Note that dryer is divided into two temperature and airflow zones lengthwise.  Each pass is a separate zone.  This gives a total of four independent zones for the dryer shown.

(Click drawing to enlarge)

(Click drawing to enlarge)

Note that the composite temperature curve shows that higher air temperatures are possible at the beginning of the drying process without resulting product temperatures that would damage a heat sensitive product.

(Click chart to enlarge)

30 years ago most conveyor dryers were single pass. This design took advantage of the drying curve by permitting different temperature zones to be used for succeeding portions of the drying process. During the constant-rate period, the dryer could be operated at relatively high temperatures and air velocities, reducing the drying time and thus the size of the dryer. The remainder of the dryer would be operated at lower temperatures and velocities to avoid product damage.

In the early 1970's Wenger introduced the two pass dryer to the petfood and aquatic feed industries. Since the air passed successively through both passes (the wettest product used the same air as the driest), it was no longer possible to use different temperature zones to fit the drying curve. However, the savings in dryer size due to shorter outside length and because the bottom pass could use deeper bed depths, more than offset this disadvantage. Single pass dryers in this industry became rare.

The Wenger TrueTemp® design permits floors between the conveyors. By ducting alternate top plenum sections (fans and heating devices) to separate top and bottom passes, it is possible for each pass to have temperatures and air flows completely isolated from each other.

Wenger has used this concept for years in snack pellet dryers. It has recently been shown to offer significant advantages in drying salmon feeds. It is known that salmon feeds suffer a loss of pigment if the product temperature rises above a value in the range of 70 - 80 °C, thus dryer temperatures must remain below 90 °C, resulting in larger dryer sizes. By having the product pass through successive temperature zones that are independent, the advantages of the old single pass dryer can be had without the disadvantages.

Please refer to the diagrams at left.. They correspond to a two pass, four section long dryer. The dryer is divided into two zones lengthwise as well as each conveyor pass being a separate zone, giving a total of 4 temperature and airflow zones. Please note as well that the direction of airflow reverses up to down and right to left as the product travels through the dryer. This is essential in providing uniform moisture content in the final product.

For salmon feeds the incoming moisture is in the range of 20 - 26%, depending on how it is made. Most commonly, it is dried to 6-9%. The critical moisture content will be around 18 - 20% moisture content (wet basis), depending on formulation.

In the four zone dryer shown, the critical moisture will be reached somewhere in the second zone. Therefore, zone 1 can be operated at as high a temperature as possible with no fear of damaging the product. Up to 150 °C is practical in a gas dryer. High air velocities may also be used to speed drying. At typical recirculated humidities, the surface temperature of the product will be about 40 - 45 °C.

Although the critical moisture will be reached in zone 2, there will still be enough evaporative cooling that the rising product temperature will not yet reach the air temperature. Up to 120 °C is practical in zone 2. Moderately high air velocities are still useful.

In zones 3 and 4 the air temperature should not exceed about 90 °C since the product temperature will begin to approach 70 °C. Air velocities here can be much lower for improved energy efficiency without affecting the drying rate.