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A new perspective on steam energy in the dryer section

Paper machine dryer sections consume large amounts of steam energy. And while significant time and effort is devoted to measuring and evaluating dryer section efficiency, traditional measures provide limited insight into the energy efficiency...


October 1, 2012
By Pulp & Paper Canada

Topics

Paper machine dryer sections consume large amounts of steam energy. And while significant time and effort is devoted to measuring and evaluating dryer section efficiency, traditional measures provide limited insight into the energy efficiency of the dryer section. In fact, one of the most commonly cited drying performance indicators, pounds of steam used per pound of water evaporated, unfairly penalizes dryer sections using high-pressure steam, rewards dryer sections using low-pressure steam, and ignores the energy consuming aspects of condensate handling altogether.
Drying paper with steam-heated dryers is a fundamentally efficient process. The steam does not give up its heat unless it is transferring it to the drying cylinder and the drying cylinder does not accept the heat unless it is able to transfer the heat to the paper. Rather than focusing on the steam that is condensed, improvement opportunities can be better identified by looking at the steam that is not condensed and the ways in which the condensate is handled.
Improvement opportunities on inefficient dryer sections usually amount to less than 15% of the total amount of energy used in the drying process. The commonly used drying performance indicator (steam-water ratio) can vary by more than 15% due to the relationship between steam pressure and steam latent heat and due to the difficulty in accurately quantifying the evaporation load. As a result, the steam-water ratio has limited value. Another limitation of the steam-water ratio is that it reveals nothing about how energy is being wasted.
An alternative method for assessing dryer section energy efficiency which addresses these limitations looks specifically at the waste energy flows and their relationship to total dryer section energy consumption. This article describes new energy-focused Dryer Performance Indicators (DPI) based on waste energy flows. These indicators allow for assessment of the efficiency of the dryer section and identification of areas within the dryer section where potential improvements can be found.

Energy-focused dryer performance indicators
For even the most inefficient dryer sections, a high percentage of the consumed steam energy goes into heating the wet sheet and evaporating water. Waste energy flows are generally small compared to the total amount of energy that is used specifically for drying paper.

1. Losses to condenser or atmosphere
When assessing dryer system performance, steam loss to a condenser is treated the same as steam loss to atmosphere – both are considered waste energy flows. This is appropriate even in cases where warm water from the condenser is reused in the process. If the steam system continually makes warm water, there is no incentive to minimize water usage or to find other less energy-intensive sources, such as from the pulp mill or heat recovery systems. It is important to note that losses to a condenser or to atmosphere include loss of blowthrough steam from the steam system. Flash steam losses resulting from the discharge of high-temperature condensate to a lower pressure vessel are assessed separately.
With proper equipment and good management of the steam system, all machines should be able to achieve a value of 2.5% or less of total steam energy consumption. On some paper machines, it is not possible to eliminate all steam flows to a condenser. For example, paper machines that make lightweight paper grades often operate wet end dryers at low pressures to avoid picking, cockle, and other quality problems.
There are, however, many other applications in which there are practical solutions to reduce or eliminate the energy losses to the condenser or atmosphere. These include:
• Converting from rotary to stationary type syphons, to reduce the amount of blowthrough steam.
• Minimize the number of dryers draining directly to the condenser.
• Replacing poor-performing thermocompressors with high-efficiency units.
• Improving management of set points on cascade steam systems to maintain a proper cascade.
• Upgrading rotary steam joints to
modern designs that are less prone to steam leaks.
• Upgrading old piping that has thin-walled areas and is prone to leaking.

