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High Tunnel Ventilation, 2015


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by Larry D. Jacobson, Professor and Extension Agricultural Engineer | University of Minnesota

Presented at the 2015 Minnesota Statewide High Tunnel Conference.

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High Tunnel Ventilation, 2015

  1. 1. Presented at MN Statewide High Tunnel Conference, Feb 17, 2015. Arrowwood Lodge, Baxter, MN Larry D Jacobson, Professor and Extension Agricultural Engineer Dept of Bioproducts & Biosystems Engineering University of Minnesota, St.Paul, MN Understanding High Tunnel Ventilation in Detail
  2. 2. Presentation Outline  Ventilation principles  Natural vs. Mechanical  High Tunnel application  Manual Control  Automated Control (by temperature)
  3. 3. Ventilation or Air Exchange of High Tunnels Needed to Remove Heat & Moisture
  4. 4. Suggested Temperatures in High Tunnels for Selected Crops 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.
  5. 5. Air temperature (F) Moisture-Holding Capacity of Air Every 18 F increase doubles moisture holding capacity
  6. 6. Air expands as it is heated and can absorb more moisture. Warm Air Holds More Moisture
  7. 7. Two Different Ventilation Systems used in HT  Natural Ventilation  Driving Forces –  Thermal Buoyancy  Wind  Mechanical Ventilation  Driving Force –  Exhaust Fans
  8. 8. Natural Ventilation – Buoyancy Driven Case W a rm A ir C o o l A ir C o o l A irThermal Buoyancy
  9. 9. Natural Ventilation – Wind Driven Case W in d
  10. 10. Natural Ventilation needs both an inlet and outlet Outlet Inlet
  11. 11. Typical Inlets for High Tunnels
  12. 12. End Wall Outlet Louver (Vent)
  13. 13. Ridge type “Outlet” for High Tunnel
  14. 14. Manual Control of sidewall Inlets  Management Tools for managing opening size (controlling air exchange)  High-low thermometers  Remote thermometer (read out in home/office)
  15. 15. Automated Temperature sensor and Inlet Controller
  16. 16. Mechanical Ventilation System – needs to have negative pressure in HT - Negative (A)
  17. 17. For successful mechanical ventilation need to seal up leaks so can obtain negative pressure or slight vacuum inside the High Tunnel Before After
  18. 18. Mechanically Ventilated HT
  19. 19. Summary  Natural Ventilation is probably preferred in High Tunnels because of lower cost and difficulty to seal up all the leaks so mechanical ventilation will work properly  Temperature control is primary need for ventilation or air exchange, 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
  20. 20. Questions
  21. 21. Inlets and Outlets