A "green" dryer is about more than simply selecting the right unit to buy. Follow these 10 tips to learn how to optimize the operation of your convection dryer while satisfying environmental objectives.
With more companies establishing green initiatives, there is increasing pressure to produce high quality finished products while lowering the overall energy required. A primary opportunity lies in the operation of your convection dryer.
Often, the dryer is the largest consumer of energy within a production line. Making sure it is operating as efficiently as possible, and evaluating energy reduction strategies, will help your dryer go green.
Tip 1: Manage Dryer Moisture Control
A common source of dryer inefficiency is overdrying the product to ensure all of it is at or below the target moisture specification. Product moisture inconsistency generally is a function of the dryer’s design and can be difficult to remedy.
Reducing the amount of overdrying has a two-fold savings if the product is sold by weight: a reduction in energy due to the evaporative load reduction, and a reduction in the amount of solid raw ingredients required in the finished product. For instance, based on a 10,000 lb/hr production rate, a 1 percent increase in finished product moisture results in a 100 lb/hr increase in water in the finished product -- and a 100 lb/hr reduction of dry solids required. Assuming the production line runs 12 hours per day, 300 days per year, this produces a savings of 360,000 lb/yr of dry material and a reduction in required energy.
Reducing the amount of solid raw ingredients needed to produce the same amount of product increases yield and lowers the plant’s carbon footprint. If your existing system cannot deliver consistent product moisture, evaluate if the potential savings justifies replacing the dryer with an improved design that produces a more consistent product.
Tip 2: Maintain Even Airflow
Even airflow is critical to even drying in a convection dryer. Improved moisture uniformity allows the dryer to be operated closer to the target moisture specification. This improves yield and saves energy. In a convection dryer that uses a conveyor for product transport, uneven airflow through the product often can be attributed to poor product loading.
Tip 3: Consider Preheating Makeup Air
Heating makeup air before it enters the dryer generally is the second largest consumer of energy. Consider implementing makeup air strategies to reduce this value. For example, if your drying operation is following by a cooling process, the cooling air exhaust will have a higher temperature than the ambient makeup air entering the dryer (assuming that the cooler is using the same ambient air as the dryer). Energy savings can be realized by using the cooling air exhaust for dryer makeup air. An alternate strategy for makeup air preheating is the use of an air-to-air heat exchanger on the dryer exhaust volume. This will preheat the makeup air for use by the dryer. The cost and return on investment of these strategies should be investigated before implementing. They become more attractive as the amounts of cooling exhaust air and dryer exhaust air increase in both mass and temperature.
Tip 4: Clean the Dryer
Production rate and dryer efficiency can be reduced due to product buildup; therefore, it is important to periodically clean the dryer. In a dryer that uses a nozzle air delivery system, clogged nozzles can reduce airflow. In any convection dryer where steam is the heat source, clogged steam coils will result in reduced airflow. Inefficiencies due to product buildup often result in increased operation temperatures.
Tip 5: Ensure Proper Exhaust Volume
The volume of dryer exhaust air is related directly to the amount of entering makeup air. The goal is to exhaust as little air as possible while still maintaining effective drying. If the evaporative load of the dryer changes often, the addition of a humidity-controlled exhaust system can be a strategy for ensuring proper exhaust volumes. When the exhaust is controlled automatically to an optimal recirculation air humidity, dryer exhaust will be minimized, thus reducing emissions and energy consumption.
For instance, if the exhaust volume is 10,000 acfm during operation and the evaporative load of the dryer is reduced by half, your exhaust volume can be reduced in half. Assuming typical air temperatures and that half of the production hours are at the reduced level, a savings of $9,000/yr could be realized.
Tip 6: Maximize Dryer Airflow
The mass of air passing the product in the dryer is a critical value and is related to the drying rate achieved. Therefore, it is important to have as much air mass as possible pass the product. Often, the dryer is being used for a product other than originally intended, or the original design did not provide enough air mass. In these situations, determine if the mass of air passing the product in the dryer can be increased. This will help increase the dryer’s production rate and improve yield.
Tip 7: Check for Proper Fan Operation
A dryer’s recirculation fans are the lifeblood of the machine, providing the heated air mass to the product. Proper fan rotation and alignment are necessary to ensure that your dryer is circulating the proper amount of air throughout the dryer and your product. Recirculation fans generally use three-phase power.
Sometimes, when a fan motor is replaced, the new motor is wired so that the fan rotates in the wrong direction. Fan rotation should be checked whenever a fan motor is replaced. A backward-inclined fan rotating in the wrong direction will reduce airflow by up to 50 percent. An axial-type fan rotating in the wrong direction will move air in the wrong direction. Reduced airflow requires the dryer to operate at a higher temperature to accomplish the same amount of drying. Also critical to fan performance is the proper alignment of the fan cone or housing. Improper alignment can reduce the fan performance by half.
Tip 8: Consider Increasing Product Drying Time
An area of opportunity for energy savings is to increase the product’s drying time. An increase in drying time results in a reduced operating temperature. This can be accomplished in a conveyor dryer by reducing the conveyor speed and increasing the product depth. Of course, do not exceed the limiting factors for stacking product or you will risk clumping and inadequate drying through the product depth. In dryers that process product in a monolayer, improvements to the product loading by placing the product closer together increase the dryer’s product rate and efficiency.
Tip 9: Optimize Heating System Efficiency
Whether you are using fluid heat exchangers such as steam or hot oil coils or a combustible gas system as the dryer’s heat source, it is important that the system operates as efficiently as possible. Make sure that heat exchange coils are free of debris, which can cause clogs that will reduce the coil’s heat transfer ability. If you are utilizing a steam coil, be sure that condensate traps are working properly. Malfunctioning traps can allow the coil to fill with water, reducing heat output. If the dryer uses a combustible gas system, “tuning” the system should be part of a regular maintenance program to ensure that the burner is set up properly and working as efficiently as possible.
Tip 10: Have Well-Trained Operators
Now that you have read the previous nine tips, you are armed with the necessary information to improve your dryer’s efficiency. There is just one problem: you are not the employee that operates the dryer on a daily basis. Having well-trained operators who understand the impact of any changes they may make on the dryer’s efficiency is imperative. Talk with the operators about the changes they make and why, and help them understand how the changes affect the dryer’s overall efficiency.
As Kermit the Frog reminds us, “It ain’t easy being green.” Following these tips should allow your dryer to go “green” by increasing the dryer’s efficiency.
This article originally was published with the title "10 Tips: Get Lean, Clean and Green" in the September 2009 issue of Process Heating.