Source: Ontario Ministry of Agriculture, Food and Rural Affairs
Fact Sheet written by: James Dyck – Engineering Specialist, Crop Systems and Environment/OMAFRA
Are you paying too much to dry your grain? A grain dryer wastes as much as 40% of the energy it uses. Reducing and reusing excess heat in the exhausted air can reduce drying costs by up to 40%!
This factsheet explains how to recover wasted energy and reduce grain drying costs. There are three steps:
- Use an efficient dryer. Dryer type can make a 30% difference in energy use.
- Run the system as efficiently as possible. Cool grain using dryeration or in-bin cooling to improve energy use by up to 30%.
- Reclaim heat from the dryer exhaust air to reduce fuel costs by 20%-40%.
Understanding Grain Dryer Energy Use
Grain Drying in Ontario
In Ontario, soybeans and wheat are often harvested at or near optimal storage moisture. By comparison, corn can be harvested as high as 30% moisture, and must be dried to 15% or less before storage. Wet grain spoils quickly in storage, whereas dry grain will store for a long time. Waiting for corn to dry in the field can delay harvest and result in a loss of quality.
Each year, Ontario grows 850,000 hectares of corn, producing 8.5 million tonnes of grain. Assuming an average moisture content of 25% at harvest, drying this much corn down to 15% moisture in a continuous cross-flow dryer will use almost 300 million L of propane and produce nearly 450,000 tonnes of CO2 emissions. Reducing Ontario’s grain drying fuel usage by at least 20% can save 60 million L of propane and reduce CO2 emissions by 90,000 tonnes (e.g., the same as taking 19,000 passenger cars off the road).
Grain Dryer Efficiency
Continuous cross-flow dryers, whether horizontal (Figure 1) or tower-style, are versatile and dry a lot of grain very quickly. They are the most common grain dryers used in Ontario today. Unfortunately, they are also the least efficient. Efficiency is the amount of energy (measured in kilojoules, kJ) needed to evaporate water from the grain.
Figure 1. A typical grain dryer. This is a horizontal continuous cross-flow grain dryer without heat recirculation. Source: Brock Grain Systems, Milford, Indiana, USA.
If grain is harvested at 25% moisture, 134 kg of water must be removed to end up with one “dry” tonne (1,000 kg) at 15% moisture. A continuous cross-flow dryer uses 860,000 kJ of energy (34 L of propane) to evaporate this much water. Burning 34 L of propane produces 52 kg of CO2 emissions. Propane is the most common grain drying fuel in Ontario. If natural gas is used, 23 m3 of fuel is needed which produces 44 kg of CO2 emissions. Approximately 5 kilowatt-hours (kWh) of electricity are used to run the dryer fan (Figure 2).
Figure 2. Approximate fuel used and carbon dioxide emissions produced to dry one tonne of corn from 25% to 15% moisture using a continuous cross-flow dryer.
A different type of dryer can change fuel use significantly. A continuous mixed-flow dryer is about 30% more efficient than a continuous cross-flow dryer (Figure 3). No-heat and low-temperature dryers are very efficient, but they only use electricity (no fuel).
Electricity is more expensive than propane or natural gas for the same amount of energy, so despite high efficiency, these dryers are more expensive to operate.
Figure 3. Approximate efficiencies and operating costs for different grain dryers. Numbers are based on drying one tonne of corn from 25% to 15% moisture. Costs include fuel (propane) and electricity. Source: Scott Sanford, M. Eng., University of Wisconsin-Madison, USA.
Operational Changes to Reduce Drying Energy
A well-maintained and well-operated grain dryer will dry more efficiently than a neglected one:
- Keep dryer screens, inlets and exhausts clean of red-dog and fines.
- Have a certified gas technician adjust the burners for maximum efficiency.
- Clean fan housings and blades, and check belt drives for any wear.
- Calibrate temperature and grain moisture sensors annually.
- Keep grain as clean as possible to ensure good air flow.
- Level the grain in a bin-type dryer for more uniform drying.
- Avoid over-drying the grain.
Staged Heat Drying
Stacked dryers with multiple heating sections can use different plenum temperatures to increase efficiency. Set the top section to hot (e.g., around 100°C) where the grain is wettest. Reduce the temperature of the lower section(s) to avoid over-heating and damaging the grain. Set the lowest heating section to 30°C-40°C above ambient air.
