Energy Efficient Grain Drying Kenneth Hellevang, Ph.D., P.E. Professor & Extension Engineer
Estimated Corn Field Drying PET=Potential Evapotranspiration EMC (%) PET (in) Est. Drying (%pt) Month Week Sep 15 4.0-5.0 18 4.5 Oct 16 2.8-3.5 11-12 2.5 Nov 19 0.8-1.2 4-5 1 Dec 20 0.5-0.8 2 0.5 Jan 21 0.5-0.8 2 0.5 Feb 21 0.5-0.9 3 0.8 Mar 19 1.3-1.6 5 1 Apr 16 3.2-4.5 16 4 May 14 6.5-8.5 30 7 NDAWN, Weather, Total PET, Estimate:1-inch = 4% drying EMC-equilibrium moisture content, GDD-growing degree days
Natural Air Drying Natural air drying, if properly designed and managed, is the most energy efficient drying. It is not efficient or effective on very wet grain or at November-March temperatures.
Effect of Supplemental Heat When NA/LT Drying Wheat 17% initial M.C., 0.75 cfm/bu, 10,000 bu Bin, $0.10 electric heat, $8.00/bu., ND Climate Adding heat increases drying and shrink cost. Only warm the air 5 F when necessary to reduce grain moisture. Use a controller. Temp ( F) RH (%) EMC (%) Drying Time (Days) Ave Sep 58 65 14.4 -- +3 Fan 61 58 13.5 31 Shrink Cost Heat Cost Overdry Cost 10 + 3 71 41 10.8 27 $2,424 $1567 $3,991 Ave Oct Cold Sep 47 65 15.0 -- +3 Fan 50 58 13.9 39 10 + 3 60 40 11.0 37 $2,248 $2,130 $4,378 Humid Sep 58 75 16.1 -- +3 Fan 61 67 14.6 40 10 + 3 71 47 11.6 28 $1,720 $1613 $3,333 5 + 3 66 56 12.9 32 $552 $807 $1359
Natural Air and Low Temperature Corn Drying 21% Initial Corn Moisture Content, Average ND Climatic Conditions Drying Time (Days) Month & added heat Temp. ( F) RH EMC 1.0 cfm/bu 1.25 cfm/bu Oct. +3 F (fan) 50 58% 13.5% 42 34 Oct. 15 Nov +3 F (fan) 37 66% 15.8% 65 52 Nov. +3 F (fan) 30 64% 16.0% 70 56 Nov. +3 F (fan)+2 F 32 58% 14.6% 65 52 Nov. +10 F 37 48% 12.5% 51 41 NA/LT corn drying works well until outdoor temperatures approach freezing, then becomes inefficient.
Natural Air & Low Temperature Corn Drying Month & added heat Spring Drying North Dakota Climate Ave. Temp (ºF) RH Corn EMC Drying Time (Days) 1.0 cfm/bu 1.25 cfm/bu Apr 42 65% 15.3% 51 41 +5ºF 47 54% 13.3% 46 37 May 56 60% 13.5% 43 34 Natural air drying is very efficient in the spring. Start fans when outdoor temperatures average about 40 F.
Fan Type Comparison Corn: 27 ft. diameter, 20 ft. deep, 10 hp fan Fan cfm Airflow Rate (cfm/bu) AF 24 (Axial Flow) 8,673 0.95 ILC (In-line Centrifugal) LSC (Low-speed Centrifugal, 1750 rpm) HSC (High-speed Centrifugal, 3500 rpm) 7,232 0.79 10,117 1.10 5,824 0.64 Typically the low speed centrifugal fan moves the most airflow per horsepower through corn, so is the most efficient.
Fan Power Required Airflow Rate Corn Depth (ft) 16 18 20 22 24 (cfm/bu) ----- hp per 1,000 bu ----- 1.0 0.6 0.8 1.1 1.3 1.7 1.25 1.1 1.4 1.8 2.3 2.9 1.5 1.7 2.2 2.9 3.6 4.5 Horsepower calculated based on a 42 ft diameter bin 42 ft diameter bin, corn 36 ft deep, 1.0 cfm/bu Fan = 180 hp, static pressure = 17-inches wg.
Stirring Batch Bin Dryer 8-15 cfm/bu. Maximum drying rate obtained with shallow depth. Use maximum temperature that will not damage the grain. Stirring limits over-drying. Drying temperature up to ~160ºF.
Continuous Flow Bin Dryer Maximum drying rate with about 6-8 ft. corn depth Again use maximum air temperature and limit grain depth to maximize dryer capacity and efficiency. Cooling is done in a separate bin.
Cross-Flow Dryer Batch Dryer A traditional cross-flow dryer requires about 2,500 Btu to remove a pound of water from corn.
Energy requirements of a conventional cross-flow dryer as a function of drying air temperature and airflow rate. (University of Nebraska) Energy required to remove a pound of water is reduced at higher plenum temperatures and lower airflow rates. Use the maximum temperature that will not damage the grain.
