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Based on: Industry Standards and Best Practices

Languages: English

Sample Transcript

The two main goals of a paper machine dryer hood air system are to remove the water vapor that is evaporated from the sheet in the dryer section and to control the temperatures, humidities, and air flows around the sheet in the pockets across the full machine width. By satisfying these two goals, the hood air system increases the drying capacity of the machine, improves sheet moisture profiles, reduces stain requirements, and improves the runnability of the dryer section. The benefits of a properly designed and operated hood air system are improved sheet quality, increased production, and improved energy efficiency.

Paper Machine Dryer Hood Air Systems

Training Time: 34 minutes

Paper and board machine dryer hood air systems have two main goals: remove the water vapor evaporated from the sheet in the dryer section and control the temperatures, humidities, and air flows in the dryer pockets across the full machine width. A properly designed and operated hood air system will increase the drying capacity of the machine, improve the reel moisture profile, reduce steam requirements, and improve the runnability of the dryer section. This module will cover the components and operation of paper machine dryer hood air systems.

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Increasing the steam pressure (and temperature) inside the can increases heat transfer by increasing the temperature difference between the shell and the sheet

With open hoods, higher building supply air flows are required to balance the higher hood exhaust air flows

Open hoods are characterized by a roof and four walls which extend down to 8' - 9' above the operating floor level

Learning Objectives

  • List the objectives of a dryer hood air system
  • Describe heat transfer and mass transfer during the sheet drying process
  • Identify and describe open and closed hoods
  • Identify and describe the components of a typical modern hood air system
  • List the goals and benefits of a hood air system
  • Explain how hood supply and exhaust air flows are typically regulated
  • Describe the functions of pocket ventilators, anti-blow boxes, and vacuum rolls in dryer sections
  • List common uses for the heat recovered from hood exhaust air
  • Explain how humidity is measured
  • List and describe the indicators that can be used to evaluate hood air system performance
  • Identify and describe safety hazards and guidelines for paper machine dryer sections
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Paper Machine Dryer Hood Air Systems FAQs

What is the difference between "open" hoods and "closed" hoods?
Open hoods have walls and a roof, but are open at the operating level. Closed hoods have movable walls at the operating level to fully enclosed the dryers, and totally closed hoods are also enclosed at the basement level.

What are the advantages of closed hoods?
Lower exhaust air flows so exhaust fan requirements are lower and heat recovery is possible. Also reduced building heating load because they have dedicated supply air systems.

For a machine with closed hoods, should the supply air flow be equal to the exhaust air flow?
Modern hoods should have 65% to 75% of the exhaust air flow supplied as dry heated air. The remainder is supplied by infiltration around the hood seals and doors to prevent hot humid air from escaping into the machine room.

How can the heat that is recovered from hood exhaust air be used?
It can be used to preheat the hood supply air, heat process water, or heat ventilation air to the building

Can hood performance be evaluated somewhat easily?
Yes. Hood performance indicators include the hood supply air flow, hood supply air temperature, and hood exhaust air humidity, which are all measurable.

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