Staged drying reduces the exhaust air temperature in the lower section(s) and prevents overheating the grain
Dryeration and In-Bin Cooling
Cooling grain in a bin is more efficient than cooling it inside the dryer. Most continuous dryers can be set up to run in “full heat” mode (i.e., no cooling). A storage bin with a large fan is needed as a standard aeration fan may not be adequate. The fan needs to produce 225 L/s of air flow for every tonne per hour of dryer capacity (12 cfm for every bushel/hour). For example, a 12.5 mt/hr (500 bu/hr) dryer will need a bin with a 2,800 L/s (6,000 cfm) fan. There are two methods to select from:
- Dry the grain until it is 1%-2% above the desired moisture level.
- Transfer the hot grain to a storage bin.
- Run the fan immediately to cool the grain and remove the final points of moisture.
- Grain can remain in this bin for storage.
- Fuel savings up to 15% and dryer throughput increases up to 30% are possible.
- Dry the grain until it is 2%-3% above the desired moisture level.
- Transfer the hot grain to an intermediate bin and let it “steep” for 4-12 hours with no air flow.
- After steeping, turn on the fan to cool the grain and remove the final points of moisture.
- Once dry, transfer grain to a storage bin. If the grain is not moved, condensation created during the steeping process will cause it to spoil.
- Fuel savings up to 30% and dryer throughput increase up to 50% are possible.
Heat Recovery Systems for Grain Dryers
The air exhausted from a grain dryer is often much warmer than the outside air. Capturing and re-using this air will reduce energy use. There are two distinct types of heat recovery systems:
- Heat recirculation, which may be factory-installed or custom-built.
- Heat exchangers, which are usually custom-built.
Heat Recirculation System
A heat recirculation system guides warm exhaust air directly back into the dryer fan inlet. Heat recirculation works best on continuous flow horizontal or tower dryers operated to dry and cool simultaneously.
Heat Recirculation Performance
For a single-stage continuous cross-flow grain dryer, the temperature and relative humidity (RH) of the exhaust air will look similar to Figure 4. Any exhaust air that is warm (at least 20°C-30°C above ambient) with low humidity (40% or less) can be recirculated to reduce energy use. Generally, this is the exhaust air from the lower half of the dryer (including the cooling section).
This profile is different for other types of dryers. A multi-stage dryer has multiple temperature “peaks”. A batch dryer has a uniform exhaust temperature that increases as the grain dries.
Figure 4. Approximate discharge air temperature and relative humidity profiles from a continuous cross-flow dryer with ambient air cooling. Warm, dry exhaust air can be recirculated to reduce energy use.
Consider a dryer with a plenum temperature of 104°C and an ambient air temperature of 10°C, as in Figure 4. By recirculating the bottom half of the dryer (the whole cooling section and the lower part of the heating section), the average recirculated air temperature will be about 40°C. The expected fuel savings is calculated as follows:
[(Recirculated Air Temperature – Ambient Air Temperature) ÷ (Plenum Air Temperature – Ambient Air Temperature)] × 100 = % Fuel Savings
[(40°C – 10°C) ÷ (104°C – 10°C)] × 100 = 32% Fuel Savings
This savings level is realistic and should be achievable, especially at colder ambient air temperatures.
Heat Recirculation Design
The type of dryer affects the design of the heat recirculation enclosure:
- Two-fan, single-stage dryers operating in heat/cool mode:
- Recapture air from the bottom 50% of the dryer (e.g., the cooling section and the lower portion of the heating section).
- Recapturing only the cooling section will choke the drying fan if extra air is not added.
- Single-fan or two-fan dryers operating in full heat mode (e.g., no cooling section), and single-fan dryers operating in heat/cool mode:
- Recapture air from the bottom third of the dryer.
- Blend some ambient air into the recirculated air, or else throughput will be reduced.
- Additional in-bin cooling of the grain is needed.
- Three-fan dryers (assuming all fans have equal capacity):
- Recirculate air from the lower fan into the middle fan.
- Recirculate air from the middle fan into the top fan.
