How much energy can be recovered from the dryer section exhaust air flow?

Essential part of energy efficient dryer section operation is a properly dimensioned heat recovery system. In green field projects the design of the heat recovery system is almost the standard, but in rebuild cases there is need for tailor-made design is needed to maximize the benefit of the heat recovery system.

Process conditions for dryer section exhaust air system

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:

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

The higher humidity level in the exhaust air the higher heat recovery rate

Generally the higher humidity level in exhaust air the higher heat recovery rate can be reached. Physics behind this fact is that higher humidity means

  • higher specific enthalpy Higher heat capacity rate I.e. higher energy flow in the stream
  • higher dew-point condensing inside heat recovery occurs on the higher temperature giving higher outlet temperature for the heated flow

Heat recovery during the cold outdoor temperature period

The overall heat recovery rate depends on the climate conditions where the paper machine is located. As an example the following heat recovery performance can be reached in the Scandinavian paper mill during the cold winter day:

Heat recovery in cold temperatures

Heat recovery in cold temperatures

From the figure above can be seen that percentage-wise the heat recovery rates compared to the overall potential are:

  • Supply air 4.9%
  • Process water: 25.3%
  • Machine room ventilation (water glycol): 33.3%

Thus giving total heat recovery rate to be 63.5% from the potential.

Heat recovery during the warm outdoor temperature period

If we update the external conditions are updated to represent the average summer day, the overall heat recovery Sankey looks like shown in figure below.

Heat recovery in warm temperatures

Heat recovery in warm temperatures

Overall heat recovery rate

To evaluate the overall energy efficiency the analysis needs to be discretized into the shorter calculation periods to consider also the changing ambient conditions. This kind of analysis will also give indication about the proper dimensioning point to avoid the under or over dimensioning of the heat recovery systems.

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