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A Summary of Research Findings on Controlling the
Natural Ventilation Process
 -----------------------------------------------
ISU Publication #: none - electronic only
Authors: Steven J. Hoff, Ph.D. and Jay D. Harmon, Ph.D., P.E.
Department of Agricultural and Biosystems Engineering 
Iowa State University
Date: 11/94
 
     For the past two years, the Iowa Energy Center has sponsored research devoted to the study of naturally ventilated buildings in Iowa.  This project, under contracts 92-07 and 92-07-01, studied the influences of weather conditions on the environmental control capabilities of three naturally ventilated buildings.  The ultimate outcome of this research is a model that can be used to help guide management decisions for naturally ventilated buildings. The model will be available January 1995 through your area Extension Agricultural Engineering Field Specialist.
 
Buildings Studied
 
     Three buildings in central Iowa were studied. Two of the buildings were swine finishing houses and one of the buildings was an intermediate turkey house.  The three buildings varied in terms of environmental control.  One of the swine finishing houses used pure natural ventilation techniques with no supplemental heating.  The other swine finishing house used mechanical ventilation assist during the winter periods and distributed fresh air during these periods using a poly-tube distribution system. Supplemental heat was supplied as required.  The intermediate turkey house used supplemental heat as well. Table 1 summarizes some of the features of each building studied.
 
Monitored Variables
 
     Each building was studied for inside humidity and temperature.  Both humidity and temperature was recorded on a 15 minute basis for a period of approximately two years. A single point in the building was monitored for humidity whereas 20-26 locations were monitored for temperature.
 
Results From Study
 
     Selected weather periods will be presented to briefly summarize the environmental control capabilities of each building type.  The time periods have been selected to help you see the expected differences between environmental control capabilities as a function of building design and natural ventilation strategy.  For all the graphs presented, "SS", "C", and "NS" correspond to animal-level temperatures at the building south-side, center, and north-side, respectively.
 
Cold-Weather Control
 
     During cold-weather periods, the advantages of mechanical assist become very evident.  Figures 1 and 2 represent building air temperature for the hybrid system and the pure natural ventilation system, respectively.  The desired temperature for the hybrid system was set at 65 oF and as can be seen, this temperature was maintained very closely. The average temperature maintained was consistent across the south, center, and north-sides of the building.  As shown in Figure 2, the pure naturally ventilated building struggled to maintain a reasonable  temperature.  Temperatures between north and south sides varied by as much as 15 oF with nearly a 30 oF temperature swing for the entire day.  As Figure 2 clearly shows, the pure naturally ventilated building was affected a great deal by the outside weather conditions.
 
     The large drop in building air temperature between 16:00 and 24:00 hours (4:00 pm to midnight) was most likely the result of a changing weather pattern.  During this time period, the wind speed increased from 7 to 18 MPH and the wind shifted from due south to due north. It appears that  the curtain and ridge control systems were unable to respond quickly to changing weather resulting in a poorly controlled building.
 
     The overall movement towards hybrid systems appears to have merit. Better distribution control over entering cold air will provide more uniform temperatures in the building provided inlets are properly positioned. However, rapid control of curtains and ridge, combined with proper placement of thermostats, will result in a pure naturally ventilated building performing much better than the example given in Figure 2. This is the concept of a current research project being conducted at ISU and funded by the Iowa Energy Center.
 
Mild-Weather Control
 
     Natural ventilation control during mild weather conditions such as the spring and fall is a difficult period because of the potential daily swings in temperature. During the day, temperatures can easily rise to 70 oF and drop below freezing at night. Thus, the building must be controlled in both a cold-weather mode and a summer-weather mode during a single day.
 
     Figures 3 and 4 show the building air temperatures for the hybrid and pure naturally ventilated buildings in late February 1994.  The outside air temperature peaked at 55 oF during the afternoon and fell below freezing near midnight. The hybrid system, still running in a mechanical assist mode, was able to maintain reasonably consistent temperatures in the building near 68 oF. The pure naturally ventilated building did not perform as well.  During this period of operation, the north-side opening was manually adjusted to an opening of roughly 1 inch which is the normal "cold" weather setting. During the heat of the day, the north-side air temperature
rose 10 oF above the south-side due to a lack of fresh air mixing.
 
     A more appropriate control of the north-side would have been to open the inlet to promote north-side fresh air mixing and thus equalize the temperatures across the building. This is a good example of the inadequacies in temperature control that result from manual adjustment of inlet openings. A simple controller combined with a power winch would have opened the north-side during the heat of the day.
 
Hot-Weather Control
 
     During warm and hot summer days, maximizing fresh-air exchange in the building becomes very important. If little fresh-air moves through the building, the inside temperature will tend to rise much above outside temperature.  With sufficient air exchange, the building air temperature will closely follow the outside air temperature as shown in Figures 5 and 6. Inside temperatures below the outside temperature imply that evaporative cooling is taking place.  This cooling was a result of spray misting in the building.
 
Recirculation
 
     The hybrid system studied with this research project provided a mechanism for recirculating inside air with fresh air. The system used a poly-tube distribution duct and combined fresh outside air with recirculated inside air.  This method will temper cold outside air before it is exposed to the animals.
 
     This practice, if not managed properly, will result in high gas, dust, and moisture levels in the building. During very cold periods, the temptation is to use proportionally more recirculation air to save on heating costs in the building. The consequence of this practice can result in a very poor working environment.
 
     Figure 7 represents the humidity conditions during a cold day in late December 1993.  The upper relative humidity curve represents the moisture levels in the hybrid system incorporating recirculation.  The lower curve represents a building using 100 percent fresh-air. The extremely high humidity levels in the hybrid building was the result of near 100 percent recirculation air being used for distribution purposes. With this practice, the air quickly becomes saturated and over long periods will be detrimental to building longevity. The end result is a building with excellent temperature control and uniformity but at the expense of high gas, dust, and moisture levels.
 
Table 1. Comparison of Buildings Studied and Control Mechanisms
 
Building Species Width Length South-Side North-Side  Ridge
one      swine  48 ft 104 ft 4 ft (R-4) 4 ft (R-4) capped
                                                     16'oc
two      turkey  62 ft 216 ft 4 ft (R-1) 4 ft (R-1) 1' cont.
three    swine  45 ft 96 ft 4 ft (R-1) 2.5' door  1' cont.
                                           (R-10)
 
 
Opening Controls
 
Building     South-Side     North-Side     Ridge
one        auto winch w/    auto winch w/   manual
           zone control     zone control
 
two        auto pneumatic     manual       auto pneumatic
           w/ zone control                  w/ zone control
 
three      auto winch-no    manual set     manual
             zone control    on 4 ft doors
 

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