Reclaiming Corn Drying Energy

Agdex#:	111/736
Publication Date:	04/83
Order#:	88-003
Last Reviewed:	01/88
History:	Revision of Factsheet "Reclaiming Corn Drying Energy", April 1983
Written by:	H. Spieser - Engineer (Pesticide Application & Grain Storage)/OMAF

Table of Contents

1. Introduction
2. Reclaiming Drying Energy
3. Heat Recirculation
4. Heat Recirculation Design
5. Heat Exchanger
6. Heat Exchanger Design
7. Summary
8. References
   

Introduction

High-temperature drying of grain corn is fast and effective but uses a lot of energy. When heating fuel energy, such as propane or natural gas was cheap, the initial cost of the drying system was more important than the operating cost. That is not true today.

With many conventional cross-flow dryers as much as 40 litres (L) of propane are required in order to produce a dry tonne (t) of corn at 15.5% m.c. from 30% m.c. (wet basis) as harvested. Thus Ontario corn producers use, directly or indirectly, the equivalent of 120 million L of propane each year to dry grain corn. This amount can be reduced by at least 20% or the equivalent of 24 million L of propane per year.

Most dryer operators are willing to spend some money if they can get it returned in three years or less. The allowable expenditure is highly dependent on the annual volume of corn dried, which generates the fuel saving as dollars to pay for the system changes.

Table 1 shows the range of fuel use reductions possible and the allowable capital expenditure for recovery in one year for a range of annual drying fuel costs. The fuel cost can be for propane, natural gas or other heating fuel.

Figure 1. High-temperature drying of grain corn requires large volumes of propane or natural gas.

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Reclaiming Drying Energy

The methods of reclaiming heat energy fall into two distinct categories; (a) heat recirculation and (b) the use of heat exchangers.

Heat recirculation works best on continuous flow dryers operated to dry and cool simultaneously.

Heat exchangers, however, can be used on either continuous-flow or batch type high-temperature dryers.

All further discussion and comment will be directed to the common thin-column dryers of the cross-flow design. The number of columns can vary from one to four, and the column while most often vertical can also be horizontal on an open mesh belt.

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Heat Recirculation

Ambient (outside) air passed through a column (or layer) of hot corn will pick up heat and be warmed. While some moisture may be added, the resultant relative humidity (RH) of the ambient air will be reduced significantly as the corn warms it during cooling. The warmed cooling air is typically 30 to 40°C over ambient air temperature with relative humidities in the 10 to 20% range.

The warmed cooling air alone can be directed to the blower/burner (drying fan) for fuel reductions of 15 to 20% (Figure 2).

However, the warmed cooling air only makes up one-half to two-thirds of the airflow requirements of the drying fan. Thus we can also consider recirculating air from the lower portion of the drying section. Air coming through the lower section will be 40 to 50°C above ambient air temperature. This air has done some drying, but will not be near saturation. RH values in it are more typically in the 20 to 30% range. Thus it can be collected with the warmed cooling air to provide the total airflow requirements of the drying fan (Figure 3).

Figure 2. Recirculation of warmed cooling air only from continuous-flow dryers can reduce energy by 16% or more.

Figure 3. Approximate discharge air temperature and relative humidity profiles from continuous flow dryer with ambient air cooling.

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Allowing for some radiated heat loss the blended recirculation air from the cooling and lower portion of the drying section should be available to the drying unit at temperatures well above ambient temperature and with an RH under 40%.

For example if we assume a drying plenum temperature of 104°C (220°F) and an ambient temperature of 10°C (50°F), we are likely to have a recirculation air temperature of about 40°C.

By ratioing the relative temperature rises we can approximate the fuel saving benefit as follows:

(Recirculated Air Temp - Ambient Air Temp) ÷ (Plenum Air Temp - Ambient Air Temp) x 100 = % Fuel Saving

For the example conditions the apparent benefit from recirculation would be about:

(40-10) ÷ (104-10) x 100 = 32%

That savings level is not an unrealistic target and should be attainable, especially at lower ambient air temperatures.

