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"GreenScreens" - What does a screening machine really cost?


Meike Schulz, RHEWUM GmbH

In addition to the investment costs of screening machines, the multi-year operating costs should also be considered when making a decision. In addition to the unavoidable maintenance, a significant, although not obvious, cost driver is the energy requirement of the type of screening machine used. In the following, the total cost of ownership of different types of screening machines is considered. 


          1. Motivation

            In times of generally rising energy costs, not only the costs of basic energy sources such as coal, oil and gas are increasing, but also those of secondary energy sources such as electrical energy. The latest developments show that, in Germany in particular, the energy turnaround that has been initiated will lead to a disproportionate increase in electricity prices, at least in the medium term. Whether the additional extraction of shale gas, which is frequently reported in the media, will noticeably counteract this price trend seems questionable. Accordingly, plant operators should generally take into account a future increase in electricity prices today. Therefore, it should be considered that capital goods such as screening machines are usually operated continuously for decades. From today's point of view, it is definitely important to consider the aspect of long-term operating costs (and thus energy costs) for upcoming new investments as well as for the replacement of existing plant and plant components. 

          2. How has the price of electricity developed and how will it develop?

            Although the liberalization of the electricity markets caused the average electricity price to fall from around 9 ct/kWh to around 6 ct/kWh between 1998 and 2000, since 2000 the electricity price has now risen to around 15 ct/kWh, i.e. almost 2.5 times higher (Fig. 1), and is now a competitive barrier in energy-intensive sectors. If the price increase of the last 14 years is extrapolated to the coming years up to 2030 (Fig. 2), an electricity price of 27 ct/kWh is realistic and would then have quadrupled. Since screening machines are long-lived capital goods, it is to be expected that a corresponding price scenario is not unlikely for a current investment decision.

            Figure 1: Electricity price development in Germany: Source - German Association for Energy and Water Technology

            Fig. 2: Electricity prices in Germany - linearly extrapolated according to recent developments

          3. Comparison of different types of screening machines

            Fig. 3: Classification of screening machines

            In order to compare the efficiency, screening machines whose basic body vibrates, such as linear vibrating screens, or double-frequency screens, flat screens with circular thrust movements, elliptical vibrating screens, which are referred to below as indirectly driven screening machines, are compared with directly excited screening machines in which only the screen cloth is vibrated. 

            In the separation of bulk materials, system-related losses must always be overcome within the scope of vibration generation:

            1. Damping of the feed material by friction
            2. Damping by screen cloth
            3. Damping by bearings (springs, rubber spring elements, etc.)
            4. Efficiency of the drive (minimization of system-related losses, ≈ 2 - 10 %)
              • Losses due to gearbox that may have to be used
              • Losses due to V-belt drive (max. 10 %)
              • Drive losses of the electric drive (unavoidable)
            5. Losses due to insufficient utilization of the screen cloth surfaces

            The mass ratio between the feed quantity and the vibrating mass of the screening machine is decisive for positive energy efficiency, in addition to the efficiency of the drive. The energy efficiency of a screening machine can be determined in a first approximation by the quotient of the feed quantity to the installed power: 

            *Aufgabemenge [DE] -> Feed quantity [EN]
              Installierte Leistung [DE] -> Installed power [EN]
              Siebeffizienz [DE] -> Sieve efficiency [EN]

            An evaluation of already supplied RHEWUM screens grouped by drive type is given in Fig. 4. Figs. 4a and 4b show common designs of the respective type.


            Directly excited screening machine

            Indirectly driven screening machine

            Ratio of masses accelerated to sieving 

            mScreen+Axes << mAask

            mMachine >> mAtask

            Average installed electrical power per screening unit

            1,8 kW

            7,0 kW

            Average of the feed quantity per sieve unit

            ca. 50 t/h

            ca.100 t/h

            Average seven energy efficiency

            ca. 28 t/(kWh)

            ca. 14 t/(kWh)

            Ratio direct to indirect driven screen



            Fig. 4a: Direct driven screening machine, type RHEsono

            Fig. 4b: Linear vibrating screen, type RHEox

            Fig. 5: Comparison of screening machine types in screening efficiency and screening capacity

          4. Effects of energy efficiency on operating costs

            Simplified, it can be assumed that a directly excited screening machine generates only 50% of the energy costs as an indirectly (conventionally) driven screening machine. The following table describes the direct energy cost savings to be expected in the life cycle of such a screening machine.

            Term (a) 

            Savings potential in euros per kW saved - Electricity prices stable scenario (15.4 ct/kWh)

            Savings potential in euros per kW saved - scenario electricity prices progressive

            Realistic savings potential per screening unit with 4-5 kW difference in drive power





















            Basis: 8,700 operating hours per year and a price development as in Fig. 2

          5. Summary

            In addition to many technical advantages of directly excited screening machines, such as static housings with fixed flange connections, little vibration transmission into the screening machine environment, and others, a commercial advantage for this type of machine can be seen in the long term.

            What needs to be considered is the total life cycle cost (total cost of ownership) rather than the initial investment cost that is often considered. Even if screening machines often play a subordinate role in the energy requirements of the overall production processes, consideration of energy efficiency will also become more important in the future, especially since these are capital goods with a very long life cycle. In addition, the replacement of existing screening machines must also be considered, since directly excited screening machines are generally more compact than machines of conventional design, in addition to the energy saving potential. The energy turnaround initiated in Germany, with energy prices expected to continue to rise sharply in the future, forces such a long-term approach. An investment that is favorable at first glance can become expensive over the entire life cycle of the machine.

          6. Quellen

            Reference list RHEWUM GmbH
            Bundesverband der Energie- und Wassertechnik: Electricity Price Development in Germany 2000-2014

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