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High Tunnel Ventilation, 2015
The presentation discusses high tunnel ventilation principles, comparing natural and mechanical methods for effective air exchange. Key considerations include temperature management for optimal crop growth, with suggested temperature ranges for various vegetables. Natural ventilation is favored due to cost-effectiveness and simpler management, though automated controls can enhance temperature regulation.
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High Tunnel Ventilation, 2015
- 1. Presented at MNStatewide 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. Presentation Outline Ventilationprinciples Natural vs. Mechanical High Tunnel application Manual Control Automated Control (by temperature)
- 3. Ventilation or AirExchange of High Tunnels Needed to Remove Heat & Moisture
- 4. Suggested Temperatures inHigh 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. Air temperature (F) Moisture-HoldingCapacity of Air Every 18 F increase doubles moisture holding capacity
- 6. Air expands asit is heated and can absorb more moisture. Warm Air Holds More Moisture
- 7. Two Different Ventilation Systemsused in HT Natural Ventilation Driving Forces – Thermal Buoyancy Wind Mechanical Ventilation Driving Force – Exhaust Fans
- 8. Natural Ventilation – BuoyancyDriven Case W a rm A ir C o o l A ir C o o l A irThermal Buoyancy
- 9. Natural Ventilation – WindDriven Case W in d
- 10. Natural Ventilation needsboth an inlet and outlet Outlet Inlet
- 11. Typical Inlets forHigh Tunnels
- 12. End Wall OutletLouver (Vent)
- 13. Ridge type “Outlet”for High Tunnel
- 14. Manual Control ofsidewall Inlets Management Tools for managing opening size (controlling air exchange) High-low thermometers Remote thermometer (read out in home/office)
- 15. Automated Temperature sensor andInlet Controller
- 16. Mechanical Ventilation System– needs to have negative pressure in HT - Negative (A)
- 17. For successful mechanicalventilation need to seal up leaks so can obtain negative pressure or slight vacuum inside the High Tunnel Before After
- 18.
- 19. Summary Natural Ventilationis 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
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Editor's Notes
- #6 A 10-degree C (18 F) rise in air temperature roughly doubles the amount of moisture air can hold (over the range of normal air temperatures). Illustration: morning fog ‘burns off’ or disappears as the air warms up (even though the amount of moisture in the air is basically the same). As the day goes on, the air warms up and takes on additional moisture (from the ground, plants, etc.). But, when it cools off again in the evening, it loses ability to hold moisture and may become ‘saturated’ (foggy) again.
- #7 Warming air increases the air’s ability to hold water vapor and visa versa.