Zero Line Control for the Dryer Hood

An overview of the dryer hood and its air system will be presented in order to understand hood balance principles and the determination of the required hood air quantities.

The quality of the vapor hood over the drying section influences the energy requirement for the drying process. Of prime importance is the sealing of the hood in order to attain the highest possible dew point temperature and exhaust pick-up humidity.

This approach yields the smallest exhaust air quantity, in turn lowering the energy demand on the air system of the dryer section as well as smaller process air system equipment.

A case study will be presented to examine the benefits of the proposed control strategy.

Contents

Dryer Hood Design
Air System Overview
Hood Balance
Zero Line Control
Case Study
Conclusion

Dryer Hood Design

The requirements for a good dryer section hood are diverse; from good process control to operability of the dryer section all while maintaining accessibility for maintenance service. From an operator comfort and safety standpoint the hood also needs to reduce heat and moisture spill into the machine room while minimizing operating noise level from inside the hood.

From an investment standpoint long durability of the hood is desirable as well as securing an energy-efficient operation of the paper drying process. The biggest influence on the energy requirement for the drying process is the quality of the vapor hood over the drying section. Of prime importance is the sealing of the hood in order to obtain the highest dew point temperature possible, which in turn leads to the highest possible exhaust humidity.

Special design considerations are required to maximize hood-sealing in order to achieve the highest possible dew point temperature and exhaust humidity. This approach also includes the minimizing of surface temperatures at thermal weak spots, e.g. minimizing structural thru-metal which provides a thermal bridge to the outside of the hood and reduces the hood's capacity to retain its heat while providing opportunity for ambient room air to create cool spots inside the hood, leading to condensation and dripping.

The most common hood design found in industry is the standard hood which can attain a dew point temperature of 135°F (57°C) representing an exhaust grain loading of 900 Grains. Dividing this value by 7000 Grains/lb of water results in exhaust humidity of 0.128 lb H2O/lb DA (Dry Air) or 128 g/kg in metric terms.

A high-humidity hood on the other hand can attain a dew point temperature of 144°F (62°C) representing an exhaust grain loading of 1200 Grains or 0.170 lb H2O/lb DA (170 g/kg). The newest and highest humidity hoods on the market can reach dew point temperatures as high as 147°F (65°C) or 1300-1400 Grains, representing 0.190-0.200 lb H2O/lb DA (190-200 g/kg).

Air System Overview

The process air system (PAS) supporting the hood plays an important role in meeting these optimization objectives. The PAS is composed of both the Supply air systems which provide hot, dry make-up air to the hood and the Exhaust air systems which evacuate the moisture-laden air after it has picked-up the water evaporated off the sheet web.

Most typical Supply air systems include make-up for a variety of sheet handling devices inside the hood such as blow boxes, blow pipes, ventilating doctors, pocket ventilation nozzles (PV) and sheet stabilizer boxes. On the other hand, the exhaust component is mainly provided by the hood plenum exhaust systems.

Hood Balance

The quantity of hood exhaust air required to properly remove all the water vapor from the drying process is calculated as follows:

Exhaust Air Quantity [lb DA/h] = Water Evaporation of the sheet web [lbs H2O/h]
Exhaust Pick-up Humidity [lbs H2O/lb DA]

- Where the Exhaust Pick-up Humidity is the total, leaving exhaust humidity out of the hood stacks minus the incoming ambient air humidity (typically the ambient air in the machine room).

From the equation above we see that the higher the humidity that the hood can handle, the lower the resulting exhaust air quantity will be; and so the smaller exhaust air quantity the lower the energy demand on the air system of the dryer section all while keeping the process air system equipment as small as possible, i.e. low capital cost and low operating cost.
Once the total exhaust air quantity is determined, then we can back calculate to determine the supply air mass required, since the latter represents 60% to 65% of the total exhaust air mass. This ratio is recommended for optimum hood balance and the ability to maintain the zero pressure line at the ideal height of about 1 meter off the machine floor.

As shown in the sketch below, this suggested zero line height is meant to prevent cold, infiltrating ambient room air from cooling off the high supply air temperatures required at the sheet handling devices (and their respective drop ducts) inside the hood.

