How to control the exhaust air rate of the dryer?

To maintain the sufficient process conditions and to optimize the energy efficiency of the dryer section the exhaust air rate needs to be adjusted. By keeping the exhaust air rate on the optimal level, the heat recovery and air system operation will be maximized, and dryer’s specific energy consumption is minimized.

Exhaust air rate to be based on the evaporation

Exhaust air is used to carry out the evaporated water from the dryer. As shown, the surrounding process machinery (like hood) typically defines the maximum humidity for the exhaust air. This information can be used for the exhaust air rate control and simple principle is:

  • Measure the humidity of the exhaust air
  • If humidity is too high increase the exhaust air flow rate
  • If humidity is too low decrease the exhaust air flow rate

However, this control strategy can lead in some air system configurations to the situation where ventilation will decrease to too low level on some dryer areas. One possible alternative is to control the flow rates based on the production or the for example the steam group pressures, because they indicate quite exactly how intense the evaporation inside the dryer is.

Mechanical solution

In practice the exhaust air flow rate can be controlled with frequency converter or with the guide vanes. How the control is done has to be taken into the consideration when selecting the fan to enable sufficient operation window. With the right selected fan and the frequency converter operated fan motor, the control can be done in energy efficient way. The humidity measurement devices are nowadays very reliable, but they must be calibrated regularly to maintain the reliable operation.


Exhaust air rate of the dryer section is valuable parameter to be optimized to improve the dryer’s energy efficiency and to improve the heat recovery system efficiency, too. To enable the control of the air system it must have controllable air flow rates and the measurement of the humidity level. Another alternative is to create the production-based system which controls the exhaust air rate based on the production value or main heating system capacity.


Kalle Riihimäki

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