Energy Tips Archive

July 2016

Is separator tank size and design critical to the proper operation of dryer cans?
Absolutely!  Undersized separator tanks generally result in high velocities of steam and condensate flow through the inlet separator nozzle, through the shell of tank, and through the outlet blow-through steam nozzle.  High velocities do not provide the basis for good separation of the steam and condensate and as a result, the blow-through steam will tend to carryover some condensate.  High velocities will also erode the baffles and impingement plates inside the tank much quicker, especially with 2-phase steam and condensate flow potentially causing short-circuiting between the inlet and outlet nozzles.  Condensate carryover will accelerate the erosion of downstream piping and thermocompressors and increase the condensate load for the syphons to remove from the dryers.  Undersized separator tanks also tend to have poor level control.  Separator tanks must be sized based on achieving a low vapor velocity through the tank and adequate liquid retention time.

The issues associated with undersized separator tanks will cause poor operation of the thermocompressor and dryer control equipment.  Venting and steam waste is common when condensate separator tanks are undersized.

June 2016

Do high-efficiency thermocompressors save steam?
High-efficiency thermocompressors do offer a higher entrainment ratio which means a lower motive steam (high-pressure) flow is required to recompress the same amount of suction steam (low-pressure) flow or conversely pull a higher amount of suction flow for the same amount of motive flow for a given set of pressure condition.  
 
A high-efficiency thermocompressor will offer an operating cost advantage if the source of the motive steam bypasses a cogeneration turbine or if the source of motive steam is produced on a high-pressure boiler separate from the source of low-pressure steam.  In these cases, the motive steam will have a higher cost than the make-up steam.  Minimizing the motive steam required will maximize the make-up steam flow and the power generation.
 
For tissue machine applications where the motive flow is limited to the Yankee's condensing rate, the higher entrainment ratio provides the benefit of increased blowthrough flow for the same amount of motive steam flow and for this reason, high-efficiency thermocompressors have become the industry standard for Yankee dryers.  
 
On booster thermocompressor applications, a high-efficiency thermocompressor will reduce the ratio of motive to suction steam flow for a fixed discharge flow. 

May 2016

Do Turbulator bars improve the energy efficiency of a dryer section?
The amount of energy required to dry a paper web depends on the amount of moisture entering and leaving the dryer section, machine speed, sheet width, and sheet weight. Turbulator bars do not directly affect the amount of energy required to dry the paper, but they can have a major impact on other operating parameters:

  • Turbulator bars increase the heat transfer capacity. This can be used either to reduce the dryer steam pressures or increase the amount of paper produced.
  • Turbulator bars improve the uniformity of heat transfer. This improves the cross-directional moisture profile.
  • Turbulator bars increase the responsiveness to changes in dryer steam pressure. This results in less out-of-spec paper being produced during grade changes.
  • The increased uniformity and drying response improve sheet runnability and machine efficiency.
  • Turbulator bars force the condensate in the dryers to rim at a lower speed. This reduces the power required to drive the dryer cylinders.

April 2016

Tracking Energy Losses From A Dryer Drainage System
One method to evaluate dryer section energy efficiency is to track the losses from the dryer drainage system.  Any steam not condensed in the dryers to dry paper should be considered a loss.  A well designed system will have a loss of no more than 1% to 3% of the total steam supply to the dryer section.
 
Many dryer drainage systems are designed with a vacuum system that condenses steam vented from the system. The energy consumed by the vacuum condenser represents the loss from the system. Measuring water flow, the water temperature in and out of the heat exchanger allows the energy loss to be calculated.  This can be done with three field instruments and provides a valuable online energy monitoring system.  The heat loss from the system can be trended against the steam flow to the dryer section to calculate the percent loss from the system.
 
This method of energy monitoring will quickly show any change in efficiency of the dryer drainage system.  A leaking vent valve or improper system setting will show up as an increase in the percent loss from the system.

March 2016

Tracking Energy Losses From A Dryer Drainage System
One method to evaluate dryer section energy efficiency is to track the losses from the dryer drainage system.  Any steam not condensed in the dryers to dry paper should be considered a loss.  A well designed system will have a loss of no more than 1% to 3% of the total steam supply to the dryer section.
 
