The flow control valve is NOT installed in the exhaust for two reasons. 1. Air from the exhaust is filled with residues and corrosive gases that will affect the medium to long term performance and service life of the valve.  This means you must remove and clean the valve to prevent sluggish operation and poor width control. 2. Because the exhaust air temperature is much hotter than the supply, the density of the air is much lower.  This reduces the control effect of the flow control valve.  Because the exhaust temperature is dependent on process variables including melt temperature, material thickness and cooling temperature, it is more difficult to tune systems to provide a similar performance when the valve is mounted in the exhaust.