Best Practices in Paper Dryer Operations

Dryer Bars

• Dryer bars should be installed in dryers on machines that are operating above the rimming speed (above 1100-1200 fpm (335-365 mpm). Bars will increase the drying capacity (increase production or decrease operating steam pressures) and improve the drying uniformity.
• If bars are installed in only a few dryers, to increase the drying capacity, then the bars should be installed in those dryers that are most effective (typically in the middle of the main section dryers, in the “constant rate” zone).
• The number of bars is selected by the supplier to produce the highest heat transfer with the syphon system that is installed in the dryer. Typically 18 rows of bars are used in a 5’ (1.5 m) diameter dryer and 24 rows of bars are used in a 6’ (1.8 m) diameter dryer.
• The bars should be rectangular in cross-section and be stiff enough to withstand the impact of tumbling condensate and tall enough so that they are not covered by rimming condensate in dryers that have heavy condensing loads (over 3000 lb/hr). Specifically, the bars should be at least 0.75” wide and 0.5” tall.
• In a dryer with a rotating syphon, the bars should extend across the dryer face, to be within about 1-2” (25-50 mm) of each dryer flange.
• In a dryer with a stationary syphon, the bars should extend across the dryer face, to about 1” (25 mm) from the edge of the stationary syphon.
• The stationary syphon can be swept back and positioned close to the dryer flange or be positioned directly below the end of the support tube.
• For maximum flexibility in adjusting the dryer shell edge temperatures, the syphon should be located below the end of the support tube and a short segment of bars (“edge control bars”) be placed just outboard of the syphon.
• Note: “Dryer bars” is a generic term. The Kadant Johnson registered trademark is “Turbulator” bars.

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Dryer Inspections

• Dryers should have routine external inspections, looking for loose equipment, steam leaks, oil leaks, and proper lubrication flow rates. Leaking steam valves, joints, and flex hoses should be replaced or repaired. Oil leaks should be repaired and oil spills cleaned, for operator and machinery protection.
• Dryers should have routine internal and external inspections. The ideal inspection frequency is every year. As a minimum, all dryers should be inspected on a 5-year rotating cycle (20% per year).
• The frequency of internal dryer inspections may be a requirement of the insurance carrier
• Observations and measurements should be documented for future reference and for follow-up.
• General external inspections should include ultrasonic shell thickness scans, magnetic particle inspections of the dryer heads, head bolt inspection, and review of pressure safety equipment.
• Internal inspections should include integrity of balance weights, condition of syphons and supports, condition of the dryer inside surface, condition of dryer bars (bars, hoops, springs, fasteners), and location of bars (if partial-width bars are used in the dryers).
• External inspections should include head flange leakage, manhole cover leakage, head condition (surface cracks), doctor blade wear pattern, and shell surface condition.

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Dryer Syphons

• Check syphon clearances during each annual inspection.
• Modern close-clearance rotary syphons should have a gap between the shoe and the shell of 0.06” (1.5 mm).
• The clearance below the stationary syphon should be set to the manufacturer’s recommendations (typically in the range of 3-8 mm). The wrong settings can adversely affect dryer performance.
• The syphon shoe (older rotary and modern stationary shoes) must face the direction of rotation. For two-tier dryers, the syphon in a top dryer will face in the opposite direction as the syphon in a bottom dryer.

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Sight Glasses

• Port-hole style sight flow indicators are recommended for condensate service. They can be valuable for start-ups and trouble-shooting.
• Most site glasses are rated for a maximum rating of 75 psig (5 bar) when used in condensate service. Above this pressure, the saturation steam temperature is high enough to cause degradation of the glass.
• Do not use dome-style site flow glasses if there is a potential for heavy debris to be entrained in the condensate that is being evacuated from the dryer.
• Site flow glass should be routinely inspected. Kadant Johnson has detailed instructions available as a guide on the installation and maintenance of sight flow glasses.

