Principles of Grain Drying
Very little temperature rise is desired when drying rice or soybeans, whereas corn is often dried with higher
temperatures. Important: As a guide to maintaining quality and avoiding risk of fire when drying corn, in-bin-
drying temperature should not exceed 120°F (49°C) for stir drying and 160°F (71°C) for in-bin continuous-flow
drying. Refer to the table below for maximum air (plenum) temperatures for in-bin drying. These are general
guidelines only, and drying temperatures may need to be lower for your specific situation.
Heater Selection
Major considerations in heater selection are temperature rise required, type of fuel, heater placement and heater
controls. Temperature rise is the difference between ambient (surrounding air) temperature and plenum
temperature. Use one of the following formulas to determine heater required:
Btu/hr = Temp. Rise (ºF) x CFM x 1.08
kW = Temp. Rise (ºC) x Cubic meters/hr x .000333
kW x 3000
Cubic meters/hr
Table 13 - Maximum Air (Plenum) Temperatures for Drying*
Maximum Air (Plenum) Temperatures
*Note: From North Dakota State University (NDSU) extension service, AE 701 (Revised), November 1994.
Please contact your local extension office for further information regarding your specific situation.
Heater Controls
Heater efficiency and cost of operation can be improved through proper selection of controls. For continuous flow
drying, use high-low or modulating valve control. The following section describes types of heater controls:
Thermostats, the least expensive but also least efficient units, cycle heater completely on and off to maintain
plenum temperature. When burner is shut off, fan blows outside air into plenum, cooling it off. The on and off
action provides an average temperature corresponding to a particular setting, but with short periods of high
and low temperatures when thermostat cycles burner on and off.
High-Low Burner Controls cycle burners from a high setting to a setting 20°F to 30°F (10°C to 15°C) cooler
to maintain a more uniform plenum temperature. This function eliminates extremes in plenum temperatures of
standard thermostats and provides economy of operation.
Modulating Valves provide the most precise temperature control by continuously regulating burner flame to
maintain constant temperatures. Modulating valves work through a capillary tube filled with gas that expands
and contracts with changes in plenum temperature. This gas moves a diaphragm controlling LP gas or natural
gas pressure to burner.
Humidistats are used with a low-temperature burner and are located in plenum of bins. Humidistats cycle
burners on and off based on relative humidity to control the humidity of drying air.
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