Figure 5 shows one possible configuration for a heat recirculation system on a two-fan continuous flow dryer operating in the heat/cool mode.
Figure 5. Example of a heat recirculation enclosure for a continuous flow dryer.
Heat Recirculation Cautions
Consider the following for heat recirculation:
- Dryer throughput is slightly reduced with heat recirculation, because the recirculated air contains more moisture than the ambient air.
- Full-heat dryers using heat recovery will have significantly lower throughput. To avoid this, blend some ambient air into the recirculated air.
- Grain coming out of a full-heat dryer or a single-fan heat/cool dryer will need to be cooled in storage.
- Do not install the enclosure too high up the dryer. Exhaust air from the top of the dryer is cooler and wetter, which will reduce the benefit of heat recovery.
- Settling areas for red-dog must be used. Recirculating too much red-dog creates a fire hazard.
- Check and clean the recirculation enclosure regularly. Remove any buildup of red-dog to maintain efficiency and prevent fires.
Factory-Installed Heat Recirculation
Some manufacturers offer factory-built recirculation kits for their dryers (Figure 6). These kits can be ordered with a new dryer, or added onto an existing dryer in the field. Contact the dryer manufacturer for available heat recovery options and pricing.
Figure 6. A continuous cross-flow grain dryer with factory-installed heat recirculation ductwork. Source: Brock Grain Systems, Milford, Indiana, USA
Factory-Installed Suction Cooling
A standard grain dryer uses two fans which push outside air through the grain for both heating and cooling (“pressure cooling”). Many dryers can also be purchased with suction (vacuum) cooling. Fresh air is drawn into the dryer through the hot grain in the cooling section, and then heated and blown through the wet grain in the drying section. The result is similar to heat recirculation, saving 15%-20% in fuel compared to a standard dryer (Figure 7).
Figure 7. Cross-section diagrams of a continuous flow dryer with pressure cooling and suction cooling. Suction cooling can save 15%-20% fuel compared to a standard, pressure-cooled dryer.
Heat Exchanger System
A heat exchanger separates exhaust air and fresh air with a waterproof divider. Heat travels through the divider from the hot exhaust air to the cold fresh air. The two airstreams do not mix, and no moisture or fines can pass through the divider. Unlike heat recirculation, the heat exchanger recovers heat from all of the exhaust air (even the most humid air), and can be used on any type of dryer.
Heat Exchanger Performance
In general, a heat exchanger is not as efficient as a recirculation system because the divider cannot transfer 100% of the exhaust heat to the fresh air. Estimating the performance is complicated, but a well-designed system will reduce energy use by up to 20%. Heat exchangers are most efficient at cold ambient air temperatures; 10°C and below (Figure 8).
Figure 8. Average ambient temperatures during the grain drying season for several locations across Ontario. Heat exchangers operate best at temperatures below 10°C. Data Source: Environment Canada
Heat Exchanger Design
Heat exchangers are usually custom-built using hollow tubes. Incoming air travels through the tubes, while exhaust air passes over the outside. Heat travels through the tube wall between the two airstreams. A tubular heat exchanger can be built as follows:
- Use corrugated galvanized metal or plastic (“Big-O”) tubes.
- Plastic has poor heat transfer compared to metal, but will not corrode.
- Corrugations create air turbulence, which improves heat transfer.
- Use tubes 100 mm (4 in.) in diameter.
- Small tubing increases the total surface area for heat transfer.
- Use 30 tubes (100 mm diameter) for every 1000 L/s (2000 CFM) of dryer air flow.
- Intake air velocity through the tubes should be 4-5 m/s.
- Use tubes at least 15 m long (50 ft).
- Incoming air should take at least 3 seconds to travel through the tubes.
- Space adjacent tubes at least 100 mm (4 in.) apart.
- Exhaust air needs to travel freely over top.
- Stack the tubes vertically in line, not staggered.
- Allows red-dog and fines to pass through much more easily, reducing maintenance.
- Use S-bends in the tubes if additional length is needed. Keep tubes counter-flow to exhaust air for best heat transfer.
- Use long smooth bends for any directional changes, to minimize friction loss.
- Locate the tubes beside or in front of the dryer.
- Do not locate the tubes above the dryer; this will create serious dripping problems.
