About ABE

Prospective Students

Undergraduate Students

Graduate Students

Industrial Technology
  Program

Faculty and Staff

Extension and Outreach

Research

Events and News

MWPS

Locator Map

Search

Controlling the Natural Ventilation Process 
----------------------------------------------------
ISU Extension Publication #: none - electronic format only
Authors: Dr. Steven J. Hoff and Dr. Jay D. Harmon 
Department of Agricultural and Biosystems Engineering
Iowa State University
Date:  11/94

 
       Natural ventilation systems have, in the past, been used primarily for larger animals where inside temperature control is not a critical factor. That is, for larger animals where temperature fluctuations between 15-20 ¡F is tolerable.  With a leaner pig, less natural body insulation (i.e. fat) is available to prevent excessive heat loss during cold winter periods.  Thus, pigs will lose energy to the environment that otherwise would be used for growth. The end result is a need for better control of naturally ventilated buildings.

       Better control implies providing better temperature, moisture, and air quality control in the building. There exist several strategies for controlling naturally ventilated buildings. A few of these strategies will be outlined below.
 
Building Width
       One enemy that exists in controlling naturally ventilated buildings is building width. As buildings become wider, it becomes extremely difficult to distribute fresh air to all portions of the building.  Realistically, a 45 foot wide building is the absolute maximum width that one could ever expect to ventilate naturally. Buildings wider than this will need mechanical ventilation assistance for delivering fresh air to the building during cold winter periods.
 
Side-Wall Openings

       Openings along the buildings length provide a means for fresh air to enter the building. Side-wall openings have evolved over the years to the use of movable curtains on the side-wall. Moveable curtains are ideal for controlling the opening size to accommodate various wind speeds and outside temperatures.  Curtains up to 400 feet in length can be controlled very easily with mechanical winches and simple thermostatic controls.      Curtains have generally been reserved for the south side of buildings where the effects of northerly winter winds are minimized. On the north side of buildings, insulated doors are common although insulated curtains are becoming increasingly popular.

       A good practice would be to supply both north and south side openings with movable insulated curtains.  Insulated curtains can be purchased with R-values as high as 12.0.  These curtains consist of multi-layers of single curtain fabric.  Prices range from $6 to $8 per foot for a 4 foot insulated (R-4) curtain.

       Insulated curtains provide two useful functions. They reduce heat loss due in part to the increased R-value and they reduce infiltration of cold outside air through the exposed portion of the curtain. Reducing infiltration will greatly decrease the heating load on the building and hence increase the ability for maintaining desired temperatures.

       Side-wall openings should be designed to allow for at least a 4 foot opening on both the north and south walls.  The curtain should be designed to move from top to bottom so that cold winter air is introduced to the building as high above the animals as possible.  This will help reduce cold drafts at animal level.

 Ridge Openings

       Ridge openings need to be designed to allow for obstruction-free airflow.  Ridge caps of any type will greatly reduce the ability of inside air from being exhausted through the ridge.  Ridge caps provide one useful function, and that is to keep rain and snow from entering the building. This same effect can be achieved with an internal trough and thus would be recommended instead of expensive ridge cap systems.

       Upstands 6 to 12 inches above the ridge line will accelerate wind across the ridge and hence increase the "pull" of stale air from the building. 
Ridge vents should be designed with a 6 inch minimum width.  For buildings up to 45 feet wide, this width should be increased to 12 inches.  Simple baffle controls can be used to adjust the airflow through the ridge.
 
Controlling Side-Wall and Ridge Openings
       Due to its very nature, naturally ventilated buildings are difficult to control because of the variations in wind speed, wind direction, and outside temperature. 
       Expecting to control a naturally ventilated building manually is unreasonable and should not be attempted. Further, providing automated control to one opening, and manual control to other openings will, in general, perform no better than if you provided manual control to all openings.  Therefore, a recommended practice would be to provide automated control for each controlled opening in the building.
 
