Is FL421XV01 the regulating valve that comes with the fan inlet? Correct. The supply of the fan U-421 includes the valve.Why the U421 outlet pipe diameter becomes smaller? What is the pipe diameter? Measured as DN250 on the graph. There is no instrument installed here on the PID diagram. Please see the attached sketch. It changes from rectangular shape (370 x 240 mm) to DN450 (silencer plus duct). It’s also necessary to provide a sampling point (coupling 2” plus plug).What is the function of the flange hole in the U421 outlet? Is there a muffler here? The diameter measured on the picture is DN300. Correct. It’s necessary to install a silencer DN450.Our plan is to change the U421 outlet duct to square and round, and then the D300 duct will exit the roof of the plant. Is this okay? If not, please come up with a detailed design plan as soon as possible. Please refer to the attached sletch.