Paper machine dryers consume tremendous amounts of energy and have a direct impact on product quality, but the drying process is highly complex and requires extensive process knowledge to optimize.
Although condensing steam to dry paper is an inherently efficient process, the drying process can be inefficient. Fortunately, new technologies and control strategies give even greater ability to manage steam to ensure optimal energy use resulting in improved drying efficiency and machine runnability. The following “tips” from Kadant Johnson answer some of the more frequently asked questions when it comes to energy efficiency in a paper machine’s dryer section.
What is a paper dryer journal insulating sleeve and what does it do?
An “insulating sleeve” is a flanged tube that extends through the bore of a paper dryer journal. The flanged end of the tube is usually bolted to the inside surface of the dryer head and sealed at the opposite end of the journal (the outboard end) by o-rings or packing in a journal adaptor flange. This sleeve creates an insulating air gap in the annular space between the tube and the dryer journal bore. This air gap greatly reduces the amount of heat that is transferred to the dryer journal from the steam that is passing through the inside of the tube. This reduction in heat transfer reduces the journal temperature and greatly reduces the risk of dryer bearing inner race cracking. It also reduces the amount of heat that goes into the bearing lubrication oil, helping it to maintain its viscosity thereby extending bearing life. Well-sealed sleeves are very effective, but even the slightest steam leak into this air gap will make the sleeve ineffective.
What is a “flooded” paper dryer, what causes it, and how do I know if I have one?
A “flooded” dryer is one that has excessive condensate in it. A flooded dryer results from a long period in which the steam is condensing in the dryer at a higher rate than the condensate is being removed. The residual condensate in the dryer may be either in a non-rimming condition (in which case the drive load will be high) or in a rimming condition (in which case the heat transfer will be low). Dryer flooding can result from mechanical problems or from steam system problems. Mechanical problems include broken internal syphons, plugged syphon pipes, eroded syphon pipes, internal steam joint leaks, plugged orifice plates, and closed shut-off valves. Steam system problems include inadequate differential pressure, reverse acting control valves, undersized separator tanks, malfunctioning level controls, and uncontrolled desuperheater stations. A systematic approach to troubleshooting flooded dryers can distinguish between these two types of problems and help to resolve them.
What are paper dryer doctors and why are they used?
A dryer doctor is basically a long beam that extends across the face of the dryer shell, holding a scraping blade against the dryer surface. Dryer doctors have a number of purposes: sheet shedding, dryer surface cleaning, threading, and air handling and control. Paper dryer doctors are often overlooked in the papermaking process, but they need proper attention to maintain efficient dryer operation. Important factors include doctor blade material, holders, doctor beam design, oscillation, alignment, and loading.
Does a dryer with higher steam pressure rating have a higher drying capacity?
Not necessarily . Higher dryer surface temperatures produce higher drying rates and the temperature of saturated steam increases as the steam pressure increases. As a result, higher steam pressures will produce higher drying rates in an existing dryer, but it is better to have a dryer that is rated and operating at a lower steam pressure than a dryer that is rated for a higher steam pressure but operating at the lower steam pressure. This is because dryers that are rated for higher steam pressures must have thicker dryer shells and the thicker dryer shells have higher resistance to heat transfer. As a result, it is possible for the drying rate of a low-pressure dryer to be higher than the drying rate of a high-pressure dryer when each of them are operating at their rated pressure. This is because the increase in steam temperature with increasing steam pressure does not always offset the loss in heat transfer that comes with the thicker shells of high-pressure dryers.
Is it okay to spray water on hot paper dryers to cool them off?
As a general rule, water should not be sprayed on hot paper dryers. This can cause high stresses in the cast iron shells and heads. The resulting cracks can be catastrophic. There is, however, a difference between spraying a large quantity of cold water against a single spot on a hot stationary dryer surface and the indirect application of a light warm spray of water over the entire dryer surface. Regular operator and safety training is needed to create the proper understanding and caution when water is used around dryer cylinders.