One of the easiest pitfalls is to separate the drying discharge air at too high a level. The higher up the drying screens, the lower the discharge air temperature and the higher the RH of that air. When kernel moisture is given up readily as it is in the upper sections of the drying column, significant heat is taken from the air to evaporate the moisture--reducing air temperature and increasing RH dramatically (Figure 3).

Most commercial cross-flow farm dryers have a cooling section that is one-quarter to one-third the total column height. Typically the cooling section airflow rate is higher than the drying section airflow rate. Thus, locating the air discharge divider above the dryer screen mid-point should be discouraged in most applications. Otherwise some of the lower quality recirculation air will have to be discharged with no benefit at all.

Four column dryers present a design challenge but not an insurmountable one. The discharge air space between the central columns can be horizontally partitioned and the air from the lower section directed around the back of the dryer.

Dryers with only a single fan for drying and cooling or dryers with more than two fans require special consideration as do two-fan dryers operated in the all-heat model.

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Heat Recirculation Design

1. For heating/cooling two-fan dryers, recirculate air from the cooling section and lower portion of the drying section to the drying fan (Figure 4). Generally this means locating the discharge divider at or slightly above mid screen height.

Figure 4. A schematic of a complete recirculation system on a continuous-flow dryer using ambient air cooling to provide energy savings and cooled dry corn.

2. Do insure that the upper nearly saturated drying air is diverted away from the recirculation system.
3. Do provide clean ambient air to the cooling fan to insure cool corn and maximum benefit from the corn heat. The air duct to the cooling fan should have at least twice the cross sectional area of the cooling fan to minimize pressure losses.
   With single fan dryers a compromise is inevitable. Warm recirculated air will not allow full cooling. Further cooling in storage with relatively high airflow rates is essential.
4. If the dryer, single or two fan type, is operated as an all-heat dryer the divider should be lowered to between one quarter and one third of total screen height and ambient air blended in to maintain dryer throughput (output) rates. Alternately a heat exchanger should be considered.
5. For three-fan dryers operated in the dry/cool mode, cascading discharge air upward is likely to be the most productive. Assuming all fans are of equal capacity, two recaptures are possible.
6. Commercially available recirculation equipment is optional for some makes/models of dryers. If available it should be considered. If not the following guidelines should be of assistance in designing a recirculation system that can also provide weather protection.
7. Construct an enclosure that is wide enough to facilitate easy movement beside the dryer for maintenance such as screen cleaning. A walkway one metre (3') on either side is suggested.
8. The enclosure side roof can abut the dryer at cut-off level if upper dryer protection is not required
   (Figure 4).
9. Extend the enclosure full width at least three metres (10') in front of the dryer to provide settling chambers for "reddog" etc. on either side of the duct bringing fresh ambient air straight into the lower cooling fan.
10. The extended enclosure need only be high enough to incorporate the drying fan. Blocking must be included to insure the forward part of the enclosure is relatively air tight to prevent saturated drying air from entering, while drawing make up air from a relatively open rear section - especially if a 4-column dryer is in place. Half of its recirculated warm air must be directed around the rear of the columns into the enclosure.
11. deally the enclosure should be insulated to prevent condensation and reduce radiant heat loss. However, condensation should be minimal if the cut-off is at the correct height and the lost heat is not likely to provide capital recovery at present (1987) fuel prices. 
    

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Heat Exchanger

Heat exchangers can permit heat reclaiming even from near saturated drying air. Heat only is transferred through a waterproof material (divider) from the hot exhaust air to the cooler incoming air. When the moisture in the hot exhaust air condenses on the cool divider, the latent, or wet heat, is also available as sensible, or dry heat, to warm the incoming cold air.

Heat exchangers can be applied to continuous flow or batch type dryers. They provide the best alternate for reclaiming heat when drying and cooling are not occurring at the same time. The heat exchanger should be considered when continuous flow dryers are operated in the all heat mode.

A constant drying plenum temperature will provide a near constant exhaust air temperature from a continuous flow dryer and an exhaust temperature that increases with time for a batch dryer.

The colder the outside (ambient) air temperature the greater the benefits will be. For a thin-column, continuous -flow dryer operating at 94°C plenum temperature the average discharge air temperature will be about 40°C. Thus ambient air coming in at 20°C will receive only a small temperature increase. Air coming in at 0°C will receive a significant temperature increase and resultant fuel reductions of about 20%. Therefore the decision to use a heat exchanger is highly dependent on projected outside air temperatures.

