Adopting counter flow theory, cold air goes inside of cooler from bottom to top, hot pellets
goes to cooler from top to bottom, as time goes on, pellets will pulse at the cooler bottom, cold air will cool
them at the bottom gradually, in this way will reduce pellet broken , if cold air also go to cooler from top,
which same as pellets, then hot pellets will meet with cold air suddenly ,then pellets will be easily broken,
especially pellets surface. In this way can cool fully and equally, pellet broken rate is smaller than 0.2%.