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

Presentation Outline

Ventilation principles Natural vs. Mechanical

High Tunnel application Manual Control Automated Control (by temperature)

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

Moisture

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.

Air temperature (F)

Moisture-Holding Capacity of Air

Every 18 F increase doubles moisture holding capacity

Air expands as it is heated and can absorb more moisture.

Warm Air Holds More Moisture

Two Different Ventilation Systems used in HT

Natural Ventilation Driving Forces –

Thermal Buoyancy Wind

Mechanical Ventilation Driving Force –

Exhaust Fans

Natural Ventilation – Buoyancy Driven Case

W a r m A i r

C o o l A i r

C o o l A i r

Thermal Buoyancy

Natural Ventilation –Wind Driven Case

W i n d

Natural Ventilation needs both an inlet and outlet

Outlet

Inlet

Typical Inlets for High Tunnels

End Wall Outlet Louver (Vent)

Ridge type “Outlet” for High Tunnel

Manual Control of sidewall Inlets

Management Tools for managing opening size (controlling air exchange)

High-low thermometers

Remote thermometer (read out in home/office)

Automated Temperature sensor and Inlet Controller

Mechanical Ventilation System –needs to have negative pressure in HT

-

Negative

(A)

For successful mechanical ventilation need to seal up leaks so can obtain negative pressure or

slight vacuum inside the High Tunnel

Before After

Mechanically Ventilated HT

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

Questions

Inlets and Outlets