Figure 9. Example of a heat exchanger system for a continuous flow dryer.
Heat Exchanger Cautions
Consider the following for heat exchangers:
- Use screens on air inlets to prevent bird entry.
- Red-dog will build up on the outside of the heat exchanger tubes, which reduces heat transfer. and creates a fire risk. Clean tubes (e.g., power wash) regularly.
- Make sure there is access above the tubes for maintenance and power washing.
- The area below the tubes will collect red-dog and condensate. Make sure this area is accessible and designed to handle a wet environment.
- Insulate the entire enclosure to prevent condensation problems and mold/deterioration.
- Construction costs are higher than a recirculation system.
- Heat recovery performance is lower than a recirculation system.
Safety Approval for Modified Grain Dryers
Safe operation of a grain dryer is very important. All grain dryers in Ontario are certified for safe use, either in the factory by an agency like the Canadian Standards Association (CSA), or in the field by the Technical Standards and Safety Authority (TSSA).
New dryers with factory-installed heat recirculation are already certified for safe operation of the heat recovery system. Adding heat recovery (or any other modification) to an existing grain dryer changes the operating characteristics of that dryer, so the safety approval needs to be re-done in the field. It is illegal to operate an unapproved dryer.
TSSA is responsible for all field approvals in Ontario. Contact TSSA (1-877-682-8772) for details. Information is available on the TSSA website.
What Does Field Approval Involve?
The dryer operator needs to submit the gas piping diagram, electrical schematics, purge calculations and other relevant information such as drawings and the user manual to TSSA. TSSA will review the documents and then inspect the dryer to make sure it meets all safety regulations. If the heat recovery system is being installed by the dryer manufacturer, they will coordinate the approval with TSSA.
The field approval process takes about a month and the cost is charged at an hourly rate for every hour TSSA spends on the approval. The total cost may be $1,500-$2,000 depending what TSSA requires.
How to Reduce the Approval Cost?
To keep TSSA costs down, have the dryer manufacturer or an experienced contractor install the system and get the approval. A good contractor knows what TSSA requires and will work to get the approval quickly. The less time TSSA needs, the less it will cost. An operator on their own may not be familiar with TSSA procedures, which means the approval could take longer and cost more.
Budget for a Heat Recovery System
Installing heat recovery increases the cost of the dryer. However, fuel savings over the long term will provide a net benefit. A new grain dryer will operate for 20 years or more. If the extra cost of heat recovery can be paid back within 5 years, this provides 15 years or more of net savings.
Use Table 1 to estimate the savings on a continuous cross-flow dryer with heat recovery. For example, drying 5,000 tonnes of corn annually from 25% to 15% moisture costs $85,000 at today’s fuel prices. Reducing energy use by 30% with heat recovery saves $25,500. Multiply by 5 to get a total 5-year savings of $127,500. Contact the dryer manufacturer to confirm pricing for a heat recovery system and compare that to the 5-year savings, to determine if it is a good investment.
To adjust for a different propane price, multiply the cost by the new price, and divide by $0.50/L. For example, if propane increases to $0.75/L, the savings for drying 5,000 tonnes of corn with 30% heat recovery will be $38,250. [$25,500 x (0.75/0.5) = $38,250]
Reducing grain drying energy saves fuel, reduces emissions and saves money. The following benefits can be achieved:
- The type of grain dryer can make a 30% difference in energy use.
- Dryeration or in-bin cooling improves dryer energy use by up to 30%.
- A heat recovery system reduces fuel costs by 20%-40% without affecting dryer throughput.
- Suction cooling provides some benefits of heat recovery with 15%-20% energy savings.
- Heat recovery can be added to most new or existing dryers.
- Adding heat recovery to an existing dryer requires field approval by TSSA. Contact TSSA for details.
- Estimate the savings over 5 years to determine a budget for installation or upgrades.
This factsheet was revised by James Dyck, P. Eng., Engineering Specialist, Crop Systems and Environment, Vineland, OMAFRA and reviewed by Steve Clarke, P. Eng., Engineering Specialist, Energy and Crop System, Kemptville, OMAFRA and Chevonne Carlow, PhD, Greenhouse Floriculture Specialist, Vineland, OMAFRA.