Many dryer drainage systems are designed with a vacuum system that condenses steam vented from the system. The energy consumed by the vacuum condenser represents the loss from the system. Measuring water flow, the water temperature in and out of the heat exchanger allows the energy loss to be calculated.  This can be done with three field instruments and provides a valuable online energy monitoring system.  The heat loss from the system can be trended against the steam flow to the dryer section to calculate the percent loss from the system.
 
This method of energy monitoring will quickly show any change in efficiency of the dryer drainage system.  A leaking vent valve or improper system setting will show up as an increase in the percent loss from the system

February 2016

Is there an accurate, reliable, and safe way to determine the out-going press dryness?
Yes, there is. The out-going press moisture can be determined from a mass and energy balance of the dryer section.  By knowing the paper weight, width, speed, and reel moisture and the amount of steam used for drying, the out-going press moisture can be calculated using an analytical model of the dryer section. This is much simpler than a water balance that requires accurate measurement of many water flow rates around the press section. It is also much safer than attempting to get a "grab" sample of the sheet on the front side of the press section. It is also less costly than a cross-machine moisture scanner.  And the result is more accurate than all of these alternatives. For a "tight" well-managed dryer steam system, the sheet dryness can be determined within 0.1-0.2 dryness percent.

January 2016

Is there a better way of measuring energy efficiency in paper drying?
Yes, there is. Rather than measuring the amount of steam that you use, measure the amount of steam that you lose. The steam consumption (amount of steam used per hour) is, by itself, an inadequate measure. A high value could be the result of either drying a lot of paper or venting a lot of steam. One is good. The other is bad. Indexing the amount of steam used with amount of water evaporated (pounds of steam per hour per pound of water evaporated) is a better metric, but significant amount of energy can easily be lost in the "noise" of this metric.

A better approach is to simply look for the steam losses. Specifically, look for steam that is vented to the atmosphere or vented to condenser tanks. Look for condensate that is dumped to a sewer and look for hot air that is exhausted from the dryer hood to the atmosphere. These are direct measurements of energy losses and they are direct indications of potential areas for energy recovery.

December 2015

What common metrics are used for measuring paper drying efficiency?
Two common metrics for measuring paper drying efficiency are drying load (amount of water evaporated per hour) and steam consumption (amount of steam used per hour). The Technical Association of the Pulp and Paper Industry (TAPPI) recommends these metrics be indexed by the dryer surface area (resulting in pounds of water evaporated per hour per square feet of dryer surface area) and by the amount of water evaporated (resulting in pounds of steam per hour per pound of water evaporated). The indexed values are better indicators of dryer performance and overall energy efficiency of the drying process. Indexed values can be compared to similar machines as a benchmark of performance.

November 2015

Are the non-condensable bleeds on thermocompressor sections necessary?
Most thermocompressor sections are equipped with a non-condensable bleed (NCB) which are are applied to thermocompressor sections to prevent accumulation of non-condensable gases in the dryers. If allowed to accumulate, the non-condensable gases will diminish heat transfer rates for paper dryers in the steam section and cause non-uniform cross direction dryer surface temperature profile. This will lead to non-uniform cross direction sheet moisture profile.

Today, most boilers generate steam with extremely low levels of non-condensable gases and some producers seem to "get by" with no NCB's on their thermocompressor sections. Still, most steam system suppliers advocate having a NCB to safeguard against costly operational problems.

The discharge from NCB's is usually directed to atmosphere or a condenser. In this case, the presence of the bleed has negative effect on energy efficiency. In some cases the need for a NCB is avoided by arranging an individually controlled dryer (that drains to a condenser) to draw its steam from a thermocompressor section. Where allowable, this is an advantageous concept.

The most prevalent improvement opportunity for NCB's is to gain an appropriate flow rate. For a typical thermocompressor section, an NCB flow of 100 to 200 lbs/hr is generally considered adequate to protect against accumulation of non-condensable gases. To ensure the desired flow rate is achieved, the bleed should be equipped with a properly sized stainless steel orifice plate.

When it comes to NCB's, avoiding excessive flow rate will save steam. Your mill might benefit from a review of your NCB's

October 2015

How do condensate traps impact energy efficiency?
Traps indeed impact the energy efficiency of a steam system. Condensate traps are applied to the drain lines of various types of steam heating equipment such as steam coils, heat exchangers, and unit heaters. They are also frequently applied to keep steam mains free of condensate.

