Feasibility of pressurized thick stock dosing compared to stuffbox

1. Design principles of thick stock dosing

Stability of the operation of approach flow system  (AFS) is essential when machine direction ( MD) profile of paper basis weight is controlled. Several reliable methods for dosing  the thick stock to the mixing at the bottom of the ww-silo silo are available. 

In this this blog we will talk about energy consumption of thick stock dosing only. Comparison is
done between stuffbox system and pressurized dosing with variable speed drive ( VSD). 

Principles of the design of AFS and thick stock dosing are presented for example at TAPPI
instructions TIP 0404-54 ( http://www.tappi.org/downloads/tips/untitled-0108040454pdf.aspx )

Picture 1: Typical Stuff Box ( TAPPI - TIP 0404-54)

2. Basis weight control of paper at stuffbox

Purpose of the stuffbox is to stabilize the incoming pressure before the basis weight valve. Constant feed pressure is arranged by running the stuffbox at constant level. Excess stock
overflows back to the machine chest. Tank levels ( machine chest, ww-silo and stuffbox) 
vary from case to case, but typical values can be seen from picture 1  ( compare also 
to TAPPI 0404-54).

Picture 2: Basis weight control with stuffbox   

In this example, we estimated the electric energy consumption of thick stock pump for a 
machine which production is 933 tpd, and thick stock flow 287 l/s ( = 4546 gpm).
Required pump capacity is 337 l/s ( = 5348 gpm) due to the circulation of stock back to
the machine chest.

Estimated total head of the pump in this example is 19 meters ( = 61 ft) . Static head is 
9 m ( = 30 ft) and pressure loss at the level control valve and piping totally 10 m ( = 33 ft).
Actual values should of cause be checked case by case.

Estimation is done with an application, that can be downloaded from KGU AppStore
( App 4.2, Feasibility of pressurized dosing compared to stuffbox , see behind  the 
link  http://www.kgu.fi/downloads.html ). 

All required input values of the calculation can be modified according to actual operation
data of the paper mill. We used in this example the default values of the application, 
because they should be near by the typical operation conditions of thick stock dosing.

Normal running load (NRL) of the thick stock pump in these conditions is  81 kW ( 108 hp).

3. Pressurized thick stock dosing with VSD

Minimum energy consumption can be achieved at pressurized thick dosing system,
where recirculation  of the stock is eliminated, and throttle loss at control valves are 
minimized with a variable speed drive ( for details, see picture 3).

Picture 3: Basis weight control with VSD system

Normal running load of the thick stock is estimated based on the flow of 287 l/s 
(=  4546 gpm), and pump total head of 9 m (30 ft), thus estimated NRL of the pump is
about  33 kW ( 44 hp).

4. Comparison of annual electric energy costs

Thick stock pump flow at VSD system is smaller, and estimated stuffbox overflow in this 
example was 15 %. 

Total head of the pump at VSD system is also smaller. Static head in this example is
only 3 meters ( 30 ft).

Difference of  NRL load is 48 KW ( = 64 hp), which equals to about 27000 € annual 
electricity cost ( 70 € /MWh). Compare also to picture 4, estimation results from
KGU app 4.2, Feasibility of pressurized dosing compared to stuffbox).

Picture 4:  Feasibility of VSD-system compared to stuff box    

5. Investment costs of required modification

Investment cost of VSD was estimated  using the same formulas as at our earlier
applications ( compare to the blog Nov 2014, HP pumps).

Cost of the required piping modification was based on assumption, that about 10 meters
( = 33 ft) new piping is required. Piping material cost was assumed to be 10 € / kg, and
the additional installation and support material cost another 10 € / kg. 

Picture 5: Estimated pay-out time of installation of VSD

6. Summary

Based on these assumptions,  pay-out time of improvement  is about one year
 ( = replace stuffbox with variable speed drive).

Of cause this estimation should always be checked based on the actual operation
data, and cost of electricity at the mill.  Actual pump characteristics should also be
always checked.

The feasibility of VSD at thick stock dosing can be estimated with a simple application
( KGU AppStore,  http://www.kgu.fi/downloads.html ). 

This time the app is not free ( we need to cover our cloud and internet costs ). However 
the savings are about one thousand ( = 1000 ) times  higher compared to the costs of
downloading the app.


KGU Engineering 
Kari U Kokkonen.

