Conventional tray type dryer are available in single pass, double pass, and triple-pass conveyor systems. The most popular dryer design used by the pet food industry is the double pass unit. That design allows more product to be dried per square foot from belt area than does the single pass unit. That is primarily because the moisture content of the product entering the dryer is between 23 and 26% and is usually held at a bed depth of 10 centimeter (4 in) on the conveyor. In a double pass dryer, the product is tumbled after it has been partially dried; then, by operating the lower conveyor at a slower belt speed, the product can be 15 to 20 centimeters (6 to 8 in) deep on the final conveyor. The tumbling of the product from the first conveyor allows a quick release of approximately 3% moisture because of the repositioning of the particles in on the tray or belt.