Paper Machine Cascade Steam Systems
Paper and board machines use steam-heated dryer cylinders to dry the sheet to its final moisture target. In a cascade steam system, fresh steam is typically introduced to the main steam group and then it is cascaded upstream to the intermediate and wet end steam groups. This arrangement automatically stair steps steam pressures (and steam temperatures) upwards as the sheet travels through the dryer section and moisture becomes harder to remove. This module describes the steam and condensate flows and controls for cascade steam systems. It also discusses their advantages, limitations, and safety hazards and guidelines.
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Learning Objectives
• List the main objectives of a paper or board machine dryer section • Identify and describe the purpose of dryer cans, dryer felts, dryer hoods, drive sections, and steam groups in a dryer section • Explain how thermocompressor systems and cascade steam systems allow steam to be reused in dryer sections • Describe the differential pressure controls that are used within and between steam groups in a cascade steam system • Identify and describe blow-through steam flows in a typical cascade steam system • Identify and describe safety hazards and safety guidelines related to paper machine cascade steam systems
Specs
| Course Level | Intermediate |
| Languages | English, French, Polish, Russian |
| Compatibility | Audio, Video |
| Based on: | Industry Standards and Best Practices |
Key Questions
What is a “cascade steam system”?
In a cascade steam system, fresh steam is introduced to one of the later steam groups, and the flash and blow-through steam from the condensate tank is cascaded back through the previous steam groups, in order.
Is a cascade steam system the only system that re-uses the flash and blow-through steam from the steam groups?
No, thermocompressor steam systems also re-use the flash and blow-through steam, except they re-use it within each group.
Why do cascade steam systems sometimes use condensers and vacuum pumps?
Because there are several pressure drops as the steam cascades through the groups, the condensate header for the wet end group is often under vacuum. A condenser can be used to supply vacuum to this condensate header. A vacuum pump is sometimes used to remove air and other NCGs from the system.
How do cascade steam systems make control of the moisture at the reel more difficult?
Fine steam pressure adjustments, such as those used for reel moisture control, are difficult to achieve because a change to one pressure will cause several other pressure changes.
How do cascade steam systems automatically account for the decrease in drying rate that occurs as the sheet dries on a paper or board machine?
“Drying rate” is the rate at which water can be removed from the sheet. In a cascade steam system, because the steam cascades from the dry end to the wet end, steam pressures and temperatures automatically stair-step upwards as the sheet dries. This naturally compensates for the decrease in drying rate.
Sample Video Transcript
In a cascade steam system, fresh high-pressure steam is the soul source to the steam header for the main steam group, often the biggest and last steam group. The flash and blow through steam from the main group’s condensate tank is directed to the steam header for the proceeding group, where some fresh make-up steam can be added to boost the steam header pressure. The flash and blow through steam from the intermediate group’s condensate tank is directed to the steam header for the proceeding group and so on. Fresh steam can be added to each upstream section steam header to boost its pressure.
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