How much increased water content in dryer section exhaust air will improve the heat recovery efficiency?

Generally, exhaust air from the dryer section has temperature around 85 °C and humidity is between 140 – 180 g H2O /kg dry air meaning dew point to be 58,5 °C – 62,9 °C. Standard recovery system recovers energy to the following target flows:

1. Supply air to dryer section (Target temperature after heat recovery +60…+65 °C)
2. Supply air to machine room (Target temperature after heat recovery +20…+25 °C)
3. Fresh water to process (Target temperature after heat recovery +40…+55 °C)
4. Circulation water to process (Target temperature after heat recovery +45…+55 °C)
5. Circulation water for machine room heating (Target temperature after heat recovery +40…+55 °C)

For these target flows the higher humidity level has great impact especially on items 2 – 5 because on these heat recovery stages the latent heat from condensation plays the most remarkable role.

Example: Effect of exhaust air humidity level for circulation water heating

Circulation water coming to the heat recovery has temperature around 30 °C and target is to heat it close to the process temperature – for example +55 °C. We assume that we have heat exchanger dimensioned for the following process conditions:

• Exhaust air:
  • mass flow: 20 kg/s dry air
  • water content: 160 g H₂O/kg dry air
  • temperature in: +70 °C
• Process water:
  • mass flow: 15 kg/s
  • temperature: +30 °C à +55 °C
  • Duty: 1 568 kW

If everything else is kept constant, but exhaust air humidity decreases to 120 g H₂O/kg dry air the duty will decrease to 1 227 kW and water temperature out from heat recovery to 50,0 °C. Duty of heat recovery is decreased by -22%

Example: Effect of exhaust air humidity level for fresh water heating

In this example we assume that incoming water temperature is 10 °C and target is to heat it close to the process temperature – for example +55 °C. We assume that we have heat exchanger dimensioned for the following process conditions:

• Exhaust air:
  • mass flow: 20 kg/s dry air
  • water content: 160 g H₂O/kg dry air
  • temperature in: +70 °C
• Process water:
  • mass flow: 15 kg/s
  • temperature: +5 °C à +55 °C
  • Duty: 3 135 kW

If everything else is kept constant, but exhaust air humidity decreases to 120 g H₂O/kg dry air the duty will decrease to 2 684 kW and water temperature out from heat recovery to 47,8°C. Duty of heat recovery is decreased by -14%

 Summary

Exhaust air humidity plays remarkable role when maintaining good energy efficiency in the drying process. It affects both specific heat consumption and efficiency of heat recovery. If humidity level needs to be decreased due to some temporary limitations on the process the available energy saving potential should give enough motivation to bring the process conditions back to the dimensioning level as soon as exceptional process situation has been overtaken.