Stock Approach – Deaeration
Entrained air in the dilute stock supplied to the headbox of a paper or board machine can cause problems with drainage, foam, sheet formation, and overall process stability. Therefore, mechanical deaeration is sometimes required. This module will identify four ways to keep entrained air out of paper and board making processes and describe the three ways mechanical deaerators remove air from stock.
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Learning Objectives
• List and describe three forms of entrained air in stock suspensions and white water • Identify four ways to keep entrained air out of paper and board making processes • Identify the negative effects of, and potential benefits of removing, entrained air in stock • Identify and describe the key equipment • Identify and describe primary process flows • Describe the three ways mechanical deaerators remove air from stock • Explain why a stand-alone deaerator might be used instead of a combination cleaner/deaerator • Describe the function of the condenser and vacuum device in the vacuum system • Identify potential causes for a sudden drop in vacuum, deaerator flooding, and low vacuum or system instability
Specs
| Course Level | Intermediate |
| Languages | English, Portuguese |
| Compatibility | Audio, Video |
| Based on: | Industry Standards and Best Practices |
Key Questions
Are there multiple forms of entrained air?
There are three forms of entrained air that can exist in stock suspensions and white water – free, bound, and dissolved.
What are the differences between free air, bound air, and dissolved air?
Free air consists of larger bubbles that can be removed with time, bound air consists of smaller bubbles that can attach to fibers and be difficult to remove, and dissolved air only presents a problem if it converts into free or bound air.
Is deaeration necessary on every machine?
The amount of entrained air that is tolerable in a paper or board making process depends on many things. So, though all paper and board machines would benefit at least a little from deaeration, not all of them use it.
How do deaerators remove entrained air from the stock suspension?
They use three means: they use vacuum to “”boil”” the stock to remove air, they spray the stock to expose more surface area to the vacuum, and they impinge the stock on hard surfaces to separate the bound air from the fibers.
Can a deaerator cause pulsations in the stock flow to a headbox?
Pulsations in the stock flow to the headbox can be caused by excessive air in the stock going to the deaerator, an undersized or underperforming vacuum system, or low stock flows to the deaerator, which can cause the level inside the vessel to rise and fall, leading to fluctuations in the stock flow to the fan pump.
Sample Video Transcript
To maximize the removal of entrained air by mechanical means, the stock must be exposed to a vacuum level that is sufficient to cause the stock to boil. This causes the free air bubbles to break and the dissolved air to come out of the water, be sprayed to maximize the surface area of the stock that is exposed to the vacuum, and impinge upon a hard surface to break the bound air bubbles away from the fibers. Most deaerators are designed to accomplish all three: boiling, spraying, and impingement.
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