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2. Energy use for heating dryer
section supply air
This indicator is useful to assess whether total energy use for the purpose of heating dryer section supply air is at an appropriate level. For many machines, air heating has significant improvement potential. The minimum amount of energy required to heat the supply air and avoid condensation and dripping depends on the insulation value of the hood, the type of false ceiling, the air supply distribution, and the dryer section evaporation load.
Since evaporation load is a factor and this load varies considerably from one machine to another, the amount of energy used for heating the supply air can be minimized by measuring the evaporation load and controlling the air system accordingly. Few machines in North America actively manage the machine ventilation system. Consequently, there are significant opportunities available for energy conservation.
The most efficient dryer sections employ heat recovery systems that preheat the dryer section supply air using hot, humid exhaust air. Heat recovery systems can reduce the amount of energy needed for heating dryer section supply air to far below (i.e., better than) the “good performance” target of 12% of total dryer section energy consumption.

3. Flash steam losses
This indicator applies to flash steam that is generated by hot condensate discharging into a low-pressure receiver that is vented to atmosphere or to a condenser. Flash steam should not be confused with blowthrough steam – the uncondensed steam exiting a dryer with the condensate. Flash steam loss is evaluated separately because eliminating it requires a different set of solutions than minimizing blowthrough steam or eliminating other steam losses.
A thorough assessment of dryer section energy efficiency requires an investigation of how condensate is handled both at the paper machine and at the powerhouse. The most efficient steam system handles condensate such that no flash steam is vented to atmosphere at any point in the process, except that needed for proper deaerator operation. Using flash steam to generate warm process water should also be avoided. Though this practice is not as wasteful as venting to atmosphere, it should still be viewed as an energy loss.

4. Energy losses from dryers
to surrounding air
The energy loss from a dryer section to surrounding air is difficult to quantify. However, it is important to characterize this indicator because it is a significant part of the total energy system. With all paper machines, dryers lose heat to surrounding air through convection and radiation. The sheet and dryer fabrics also lose heat through convection and radiation. The heat losses from the dryers, sheet, and dryer fabric impact dryer section steam consumption. Even though these energy losses ultimately increase dryer condensing rates, they cannot easily be distinguished from “drying energy” on operating paper machines.
The “good performance” level for energy losses from dryers to surrounding air is 10%.  This is the expected loss for a dryer section with an enclosed hood operating with proper exhaust flow and a normal level of make-up air flow and temperature.
Open hoods require more exhaust and normally operate with much lower exhaust humidity. The higher level of exhaust flow and the hood construction lower the ambient air temperature around the dryers and the sheet resulting in increased heat loss from the drying process. While a closed hood is expected to operate with total heat loss of approximately 10% of the total dryer section energy consumption, an open type hood typically operates with a loss of approximately 13%.

5. Assessing high pressure < br/>motive steam use
Thermocompressor steam systems use high-pressure motive steam for recompressing blowthrough steam. High-pressure motive steam can be more valuable than low-pressure steam because of its power generating capability. In mills that have power generating turbines that are not running at their maximum output, it is important the steam system does not use excessive amounts of motive steam, but rather use that motive steam to generate power.
When assessing whether motive steam consumption is excessive for a given machine, consideration must be given to both. Methods to reduce motive steam consumption include:
• Replace inefficient thermocompressors with units capable of higher entrainment ratios.
• Convert from rotary to stationary syphons to reduce the amount of blowthrough that must be recompressed.
• Ensure the syphons are correctly sized for the current operating conditions. Many machines have oversized rotary syphons resulting in excess blowthrough that may require additional motive steam.
• Improve accuracy of differential pressure transmitters so that differentials can be minimized.
• Actively manage differential setpoints as a function of machine operating conditions.

Conclusions
The first step to improving dryer section energy efficiency is to evaluate it. The dryer performance indicators outlined in this paper focus these efforts on identifying and quantifying the losses, with a reasonable level of accuracy. These indicators reveal where improvement potential lies. The deliberate focus on these five energy-specific indicators provides greater insight into dryer section performance and areas for improvement. When applied properly, this method will yield greater results for optimizing dryer section performance and energy utilization while supporting capital investment decisions that drive real value into papermaking operations.

Mike Soucy, P.Eng, is president of Kadant Canada Corp.