Field Report - Dryer steam and condensate system tips

The purpose of a paper machine steam and condensate system is to control steam pressures for optimum drying, provide stable control over the entire operating range and efficiently remove condensate from inside the dryers. The comments below are a collection of performance tips for keeping your S&C system up & running at maximum capacity.

Dryer cylinders

Dryer can thermal profiles can cause moisture streaks in the sheet. Dryer cans need to be level for equal condensate thickness.

Steam systems

Higher saturation steam pressure = higher can surface temperatures. Can surface temperatures normally increase as sheet moistures drop. Plot can surface temperature and steam temperatures to identify issues with cans. This difference should not exceed 40°F and cans in the same section should be within 5°F of each other. Higher differences indicate dryer drainage issues.

Superheated steam does not contribute to can surface temperature. Too much superheat actually reduces the condensing rate and can cause steam joint problems. Steam should be close to its saturation temperature. This is not a problem with cascading steam systems as flash steam is at its saturation temperature. For noncascading systems though, fresh steam from the desuperheater needs to be controlled to optimize heat transfer and condensing rates. If steam temperature gets too hot, check operation of the desuperheater.

Steam temperatures are normally graduated down the machine and lower in early sections to prevent picking, cockling and sheet release problems from the cans (the sheet wants to stick to hot cans). Valmet has, however, found one machine that operates in the opposite way, running as much as 60 psi in the first dryer section. This is rare though as blowthrough needs to be sent to later drying sections or goes directly to a condenser, the latter of this leading to high condenser loads.

Steam valves to atmosphere or to the condenser should be off under normal running conditions, except for the 1st dryer section blowthrough valve to the condenser.

Condensate systems

Poor condensate removal can cause the ends of dryer cans to be hotter than the center due to more turbulence at the ends. This can cause moisture frown profiles.

Condensate layer is 70x slower at conducting heat as the dryer can shell. The target condensate layer thickness is 3mm.

Condensate cascades at speeds less than 1000 fpm, and rims at higher speeds.

Dryer bars are used to increase turbulence and heat transfer at condensate rimming speeds. They also help minimize the can surface temperature difference where the stationary siphon groove is.

Unorun dryer cans have high condensing rates due to low sheet temperature coming from press section.

Blow through amount is about 15-20% of condensing load to keep the can clear and determines siphon and condensate line sizes. Too large of a siphon requires more blow through.

Steam and condensate headers are used to control blow through in a dryer section. Pressure transmitters have to be calibrated accurately. An inaccuracy of as little as 2 psi can cause evacuation problems.

Condensing rate and pocket humidity

Felt wrap and tension decrease boundary air and can temperature, and increase heat transfer and condensing rate. High pocket humidity reduces condensing rate. The first pocket after unorun dryers tends to have higher pocket humidity as water vapor is released from the hot wire side of the sheet.

Heat applied in a dryer section includes:

  • heat to bring water and fiber to evaporation temperature
  • heat to evaporate the water (~985 Btu/lb)
  • heat to raise the sheet temperature to exiting temperature
  • heat lost to radiation from heads and non wrapped can surface (minor)
  • heat lost to dryer fabric (minor)

For assistance with your S&C systems, contact your Valmet representative.