Operational Procedures for High Capacity Dryers

Edição XIV | 06 - Nov . 2010

Adair Luiz Sulzbacher - alsulzbacher@uol.com.br | Francisco Amaral Villela - francisco.villela@ufpel.edu.br | Victor Goltz - victorgoltz@bol.com.br

    The operation of grains and seeds drying is an essential step in receiving agricultural products and their correct application ensures lower qualitative losses during the stages of processing and storage until the final use, either for seed or for grain purposes.

    Basically, drying is intended to promote the maintenance of seed quality and nutritional and physical attributes of grain used in industry. The reduction of biochemical processes, which promote the deterioration of seeds and grains, can be obtained by reducing the moisture, leading to control of fungal attack and the reduction of the respiratory process. Thus, it is possible to achieve the following benefits:

- Lower dry matter consumption by lowering down respiration, so preserving the amount harvested;

- Provides an harvest anticipation;

- Allows storage for longer periods of time; and

- Reduction of insect infestation.


Drying methods
    Drying of agricultural products may be done in a natural or artificial way, and this can be with low or high temperature. In this work, it will be shown the artificial drying using high temperature (Figure 1).

Drying at high temperature
    Dryers of this type operate with high temperatures of the air flow (between 50-60 °C and 100-120 °C). They can be classified according to two criteria.
    First, as to directions and ways of drying air flow and product mass, the dryers can be of:

-Fixed-bed;
-Crossflow;
-Countercurrent flow;
-Concurrent flow;
-Mixed flows - cascade dryer type.



Drying methods


    Second, as to the operation way, dryers are classified into continuous and intermittent. In continuous, the product needs to pass only once through the dryer to achieve the desired water content. While in the intermittent, the product needs to recirculate more than once in the body of the dryer.

Components of a dryer

1-Silo 

    It is a grain deposit, located on the top of the dryer, designed to offset the volume reduction of the grain mass during drying. It has the function to receive the product and distribute it evenly in the drying tower, preventing that the heated air from the furnace passes directly through the ducts, affecting the operation of drying and causing fire risk. In addition, it serves as homogenization chamber (equalization) in intermittent drying.
    In the silo you will find the level control to indicate the full load of the dryer. Moreover, it is endowed with a platform for maintenance and inspection door, which allows access to the interior.

2- Drying Tower

    It consists on panels and ducts in a “V” inverted form or columns, where the grains move, getting the drying air and losing moisture, in other words, where the drying is processed.
    All capacity for drying may be used or 2 / 3 for drying and 1/3 (bottom) for cooling.
    The thermometry system is installed in the drying tower, which monitors the drying temperature as well as ladders and platforms for cleaning and maintenance.

3 – Discharge

    Comprised of rotating shafts with fins (locks) driven by a motor and reducer, monitored by a frequency inverter, which allows to control the discharge speed in order to change the flow of the product output. Coupled with the discharge is the funnel that directs the product on the discharge conveyor.

4 – Shutdown

    External closure of the dryer that contains the ventilators and from where circulates the drying and cooling air. It has internal partitions that allow the dual use of the drying air.

5 – Ventilator

    The ventilators are usually axial, sized to provide an air flow with output and pressure to achieve adequate drying.

6 – Furnace

Local generation of heat required to heat the drying air which promotes the removal of grain moisture. It can be replaced by gas burners, oil, heat exchangers, etc.

Procedures before the drying beginning
    
Before connecting the equipment, the following safety procedures should be considered:

1 - If the ventilators are unobstructed and running freely;
2 - If the ventilators are with rotors turning in the right direction according to the arrow located on the helix;
3 - If the dryer is clean with no residue or products on ducts;
4 - If the dryer is clean, with no product on the bottom, on shutdown or in the inner part;
5 - If the locks of the discharge are clean, unobstructed and turning in the right direction;
6 - If the trays are locked (locking pin);
7 - If the current discharge are clean / lubricated and properly tensioned (leave the discharge running for a few moments);
8 - If the thermometers are working properly;
9 - If the product level control is operating perfectly;
10 - If controls of air cooling and reuse are in the correct position.


 
Schematic representation of mixed-flow dryer


Specific considerations at the drying beginning 

1 - Continuous Drying

    It is a type of drying, where the product enters humid (maximum 18% wb) and gets out dry (13%) and cold (5-10 º C above the ambient temperature), ready for storage.
    The dryers were designed considering the following parameters:

Exemple for soyabeans
Inicial moisture: 18% w.b.
Temperature of ambient air: 20° C
Air relative humidity: 60%

    They are designed for different products with different moisture or ambient air. These conditions affect markedly the drying capacity of the equipment.

Operations Roadmap

1. Regulate the flow, as directed in the item beginning of the operation;
2. Light the fire during loading of the dryer to speed the drying operation. In the furnace, keep the ashtrays doors open, the power ports of the fuel (firewood) must remain closed and the shutters of mixed air of the dryer open;
3. Place the system flow so that the product that is out of the dryer still humid returns to loading funnel;



Schematic representation of the dryer parts


4. Turn on the loading lift and other equipment that supports the lift in the flow of the wet product;

5. When the product reaches about half of the tower, activate the discharge, in order to avoid formation of holes in the body of the tower;

6. Upon reaching the full capacity of the dryer, the level control will act indicating this condition;

7. Close the registry of the hopper;

8. Turn off the machines that precede the dryer lift to prevent the bushing;

9. Turn on the ventilators;

10. Start drying intermittently, in other words, make the product that leaves the dryer to come back to the load funnel. The operator must collect samples to determine the time that the first part of the product that comes out of the dryer, has already done one full circle and hence is already dry. From this moment, we can start the continuous drying.