Heat and Cool Batch Inside wall hotter. (Loss of grain quality and efficiency)
Pressure Heat, Pressure Cool
Full Heat Continuous Flow Best capacity, bu./hr., for dryer size and cost Heat in grain drives out moisture in the bin In-bin cooling - Needs full floor aeration and moisture management Better efficiency and quality than pressure cool
Two Heat Continuous Flow Slightly better quality than single zone Efficiency loss in lower plenum Same dryer can also do pressure cool FH needs aeration and moisture management
Pressure Heat, Suction Cool Capacity loss to full heat Cooling air is recycled to fan inlet Efficiency gain over full heat Partial cooling with aeration Needs little management
Vacuum Cooling Vacuum cooling increases drying energy efficiency by 20% to 30%. This may be more beneficial in cold northern climates.
Pressure Heat, Suction Cool with Heat Reclaim Lower heat zone recycled Best efficiency. Needs little management
Fuel Cost Cost per bu. $1.85 gal. Propane, $.10/kWh 5% Pt. Removal Pressure Heat Pressure Cool 16.0 Full Heat 13.1 Pressure Heat Vacuum Cool 12.5 Pressure Heat Vacuum Cool Heat Reclaim 9.0
Dryer Improvements Staged Temperature Grain Turner or Inverter Using a higher plenum temperature on the wettest grain reduces energy consumption. A grain turner or inverter reduces over-drying, moisture variation and excessive kernel temperatures.
Differential Grain Speed Dryer Increasing the grain flow rate near the plenum reduces excessive grain temperatures and creates a more uniform grain moisture content coming from the dryer.
Moisture Equalizers Moves inside wall faster Less variation of moisture across column Better test weight (1 to 4 lbs./bu.) Able to dry at 10 to 20 deg F higher temps with same grain quality (10 deg = 4% energy savings and 8% capacity increase )
Variable Width Column Saturated air 8-12 Heat Zone 12 Heat Zone 12 Cool Zone
Efficiency Fan Selection Important 85% 80% 75% SQ8 VA Corn Design range Wheat 70% 65% 60% SQ12 VA SQ16 VA SQ16 Cent Brock Q VA Centrifugal 55% 50% 45% 40% 35% 30% 25% Vane Axials 20% 15% 10% 5% 0% 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Static Pressure
Mixed Flow Dryer A mixed flow dryer uses a lower airflow rate per bushel than a cross-flow dryer which increases energy efficiency. The moving grain design minimizes exposure to the hot plenum air which reduces the potential for grain damage. Airflow Rate=40-45 cfm/bu.
Drying Cost Increases at Colder Temperatures High Temperature Drying @ $1.10/gal propane 2.4 /bu-pt. 21% to 15.5% 2,500 Btu/lb of water Outside Temp. % Increase Cost /bu 40 --- 13.2 20 114 15.0 0 129 17.0-20 142 18.7 With Air Recirculating 2,000 Btu/lbw 1.9 /bu-pt. @ 40 F 10.5 /bu Dry when it is warmer if possible, since it takes more energy to dry at colder temperatures.
Heat of Vaporization Drying Cereal Grains Brooker, Bakker-Arkema, Hall Water Heat of Vaporization = 1,070 Btu/lb @ 40 F Minimum energy to evaporate water from corn is about 1,200 Btu per pound. Realistic dryer minimum is probably about 1,500 Btu.
Dryeration Dump hot, temper without airflow 4-6 hrs, cool Extensive condensation - must move corn to another bin Moisture reduction: 0.25%/10 F cooled, 2.5% Increases dryer capacity 50%-70%, Reduces energy by about 15%-30% Dryeration reduces energy consumption by about 25%, but it is imperative to move the corn to another bin for storage to prevent storage problems.
In-Storage Cooling Immediately cool, Airflow rate 12 cfm/bu-hr of fill rate About 1-1.5 percentage point moisture reduction (0.1 0.15 / 10ºF) Reduce condensation if below 50ºF by partial cooling in the dryer typically to about 90 F In-storage cooling requires rapid cooling and cooler initial grain temperature to limit condensation. Slow cooling saves more energy, but storage problems typically occur near the bin wall.
Grain Dryer Energy Audit Existing energy use Expected new system energy use Energy savings Simple payback
Existing Energy Usage Bushels of corn dried : 239,592 bu. dry bushels Average initial moisture content: 23.69% Average final moisture content: 14.72% Water removed from the corn per bushel: 6.52 lb. Corn weight: 62.01 lbs. at 23.69% moisture 55.49 lbs. at 14.72% moisture content This is not a normal percentage or shrink calculation!!
Energy per Pound of Water Corn dried = 239,592 dry bushels Fuel Used = 45,916 gallons of propane Propane per bushel = 0.1916 gal. Energy per bushel = 17,554 Btu Water removed per bushel = 6.52 lb. Energy per pound of water removed 17,554 Btu / 6.52 lb = 2,692 Btu/lb water
New Dryer Estimated Energy Usage
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