PM HP showers - Fresh water consumption and variable speed drive

Abstract

This article describes a method for estimating fresh water consumption and energy savings and investment costs of installation of variable speed drive to PM wet end HP showers.

The estimation can be done  with application 4.1,  PM wet end high pressure showers. It  can be loaded free of charge from our homepages  ( http://www.kgu.fi/downloads.html ). 


Fresh water consumption of HP showers is typically about 1 m3/tn, and it is one of the biggest fresh water consumption positions at the PM. Water consumption is also estimated based on the given nozzle size and shower operation pressure. 

1. Background

At the beginning of 90's we made a study, and concluded that VSD was not feasible
at some positions of PM wet end process systems. When we repeated this study 15 years later, the  result was different. 

Today VSDs are used extensively at new paper making lines, but most of the machines is operation in Europe have been designed during the 20th century. Thus. energy efficiency of these older lines is not necessarily up to date.

At earlier blogs we have been talking about PM vacuum system efficiency  improvements. Similar energy saving potential can be found also from other systems. PM wet end shower water system high pressure  pumps are a good example of at position, where the installation of VSD can today be feasible. 

2. Development of electric energy price in Europe

Electric energy price in Europe has increased about 40 % during last 14 years.
   - compare to picture 1, index of real electricity prices in Europe /1/.  

               Picture 1: Index of real electricity prices, 2008 = 100, reference  /1/.               

3. Other conditions effecting the feasibility of VSD

Frequency converters are standard solution at many positions of paper production lines ( for example PM drives, fan pumps etc...). Due to the increased energy prices, installation of VSD is today feasible at many applications, where it was not feasible before. 

The costs of frequency converters have been stable during the last decade. When equipment costs today ( 2014)  were compared to price level in 2006, the prices today were more or less the same as earlier.

Typically there are about 150 centrifugal pumps at the paper mill. When the operation conditions of the pump vary during the time ( depending on the paper grade, felt age, etc.) feasibility of VSD should always be studied. It is important to check the situation also at  older machines, because external conditions have changed since the machine was designed.

A good tool for estimating the feasibility of VSD can be found for example from the homepages of  Vacon  /2/ . 

Picture 2: Estimation of energy savings with VSD compared to throttling  /2/. 

4. Paper machine high pressure showers

Operation conditions of high pressure needle showers at PM wet and can vary a lot. Design pressure can be pretty high ( up to 40 bar), but  operation pressure in practice can be limited to less than 20 bar due to the wearing of felts and fabrics.

Picture 3: High pressure needle shower /3/.

Sometimes the operation pressure of HP showers at press section is also controlled depending on the age of the felt. Even though VSDs are commonly used at the HP pumps of new PM lines,  many older machines are still running the pumps at constant speed.

5. Estimation of the feasibility of VSD

An application was developed to estimate the feasibility of installation of frequency converter to HP pumps, and it  can be found from the homepages of KGU Engineering /4/ . 

Estimation is based on the following parameters

• Total amount of HP shower pipes,
• Nozzle size,
• Pump design pressure,
• Normal operation pressure of the shower.

These parameters can be varied to find out annual operation costs, fresh water consumption, energy saving potential and pay-out time of installation of VSD. 

5.1 Required input data of the application

The feasibility of installation  of new VSD can be estimated when pressure levels and total water flow to showers are available. 

Water flow can be estimated based on nozzle diameter, spacing of nozzles, machine width and amount of shower pipes ( compare to picture 4 ).

Picture 4: Input data for app HP shower pump /4/.                      

5.2 Estimation of energy consumption

Motor load is calculated with the formula 1.

         P (kW)  =  H (m)  *  V (l/s)  /  E (%)                                                     ( 1 ) 

         where   P is calculated motor load, kW
                      H is pump head (pressure), meters water column
                      E is efficiency of the pump, %

If operation pressure of the shower is controlled with a throttle valve, design pressure is used at the estimation of motor load ( 35 bar or 25 bar in this example). 

If VSD is available, operation pressure of the shower pipe is used at the estimation of motor load. In practice, there can be static height difference between the HP pump and shower pipe, but because pressure levels in this application are so high, it is ignored in this application.

Efficiency of HP pump is typically at the level of 45 ... 70 %.  Default value is 65 %, but the user can change it, in case the exact value is available. For feasibility purposes the default value is normally OK.