OBS: Start the dryer load with a product pre-clean, because the impurities in excess, besides decreasing the dryer yield, influence the internal grain flow and can cause fires.

NOTE: In the first dryer load, part of the product may fall off the tower. The dryer, however, should not be put into operation before doing the cleaning. The product could overheat and even ignite, which could cause fire in the dryer.


2 - Intermittent Drying

    Used for products with moisture above 18% or for those which require drying temperatures milder, such as seeds.
    The dryer should be loaded and the product repeatedly circulated through the body of the dryer until the moisture reaches the desired level for storage.

Operations Roadmap

1. Repeat the roadmap as for continuous drying;

2. Start the drying making the product which comes out of the dryer, to return to the load funnel. The operator, from the first full round, must collect samples to determine the time that the entire mass reaches the desired moisture. Upon reaching the desired level, unload the dryer, sending the dry seed to storage.

OBS: The operation of unloading of dry product and loading of wet product can be repeated up to three times, after that  the dryer should be cleaned.

    Before starting the unloading, disconnect the discharge of the sluices; complete the load of the dryer with wet product until it starts to return; adjust the discharge in the panel with level control; put in continuous operation, entering wet product and leaving it dry; monitor the discharge and when it start coming out moist, stop the unloading and start making the product circulate until it comes out dry.

3 - Drying Whole Column
    The drying in whole column, is employed when all of the dryer tower is subjected with heated air.
    This type is used for products with more than 18% moisture, which requires more than one passage in the dryer. This procedure avoids the cooling of the product and the additional energy to warm it again when returning to the drying tower. It is recommended that in the last passage of the product through the dryer, the records of cold air intake are open to process the cooling. You can also keep the controls closed to increase the capacity of the dryer and cool the product in a silo or warehouse with the use of aeration.

FIRE PREVENTION
    There can be many causes of fire in dryers. So, it is important that the operator has the knowledge to avoid the occurrence.

1 – Impurities

    One of the most common causes is the excessive impurities inside the dryer, that by not moving with the same speed of the grains, will suffer overheating until they combust, causing fire.
    The solution to this problem is periodic cleaning (every three days) of all the entire drying tower, removing straw and other impurities that have been accumulated.

2 - Improper operation of the furnace

    If the operator closes all furnace openings with the aim of reducing the consumption of firewood, it will result in an incomplete combustion and in a reduction of the air flow. It may also pressurizes the system, causing drag sparks, that when finding the hottest grains or straw already overheated may cause a combustion.

NOTE: It is worth to highlight to the operators that drying occurs through a certain volume of heated air, designed for this purpose. If the mixing control, designed with this function is closed, there will not be sufficient air volume for drying and thus decrease the efficiency of drying. In other words, the product has to spend more time in the body of the dryer to remove the same amount of water than if it has been operated properly with a higher volume of heated air. Therefore, closing the vents does not result in saving fuel and also increases the consumption of electricity. The system operates longer than necessary.

High capacity dryer - actual size


3 - Thermometer with Defect

    If the thermometer is not marking the temperature correctly, the furnace operator can raise excessively the temperature of the drying air.
    Before going into operation, you should always check if the thermometer is working properly.

4 - Grains that fall from the tower during the loading

    As has already been explained, it is common to have seeds falling from the tower during loading. If these seeds are not removed before the beginning of the drying operation, it could ignite.
    You should always carry out the cleaning of the dryer, before starting to  operate.

PROCEDURE TO PREVENT FIRE
    The operator must close all vents of the air dryer, because without oxygen there is no combustion. For this, immediately turn off the ventilators, stop feeding the furnace with firewood, close all controls of air dryer and furnace, turn off equipments.
    By stopping the fire, unload the product and rigorously clean the dryer.
    If the outbreak has reached major proportions, stifle the dryer as indicated earlier, but making simultaneously the unloading of the product.

IMPORTANT!
    Never unload the dryer that is with focus of fire, without deleting it. This prevents the formation of empty spaces in the tower, because with the arrival of impurities, you can lose control of the fire creating a situation difficult to control. If necessary, start the unloading and at the same time keeping the load with moist, in other words, unload dry product and load with wet one, simultaneously, remaining the dryer always full.

PROCEDURE IN CASE OF POWER FAILURE
    If the dryer is running, open all the shutters of all openings in the furnace (firewood door, inspection door). Herewith, you avoid overheating.

CLEANING THE DRYER
    The periodic cleaning of the dryer has a fundamental importance. The main points to be considered are:

• Tower of drying: you should clean it periodically (every three days), avoiding that the product and impurities eventually retained, suffer overheating and come into combustion;
• Discharge: it should be cleaned weekly or immediately after drying, in case you note the presence of stones, wood or other objects which might damage the operation of the locks;
• Shutdown: the area of the shutdown should be clean when it contains grains or
waste. Also, you should check and clean the daily internal partition located just below the ventilators;
• Ventilators: periodically check if the ventilators are clean, with no straw or dust buildup.


    The natural or artificial drying of grain or seeds, depends on environmental conditions, method or model of dryer. It constitutes a technically and economically alternative to allow seed and grain production in quantity and quality. Special care in the operation of artificial dryers should be taken into account to ensure getting a quality product and safety procedures, maintaining operational efficiency of the unit, extending the useful life of the equipment.

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