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Steam Systems

• Conduct routine operations inspections of the steam system (tanks, pressure gauges, pumps, flow meters (orifice plates and transmitters).
• Check vacuum condenser (receiver) to insure that adequate vacuum levels are being achieved.
• Heat exchanger tube bundles should be inspected and cleaned on an annual basis.
• Turn down steam temperatures during sheet breaks (automated systems for turn-down are most effective). Turn-down adjustments should be based on the required change in dryer surface temperature, not fixed adjustments in steam pressures.
• Adjust steam pressures in advance of grade changes, to reduce the delay in establishing the new dryer surface temperatures.
• Monitor valve and transmitter condition (calibration, response, accuracy)
• Check transmitter connections (inspect lines). Properly operating transmitters are critical for good system performance.
• Every 2-3 years, the internal condition of thermocompressors should be checked for wear.
• Maintain records of steam system operation as a benchmark for performance monitoring. Include operating steam pressures, differential steam pressures, separator rise time times (condensing loads), machine speeds, sheet weight, and sheet trim.
• Monitor steam system to be sure that vent valves are not open during normal operation and have limited venting during sheet breaks. In a good steam system, there is no steam venting, during sheet breaks, grade changes, and particularly not during normal operation.
• For stationary syphons, the dryer differential steam pressures (inlet-to-outlet) should be in the range of 2-5 psi, depending on the machine speed and dryer condensing load.
• For rotating syphons, the dryer differential steam pressures (inlet-to-outlet) should be in the range of 5-15 psi, depending on the machine speed and dryer condensing load. Syphon suppliers should be contacted for their recommendations based on machine operating conditions.
• Dryer surface temperatures should be checked periodically, particularly before and after major dryer section modifications. The dryer surface temperature should normally be less than 35-45 F below the steam temperature in effective dryers.
• Dryer head temperatures should be checked with an infrared sensor on a periodic basis, to determine if any of the dryers are flooded. This is particularly important if the dryer drive loads are found to be running higher than normal.

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Oil Leaks

• There are a number of sources of oil leaks on paper making machinery. Some were related to design, some to installation, and others to maintenance. New product designs have fixed most of the sources of the leaks. Some of the existing leakers can be fixed. Some of them you just have to live with.
• The oil leaks from piping can usually be addressed by proper piping (special fittings, welded couplings, etc.)
• Oil leaks from sealed bearings are generally eliminated by keeping the oil temperature controlled (cooled or heated, as needed), by insuring that the gravity drain lines have generous slopes, no hose kinks, and adequate sizes, and by maintaining the roll seals. It is important to clear paper wraps from the roll journals, particularly those with labyrinth seals. "Chip slots" or drain channels must be provided and located on the lower side of the labyrinth grooves to insure that the oil drains back into the housing.
• Bearing housing and gearbox vent caps should be kept clean and open.
• Oiler sight-flow units should be positioned above catch pans (to contain oil during maintenance as well as to catch leaks).
• Quick disconnect lines should have the junctions located above catch pans (to contain oil during maintenance as well as to catch leaks).
• The oil leaks from pocket ventilating roll bearing housings were solved with new designs and conversion kits in the late 1980s. The upgrade kits are available from the machinery builders.
• One of the more difficult leak areas to fix are the junctions between gearcase components in enclosed gear driven dryer sections. In some situations, replacing gaskets (if they exist) will help. In other cases, it is possible to disassemble and re-seal the joints with modern sealants (the LockTitle product Hylomar has proven to be fairly effective). Cleaning, repainting and caulking leaky joints may help, but it is often only a temporary solution.
• Some of the old dryer gearcases used rubber boots to seal the gaps between gearcase units. These gaps can be sealed up today with newer materials that are strapped over the gaps. These are generally available through the original equipment manufacturers.

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Ventilation Systems

• Check pocket humidity values during normal operation. Suppliers can assist with these measurements. Pocket humidities should be below 0.24 (pounds of water vapor per pound of dry air).
• Check and clean / replace air filters and heating coils in ventilation system.
• Check fans, belts, and air dampers for proper operation.
• The hood should be balanced such that the “zero point” is about 6’ above the operating floor level.
• The dryer access doors and panels should be closed during normal operation.
• Inspect the hood for signs of sweating, particularly in the hood corners.

