Hello!
An OptiX OSN 9800 M24 subrack are divided into two sections: A and B. Every two adjacent sections have an overlap of a board. Four dual-fan conjoined modules are designed for each section to dissipate heat in the section.
The fan tray assembly is located in the upper part of a subrack. It draws external air into the subrack and dissipates heat by blowing air out of the subrack, forming an air duct from bottom to top.
The boards in each section are inserted vertically so that the boards and the air duct are horizontal, ensuring reliable heat dissipation.

If the temperature is too high, check whether there is too much dust on the air filter of the fan. If there is too much dust, clean the air filter periodically.
Thank you!