Winter Ventilation Control
       The best place to start a discussion of natural ventilation control is how to control the building during cold winter periods. During winter months, moisture and gas removal become the governing criteria for ventilation.  In general, 4 air exchanges per hour are needed to maintain moisture and gas levels in a building. This air must enter the building and be distributed throughout the building to eliminate pockets or zones that are stagnant and hence high in moisture and gas levels.
       A common practice is to completely close off the north side-wall opening and to use the ridge vent and south side-wall opening to control the ventilation process.  As buildings increase in width, the north side of the building will lack in fresh-air exchange because of inadequate fresh-air exchange in this region.
       A better control strategy for winter ventilation control would be to provide minimum openings on both the south and north sides, and to use the ridge to control the fresh-air exchange rate in the building.  The ridge can be very powerful in controlling ventilation during cold winter ventilation
periods. Stale air will be exhausted at the ridge and fresh air will be drawn in through both side-wall openings. If the air is drawn in near the eave, sufficient warming of the air will occur before the air reaches animals. More importantly though, fresh air will be drawn in on both sides of the building thus increasing the chances for equal distribution of fresh air.
       A control strategy that should be avoided at all times is one where only one opening is active.  For example, assume that we are so concerned about a cold front moving through that we close off the ridge and north side-wall opening. To control the ventilation process, we are left with only the south side-wall opening. This opening will behave as both an inlet and an outlet with very detrimental consequences.
       The bottom portion of this opening will behave as a fresh-air intake and the top portion will behave as an exhaust.  The end result is a short-circuiting loop that will in essence provide little or no fresh air to the building because fresh air is immediately circulated back outside the building.  The building will build-up in moisture, gases, and dust and will, in general, become a miserable working place for the employee and a respiratory nightmare for the animal.
       Now, if one were to open the ridge, the ridge will add an additional exhaust point for the building and less of the side-wall opening will behave as an outlet thereby reducing the short-circuiting phenomena.
 
Summer Ventilation Control
       Any time the outside temperature is above the desired inside temperature, the openings should be controlled to maximize the rate at which fresh air enters the building. In many respects, summer ventilation control is a simple process and can be handled manually. 
       During summer periods, the main objective is to maintain inside temperature at a level that is no more than 3-5 oF above the outside temperature. To accomplish this, many air exchanges are needed.  In fact, for most building arrangements, assuming no supplemental cooling, 70 air exchanges per hour are needed to meet this objective. A full 18 times the rate required for moisture and gas removal.
       In order to achieve this air exchange rate, all side-wall openings must be fully opened and the building needs to be oriented to take full advantage
of predominant summer winds. During hot, calm days, air exchange rates will be very low and the tendency will be for inside temperatures to rise much above the outside temperature. Providing drip/spray coolers will help remove excess heat from the animals.
 
Fall/Spring Ventilation Control
       Providing adequate control of openings in naturally ventilated buildings is most important during spring and fall weather conditions.  Days can be warm and much above desired inside building temperature with nights can be cold and much below desired inside temperature.  Therefore, the openings must be controlled in a "summer" mode for a portion of the day and in a "winter" mode for the remaining portion of the day.  It is during this period where manual control of the natural ventilation process fails miserably.
 
Hybrid Ventilation Control
       Many companies and individual producers have, for many years, recognized the difficulty in controlling naturally ventilated buildings for the weather extremes that exist in Iowa. As a consequence of this, many naturally ventilated buildings are being designed or retrofitted to provide mechanical assist for the ventilation process during cold winter periods and in some, during the unpredictable periods of fall/spring.  The end result is a combined natural and mechanical, or "hybrid", ventilation system.
       In general, a hybrid system will alleviate some of the winter time air distribution problems present in pure naturally ventilated buildings. Incorporation of hybrid systems will allow for much wider buildings because fresh-air intakes can be strategically placed to accommodate all portions of the building. 
       For hybrid systems, the need for a ridge vent system is eliminated.  Remember, the ridge vent becomes most important during cold winter periods where we want thermal buoyancy to "drive" the ventilation process. If we mechanically ventilate the building during these periods, then the ridge vent becomes redundant.
       It is felt however, that improved controller designed can provide adequate opening control so that a pure naturally ventilated building will perform as desired for all portions of the Iowa season. A controller of this nature, is currently being developed by the authors, under the financial support of the Iowa Energy Center. Details will be presented at a later time.
 

IOWA STATE UNIVERSITY
University Extension
 
Ames, Iowa
 

...and justice for all
Iowa State University Extension programs and
policies are consistent with pertinent federal and
state laws and regulations on nondiscrimination
regarding race, color, national origin, religion, sex,
age, and disability.
 
Cooperative Extension Service, Iowa State
University of Science and Technology, and the
United States Department of Agriculture
cooperating. Robert M. Anderson, Jr. director,
Ames, Iowa.  Distributed in furtherance of the Acts
of Congress of May 8 and June 30, 1914.
 
11/94       

Back to Livestock