At the end of this module, you will be able to:
- Describe the design of Yankee hoods
- Describe Yankee hood air system equipment and flows
- Describe cross-machine moisture profiling in Yankee hoods
- Describe supply, recirculation, and exhaust air flows
- Identify heat recovery
- Gas burner modes, permissives, and interlocks
- "Balancing" the hoods
The following key questions are answered in this module:
What is the purpose of the Yankee hoods?
Hoods help dry the sheet by directing hot, dry air onto the sheet. The hoods also removes the humid air that contains the water evaporated from the sheet.
What are the components of the hood system?
The hood and air system includes supply and exhaust fans, gas burners, ducts, heat exchangers, profiling dampers, and blowboxes.
What are the possible air system configurations?
The air system can be parallel, cascade, or mono. A system may be have crossover ducts so it is capable of changing configurations.
What is the purpose of profiling dampers?
The profiling dampers can vary the air flow from the supply header to control the moisture profile across the dryer.
What happens with exhaust air from the hoods?
The hood exhaust air is used to heat the incoming make-up air and sometimes it can also heat water or machine room air.
Below is a transcript of the video sample provided for this module:
In most Yankee hood air systems, the air supply fans pull in fresh preheated makeup air, combine it with some of the still hot recirculated exhaust air and the resulting mixture flows to gas burners. The gas burners directly heat the supply air to the desired temperature. Each supply fan blows this heated supply air through the supply duct and into the hood via an articulating flexible joint. Inside each hood, a supply header extends across the machine and profiling dampers are often installed to vary the airflows across the machine. The hot supply air flows through the supply header and profiling dampers and into a single plenum nozzle blow box or multiple cross nozzle blow boxes. The blow box or blow boxes direct the hot air out through holes or nozzles and down onto the sheet surface. Note that the supply fan speed controls the airflow to each hood, and therefore, the velocity of the air impinging on the sheet, commonly referred to as the air impingement velocity.
Use the additional resources and links below to learn more about this topic: