Dryer Fabric Uneven Cross-Machine Air Permeability: Causes and Solutions

In the drying section of a paper machine, cross-machine direction (CD) non-uniformity in dryer fabric permeability is an issue that is often underestimated. On the surface, it appears to be uneven air permeability, but in essence, it is often due to misalignment of the air path, heat path, and moisture path. Even when the dryer fabric surface appears clean, the spray system functions correctly, and the fabric has not been in service for an excessively long time, issues such as inconsistent cross-machine drying, localized high steam consumption, fluctuating drying loads, and instability at higher speeds can still occur.

Causes of Non-uniform Cross Machine Air Permeability in Dryer Fabric

Typically, the initial reaction is to attribute this issue to dryer fabric clogging, ineffective spraying, or problems with the drying cylinders or vacuum levels. However, simply treating the issue as a case of a dirty fabric often fails to reveal the root cause. Uneven air permeability of dryer fabric is often not a single fault. It may be the result of the superposition of three systems: uneven air pressure in the hood, local blockage of the fabric, and uneven layout and heat distribution of the dryer cylinder.

Uneven Air Permeability is Not Necessarily a Problem With The Dryer Fabric Itself

When this issue arises, it is often attributed to “dryer fabric clogging”; however, in reality, uneven cross-machine direction (CD) permeability readings for a single dryer fabric can stem from three categories of causes: variations in the inherent permeability of the dryer fabric itself, differences in the thermal and moisture conditions across the fabric surface, and localized environmental variations caused by the hood and drying cylinders. In other words, a discrepancy in air permeability between the left and right sides does not necessarily indicate a difference in mesh structure; it could result from localized variations in temperature, humidity, airflow, or moisture content, all of which influence the observed surface characteristics.
Therefore, when troubleshooting, one cannot simply look at whether the dryer fabric is broken. To truly troubleshoot uneven banner distribution, one must also consider how the airflow patterns, heat distribution, and moisture removal.

 

Air Hood Inspection

First check the air hood. It’s not enough for there to be wind; the air pressure also needs to be uniform. In fact, the root cause of many issues in the drying section is inextricably linked to the hood.
The hood serves functions beyond merely blowing air and exhausting moisture; its more critical role is to stabilize the air conditions surrounding the drying section. If the air pressure on the left and right sides of the air hood is uneven, it will lead to different local dehumidification capabilities.

This may also lead to:

• One side dehumidifies faster, while the other side retains more moisture.
• The dry mesh on one side is more prone to moisture, dampness, and dirt buildup.
• Local wind pressure differences are created in the banner.
• Apparent non-uniformity in air permeability readings, caused initially by environmental non-uniformity.

How to troubleshoot:

• Measure whether the wind pressure, air volume, and temperature on the left and right sides are consistent.
• Check if the nozzle is blocked, deformed, or deflected.
• Check if the relative positions of the air hood, paper web, and drying wire are off.
• Check the exhaust system for smooth operation and balanced flow (i.e., ensure there isn’t excessive extraction on one side and insufficient exhaust on the other).

Unevenness across the dryer fabric is sometimes caused not merely by variations in the fabric’s permeability, but by the air hood creating differences in the microenvironment between the left and right sides.

Is There a Lateral Pattern to The Clogging of The Dryer Fabric?

Clogging of dryer fabric is a common occurrence in papermaking, but particular attention is required if the clogging exhibits a cross-machine direction pattern. If the entire dryer fabric is dirty and the blockage is uniform, then the problem is relatively simple. If one side is more blocked or certain wide areas are abnormally blocked repeatedly, it means that the problem is not random but limited.

Common causes include:

• Uneven coverage of the dryer fabric cleaning showers.
• There are localized abnormalities on the surface of the roller.
• Abnormal re-wetting of the sheet edges.
• Deposition of horizontal fibers and fillers.
• Deviations exist in tension, contact, and deviation.

The clogging of dryer fabric is not merely a matter of cleanliness; it is a question of whether the deposits exhibit a specific orientation. If the clogging is directional, there must be a directional source.

The Heat of The Dryer Cylinder is Not Necessarily Better The Higher It Is, It Should be Spread Evenly Along The Width

When checking for uneven air permeability, do not only check the dryer fabric and ignore the dryer cylinder. In the drying section, the layout of the dryer cylinders has a crucial impact on the condition of the banner. If the steam distribution, condensate discharge, and surface temperature of the dryer cylinder cause lateral differences in the contact state, it will result in:

• Different drying speeds on the left and right sides
• Different moisture contents in certain areas of the paper.
• Changes in the air permeability of the dryer fabric due to heat exposure.
• The degree of fiber compaction and the adhesion state of the mesh surface are different in some areas.

The final test result may not be simply the air permeability of the dry web, but rather the result of the combined effects of heat, moisture, and pressure. Key checks include whether there is a significant temperature difference between the left and right sides of the dryer cylinder, whether the drainage system is unobstructed, whether there are localized cold spots on the surface of the dryer cylinder, whether the heat on the transmission side and the operating side is consistent, whether the heat in the front and rear sections is consistent, and whether the heat load is unbalanced.
The uneven air permeability of some dryer fabric is caused by the dryer cylinder creating lateral differences in the moisture content of the paper web, which are then amplified by the dryer fabric.

Troubleshooting Sequence and Adjustments

The sequence of troubleshooting is crucial; in many instances, immediately replacing the dryer fabric, washing the fabric, or adjusting the airflow merely addresses surface symptoms without identifying the root cause.

First, determine if it is a measurement problem. It is recommended to perform repeated tests at a unified point, perform multiple retests at multiple points on the left, middle and right sides, compare different working conditions, and eliminate instrument error and measurement point error.
Next, check the air hood to see if the left and right air pressure is balanced, if the symmetry of dehumidification is abnormal, and if the airflow is impacting the local surface of the dryer fabric. Next, check if there are any abnormalities in the dryer fabric itself, whether the deposition on the net surface is lateral, whether the spraying coverage is in place, whether the deviation, tension, and wear are consistent.
Finally, check the layout of the dryer cylinder, temperature difference, water drainage, surface contact condition of the dryer cylinder, and distribution of drying load before and after.

Be Wary of System Aging

If the unevenness of the air permeability of the dryer fabric gradually worsens, it indicates that it is not a sudden problem, but that the system is gradually becoming unbalanced (air hood airflow deviation, dry net surface cleaning efficiency decrease, dryer cylinder drainage condition deteriorates, long-term operation leads to tension and bonding deviation). These problems are most afraid of local repairs. If the dryer fabric is adjusted today, the air hood will still be off tomorrow, and the dryer cylinder heat will still be uneven after the spray nozzle is replaced. The problem will recur, eventually leading to an increasingly passive situation in maintenance work.

Uneven cross-machine air permeability in the dryer fabric cannot simply be addressed by treating it as a clogging issue; A truly effective investigation requires first determining whether it is caused by uneven airflow, uneven mesh permeability, or uneven distribution of heat and moisture content in the dryer cylinder. What may seem like a problem with the dryer fabric is actually essentially a misalignment of the air, heat, and humidity systems.