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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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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
Ventilation and Air Flow in High Tunnels
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Presentation Outline Ventilation principles
Natural vs. Mechanical
High Tunnel application
Manual control Automated
(temperature) control
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Ventilation of High Tunnels Needed to Control Heat & Moisture
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Target TemperaturesTable 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.
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Air expands as it is heated and can absorb more moisture.
Warm Air Holds More Moisture
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Air temperature (F)
Moisture-Holding Capacity of Air
Every 18 F increase doubles moisture holding capacity
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Ventilation principles Natural Ventilation
Driving Forces – Thermal Buoyancy Wind
Mechanical Ventilation Driving Force –
Fans
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Natural Ventilation –Buoyancy Driven Case
Warm Air
Cool Air
Cool Air
Thermal Buoyancy
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Natural Ventilation –Wind Driven Case
Wind
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Natural Ventilation needs both an inlet and outlet
Outlet
Inlet
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Inlets for High Tunnels
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End Wall Outlet Vent
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Ridge type “Outlet” for High Tunnel
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Manual Control of sidewall Inlets
High-low thermometers
Remote thermometer (read out in home/office
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Automated Temperature sensor and Inlet Controller
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Mechanical Ventilation –Negative Pressure
-
Negative
(A) © 2009 Regents of the University of Minnesota
Seal up corners (leaks) so can control roll up sidewall “inlets”
Before After
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Mechanical Ventilation
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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
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Questions
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Inlets and Outlets
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