Can single-loop controllers be used for controlling dryer steam systems?
Yes, but not efficiently. A 747 airliner requires fewer than a dozen process control loops to land and all of these systems are under automatic control. The steam system for a dryer section, by comparison, requires over 50 interrelated control loops to adjust steam pressures, flow rates, differential pressures, and dryer section cascades. The dry end operator is expected to manage all of these control loops simultaneously and manually. This is a difficult process under steady operating conditions and nearly impossible during sheet breaks and grade changes. A supervisory control system is a near-necessity on modern, high-speed papermaking machines that are expected to run at peak operating efficiencies.
How can upgrades to paper drying equipment be benchmarked?
Many products are purchased with no more than a price check. But important purchases require more. One way to benchmark the performance of proposed equipment upgrades is to compare the performance of the existing machine to the performance of a machine that has the new equipment and operates under similar conditions. In the paper industry, however, most buyers of major capital equipment need more than a price check and more than a reference check. They want to observe the performance of exactly the equipment they are buying and operating under exactly their own operating conditions. Major equipment suppliers have demonstration facilities capable of simulating a wide range of conditions using different hardware configurations. This can provide a direct quantification of the potential improvement to justify the capital expenditure.
Does the temperature of the seal water in a liquid ring vacuum pump affect the vacuum pump performance?
Very definitely. Liquid ring vacuum pump manufacturers publish capacity curves (CFM and vacuum level) assuming that the seal water is at a fixed temperature (typically 60 F). When the temperature of the seal water is lower than 60 F, its vapor pressure decreases and the pump capacity will increase. When the temperature of the seal water is higher than 60 F, its vapor pressure increases and the pump capacity will decrease. The capacity curves have correction factors to account for these temperature variations. If the seal water temperature is too high, the liquid ring pump will be unable to achieve a low vacuum level.
Is the drive power for a dryer with dryer bars higher than the drive power for a dryer without bars?
It depends. The drive load increases exponentially as the dryer speed increases, until the condensate goes into a rimming condition. If the amount of condensate in the dryer is the same, the dryer with dryer bars (“spoiler bars”) will have higher drive power if the condensate is not rimming. The dryer bars, however, cause the condensate to go into a rimming condition at a much lower speed than it would in a dryer without bars. So, until the dryer without bars gets to a high enough speed for the condensate to rim, the dryer with bars will have a lower drive load. Once the condensate is rimming, the drive loads drop to a relatively low level, with the drive load for the dryer with bars being only slightly higher than the dryer without bars.
What happens to the dryer section drive load if one of the dryers is flooded?
It depends. If the dryer is running at a high speed (say, 3500 fpm) and the condensate is rimming, there may be no noticeable change in dryer drive power. At a low speed, however, where the condensate is not rimming, the drive load could be over 5x higher than a dryer with proper condensate drainage. This increase in drive load can be enough that just one flooded dryer will trip the drive motor. Consistent, stable, and reliable dryer drainage is important to machine operating efficiency.
Is there any value in the humid air that is exhausted from the dryer ventilation system?
Yes, the exhaust air has more volume and more value than most people realize. To put this in perspective: A machine that makes 1,000 tons per day (tpd) of fine paper will be evaporating 2,000 tpd of water, condensing 2,700 tpd of steam in the process, and exhausting 25,000 tpd of hot, humid air! The amount and temperature of the exhaust can be reduced by enclosing the dryer section in both the machine room and in the basement, keeping the hood doors closed, installing a fullwidth false ceiling, improving the air distribution, increasing the amount of air recirculation, modulating the supply fan to match production, and lowering the air temperature. Once these items are done, recover the energy from the remaining exhaust with air-to-air or air-to-water heat exchangers, with the recovered energy used to meet various low-level energy demands in the mill.
Kadant Johnson has performed more than 1,000 dryer audits and each process engineer has more than 15 years of experience in dryer applications. If you would like further information about information presented in this story, visit: www.kadant.com.
This article originally appeared in PaperAge Magazine’s July-August 2020 edition.