative to outside air pressure. A positive pressure system
operates above normal outdoor air pressure (fans blowing into the
enclosure). Negative pressure systems operate below outdoor air
pressure (fans exhausting from the enclosure). Neutral pressure
systems use fans both to supply and exhaust ventilation air at
approximately the same rate. Any of the systems may use air
recirculation ducts or fans to mix and distribute incoming air.
Figure 2 illustrates a negative pressure system with slot
air inlets. Fresh air is drawn into the building through the air
inlets. Like all ventilation systems, incoming air must be well-
distributed and properly mixed, or blended, so that it can remove
moisture and/or heat, and do it without creating drafts. Since a
pressure difference must be maintained between inside and out-
side, it is especially important that negative pressure buildings
be tightly constructed so that fresh air enters only through
planned vent openings.
Figure 3 illustrates a negative pressure ventilation system
with air recirculation. A recirculation fan (usually mounted near
the ceiling and away from the wall) keeps the air flowing con-
tinuously through the duct or tube. The motorized wall intake
shutters are controlled by the same thermostats that control the
exhaust fan, opening automatically to draw in fresh air from
either the outside or from the attic area. When the thermostat
turns the exhaust fan off, the motorized intake shutters also
close, and the fan simply circulates room air through the distri-
bution duct, or tube. Timers are often wired in parallel with
cold weather thermostats to ensure a minimum ventilation rate
during cold weather especially in buildings with small animals
which produce little body heat.
Figure 4 illustrates a push-pull system which operates at
near neutral pressure. This design uses a fan and pressurized
duct to bring fresh air into the building. Additional fans
operate in conjunction to exhaust stale air.
Positive pressure systems use fans to blow fresh air into
the building (Figure 5). This increases the static pressure
inside the room relative to the outside causing stale air to be
exhausted through vent openings. Uniform ventilation depends on
proper design of the air distribution system and on proper loca-
tion of exhaust vents. Positive pressure ventilation is more
effective than negative pressure systems in older, less tightly
constructed buildings.
Under-Slat Ventilation
Ventilation of the space between the liquid manure and the
slats is common where liquid manure is stored in pits beneath
slotted floors. The cold-weather fans are located so as to
exhaust air from the pit to aid in removing gases and odors.
An exhaust duct, with properly sized and spaced openings
installed under the slats or alongside the manure pit walls, aids
in collecting the air uniformly over the full length of the pit
instead of only near the exhaust fan. Size the pit exhaust fan to
handle the cold weather rate (Figure 2).
Fan Systems
Ventilation air requirements (Table 1) vary from a minimum
at the cold weather capacity to many times this value in hot
weather. Continuously operating fans that exceed the rate needed
for moisture removal in cold weather waste heat. During the hot-
test weather, fan capacity is increased to the maximum rate to
remove as much heat as possible and to provide more air movement
around the hogs. To prevent rapid temperature changes, it is
desirable to increase or decrease ventilation rates in several
small increments rather than in one large increment.
The range of air capacities in the ventilation system can be
provided in various ways: (1) a small continuously operating fan
with ``minimum'' cold weather capacity plus larger fans, con-
trolled in increments by thermostats which engage fans as room
temperature increases and disengage them as temperature
decreases, (2) variable-speed fans operated below their full-
speed capacity. Variable-speed fans are modulated by a solid-
state control which regulates the voltage going to the
capacitor-start, capacitor-run motor, or (3) a single-speed fan
controlled by a percentage or 10-minute interval timer to allow a
wide range of capacities.
Care should be taken to protect variable speed fans from
outside wind pressure when operating the fan at low speed if a
uniform ventilation rate is to be maintained. Ideally, they
should not be operated below 40% of their full-speed capacity if
providing the cold weather ventilation rate. Timer-operated fans
also have disadvantages for cold weather use. Their ``on-off''
operation can cause wide temperature and humidity fluctuations
and back-drafting at the intakes when the fan is off.
