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.
Presentation Outline
 Ventilation principles
 Natural vs.
Mechanical
 High Tunnel
application
 Manual control
 Automated
(temperature) control
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3.
Ventilation of High Tunnels Needed
to Control Heat & Moisture
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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.
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5.
Warm Air Holds More Moisture
Air expands as it is heated and can absorb more moisture.
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6.
Moisture-Holding Capacity of Air
Every 18 F increase
doubles moisture holding
capacity
Air temperature (F)
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7.
Ventilation principles
 Natural Ventilation
 Driving Forces –
 Thermal Buoyancy
 Wind
 Mechanical Ventilation
 Driving Force –
 Fans
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8.
Natural Ventilation –
Buoyancy Driven Case
Warm
Air
Cool Cool
Air Thermal Buoyancy
Air
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9.
Natural Ventilation –
Wind Driven Case
Wind
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10.
Natural Ventilation needs
both an inlet and outlet
Outlet
Inlet
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11.
Inlets for High Tunnels
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12.
End Wall Outlet Vent
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13.
Ridge type “Outlet” for High Tunnel
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14.
Manual Control of sidewall Inlets
 High-low thermometers
 Remote thermometer
(read out in home/office
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15.
Automated Temperature
sensor and Inlet Controller
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16.
Mechanical Ventilation –
Negative Pressure
-
Negative
(A)
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17.
Seal up corners (leaks) so can
control roll up sidewall “inlets”
Before After
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18.
Mechanical Ventilation
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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
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20.
Questions
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21.
Inlets and Outlets
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