The water flow is estimated based on Trial HP needle shower nozzle. Total water flow is calculated based on the nozzle diameter, operation pressure and length of the shower pipe. 

Picture 5. Shower pipe nozzles / 3 / .                          

Totally five different showers (pumps) can be added to this application. Results of the the estimation and operation costs of selected HP pump with, and without VSD,  can be seen from different pictures ( compare to the pictures 6 and 7). 

Picture 6: Operation costs of HP pump with, and without VSD.      

In this example motor load decreased from 52 kW down to 24 kW if VSD is installed to the pump of forming section HP needle showers. Annual saving is about 16.000 / year. Nozzle size of the shower pipe was 1.0 mm. 

Picture 7: Operation point of HP pump with and without VSD.       

5.3 Estimation of investment

For feasibility purposes the total investment costs have to be estimated. At our application, we have used total equipment cost of 100 €/kW for new frequency controller and motor.  New motor is not necessarily always needed, and equipment costs are also typically smaller, but we always want to keep our feasibility estimates at the safe side.

The costs of mechanical installation and cabling in this application is estimated to be 60 % compared to the component prices.

In addition to component and installation costs, a lump sum (reservation) of 5000 € and been added to the costs. This should cover the projecting and engineering costs, as well as possible new  I/O cards to the DCS system.

The costs of new nozzles or shower pipes ( if needed) are not included in the investment costs in this application.

6. Summary and conclusions

Energy saving potential and estimated pay-out time of investment are shown at the last page of the application (compare to the picture 8). 

Picture 8: Summary and pay-out time of installation of new VSD   

In this example the pay-out time of the investment was about 1 year. Feasibility of investment depends on energy price and operation conditions of HP showers, and can be preliminary estimated with this application.  

It is also important to remember, that the HP showers are one of the biggest fresh water consumers of the paper mill. For example in this example the consumption was 1.3 m3/tn. 

If the cost of fresh water and effluent treatment  is 1.0  €/m3,  total costs of water used at HP showers in this example is about 1500 €/day and  0.5 M€ per year. 

Optimization of the operation pressure and nozzle size at HP showers has a clear effect to the operation costs of the paper production line. 

References:

1.  Energy prices and costs in Europe, 
      European commission, SWD (2014) 20 final/2, p. 183 
      http://ec.europa.eu/energy/doc/2030/20140122_swd_prices.pdf

2. Vacon Oyj,  
      http://www.vem.fi/en/downloads/vacon-oyj/selection-and-pc-programs 

3. Trial Ab,
     http://www.trialab.fi/ 

4. KGU Engineering Ky,
      http://www.kgu.fi/

Efficiency of uhlebox at felt dewatering

Abstract

Energy consumption of felt conditioning is about 30 % compared to the whole vacuum system of the paper mill, and  specific energy consumption (SEC) is 20 - 30 kWh/tn. Estimated electric energy costs of felt conditioning are 1.0 - 2.0 €/tn, thus it is important to be sure  this amount of money and energy is used in the optimal way. 

A method for benchmarking and estimating the efficiency of uhleboxes is presented in this article. Felt moisture level, dewatering  and operation conditions are compared to the typical running  data of  felts.

Corresponding application can be downloaded from KGU AppStore free of charge. 

For details, please compare to:  http://www.kgu.fi/downloads.html 

1. General

Even though it is difficult to describe the efficiency of uhleboxes with simple mathematical models, we concluded, that a simple benchmarking method was needed to evaluate the operation of felt conditioning. 

Traditionally similar boxes have been selected to all felts of the press section, even though capacity requirements can vary significantly at different positions. Felts, have also developed during the years, thus sometimes it can happen, that even at the same machine some felts are running too wet, while the other ones are running dry.

Felt aging has also a strong effect on the operation of the felt, thus modeling methods can never be very exact. Energy consumption of felt conditioning is anyhow so high, thus it is important to be able to estimate efficiency, even with a rough method.

We made a simple application (Excel file), which can be downloaded from the KGU Appstore "Efficiency of uhleboxes".  In the following we will show how to use this application, and with a simple example explain how conclusions can, or can not, be made based on it. The theory and development of the model is described in more detail at our earlier blog  (published in  Feb, 2014 ). 