Fan motors should be:
o Totally enclosed and suited for use in a corrosive,
moist, dusty environment.
o Rated for continuous operation.
o Sealed ball bearing type.
o Equipped with thermal overload protection.
o Wired with separate circuits from opposite sides of the
230 V service.
Air Inlets
In a mechanical ventilation system, the size and operation
determine the rate of air change. The uniformity of air distribu-
tion, on the other hand, depends primarily on location, design
and adjustment of the air intakes in exhaust systems or the air
outlets in pressure systems. It is especially important to main-
tain enough inlet velocity during cold weather so that incoming
air is mixed or blended with the warm room air before it reaches
animal level. The opening, or cross-sectional area, of the air
inlet should be based on the capacity of the fans. If the inlet
area is too great, cold air enters at low velocity, causing it to
``dribble'' in, settle to the floor and induce drafts.
Air intakes should be designed and constructed so that a
negative static pressure of 0.04 - 0.06 in. of water gauge is
created at the inlet shown in Figure 6. This will assure veloci-
ties of 800 to 1000 ft. per min. A common rule of thumb is to
size intakes at 1 sq. ft. for each 600 cfm of fan capacity.
Table 2 gives the rate of air flow through 1-ft. long venti-
lation slots for two pressure levels.
Table 2. Rate of air flow through ventilation slots one foot
long.
_________________________________________________________________
Inches
slot Static pressure, water gauge
width 0.04 In. 0.125 In.
_________________________________________________________________
---cfm---
1 50 100
2 100 200
3 150 300
4 200 400
_________________________________________________________________
* Source: Pennsylvania State University
Example: A 20-sow, 24 ft. x 60 ft. farrowing house is ven-
tilated in winter with a single speed fan having capacity of 400
cfm. Additional fans will provide a total of 10,000 cfm in hot
weather. What size should the slot inlets be assuming the inlet
runs the full length of both side walls (2 x 60 = 120 ft. long)?
Each foot length of slot will need to provide 10,000 cfm/120
ft. = 83.3 cfm at maximum airflow. At minimum airflow, each foot
length of slot opening will provide 400 cfm/120 ft. = 3.3 cfm.
Assume a 1 in. slot width and an operating static pressure of
0.04 in. From Table 2, each foot of 1 in. slot will admit air at
the rate of 50 cfm. The slot will require an opening that is
adjustable between 83.3/50 = 1.7 in. and 3.3/50 = 0.07 in.
Power-operated inlet systems can automatically adjust the opening
area, to maintain a preset pressure difference between inside and
outside.
Adjustment of ventilation systems to provide the optimal air
distribution at the various air flow rates must be understood by
the operator. Several methods will work if properly installed and
operated, including:
1. Separate air paths for winter and summer. This allows
air to be drawn from the attic in cold weather and
directly from the outside the rest of the year (Figure
7).
2. Manually adjustable hinged or vertically adjustable
baffles (Figure 7) under ceiling slot intakes (Figure
8).
3. Gravity or spring-loaded curtain or damper at the inlet
(Figures 9).
4. Power-operated adjustable baffles under ceiling slot
intakes.
Other important considerations in planning fresh air intakes
include:
1. Ensuring there are no unplanned openings into the
building. All openings, including doors, windows, feed
drops, cracks around doors, and other leaks should be
tightly closed.
2. Providing insulated baffles under the intake slots or
holes to direct the air. In winter, the incoming cold
air is directed across the ceiling where it is warmed
and mixed with the warmest air in the building. Make
sure the ceiling liner is smooth with no obstructions
that can deflect incoming cold air. This also prevents
heavier cold air from dropping or settling to the floor
where it could cause drafts that could chill pigs. In
summer, the baffle can be lowered to deflect the air
directly onto the animals.
