One common source of dryer inefficiency is inconsistencies in output moisture throughout the product bed. Have you ever measured this? If moisture variation is significant - I1% or even I0.5% - from side to side and top to bottom, it can lead to poor product shelf life, lost production, poor product palatability and mold problems.
For example, assume you are overdrying your product by approximately 2% to provide a safety factor for out-of-spec goods. Based on a 10,000 lb/hr production rate, you must use an additional 200 lb/hr of dry material to replace the water lost in overdrying. This translates into the need for an additional 1.2 million lb/hr of dry material per year (20 hr/day times 300 production days). Now, multiply this by your raw material costs per pound, and it quickly becomes clear how small inconsistencies can add up to big dollars. Follow these 10 tips to prevent conveyor dryer problems.
Tip 1: ELIMINATE AIRFLOW SHORT-CIRCUITS
Heated process air will find the path of least resistance. If process air has the chance to bypass the product bed rather than passing through it, it will (figure 1). The result is reduced capacity, increased energy usage and re-duced moisture consistency.
Tip 2: PROVIDE PROPER EXHAUST
If the exhaust fan is not removing enough moisture-laden air from the dryer, your dryer will experience inefficient energy usage, reduced processing capacity or inconsistent product quality. Moisture-laden air must be removed to maintain optimal recirculation humidity to carry out efficient drying. Conversely, an excessive exhaust situation does not allow the process air to reach its full water-removal potential.
Tip 3: IMPROVE DISTRIBUTION OF RECIRCULATED AIR
If the dryer system setup is not promoting even distribution of recirculated air across the length and width of the dryer, you will have inconsistent product quality, moisture content and temperature.
Tip 4: PROPERLY MIX HEATED PROCESS AIR WITH RECIRCULATED AIR
Strategic areas for mixing must be present to ensure that the 3,000 degrees F (1,649 degrees C) flame is properly mixed with the recirculated air before it passes through the product bed. If these areas are not present, reduced capacity, inconsistent product quality and burnt product may be the result.
Tip 5: ENSURE THE DRYER HOLDS DRYING TEMPERATURE SETPOINTS
If the existing heat source is not functioning properly or is not providing sufficient heat to meet current production demands, dryer production capacity is reduced.
Tip 6: PROVIDE GOOD BED LOADING
Poor product distribution across the width of the dryer can cause overdrying of the shallow areas and rob production capacity. This also can create an airflow short-circuit. The result of poor bed loading is inconsistent product quality.
Tip 7: KEEP BEDPLATE OR APRON PERFORATIONS CLEAR
Clogged or blinded bed perforations restrict airflow, which causes reduced and uneven airflow through the product.
Tip 8: ELIMINATE PRODUCT CLUMPING OR CLUSTERING
Recirculation air dryers rely on maximum product surface area exposed to the process air to promote efficient and consistent drying. When products cluster together, the available surface area is limited and moisture is difficult to remove.
Tip 9: SELECT A SANITARY DESIGN
Ledges, crevices and traps provide product-accumulation sites within a dryer. This can lead to cross-contamination from one product run to another, create a site for microbe growth or create a fire hazard. Access is important - if it cannot be seen, it will not be cleaned.
Tip 10: HAVE A WORKING KNOWLEDGE OF THE DRYER'S BASIC OPERATION
Most operational problems and overall lack of success in efficiently operating a dryer revolve around the lack of basic knowledge of the operation and functionality of the equipment.
With the drying phase of most products representing a significant portion of a company's total energy bill, better focus on these 10 tips could pay dividends for your company.
SIDEBAR: Combating Dryer ProblemsTo fight typical conveyor dryer problems, understand your dryer's airflow. Where does air enter the dryer? How is air exhausted from the dryer? The following tips can help ensure problem-free dryer operation.
- Concentrate on excellent bed loading. Good dryer performance begins with good conveyor bed loading.
- Spend approximately $200 on a good hand-held, dual-input digital temperature meter to allow you to measure wet and dry bulb temperatures within your drying system. Do you lose dryer productivity during the summer? If so, humidity could be the culprit and an easy solution could be at hand.
- Train your people. Conducting a drying class with your operators, engineering and maintenance people can blend a bit of drying theory with a lot of hands-on troubleshooting guidance.
- If not already in place, develop someone within your company who has a good understanding of basic dryer operation. This person should be comfortable making day-to-day inspections and corrections.
- Monitor the dryer's moisture load. Remember that your dryer is affected by how many tons of water it removes vs. how many tons of product it produces.
Convection conveyor dryers are large consumers of production line power and gas. Therefore, small increases in operational efficiency can reflect large savings and increased capacity.