The purpose of condensate traps is to allow condensate to drain to a lower-pressure condensate collection system while disallowing the flow of steam to the lower-pressure system. There are many types of condensate traps, but nearly all are designed such that if they fail, they fail open. If a trap fails open, the equipment served by the trap will normally continue to function trouble-free. But, the failed trap will allow live steam to flow to the downstream condensate receiving system. This normally has negative impact on energy efficiency because the live steam going to the receiving system usually is not put to good use. It might be vented to atmosphere or, worse yet, it might cause the condensate receiving system to pressurize which will potentially cause drainage problems for other users leading to various operational problems.

To ensure condensate traps are functioning properly, they should be surveyed every two to four years by a trained professional. Most of the trap suppliers offer trap survey services.

September 2015

What is the role of monitoring in optimizing a dryer section?
Optimizing a dryer section begins with monitoring the dryer section operation and developing a baseline for dryer section performance. The baseline can be compared to the performance of similar machines producing similar grades. This baseline can also be used to evaluate the impact of changes made on the machine.  Improvements, however, require an active program that follows and acts on the data. Daily production meetings should include a review of energy efficiency readings. This will help to identify losses as they occur.

For example, an increase in steam going to a condenser may indicate the differential steam pressures are too high. A vent valve opening up may indicate a thermocompressor is running in a choked flow condition.  An increase in the dryer steam flow (per ton of product) may indicate the exiting press moisture is increasing and it is time to change one of the press felts. An increase in steam pressure to the air heaters may indicate coils are fouled and need to be cleaned. Again, the first step is monitoring. The next steps involve acting on the results.

August 2015

Where do you start in optimizing a dryer section?
The dryer section consumes large amounts of energy and it can have a dramatic impact on both sheet quality and profitability. The drying process, like other papermaking unit operations, is a complex matrix of interacting systems: Pocket ventilation, steam system, drive system, tail threading, sheet handling, energy recovery, and fabric conditioning, guiding, and tensioning. 

A good way to start an evaluation of the dryer section is to contact a drying systems supplier, consultant, or engineering firm for a dryer audit.  A professional dryer audit should include a complete set of dryer section measurements, an analysis of equipment sizing, recommendations for operational improvements, recommendations for equipment upgrades, lists of short-term and long-term improvement projects, and an analysis of the expected return for each project.  

Typical returns come from energy savings, increased drying capacity, reliable dryer drainage, reliable dryer drainage equipment, improved dryer section runnability, and enhanced drying uniformity. A comprehensive report identifies improvement opportunities and quantifies the associated benefits.

July 2015

What is the correct operating differential pressure for a dryer section?
The correct operating differential pressure for maintaining dryer drainage depends on a number of parameters. Some of the more important variables are: dryer section steam pressure, steam condensing rate, type of dryer syphon (rotating, stationary, or scoop), syphon pipe sizes, machine speed, and dryer diameter. When it comes to setting the operating differential pressure, unlike the Ron Popeil 4000 rotisserie, operators cannot just "set it and forget it". The operating differential pressures must be adjusted to reflect changes in operating conditions that come with start-ups, shut-downs, grade changes, and sheet breaks. Operators can make these changes manually, or they can use a supervisory control system, to make the changes automatically.

June 2015

Is it important that differential pressure transmitters be calibrated?
Proper calibration of instrumentation is at the heart of any efficient dryer section operation. If a process parameter cannot be properly measured, it cannot be properly controlled. Differential pressure transmitters are particularly important. Dryer drainage depends on it. Dryers with rotary syphons will flood if the differential pressure is not high enough and they will pass excessive amounts of blow-through steam if the differential pressure is too high. Even with modern stationary syphons, accurate differential pressure measurement is very important. Only 3-4 psi differential pressure is needed to drain the dryers with a stationary syphon, but with a measurement error of only 2 psi, the blow-through flow will easily be either inadequate or excessive.

It does not end there, the differential pressure transmitters may be properly calibrated, but the pressure sensing lines may not be delivering the right pressure to the transmitter. Over half of the dryer sections surveyed by Kadant Johnson had sensing lines that were improperly designed, installed, and maintained.