Fundamentals of drying aquafeeds How manufacturers can excel in a competitive market by Aaron L Norris, President of Norris Thermal Technologies Inc., USA
s the aquafeed markets grow worldwide, so does competition for manufacturing quality aquafeed. Regarding quality, the formulation, pelleting, and extrusion of aquafeed receive a great amount of attention. In addition, a good aquafeed dryer has a significant impact on the feed quality and a tremendous effect on the cost to produce. A well-designed dryer can enhance your production and market presence. This makes the understanding of the drying fundamentals vital to your overall success. Knowing why it works is basic to understanding how it works. When you properly deliver the correct amount of energy and time within the dryer, drying of your aquafeed will occur efficiently. These elements can be broken down into two parts; energy and time.
Energy; Air and BTU’s
Energy within the dryer is also made of two parts, airflow and BTU’s. Air is the vehicle that carries the moisture away from the product and out of the dryer. Without such a vehicle no drying can occur. Most drying problems can be summed up into two words, “poor airflow”. The amount of airflow through the product will be the determining factor in how much work (drying) is done. It is like pumping water; the size and speed of the stream of water determines how many gallons of water are pumped. Consider how much airflow is needed to remove the necessary amount of water from the product. The cubic feet per minute (cfm) of air needed depend upon the required BTU (British Thermal Unit). The required BTU depends upon the amount of water to be removed at a given volume of product. A quick rule is the “factor of 18”. For example, 10,000 cfm x .018 = 180. 180 x 170°f (heat rise) = 30,600. 30,600 x 60 minutes = 1,836,000 BTU per hour. If we know that it takes 1,500 BTU to remove one pound of water, 1,836,000 BTU will remove 1,224 pounds of water per hour. All the air must contact the product to be dried. Since the air is what accomplishes the job, the more air contacting the product, the more work will get done. This is regardless of whether the
job entails drying and/or cooling. However, there is a limitation to the velocity of air that can be used. This will vary in different styles of dryers and different product densities. Too high of an air velocity in conveyor dryers will lift the product off of the conveyor. This will create a hole in the product bed causing no work to be accomplished. Air will always take the path of least resistance. In this case, the air will travel through the hole and the capacity and efficiency of the dryer will suffer. Airflow is important to product quality. Too low of an air velocity through the product may allow the product to clump and stick. This is a major cause of inefficiency and moisture variation. This is especially true with wet extrusions. A constant, consistent, and sufficient air velocity should be maintained in all stages of drying. If insufficient airflow is used, the job will not be accomplished; it takes certain airflow with the correct amount of heat to carry the desired amount of moisture away and produce the best product quality. Each cubic foot of air can only hold so much water. These laws cannot be by-passed. Air is critical, and in fact “poor airflow” is typically the main problem with underperforming dryers. Providing the right amount of air in the right place will accomplish the job.
BTU’s is the second part of energy. BTU, by definition, is what it takes to raise one pound of water, one degree Fahrenheit. It is the energy that drives the moisture away. Ambient air and the product to be dried always contain some residual BTU. The warmer the ambient air and product the more BTU it contains. The BTU value in some products is considerable and should be taken into account, while in other products the value may be irrelevant. In perfect conditions, which are not achievable, it takes roughly 1,020 BTU’s to evaporate one pound of water. When calculating losses, such as combustion and convection, it takes a minimum of 1,300 BTU to evaporate one pound of water. Many processes with efficient dryers budget 1,500 BTU to evaporate one pound of water. All products release moisture at different rates. Understanding this is basic to understanding the required BTU needed. A product with many internal exposures to airflow releases moisture quicker than product with solid surfaces. The distance the moisture must
22 | February 2017 - International Aquafeed