2. PM production data

In our example we study the operation of felt conditioning an a two ply liner machine. Press section has two nips, shoe at the first nip. As input data, the application needs PM production data and web moisture before and after press section. For details, see picture 1.

Picture 1: PM production data needed at the application

Total removed water amount is calculated based on the production data. It can be used to verify the felt moisture measurements and estimate the ratio of nip and uhlebox dewatering. Today many new machines have water measurements, but most older machines do not.  

3. Moisture measurements of felts

Felt moisture measurements are also needed to estimate the efficiency of felt conditioning, as well as felt dry basis weight and uhlebox operation conditions. Compare to picture 2.

Picture 2: Required data for the estimation of dewatering 
efficiency of uhleboxes.

Felt measurements typically include more information of moisture profiles and permeability of felts, but in this application we need only average moisture before and after nip ( = after and before ulhebox). Dry felt basis weight is needed to calculate the relative moisture.

In can often be seen, that the uhlebox vacuum is shown and commented at felt measurement reports, but uhlebox slot widths are not commented at all. However open area of the box is as important as vacuum level, and at typical operation conditions they more or less compensate each other ( = 10 % increase in the vacuum level equals 10 % bigger open area). 

As an example, one Middle European mill had overloading problem at vacuum system. This problem could not be solved with a desktop study, thus mill visit was needed. During the visit It was noticed, that PU felt and 3rd press felt piping were cross-connected.  The big pump of PU felt was connected to the 3rd press felt, where open area of the box was only half compared to the PU felt. Vacuum level control did not exist, thus vacuum level was very high ( most of the time over 65 kPa) and that's why the motor was overloaded. Luckily felt conditioning was so much "over-sized" , that required felt moisture could be achieved, and only the motor overload was an issue for the mill.

4. Water balance of felt.

Based on the data given at pictures 1 and 2,  water balance of the felt can be calculated (Compare to picture 3).

Picture 3: Felt water balance ( 2nd nip top felt)

Water content of the felt, relative moisture and uhlebox operation conditions can be seen from the water balance of the felt. The operation values of one felt can be seen at a time, and the felt can be selected at the right top corner of the picture.  

The product of vacuum (kPa) and  retention time (ms) correlates directly with the energy consumption of the vacuum pump or blower. Thus, it is logical, that felt moisture also depends on the product of vacuum level and retention time ( = slot width) of the box. Uhlebox dewatering and vacuum*time are shown at the bottom of the balance sheet. 

5. Benchmarking of uhlebox efficiency

When the uhlebox dewatering is plotted as function of felt moisture before the box ( = after nip), we can benchmark the operation conditions of each felt.

Picture 4: Dewatering efficiency of uhlebox (benchmarking).

In our example, we selected a machine, where the operation conditions of felts differ from each other, and thus the conclusions are easier to do.  

Red dot in the graph indicates the felt, which is shown at the water balance ( = 2nd top felt).

In this kind of press section concept, the first nip removes most of the water. Thus relatively small amount of water is available at the 2nd nip. Compared to the required dewatering at the uhlebox, a lot of energy is used at the 2nd nip top felt. As a result of this, felt is running dry.

On the other hand, operation conditions of the 1st bottom felt are totally different. Compare also to picture 5.

Picture 5: Operation conditions of 1st bottom felt

The 1st nip of the PM is equipped with shoe and thus dewatering at the nip is high.  For some unknown reason, the slot width of uhleboxes of 1st bottom felt felt were small compared to the other felts. As a result of it, felt was running wet, and caused operation problems at the PM.

PU felt and 2nd nip bottom felts in this example are at the middle area, and no clear conclusions could be made.

In this example, differences were clear, and conclusions easy to do. However, the mill had been operating in the same  way for ever. Thus, even if this method seems simple, it is sometimes important to list all basic operation parameters, and check if there exists easy improvement possibilities. In this case the solution was to change the uhleboxes between 1st bottom and 2nd top felt, and modify vacuum system capacity correspondingly.

6. More examples

The previous  example was from the other side of Atlantic ocean. Attached two more examples, one from Scandinavia, and the other one from Far East.

The conclusions of these examples can be explained based on picture 6 ( compare to blog Feb 2014). When energy consumption at the uhlebox  is increased ( = higher vacuum and / or higher air flow and retention time), felt moisture decreases. However, the curve "saturates" when relative moisture of the felt is low enough, and the moisture content can not be decreased, no matter how much energy is used. In this example the curve becomes "flat" when Vacuum*time is over 100 - 120 Pa*s, and felt moisture about 650 g/m2.     