Note: in the sketch above the exhaust ducts are in yellow while the supply ducts are in red.

The diagonal curve super-imposed on the vertical axis represents the hood pressure line. The hood zero line is then defined as the point where the diagonal pressure line crosses the vertical axis, approximately 1 m off the operating floor. Above the zero line the hood is exposed to positive outward pressures, while the negative pressure below accounts for the infiltration of the ambient room air.

Zero Line Control

Following the description above, the zero line can then be defined as the line between infiltration and exfiltration of hood air. A zero line control system can then consist of a combination of temperature sensors and humidity sensors in the arrangement suggested in the flow diagram below.

By inserting a series of temperature sensors along the vertical axis of the hood we can detect the difference between these two zones. The sensors in the upper portion of the hood plenum will be exposed to hot exhaust air which is typically above 180°F (80°C) while the sensors in the lower part would see ambient room air below 90°F (30°C).

This temperature differential is large enough to allow for reliable measurement and control by the zero line system. The signals from the temperature sensors can then be fed to a DCS in order to control the zero line position.

The DCS, in turn, can then control the speed or frequency of the supply fans via their VFD. The following strategy is recommended:

Lower the supply fan speed to raise the zero line, conversely,
Raise the supply fan speed to lower the zero line

The exhaust fan on the other hand can also be on speed-control via the signal from the humidity sensor in the exhaust stream. Since the zero line can vary from section to section and with hood design, an ideal control scenario would utilize a zero line control system for each dryer section. An air systems audit by qualified field service personnel can help determine the correct set-up and number of systems required. With such an automated control loop the zero line can be optimized per grade as well as where the highest water evaporation is occurring within the hood.

While dew point control is highly recommended as a closed-loop experience has shown that zero-line control can work best as a manual control in certain cases (i.e. an alarm with a manual "to-do" indication). The reason behind it is that the dew point is more important for the energy consumption and a dew point control would always have priority over a zero line control. But if the dew point is controlled, i.e. the exhaust air fan is reduced as much as possible; one is usually limited with the reduction of supply air because of the need for sheet stabilizer air knives or the need for good pocket ventilation and even moisture profile. For best control strategy each situation must be studied case by case.

A zero-line of 1m height is desired but only as a guideline since it can also vary from case to case. The zero-line is not so much a "line" (i.e. the same all over the drying section) but rather a "plane" which typically is inclined and lower towards wet end (return loop of drying fabrics pushing air towards WE). It is also often inclined from TS to DS which could result in a skewed moisture profile. It is always lower in single tier sections than in double tier. This is not only due to the single tier being in WE but also due to the high amount of suction on the stabilizer rolls.

Any change to the hood enclosure will also change the pressure condition inside the hood. If for e.g. a sliding door in the basement is being opened this will result in a lower zero-level. But it should not result in a change of supply air flow because this could influence the dryness profile and runnability of the machine.
Hall ventilation also has a big influence on the zero line. A machine hall that is in underbalance (negative pressure) towards outside of building (e.g. no supply air but only exhaust) will also see a lower zero line than a balanced building. This can easily be verified by switching off supply air units and measuring the zero line before and after.

Case Study

The following case study examined the benefit of dew point control on energy consumption and heat recovery potential by increasing the dew point from 133°F to 144°F (56°C ? 62°C). The test was conducted on a machine producing 35# linerboard (170 gsm) at 4000 fpm (1220 m/min) and a production rate of 117 tph.

The results show that the required exhaust mass flow was reduced to 67% of its original mass which increased the enthalpy (h) of the exhaust stream by 77%.
The net impact of this change provided an increase of heat recovery capacity of 19% all while reducing electrical power consumption by 70% on the fans.

This benefit is not specific to the grade that was run and can be duplicated on any grade as long as the same dew point temperature differential of 11°F is available (e.g. 133°F to 144°F in the case above).

Conclusion

To optimize the energy efficiency of the dryer section hood and air system a zero line control system is recommended to maintain optimum hood balance. In conjunction with humidity sensor control the exhaust humidity can be maintain at maximum in order to ensure optimum drying energy costs. Case by case study is recommended in order to develop the best control strategy.