HOUSING                                           PIH-60


              Mechanical Ventilation of Swine Buildings

James P. Murphy, Kansas State University
Don D. Jones, Purdue University
Leslie L. Christianson, University of Illinois

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

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
 slot                        Static pressure, water gauge
width                   0.04 In.                     0.125 In.
  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

     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

     2.   Manually adjustable  hinged  or  vertically  adjustable
          baffles  (Figure  7) under ceiling slot intakes (Figure

     3.   Gravity or spring-loaded curtain or damper at the inlet
          (Figures 9).

     4.   Power-operated adjustable baffles  under  ceiling  slot

Other important considerations  in  planning  fresh  air  intakes

     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


     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

     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-

     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

     6.   Set up  a  regular  ventilation  equipment  maintenance

     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-
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
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

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

Cooperative Extension Work in  Agriculture  and  Home  Economics,
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culture Cooperating. H.A. Wadsworth,  Director,  West  Lafayette,
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It is the policy of the Cooperative Extension Service  of  Purdue
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             access to our programs and facilities.