Picture 6: Development of felt moisture

As an example, one Scandinavian mill wanted to improve the efficiency of uhleboxes.  Felt was running wet, and  according to benchmarking the operation point was near the red line at picture 7 ( = blue dot). Open area of the uhlebox was increased, but unfortunately it  was connected to a vacuum pump ( "Nash pump"). Vacuum level control did not exist,  thus vacuum level dropped and felt dryness did not improve. 

Picture 7: Benchmarking of  felt Runing conditions 

Another example is from Far East, where mill also wanted to improve dewatering at the uhleboxes of PU and 1st bottom felt. Red dot at picture 7 indicates the  operation point of the felts. Increase of vacuum level could however have only marginal effect to the felt moisture content, because the felt moisture content was already low ( = operation was at the "saturated flat area"). Compare to picture 6.

So far we have been talking about uhlebox dewatering only. Minimizing felt moisture before nip is however not necessarily the optimum operating strategy. Important is to maximize total water removal including also dewatering at the nip. 

7. Dewatering at the nip

Especially with printing and writing papers ( and also with lower basis weight boards) significant part of the water is removed at the nip. In these machines purpose of the uhlebox ( if used at all) is to equalize moisture profiles, condition the felts and control felt moisture before the nip to the targeted level. 

When nip dewatering is targeted, it is important to avoid over-drying of the felt. Felt suppliers should be able to inform, what is the optimum moisture for different felt types. It depends on the felt type and age, and sometimes even felt moisturizing showers have been used to speed up the start of the felt. Compare also to picture 8.  

Picture 8: Examples of operation conditions when
nip dewatering is targeted

8. Felt operation summary

Summary of felt running and operation data is displayed end of our application. This table summaries the most important operation parameters of all felts. See picture 9. 

Picture 9: Summary table of felt operation conditions

9. Conclusions

When the operation of PM vacuum systems are studied, often the target is energy savings, but in many cases different operational issues are also studied. Other articles concerning benchmarking and efficiency losses of PM vacuum systems have been described at earlier blogs, and all applications can be found from our AppStore. 

This application (3.5 Efficiency of felt conditioning) can be loaded free of charge from our homepage (  http://www.kgu.fi/downloads.html ).

This method of benchmarking the felt conditioning is simple, but it gives us (process engineers) a simple method to estimate vacuum requirements at the different PM concepts, and we hope it  could be useful for the paper makers also.   

   

Efficiency loss because of throttle at PM vacuum systems

Abstract

Even though variable speed drives are today more common than before, there exists many older paper machines, where vacuum pump or blower capacity does not match with PM requirements, and vacuum levels are controlled with a throttle valve. Pressures losses can also take place at the suction piping if velocity of  the air too high.  

Sometimes "hidden" energy saving potential can be found from the vacuum systems, because of the pressure loss.  If air flow (m3/s) and pressure drop (kPa) are available, the efficiency loss can be estimated.

A method for estimating the efficiency loss ( kWh/a and €/year) because of pressure drop at suction lines of vacuum system is described in detail in this article. 

1. Expansion if the air (ideal gas law)

Even though simplification of the ideal gas law ( p * V = Constant )  should be well known, it seems to be difficult notice when ( or how ) to apply it.  As an example, when vacuum level is increased from 40 kPa up to 60 kPa, the volume of air increases by 50 %. 

Picture 1: Expansion of air as function of vacuum level

Thus, if vacuum at the paper machine is 40 kPa, and pressure loss is 20 kPa, required capacity of vacuum pump or blower is 50 % higher compared to what is required. In practice, some pressure loss at the piping must anyhow be accepted ( typically about 3 kPa ).  

There exists many examples from paper mills around the world, where valves have been throttled at the suction lines, pressure losses exist at the piping and efficiency losses  because of expansion are high. The reasons vary a lot, but they can be categorized as follows:

• Variation of raw material quality ( recycled fibers ),
• Development of felts and and operation conditions of uhleboxes,
• Grade change of the PM ( high basis weight ),
• Rebuild of the machine has changed  vacuum requirements, but vacuum system not updated,
• Traditional design of automation at vacuum system ( low automation level ), 

Design figures of machines are typically also on the safe side. If losses take place, energy consumption of vacuum equipment ( pumps or blowers) is higher compared to requirements. 

