Let's talk dryer hoods. My primary focus will be on Yankee dryer hoods but a lot of the same principles apply to flat paper exhaust hoods. The flat paper machine hood also has it's own unique problems.

The hood of a typical Yankee dryer paper machine performs two functions. It contributes significantly to the drying process and evacuates the moisture removed from the sheet after the pressure roll. With the demands of increased productivity and energy efficiency the Yankee hood is a key component in optimizing paper machine performance.

There are many variables that impact the operation of a Yankee hood. Achieving optimum performance requires key measurements and a thorough understanding of the thermodynamic principles that govern hood performance. Sheet quality issues can also be impacted by the hood affecting runnability on the machine and in the converting plant.

Flat paper machine dryer hoods usually contribute little or no energy to the drying process and primarily function as exhaust hoods for the dryer section. Also, they contribute to reducing humidity in dryer pockets.

How does a modern paper machine Yankee hood work?

The primary drying mechanism of a paper machine hood is convection. Air is heated to a high temperature, perhaps 800 deg-F or more, and is blown on the sheet at high velocity. This air impinges on the sheet at 24000 ft/sec or greater. It comes out of a nozzle box that contains many holes (approximately 2% or less open area) that are usually 1/2 inch or less in diameter.

After the air impinges on the sheet it is evacuated from the area usually through slots in the hood that lie between the nozzle boxes. For energy efficiency purposes some of the air is re-circulated through the system and some of it is evacuated from the building.

What does "balancing a hood," mean?

There are usually a number of dampers in the ductwork of a Yankee hood used to control airflow through the duct. "Balancing a hood" in a general sense usually refers to adjusting these dampers so that the exhaust flow is equal to the in- flows (i.e. make-up air flow, combustion air flow and gas flow). This creates a condition where the hood is neither drawing air out of the machine room nor is it blowing hot air into the machine room.

Why do I want my hood balanced?

This is usually the preferred running condition for the hood. In this condition the hood is not spilling hot air into the machine room. Also, it is not drawing cool, ambient air along the edges of the sheet and creating drying difficulties on the edges. A well balanced hood usually provides the best runnability and sheet edge quality.

A hood can be balanced to a range of exhaust flows. A balanced hood is not necessarily the most energy efficient or the most productive. However, efficient, highly productive hoods are generally well balanced. Also, for each different grade or speed that the machine runs at damper settings are changed and the hood must be re-balanced.

Is the gap between the hood and the shell of the Yankee important?

Convection drying is a function of both heat and airflow. The nozzle design of most hoods produces maximum air velocity out to about ¾" to 1" from the nozzle. Beyond that distance the air velocity from the nozzle drops off rapidly and drying rate diminishes geometrically. If a paper machine is bumping up against drying limits a good place to start looking for additional drying capacity is to measure the gap between the hood and the shell. If it is beyond 1 inch every effort should be made to move the hood closer to the shell. In some cases wear and tear has caused the hood to be distorted. Closing the overall gap as much as possible is still the best course of action.

How do I know if I'm getting the most out of my burner?

Using the design criteria for your burner and hood an engineer can go through the complex thermodynamic calculations using difficult to acquire data to determine if a burner is performing correctly. A simpler way to check burner performance is to measure the carbon monoxide in the supply duct before recirculation air is added. If CO is high, and adjusting the combustion air does not reduce the CO than likely there is a problem with the burner or it's control system.

High CO is a direct indicator of incomplete combustion. Incomplete combustion will reduce the drying capacity of a hood and may create a health hazard especially if a hood is not well balanced and blowing out on the machine floor.

What is the correct way to use profiling dampers?

Profiling dampers reduce airflow in zones in the cross direction of the hood. Since the typical CD drying problem is wet streaks in order to use profiling dampers you must have extra capacity in the hood. The typical profiling damper application would require the dampers to be uniformly throttled back and then one or more dampers would be opened in the area of the streak. Fundamentally this scenario requires excess drying capacity in the hood. If you are currently operating at the upper drying limits of the hood profiling dampers would be of little use.

One word of caution - profiling dampers can be the source of wet streaks. Because of the adverse environment that they operate within dampers and damper positioning mechanisms can become faulty. Good PM practices around profiling dampers are essential for optimum hood performance.

What does optimizing a hood mean?

This is a complex process that varies by the machine and depends on your performance objectives. Absolute humidity is a key indicator of drying performance and will usually be about 0.4 to 0.6 #H2O/#Dry Air in the exhaust flow of a well running paper machine. If energy efficiency (i.e. lower energy costs) is the objective than usually higher exhaust humidities would be desired. This increases the amount of recirculation and maintains higher supply temperatures for a given amount of fuel consumption. Conversely, if there is burner capacity available and maximization of production is desired humidities in the lower end of the range are desired. This is due to the thermodynamics of convection drying. Air density, static pressure in ducts, and nozzle velocity all play a part in this relationship.

In order to identify a true optimum a large amount of difficult to acquire data must be gathered. The entire drying process must be characterized thermodynamically. This is accomplished with a hood survey.

What is a hood survey and why do it?

A complete hood survey includes airflow measurements in all of the hood ducts. These measurements include humidity, temperature, static pressure, and velocity. This usually requires the installation of ports in the ducts at appropriate locations. Supply duct air quality tests are also made to determine the condition of the burner and control system. Sheet moisture into and out of the dryer must be acquired. Yankee steam and condensate flow data must be measured.

Given this hard to acquire data an engineer can then make the necessary thermodynamic calculations (mass flow balance) to characterize the current operating state of the hood. Optimization can then usually be achieved after a few iterative adjustments to the machine.

A good thorough mechanical inspection should be performed prior to the optimization process. The mechanical inspection done after the survey is usually more effective because the survey will usually provide indications of problems that should be checked out.

How does a flat paper machine dryer hood work?

The primary function of this type of hood is to provide a means to exhaust the moist air from the dryer section. In some designs the make-up air is preheated to increase the capacity of the air to hold water. Air distribution through this kind of hood is usually much more complicated than in a Yankee hood. Poorly designed or unbalanced hoods sometimes result in "dead" pockets of air - stagnant air- or dryer pockets that have poor circulation and are saturated.

How do you improve performance of a flat paper machine hood?

A dryer section survey is usually the first step towards improving performance in this type of hood. Humidity measurements in the dryer pockets provide information as to whether the pockets are saturated or not. Airflow measurements in the ducts determine if enough airflow and air exchange is available for the drying needs of the machine. Examining the patterns of the airflow using smoke sticks or smoke machines help identify poor circulation problems in the hood.

Essential to a good survey is a thorough inspection of the hood. Dampers, ductwork, baffles, fans, and dryer pocket ventilation systems must all be in good working order for the hood to perform well.

After the survey, the key to improving hood performance is improving airflow through the hood. Adjust the dampers to achieve a slightly negative pressure in the hood. Balance the hood so areas furthest from the exhaust fans receive good circulation. Adjust pocket ventilation system to reduce pocket humidities, especially in pockets that have absolute humidities above .2 #H2O/#Dry Air. Above this humidity the pocket is considered saturated and drying performance suffers.