2. Countercurrent hot-air dryers

The variables are set up in figure 6 for a countercurrent dryer. In this case, the moist product P ₁ enters on the side where the air A ₂ exits, colder and wetter than when it entered. Here, air A ₂ exits facing the wettest incoming product (a _ws ≍ 1), so it can exit close to saturation, unlike in the cocurrent case. This explains why better use is made of the energy Q ₁ supplied to the dryer to heat the air in countercurrent than in cocurrent (for the same value of Q received by the product in both cases).