2. Estimation of the cost of efficiency losses 

In order to estimate the energy savings, the characteristics of pumps or bowers should be available. It is however possible to estimate the energy saving potential also based on the specific energy consumption ( = SEC)  of vacuum equipment  ( compare to blog March, 2013 and KGU application 3.2 :  http://www.kgu.fi/downloads ). 

If we use the same figures as before,  SEC of vacuum equipment  increases when the vacuum level is increased from about 50 kW/(m3/s) up to 70 kW/(m3/s).  Five  kPa pressure loss reserved for the piping. 

Picture 2: vacuum ssytem equipment SEC ( kW/m3/s)

As a summary, if pressure loss is  20 kPa ( vacuum at PM  40 kPa), required energy at vacuum system doubles compared to ideal situation.

3. Estimation of air flow

It is important to be able to estimate the economical aspect of efficiency losses. In order to estimate the energy loss and costs, we also need to know air flow ( = not only SEC ). If pump or blower capacity ( = air flow) is known, it should be used in the estimation. Also the design air flows ( from PI diagram or machine supplier) can be used. It is important to notice, that the design figures often are on the "safe" side ( = maximum figures for design).

There exists also one more option. If vacuum level is controlled with a throttle valve, the air flow can be estimated based on the valve size and opening angle. This method can be used, if other data is not available. For details compare to picture 3, air flow through a typical butterfly valve ( Source: Nelprof valve sizing program).

Picture 3: Example of valve characteristic, Butterfly DN-400

At some cases air flow measurements may be needed to verify the air flows. For example when several suction lines are connected to the same header, and individual flows from different positions of the PM are needed, or pump and blower capacities needs to be checked.

4. Annual energy losses

Annual operation  hours and price of energy are needed to estimate the cost of the throttle loss. Typically the market price of electric energy can be used at the estimation, because often mills buy or sell electric energy depending on their own consumption of it.  

The operation conditions and annual costs of the efficiency loss are shown at the summary page of our applications. (For details, compare to KGU AppStore, App 3.4 Efficiency loss because of throttle, http://www.kgu.fi/downloads). 

Picture 4: Annual costs because of throttle loss

Our applications can store up to twelve calculated  positions in the same calculation file, and the flowchart can be displayed one by one.

5. Summary

Throttle losses are often much more difficult to identify compared to bleed losses or efficiency of equipment. That's why they often stay "hidden" and can cause significant unnecessary energy consumption and operation  costs at PM vacuum systems.

We hope our applications will help to identify efficiency losses and estimate feasibility of improvements. For details, see our home pages ( http://www.kgu.fi/ ).

In the next blog we will be talking about efficiency of  felt conditioning at the press section.

Efficiency losses at vacuum level control of PM vacuum system

Abstract

Vacuum level is traditionally controlled at the vacuum pump system with bleed air. When the pump capacity does not match with PM requirements, these bleed air valves are open, and system efficiency is poor. 

At a blower system the bleed valves can also be open, if PM operation conditions have changed and the blower is operating at the minimum load.

This article describes a method for estimating the efficiency loss at vacuum level control of PM wet end vacuum system. Corresponding application can be found from KGU AppStore, application 3.3.

For details, see:  http://www.kgu.fi/downloads.html 

1. Vacuum level control with bleed air

Paper mill vacuum systems can be divided into two categories:
   - Liquid ring pump ( LRP / Nash ) systems, and
   - Blower systems.
   
Traditionally the vacuum level control and operation of pump systems has been designed to be as simple as possible. Typically one suction position at the paper machine is served with one pump, and the vacuum level is controlled with an automatic "bleed" valve ( = pump systems). 

Picture 1: Principle of vacuum level control ( pump system)

Operation principle of blowers is different, and bleed losses are not so common. Diffusor control enables operation in a wide capacity range and simultaneously keep the vacuum at required level (  compare to MAN Turbo RT blowers ). 

However, sometimes requirements at the PM have changed, and also the blowers may operate at the minimum load, and bleed valves at the suction side of the blower can be open. This may happen for example when the running conditions of uhleboxes  have changed during the years, vacuum levels are lower compared to the design ( = nip dewatering ).

