by Larry D. Jacobson, Professor and Extension Agricultural Engineer | University of Minnesota Presented at the 2015 Minnesota Statewide High Tunnel Conference.

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. Presentation Outline
 Ventilation principles
 Natural vs. Mechanical
 High Tunnel application
 Manual Control
 Automated Control
(by temperature)

3. Ventilation or Air Exchange of High
Tunnels Needed to Remove Heat &
Moisture

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. Air temperature (F)
Moisture-Holding Capacity of Air
Every 18 F increase
doubles moisture holding
capacity

6. Air expands as it is heated and can absorb more moisture.
Warm Air Holds More Moisture

7. Two Different Ventilation
Systems used in HT
 Natural Ventilation
 Driving Forces –

Thermal Buoyancy

Wind
 Mechanical Ventilation
 Driving Force –

Exhaust Fans

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. Natural Ventilation –
Wind Driven Case
W in d

10. Natural Ventilation needs both
an inlet and outlet
Outlet
Inlet

11. Typical Inlets for High Tunnels

12. End Wall Outlet Louver (Vent)

13. Ridge type “Outlet” for High Tunnel

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. Automated Temperature
sensor and Inlet Controller

16. Mechanical Ventilation System –
needs to have negative pressure in HT
-
Negative
(A)

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. Mechanically Ventilated HT

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. Questions

21. Inlets and Outlets