Ventilation and Air Flow in High Tunnels


   Presented at 2009 Minnesota Statewide High
      Tunnel/Season Extension Con...
Presentation Outline
   Ventilation principles
       Natural vs.
        Mechanical
   High Tunnel
    application
  ...
Ventilation of High Tunnels Needed
   to Control Heat & Moisture




       © 2009 Regents of the University of Minnesota
Target Temperatures
Table 2. Approximate Temperatures for Best Growth and Quality of Selected Vegetable Crops.*


        ...
Warm Air Holds More Moisture




Air expands as it is heated and can absorb more moisture.




          © 2009 Regents of...
Moisture-Holding Capacity of Air




    Every 18 F increase
    doubles moisture holding
    capacity




               ...
Ventilation principles
   Natural Ventilation
       Driving Forces –
            Thermal Buoyancy
            Wind
 ...
Natural Ventilation –
          Buoyancy Driven Case


                        Warm
                         Air
Cool     ...
Natural Ventilation –
         Wind Driven Case

Wind




       © 2009 Regents of the University of Minnesota
Natural Ventilation needs
both an inlet and outlet


                                             Outlet




Inlet




  ©...
Inlets for High Tunnels




   © 2009 Regents of the University of Minnesota
End Wall Outlet Vent




   © 2009 Regents of the University of Minnesota
Ridge type “Outlet” for High Tunnel




      © 2009 Regents of the University of Minnesota
Manual Control of sidewall Inlets

   High-low thermometers

   Remote thermometer
    (read out in home/office




    ...
Automated Temperature
sensor and Inlet Controller




  © 2009 Regents of the University of Minnesota
Mechanical Ventilation –
        Negative Pressure




                  -

             Negative


                (A)
 ©...
Seal up corners (leaks) so can
control roll up sidewall “inlets”
   Before                                     After




 ...
Mechanical Ventilation




   © 2009 Regents of the University of Minnesota
Summary

   Natural Ventilation is probably preferred
    in High Tunnels because of cost and
    difficulty to seal up a...
Questions




© 2009 Regents of the University of Minnesota
Inlets and Outlets




   © 2009 Regents of the University of Minnesota
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Ventilation and Airflow in High Tunnels

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Presented by University of Minnesota Professor and Extension Engineer Larry D. Jacobson at the 2009 Minnesota Statewide High Tunnel Conference in Alexandria, MN on Dec. 2-3, 2009.

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Ventilation and Airflow in High Tunnels

  1. 1. Ventilation and Air Flow in High Tunnels Presented at 2009 Minnesota Statewide High Tunnel/Season Extension Conference, Dec 2 & 3, 2009. Alexandria, MN Larry.D.Jacobson, Professor and Extension Agricultural Engineer Dept of Bioproducts & Biosystems Engineering U of Minnesota, St.Paul, MN © 2009 Regents of the University of Minnesota
  2. 2. Presentation Outline  Ventilation principles  Natural vs. Mechanical  High Tunnel application  Manual control  Automated (temperature) control © 2009 Regents of the University of Minnesota
  3. 3. Ventilation of High Tunnels Needed to Control Heat & Moisture © 2009 Regents of the University of Minnesota
  4. 4. Target Temperatures Table 2. Approximate Temperatures for Best Growth and Quality of Selected Vegetable Crops.* Temperature (ºF) Crop Maximum Optimum Minimum 85 55-75 45 Onion, Garlic 75 60-65 40 Beet, Broccoli, Cabbage, Chard, Radish 75 60-65 45 Carrot, Lettuce, Pea, Potato Snap Bean, Lima Bean 80 60-70 50 90 65-75 60 Cucumber 80 70-75 65 Tomato, Sweet Pepper 95 70-85 65 Eggplant, Hot Pepper, Okra *Adapted from Knott's Handbook for Vegetable Growers. Lorenz and Maynard. 1988. P 70. © 2009 Regents of the University of Minnesota
  5. 5. Warm Air Holds More Moisture Air expands as it is heated and can absorb more moisture. © 2009 Regents of the University of Minnesota
  6. 6. Moisture-Holding Capacity of Air Every 18 F increase doubles moisture holding capacity Air temperature (F) © 2009 Regents of the University of Minnesota
  7. 7. Ventilation principles  Natural Ventilation  Driving Forces –  Thermal Buoyancy  Wind  Mechanical Ventilation  Driving Force –  Fans © 2009 Regents of the University of Minnesota
  8. 8. Natural Ventilation – Buoyancy Driven Case Warm Air Cool Cool Air Thermal Buoyancy Air © 2009 Regents of the University of Minnesota
  9. 9. Natural Ventilation – Wind Driven Case Wind © 2009 Regents of the University of Minnesota
  10. 10. Natural Ventilation needs both an inlet and outlet Outlet Inlet © 2009 Regents of the University of Minnesota
  11. 11. Inlets for High Tunnels © 2009 Regents of the University of Minnesota
  12. 12. End Wall Outlet Vent © 2009 Regents of the University of Minnesota
  13. 13. Ridge type “Outlet” for High Tunnel © 2009 Regents of the University of Minnesota
  14. 14. Manual Control of sidewall Inlets  High-low thermometers  Remote thermometer (read out in home/office © 2009 Regents of the University of Minnesota
  15. 15. Automated Temperature sensor and Inlet Controller © 2009 Regents of the University of Minnesota
  16. 16. Mechanical Ventilation – Negative Pressure - Negative (A) © 2009 Regents of the University of Minnesota
  17. 17. Seal up corners (leaks) so can control roll up sidewall “inlets” Before After © 2009 Regents of the University of Minnesota
  18. 18. Mechanical Ventilation © 2009 Regents of the University of Minnesota
  19. 19. Summary  Natural Ventilation is probably preferred in High Tunnels because of cost and difficulty to seal up all the leaks  Temperature control is primary need for ventilation, moisture control secondary  Manual ventilation control challenging but possible  Simple aid might be remote thermometer  Automated controller for sidewall inlets would yield tighter temperature control © 2009 Regents of the University of Minnesota
  20. 20. Questions © 2009 Regents of the University of Minnesota
  21. 21. Inlets and Outlets © 2009 Regents of the University of Minnesota
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