2. Estimation of bleed air flow at vacuum level control

Air flow through the valve can be calculated based on the valve characteristics. For example NelProf is one good tool for this estimation ( compare to picture 2).

Picture 2: Example of valve characteristic ( NelProf)

Valve type, opening angle and vacuum level are needed to calculate the air flow. We are using butterfly valve at our air flow calculation (KGU Applications), because the butterfly valve is very common at the vacuum system. Our model gives a little smaller flows compared to Nelprof, but correlation is good ( = we want to be on the safe side and we do not want to over estimate the energy saving potential at our applications).

3. Electric energy saving potential

Energy loss of the bleed air flow can be estimated based on the specific energy consumption of the vacuum pump  or blower (compare to the picture 3).  Our applications use  in feasibility estimates the red curve, which gives the best available ( =lowest ) SEC.  In practice, the efficiency of vacuum pump often lower, but again we want to be on the safe side in our energy saving estimates.     

Picture 3: Energy consumption of vacuum equipment

4. Calculation of the efficiency loss

Efficiency loss because of bleed  at vacuum level control can be estimated based on the specific energy consumption of the pump or blower ( picture 3) and bleed valve operation data. 

Calculation is done with the KGU application 3.3 ( Efficiency loss at vacuum level control).

This simple software calculates the bleed air flow, estimated energy loss (kW)  and annual energy cost saving potential ( Euro/year). Compare to the picture 4.

Picture 4: estimated bleed losses ( kW )

5. Summary and conclusions

Many paper mills have been designed during the 20th century, when energy price was low, and often investment cost and level of automation at the mill have also been minimized. 

Today, when environmental issues are more important and energy cost are higher, many new technologies can be feasibly utilized to reduce energy consumption and operation costs. Especially at PM rebuilds and grade change projects it is important to check also the efficiency of existing vacuum system.  

This is one of our first applications , which can be used to identify energy saving possibilities at paper making lines.  In our next article will talk about throttle and pressure losses at vacuum system.

We hope our simple applications will make it easier to identify feasible energy saving improvements  at paper making lines.


Kari U Kokkonen,
M.Sc,  Process technology
CEO,  KGU Engineering Ky
Homepage:   http://www.kgu.fi/home.html     

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On-line benchmarking of PM line electric energy consumpion

Abstract

Due to environmental issues and profitability of paper production,  energy consumtion of the PM line is normally followed carefully during production. However, what consumption level is high or low, can be find out only by comparing the consumption to other machines, favourably producing similar grades.

A new tool for the comparison ( benchmarking) of electric energy consumption on-line is now available at  www.kgu.fi . Basis of this new on-line benchmarking method is summarized in this article.

This new on-line benchmaring can always be executed free of charge, and the target has been to make it as simple as possible, thus everyone interested in energy costs of the paper production line can easily execute it. 

1. Required data for the on-line benchmarking

 

  1.1 Mill data

Mill name, location and machine number can be stored in the database, but it is optional, and benchmaring can also be done anonymos. However, if a more detail study or comparion is needed at a later stage, it is not possible, if the machine can not be identified.

1.2 PM line production

Calculation of electric energy consumption is based on 100 % efficiency of the line. Thus the production is simply calculated based on the machine basis weigh, speed and width, which must be given to calculate the production ( @ 100 % efficiency).

      Prod (t/h) = Basis weight(g/m2) * Reel trim (mm) * Speed ( m/min) * K          (1)

1.3 Energy energy consumption ( SEC)

Energy consumption is calculated based on the normal running load (NRL) of the line during production, which is thus needed for the benchmaking.  Specific energy consumption is the ratio of NRL-load and production.

      SEC ( kWh/tn) = NRL-Load ( MW) / prod (t/h)                                            (2)

The price of enercy ( €/MWh) is also needed for the estimation, compare also to statistics of electric energy prices in Europe /1/.

1.4  Production line efficiency and operation time

At the end, the energy costs of packed production and annual energy costs are of cause important. Thus production line efficiency and annual operation time are needed.

                        

                Picture 1: Parameters of the PM line efficiency

2. Calculation results of PM line energy consumption

Based on the data given above, several specific consumption and cost figures can already be calaculated, such as:
   - Line Specific energy Consumption ( SEC, kWh/tn),
   - Specific energy costs ( €/tn),
   - Annual electric energy costs ( M€/a),
   - etc...

