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 feasibleat 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/.
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| 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/ .
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| 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 ).
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| 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.
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| 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).
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| 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.
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| 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).
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| 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/
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