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Dryer Fabrics

• Dryer fabrics should be tensioned to their heat-set values, which should be close to the design tension levels for the paper machine. Typical felt tension should be in the range of 12 to 18 pli.
• Dryer fabrics should be checked during each shutdown. Items to check include operating felt tension values, seam (trade-line) distortion, fabric surfaces wear and abrasion, and fabric edges (loose yarns and wear).
• Maintain felt record book, recording the manufacturer, felt model and serial number, size and length, permeability, dates of installation and removal, and reason for removal.
• Fabric permeability should be checked during extended shutdowns. Fabric permeability should be in the range of 75-90 cfm/ft2 for single-felt sections, around 140-160 cfm/ft2 for high-speed paper grades, and up to 400-600 cfm/ft2 for slower, heavyweight board machines.

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Other Items

• Monitor press moisture, to provide an indication of changes in furnish and press performance, to help maintain machine runnability.
• Prepare routine checks of drying rate, U-factor, and energy usage. This can be done using modern monitoring systems. Drying rates can be compared to TAPPI standard rates.
• Record normal operating dryer drive power, as a benchmark for performance monitoring.
• Monitor and record the total drying energy, as a ratio to the amount of water that is being evaporated. A good benchmark for efficient operation is a steam usage of about 1.3 pounds for each pound of water that is evaporated.
• Wet end dryers should remain clean during operation, without picking and dusting. Wet end steam pressures should be reduced if problems are encountered. A proper steam system will allow dryer turndown without the need for valving off dryers.

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Dryer Flooding

Background

Many paper dryers experience periodic problems with flooding. A “flooded” dryer is one that has an excessive amount of condensate in it. The condensate 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 rate will be poor).

“Flooding” occurs when condensate is being evacuated from a dryer at a rate that is less than the rate at which steam is condensing in the dryer.

A brief outline of some of the normal causes of dryer flooding is given below. This should help in local trouble-shooting problems with flooded dryers.

Overview

Most problems with dryer flooding are related to either problems with the steam and condensate control systems or with mechanical / installation problems. These problems are outlined separately below.

Mechanical Problems

1. Syphon shoe design. Some older design syphon shoes did not have a “scoop” contour. These syphon shoes require higher differential pressures and they are more susceptible to dryer flooding.

2. Stationary syphon shoe installed backwards. Scoop-style stationary syphons must face opposite the running direction of the dryer. If they do not, they are very inefficient in removing condensate.

3. Eroded syphon shoe. Syphon shoes can erode over time. If the tip of the shoe is eroded away, the shoe becomes ineffective in removing condensate and dryer flooding results.

4. Loss of syphon shoe clearance. If the opening under the tip of a rotating syphon is closed off, the dryer will not drain. The clearance can be closed off if the shoe is embedded into the shell or if the rotary syphon spacers were not installed.

5. Internal joint leakage. Some styles of steam joints can have internal leakage inside the joint, between the steam supply chamber and the condensate discharge chamber. This can cause dryer flooding, even though there is a sufficient differential pressure across the joint and the amount of blow through is high.

6. Broken syphons. If either the horizontal or vertical syphon pipes break off inside the dryer, the syphon will not drain the condensate. Syphon breakage can be caused by corrosion, loose balance weights, shifting dryer bars, and contact between the shell and a stationary syphon.

7. Undersized syphon pipes. The internal syphon piping (vertical and horizontal pipes and shoe) may be too small for the current operating conditions (dryer speed, condensing load, and pressure). If so, higher operating differential pressures will be required, perhaps resulting in undersized steam piping.

8. Syphon elbow erosion. If the internal dryer syphon elbow is eroded, blow through steam will by-pass the syphon shoe and the dryer will not drain. Cast iron and ductile iron elbows are particularly susceptible to erosion. It may be difficult to see the erosion during dryer internal inspections. A ball peen hammer can be used to “sound out” the elbows during internal inspection.

9. Orifice plate is plugged. Mechanical debris (gaskets, washers, capscrews) that covers a portion of an orifice plate in the condensate drain line will increase the flow resistance and prevent proper condensate drainage from that dryer.

10. Condensate isolation valve is closed, is not fully open, or does not have a full-bore flow area. If the dryer has isolation valves, they must be full-bore valves and must be kept fully open, particularly in dryers with stationary syphons. Normally, the valve position can be indicated by the position of the valve stem or handle, but this is not always the case. The handle or stem may be broken off.

