From the perspective of energy utilization, we always hope that the higher the exhaust temperature of the dryer is, the better. It is better to be the same as the inlet temperature. At this time, "waste cooling" is zero. But in fact, this can not be achieved. It is not uncommon that the temperature difference between the inlet and outlet of the dryer is more than 15 ℃. This is because: ① In the process of energy transmission and exchange, there will inevitably be losses; ② The heat transfer coefficient of homogeneous gas with low temperature and small temperature difference is small in the convective heat transfer between walls (especially in the case of forward flow heat transfer); ③ In the process of heat exchange, the temperature drop of saturated hot air flow must be accompanied by phase change. A part of the total cooling capacity absorbed by it must be used to pay for the latent heat of phase change, so the temperature drop of hot air flow is limited. This, in turn, limits the rise in the temperature of the cold air flow (especially in the case of forward flow heat transfer)