3. Bringing air through the attic in cold weather reduces
the effect of wind and allows the air to be tempered
somewhat before it enters the housing area. The air
intakes from the outside to the attic (accomplished by
slot inlets under the eaves or by screened louvers at
the gable ends) should have a net free area of 1 1/2 -
2 sq. ft. for each 1,000 cfm of fan capacity (Figure
7).
Controls
Accurate, properly located sensors and controls are neces-
sary for the satisfactory and automatic operation of a ventila-
tion system.
The most common control is the line thermostat which con-
tains a temperature-sensing element and a switch. The sensing
element is usually a gas-filled coil or a bimetallic strip that
expands or contracts to open or close the electrical circuit to
the fan motor or motorized shutter. The thermostat is a rela-
tively economical and reliable control. Controls used in live-
stock buildings should be corrosion resistant, watertight, dust-
tight and UL listed.
Humidistats, which have an element sensitive to the moisture
content of the air, are not suitable as a fan control device in
livestock buildings unless maintained on a frequent basis since
dust accumulations on the sensing element greatly affect their
accuracy.
Ten-minute interval timers, sometimes called percentage
timers, are wired in parallel with thermostats to provide an
average ventilation rate equal to the desired minimum rate.
Interval timers are adjustable to operate for any desired percen-
tage of time, such as two minutes out of ten, or 20% of the time.
Timers are unpopular with many engineers because their larger
capacity and on-off operation result in either overventilation or
underventilation at all times.
The control of variable-speed fans is accomplished with a
solid-state electronic speed control and thermistor heat sensor.
The control regulates the voltage to the fan motor, reducing vol-
tage and fan speed as temperature in the building declines, and
increasing voltage and fan speed as the temperature increases. At
the temperature setting on the control, the fan will be operating
at about one-half capacity. It typically reaches maximum speed at
about 4o F above the setting and minimum speed at about 4o F below
the setting.
A safety thermostat can be incorporated with the cold
weather fan to turn off the fan when the temperature declines
below the minimum to be maintained in the building. This should
only occur if the supplemental heater fails. Properly designed
standby power and an alarm system that can alert the manager
should activate quickly if this happens. Information on instal-
ling a warning system, or emergency ventilation, in case of power
failure is available from equipment suppliers, power suppliers
and state university Extension Offices.
Locate controls where they will sense the average conditions
at animal level. Never locate controls on outside walls or where
they may be affected by sunlight, drafts from air intakes or out-
side entrances, heating devices or other abnormal conditions.
Accuracy of temperature-sensing controls can be checked with a
thermometer located next to the control. Sensors should be
located within the animal zone but out of their reach. Locate
controls where the operator can easily read the temperature set-
ting and adjust it. The controls may be grouped in a convenient
location, often in a central aisleway.
Proper electrical design is required for dependable perfor-
mance and often is a prerequisite for building insurance. Select
sensors and controls that will hold up in a moist, dusty, corro-
sive environment. See MWPS-28 and PIH-110 for information on
recommended farm wiring practices.
Maintenance of the System
Good-quality fans, inlets and controls do not require a lot
of attention. However, regularly scheduled maintenance will pro-
vide more efficient performance and longer life of the equipment.
Periodic cleaning, lubrication and adjustment will assure reli-
able performance of the system.
Rust and corrosion are inherent problems in ventilation
equipment. Some manufacturers provide fiberglass housings for
their fans, (Figure 10). Others use stainless steel or
polyethylene for the fan frames and hoods, or special protective
coatings to prevent rusting and corrosion. Keep controls, fans,
housings, hoods, shutters, and other components clean and perform
regular maintenance to minimize deterioration and increase the
performance and life of the equipment.
Operator Checklist
1. NEVER operate a mechanically ventilated building with
power provided to only a single fan. ALWAYS provide
some backup ventilation protection, so that if a single
fan or circuit fails, an increase in temperature will
quickly activate another fan.
2. Fans should be selected and operated to provide the
range of air movement needed for animal comfort
throughout the year.
