How much exhaust air is needed for a certain evaporation?

Dryer exhaust air flow rate is one of the ways to maintain constant process conditions. However, due to the different boundary conditions set by process or machinery the flow rate can differ remarkably from the design point.

Dimensioning of air system for dryer starts from dimensioning of the exhaust air flow rate (qm,exhaust). To evaluate the exhaust air flow rate three parameters are needed:

  • Evaporation to be evacuated from the dryer [qm,evap]
  • Water content for exhaust air [xexhaut]
  • Water content for supply air [xsupply]

With these values the exhaust air flow rate can be calculated as follows:

Exhaust air flow rate calculation

Example dimensioning

We can calculate a specific exhaust air consumption which will indicate how much we need dry air to evacuate one unit of evaporated water. We assume that we have a standard paper machine dryer section where exhaust air humidity is 160 g H2O / kg dry air (dew point 60,9 °C). Supply air is assumed to be drawn from the machine room → Humidity is estimated to be 18 g H2O / kg dry air. With these figures we get

Exhaust air consumption example calculation

Result means that to carry one kilogram of evaporated water out from the dryer section we need to evacuate 7.04 kg dry air for that purpose.

Sensitivity for exhaust air humidity

If we assume that exhaust air humidity is varied due to the process conditions the specific exhaust air rate to evacuate 1 kg evaporated water varies according to the following chart. Supply air humidity is assumed to remain constant 18 gH2O / kg dry air.

Example: Operator is forced to operate dryer section with exhaust air humidity 130 g/kg. How much additional exhaust air needs to be evacuated compared to dimensioning point 160 g/kg?

Calculation: In the dimensioning point the exhaust air consumption is 7.04 kg/kg and in the new process point (130 g/kg) 8.93 kg/kg. Exhaust air flow has to be 8.93/7.04 = 1.27 bigger compared to the dimensioning point.

Sensitivity for supply air humidity

Similar analysis can be done for the supply air water content as shown in figure below. In this figure exhaust air humidity is assumed to be constant.


Increase in exhaust air humidity and decrease in supply air humidity decrease the exhaust air consumption. Smaller exhaust air rate gives several benefits for overall energy efficiency, but maximum value for the humidity is set by the process and machinery around the dryer. If humidity level is increased too much it will limit drying and also too high humidity will cause condensation and other problems on the dryer.

However, for overall efficiency it is important to operate process close to the dimensioning values so that all related equipment (fans, heat exchangers, steam coils etc.) are operated close to their dimensioning values.

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