Paper Technology International 2020 - Journal - Page 41

PAPERTECHNOLOGYINTERNATIONAL
Paper machine drying hood and heat recovery improvements
Runtech analyzes the current state of the
paper machine’s hall and dryer hood ventilation to
gain insight on how to improve system ventilation
and to enhance the machine’s ambient and working
climate. The findings are also used to reduce mold
and corrosion.
The measurements include air flows and
leakages, humidity levels and temperature profiles.
Based on the findings, we provide recommendations
and solutions, ranging from new closed dryer hoods,
heat recovery systems to dryer section exhaust
and supply air systems, steam batteries, pocket
ventilation, web stabilizers or full dryer section
modifications. The final solutions are always tailored
to customer-specific needs.
Machine hall ventilation audit
The general purpose of hall ventilation is
to prevent moisture from damaging the building or
process equipment as well as to remove exhaust
heat, moisture loads and airborne impurities. During
the winter season, the building is heated with the
machine hall ventilation to prevent any damage to
buildings and equipment caused by condensation
and freezing. The hall ventilation also enables a
healthy and comfortable working climate for the
operators.
A typical way to ventilate the machine hall is
to supply clean air to the working zones at low speed.
The supply air ventilates working zones without a
draught. Due to the thermal loads from the process
and its equipment, the supply air temperature
increases, rising up toward the machine hall ceiling
from where it is released. Temperature, humidity and
concentration of impurities are always higher at the
upper parts of the machine hall than the lower parts.
The audit focuses on air and water balance
measurements, machine hall temperature and
humidity levels, misting around the paper machine’s
wet end, hood air and water balance calculations,
hood zero level along with the water and humidity
load to the machine hall. Fans, hoods, ducts, valves,
hatches, heat recovery systems and auxiliary
equipment are all visually inspected.
Machine hall humidity levels
Air humidity (x, water content) inside the machine hall compared to the supply air humidity gives
a quick and clear picture about how the machine hall ventilation is
functioning.
When the ventilation system is correctly designed and functioning properly, humidity in the working zones should not be more
than 5 gH2O/kg of dry air above the supply air humidity.
To minimize the risk of condensation in the upper parts of
the machine hall and ceiling constructions, the air humidity should
be less than 25 gH2O/kg of dry air above the supply air humidity.
However, the value depends heavily on the heat insulation capability
of the construction as well as the temperatures both inside and
outside the machine hall.
Figure 6: During machine hall ventilation audit air humidity is
measured in the paper machine hall.
High humidity loads, leakages and cold bridges cause
condensation to the machine hall. Due to the condensation, metal
frames corrode, concrete blemishes and bricks darken, among other
things.
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