The cost of a heat exchanger will be significantly higher than for a recirculation system. The reason is two fold: One a heat exchanger system is more complex and; Two, the ducting from the dryer and over the heat exchanger must be insulated to prevent significant condensation problems and associated deterioration.

Values in Table 1 are also valid for the heat exchanger cost effectiveness.

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Heat Exchanger Design

While the heat exchanger system can take on many configurations, a few points are offered here to avoid some potential design errors.

1. Heat exchangers should only be considered for high temperature dryers operating at outside temperature likely to average 10°C or less.
2. Air intake velocities must be in the range of 4 to 5 meters/second (m/s) to eliminate dead air (insulating) layers next to dividers. Exhaust air velocities are not as critical and can be much lower. 
3. Total air intake length should be at least 15m to provide a minimum air retention time of 3 seconds. The intake air stream can be serpentined, but should be counter flow in the last pass for maximum temperature difference and heat transfer. 
4. Spiral corrugated galvanized tubing to divide the air streams provides a desirable turbulence in the high velocity intake air with minimal friction (head) losses.
   If tubing is used it should not exceed 100 mm in diameter to maximize surface contact area for exhaust and intake air.
5. A stacked tube arrangement is self-cleaning as the condensate washes the particulate matter (reddog) from the outer surface of the tubes. Provision must be made to remove the collected, wet particulate matter at the end of the drying season.
6. Directional changes of high velocity intake air must be made with long smooth curves to minimize air friction losses and to insure uniform flow in all tubes or air channels.
7. Provision must be made to collect and direct the condensate away from the drying system.
8. A remote air-intake will minimize the pickup of particulate matter. However, the "spent" exhaust air, which will be fully saturated, must be directed away from the intake to prevent recycling.
9. Located heat exchangers at the fan (front) end or at the side of dryers. They will create serious drip problems if located directly over the dryer.

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Figure 5. A complete recirculation system on a continuous-flow dryer using ambient air cooling.

Figure 6. Heat exchangers can be located at the intake end or side of dryers - but never above.

Figure 7. Spiral corrugated galvanized tubes in a stacked arrangement performed well. Not the smooth double radius air turner below.

Figure 8. Access must be available to remove "reddog" washed down by condensate.

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Summary

1. The amount of heat energy (fuel) for high temperature, thin column dryers can be reduced by 20 to 40% without affecting dryer throughout.
2. The allowable capital expenditure will vary with the potential annual fuel savings and the time period considered acceptable for capital recovery.
3. For continuous-flow, thin-column dryers incorporating cooling, recirculate all the air from the cooling section and the lower one-sixth to one-quarter of the drying section. A divider at screen mid-height will normally provide total airflow requirements for the drying fan.
4. For all-heat continuous flow dryers recirculate only the lower one-quarter of the drying air and blend in ambient air or consider a heat exchanger.
5. Make sure recirculation ducting is large enough to minimize pressure losses.
6. Provide remote air intake, with adequate duct size to cooling fan to insure full cooling of dried grain. If corn is removed hot from the dryer insure that adequate cooling equipment is available in tempering/storage bins.
7. When designing a recirculating enclosure make provision for one or more swirl chambers to collect 'reddog' thus preventing accumulations in plenums. 

References

1. Winfield, R.G., and J. Hart, 1983. Reclaiming heat energy for grain corn drying. A report for the Agricultural Energy Centre, Ontario Ministry of Agriculture and Food, Toronto, Ontario M7A 2B2, March, pp. 41.
2. Spieser, H., and G. Vanderwyst, 1986. Economics of Heat Recovery on a Bell Camp Dryer. February. 
3. Spieser H., and N. Cook, 1986. Economics of a plate-type heat exchanger on an M & W gear co. model 250 dryer. July.
4. Spieser H., and L. Goohill, 1986. Economics of recirculating warm air on a continuous flow grain dryer. February.
5. Spieser H., 1983. Economics of heat recovery on two behlen model 380 grain dryers. December.
6. Spieser H., and S. Willemse, 1984. Economics of a heat exchanger on a continuous-flow grain dryer. February.

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