These figures are calculated,  by clicking the "Calaculate PM line SEC" button ( see example  2)

                    

                Picture 2: Example of PM SEC calculation

3. Benchmarking of energy consumption

 

3.1 Standard deviation

Standard deviation is defined according to the formula shown at the picture 3.  

                  
                Picture 3: Standard deviaton
    

3.2 PM line categories ( based on SEC of the line)

Production lines are divided into four (4) categores, based on the specific energy consumption according to the table 1.

                      Table 1: Production line categories based on SEC
                      -------------------------------------------------------------------------------
                      Category    SEC (kWh/tn)                         Comment
                      -------------------------------------------------------------------------------
                         1            SEC < Average - Deviation      Low consumption   
                         2            SEC < Average                       Lower than average
                         3            SEC > Average                       Higher than average
                         4            SEC > Average + Deviaton      High consumption
                      -------------------------------------------------------------------------------  

The distiribution ( of SEC) can be described according to picture 4. Thus for example, about one third of production lines will land to categories 2 or 3 ( production lines, where SEC is near to the average). Thre rest of the lines will belong to the categories 1 or 4 ( relatively high, or low electric energy consmption).

                       
                Picture 4: Standard deviaton

Our database is "live", which means that the average and distribution levels will develop during the time when new data is enetered to the database.

4. Benchmarking of electric energy consumption of PM lines

The average electric energy consumption of all PM lines at our database is about 550 kWh/tn ( May, 2014), which  equals to the energy cost of about 38 €/tn ( energy price 70 €/MWh).  Differences between  paper grades are relatively high ( see table 2 ).

Of cause we hope these figures will slowly decrease in the future, and up to date average and deviation figures can be seen, when the benchmarking is executed on-line ( http://www.kgu.fi/benchmark_pm_sec.php )

Table 2: Average SEC levels at the database ( Compare also  /2/ and /3/ )
     

Grade	Ave SEC	StdDev.	Ave cost
	kWh/tn	kWh/tn	€/tn
Newsprint	534	60	35,2
SC (magazine)	607	78	42,5
LWC / MWC	562	83	39,3
WFC ( Coated Fine)	567	78	39,7
WFU ( Uncoated fine, Copy)	552	79	38,7
Liner/Fluting < 150 g/m2	455	86	31,8
Board_over > 150 g/m2	460	100	32,2
Averages	546	77	37,9

5. Artifical intelligece

On-line benchmarking includes also verbal analysis. Several comments are given in this "artificil intelligence" section of the program. Energy consumption and annual energy costs area compared to average, and good ( =average-deviation) levels. Comments are displayded depending on the theoretical energy saving potential compared to the other machines based on four (4) different parameters:

          1. What category PM line belongs to ( 1-4),
          2. SEC ( kWh/tn) compared to the other machines,
          3. Annual energy consumption ( MWh/a), and
          4. Annual energy cost of the PM line ( M€/a).

All parameters are further divided into several subcategries, thus there exists theoretically more than one hundred variations of different comments that can be given for the user.

                
                Picture 5: PM line categories based on SEC.

 6. Comments and conclusions

Benchmarking alone can of cause not give detail solutions for possible improvements, but it can indicate overall saving potential compared to other lines ( producing similar grades). The only way to find out the feasible improvement positions is to study in more detail the consumption of various sub systems.

Typically there does not exist only one single solution to achieve a good, low level of energy cnsumption. Low SEC can only be achieved by studying process systems one by one and make the improvements where needed.

We will discuss the feasible improvement possibilities of PM subprocesses in the future articles in more detail. We hope  this PM line total electric energy benchmarking will help to assess the importance of the energy saving  improvements of various sub systems at the mill.

References

1.  Energy price statistics by Eurostat / European Commission
      http://epp.eurostat.ec.europa.eu/statistics_explained/index.php/Energy_price_statistics

2.  Pulp and paper industry, energy bandwidth study - jacobs / IPST
      http://energy.gov/sites/prod/files/2013/11/f4/doe_bandwidth.pdf

3.  Energy efficiency in U.S, manufacturing / The Case of Midwest Pulp and paper Mills
      http://www.wri.org/sites/default/files/energy-efficiency-in-us-manufacturing-midwest-pulp-and-paper_0.pdf