3. Select fans according to their AMCA (Air Movement and
Control Association) capacity at 1/8 in. static pres-
sure to assure rated delivery under all weather condi-
tions.
4. Install and adjust inlets to maintain uniform distribu-
tion of fresh air without causing drafts.
5. Add supplemental space heat only when operating the
ventilation system at the cold weather rate. Heating
and ventilating system controls should be coordinated
to prevent unnecessary heat removal by the ventilation
fans.
6. Set up a regular ventilation equipment maintenance
schedule.
7. Keep all fan information and warranties in a separate
and accessible file. Complete packaged systems should
have an ``owner's and operator's manual.''
See Pork Industry Fact Sheets for information related to ventila-
tion.
PIH-41, Maintenance and Operation of Ventilation Fans for
Hog Barns.
PIH-54, The Environment in Swine Housing.
PIH-84, Troubleshooting Mechanical Ventilation Systems.
PIH-110, Electrical Wiring for Swine Buildings.
PIH-120, Non-mechanical Ventilation of MOF Swine Buildings.
Information and engineering designs for swine ventilation
systems are available for a nominal charge from the Midwest Plan
Service, 122 Davidson Hall, Iowa State University, Ames, Iowa
50011.
MWPS-8, Swine Housing and Equipment Handbook
MWPS-28, Farm Buildings Wiring Handbook
MWPS-31, Heating, Cooling and Tempering Air for Livestock
Housing Handbook.
MWPS-32, Mechanical Ventilating Systems Handbook.
MWPS-33, Natural Ventilating Systems Handbook.
________________________________
Reference to products in this publication is not intended to be
an endorsement to the exclusion of others which may be similar.
Persons using such products assume responsibility for their use
in accordance with current directions of the manufacturer.
REV 12/90 (7M)
Figure 1. Winter ventilation in a controlled environment. Source:
MWPS-32, Midwest Plan Service Mechanical Ventilating
Systems Handbook.
Figure 2. A type of negative pressure (exhaust) ventilation system
with pit ventilation. The cold weather ventilation rate
is exhausted through properly sized openings into the
under-aisle duct.
Figure 3. Negative pressure intake and air distribution. The duct
is typically sized to mix one part of incoming air with
four parts of recirculated air to prevent condensation on
the duct during cold weather use.
Figure 4. Neutral pressure, or push-pull ventilation. Cold weather
ventilation is distributed through the pressurized duct
while mild and hot weather ventilation is typically
operated as a negative pressure system.
Figure 5. Positive pressure ventilation. Air outlets are sometimes
located under slotted floors to provide an effective pit
ventilation system.
Figure 6. Air intake velocity and static pressure relationship.
Figure 7. Typical eave and baffled slot intake. Source: Midwest
Plan Service, MWPS-32, Mechanical Ventilating Systems
Handbook.
Figure 8. Baffled air inlets for ``across-the-ceiling'' airflow.
Do not mount pipes or lights within 4 ft. of inlets-a
smooth ceiling prevents cold air from being deflected
down onto animals.
Figure 9. Gravity curtain inlet. The curtain is weighted in order
to restrict the air inlet size and thus maintain a high
incoming air velocity to promote mixing and distribution
of fresh air.
Figure 10. Materials used for fan housings and hoods, such as
fiberglass, stainless steel, and special surface
treatments on steel, are more resistant to corrosion and
rust than ordinary painted or galvanized steel. They
still require periodic cleaning, however, to operate
effectively.
______________________________________________
Cooperative Extension Work in Agriculture and Home Economics,
State of Indiana, Purdue University and U.S. Department of Agri-
culture Cooperating. H.A. Wadsworth, Director, West Lafayette,
IN. Issued in furtherance of the Acts of May 8 and June 30, 1914.
It is the policy of the Cooperative Extension Service of Purdue
University that all persons shall have equal opportunity and
access to our programs and facilities.
.