11. Steam supply isolation valve is closed, is not fully open, or does not have a full-bore flow area. If the dryer has a steam supply isolation valve, it must be also be kept fully open, particularly on machines with stationary syphons that are operating with low differential pressures. If the pressure differential is low and the pressure drop across the steam isolation valve is higher on one dryer than the others, then the differential pressure across the dryer cylinder will be disproportionately low and the dryer will not drain properly. If the supply valve flow resistance is too high, the dryer pressure will be less than the condensate header pressure and condensate / steam will flow back into the dryer through the drop leg. See the attached sketch.

12. Small supply and drain pipes. If the drop-legs from the steam supply header and from the dryer cylinders are too small, the differential pressure across the dryer may be inadequate to drain the dryers, even though the differential between the headers is at a reasonable level.

13. Modified piping. The addition of piping tees and elbows to an existing steam system can cause an increase in flow resistance and improper dryer drainage. The changes may not seem to be significant, but changes should be carefully checked.

14. Piping obstructions. Occasionally, debris is left in the dryer piping (pipe plugs, cans, rags, etc). This can greatly reduce the flow capacity and cause dryer flooding.

15. Improperly valved-off dryers. If a dryer is to be valved off to take it out of service, positive shut-off must be maintained for both the steam and condensate lines. This can be done by disconnecting the flex hoses to the joint, putting blank plates between pipe flanges, or having two isolation valves with an open vent line between them, for both the steam and the condensate lines.

Systems Problems

1. Low differential pressure. Most system problems are related to a running a low differential pressure. This may be the result of setting the DP too low or from controlling to an inaccurate DP signal. The best way to trouble-shoot steam system problems is to directly measure the differential across the dryer (not just between the headers). This requires pressure taps on both the steam supply side and the condensate drainage side of the joint. (Care must be taken to do this work safely).

2. Temporary loss of differential pressure. This problem is most common on high-speed machines operating with rotating syphons. If there is a loss in differential pressure (even a temporary loss), the condensate will cover the clearance between the syphon and the shell. If the differential pressure is restored, but is not above the flood-recovery differential pressure, then the condensate will not be removed from the dryer.

3. Note: The addition of an aspirator hole in the rotating syphon will significantly improve the flood-recovery performance of the syphon.

4. Note: It is not uncommon to have a temporary loss in dryer differential pressure during a brief stop or during a sheet break, particularly with older steam and condensate systems.

5. Differential pressure too low. If the normal dryer operating differential pressure is set and controlled to a value that is too low to handle the current operating speed, steam pressure, and condensing load, then the dryer will tend to load up with condensate.

6. Note: Tappi Water Removal Committee has published a Technical Information Paper on recommended operating differential pressures. TIP 0404-31.

7. Desuperheater flow. Water is sprayed into the steam supply piping to reduce the temperature of the steam to a value closer to the saturation steam temperature. If the instrumentation, controls, or water control valves are not properly functioning, an excessive amount of water can be dumped into the steam header. This will greatly increase the amount of water that has to be evacuated from one or more of the dryers in that section.

8. Improper orifice plate size. If the condensate orifice plate is too small, the pressure drop will be higher than expected and the dryer will not drain at the normal operating differential pressure.

9. Incorrect differential pressure indication. This is one of the most common problems. If the indicated differential pressure is higher than the actual differential pressure, the dryers may flood even though the differential pressure appears to be adequate. There are a number of causes of incorrect DP readings:

a. Transmitter is not functioning
b. Transmitter is not properly calibrated
c. Water legs on each side of the transmitter are not equal
d. Water leg on one side of the transmitter has air in the line.
e. Seal pot is located below one of the headers with poor connection lines.
f. Differential pressure is calculated from two pressure readings (double calibration errors and larger range for errors).
g. Pressure and DP readings are “calibrated” against inaccurate gauges.

10. Improper DP valve operation. An inoperative DP valve can cause dryers to flood. If the controls indicate 50% opening, for example, and the DP valve goes to 20% or if it closes when it is supposed to be opening (reversed operation), the dryers will flood.

11. Choked thermocompressor. If the dryer thermocompressor is undersized, it may be running in “choked flow”, where an increase in valve position will result in reduced blow through suction flow rate.

12. Improperly sized thermocompressor. If the thermocompressor is undersized, it will not be able to generate sufficient differential pressure and recirculate the blow though steam.

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