Customized solutions for pneumatic conveying

1 Introduction
If a company can use approximately 99.5 % of the compressed air generated by an oil-free operating screw compressor as pneumatic conveying air, and if only about 0.5 % have to be blown off into the atmosphere to maintain a constant pressure, the system is operating with an efficiency that is unsurpassable in practice. The Sulzheim-based Südharzer Gipswerk GmbH, located south of Schweinfurt, has been using exclusively screw compressors and rotary compressors from the Hamelin-based Aerzener Maschinenfabrik for more than 20 years. For the time being 18 units (Figs. 1–3) from Aerzener are running in Sulz­heim, three of them in a central station. The latest plant for pneumatic conveying was commissioned in February 2008, i.e. an oil-free operating screw compressor, type VML 18 R-G5 of the new Delta Screw Generation 5 (Fig. 4).
 
2    In production for 60 years
For approximately 60 years Südharzer Gipswerk GmbH has been producing gypsum in Sulzheim, one of the company’s four production sites. Gypsum is mined there in the immediate vicinity. Special types of gypsum are produced there, which are used, amongst other things, to make pottery plaster and chalk for schools, which in part are sold worldwide. Furthermore, machine gypsum, manual gypsum plaster and plaster of Paris are also produced, mainly for the German market. At present the annual total production of the Sulzheim plant amounts to approx. 150 000 t. “We need compressed air for two purposes: firstly, in small quantities but with higher pressure for the control of our production facilities, and secondly, in large quantities but with a low pressure for the pneumatic conveying of our pulverulent products in an extensive piping system,” explained works manager Helmut Weiss (Fig. 5).      
 
3    Oil-free compressed air up to 3 bar (abs.)
This conveying air must meet two requirements. It must have been oil-free compressed to achieve high quality products. For special gypsum products a temperature of the compressed air of approx. 50 to 70 °C must be ensured all year round. With this temperature gypsum does not lose water, i. e. it does not dehydrogenate. Furthermore, it is ensured that the gypsum will not suffer a cold shock, in particular in winter. To produce this conveying air, rotary blowers are used, which are installed decentralized on the works premises. Furthermore, two oil-free screw compressors and one more rotary blower are running in a central compressed-air station. All units were supplied by Aerzener Maschinenfabrik located in Aerzen near Hamelin and have been operating, in part, for more than 15 years.
 
The central station has been arranged in the immediate vicinity of a so-called source silo, from where the already powdery raw material is conveyed via a piping system to the individual further processing plants at a distance of up to 200 m. Works manager Weiss does not see any alternative to this pneumatic conveying system, because there is no solution operating with such a high efficiency, profitability and ease of maintenance. The oil-free compressed air is fed with max. 3 bar (abs.) to the conveying line. In a transfer station the precisely proportioned, pulverulent gypsum enters this conveying line below the source silo. Works manager Weiss explains: “The trick is to adjust the amount of compressed air and the feed rate of the different types of gypsum with different bulk densities as optimally as possible to each other. The goal is to transport a maximum amount of gypsum with a minimum amount of compressed air up to the end of the conveying lines with a length of up 200 mm.”
 
4    Central station of oil-free compressed air
of up to 3 bar (abs.)
At present, a belt-driven screw compressor, type VML 18  R-G5, from Aerzener Maschinenfabrik has been running in the central compressed-air station, which has access from outside, since February 2008. This plant of the new Delta Screw Generation 5 supplies an intake volume flow of approx. 1000 Nm³/h with a discharge pressure of max. 3.0 bar (abs.). It is driven by a particularly energy-efficient motor of the class EEF1 with a rated power of 75 kW. Since the compressed air is only fed to the transport system below the source silo, it must be ensured that the gypsum is always in motion in order to avoid build-ups that would lead to clogging in the piping. An overflow controller will prevent the compressor from being switched off due to possible build-ups. This controller depends on the amount of the material being conveyed in the piping. The compressor compresses “against the system”, i.  e. if the pressure drops in the conveying system the output of the compressor will be reduced in parallel. In this particular case the motor has a rated power of 75 kW, however, its maximum coupling power only amounts to
57 kW. The pressure is specified based on the ­resistance of the piping ­system. The power consumption of the compressor develops correspondingly, i.e. if the piping requires 10  % less pressure, the power consumption (if necessary, minus the ­efficiency) is reduced analogously from 57   kW to approx. 51 kW.           
 
The new, more efficient compressor plant from February 2008 has replaced a 16-year old screw compressor from Aerzen, type VM 137  D,
with a plant volume flow of approx. 850 Nm³/h.  The identical plant, which is still in existence, also a 1992 model, is usually used for another plant section. However, in special cases it can be used as reserve plant for the new screw compressor VML 18  R-G5.
Furthermore, an Aerzen rotary blower, 1992 model, is running in the central station. The discontinuously generated compressed air from this plant is blown into the silo via a separate piping system. There it prevents the material from compacting in the discharge area and, consequently, from clogging of
the outlet. The incoming air required for the compression ­
enters the station, which is frost-free all year round, from
­outside via a duct system. It is not necessary to treat (e.g. dry) the oil-free compressed air generated for pneumatic conveying in Sulzheim. However, in other fields of application and
with more hygroscopic products (e.g. sugar) it may become necessary.
 
5    Reliability and low maintenance
So far there is no alternative to the equipment from Aerzener Maschi­nenfabrik for works manager Helmut Weiss: “For more than 15 years rotary blowers and screw compressors from Aerzen have been used in our plant for pneumatic conveying. We think highly of the great professional competence of the staff, the tailored implementation of all projects and the high reliability of the products. Due service work has always been carried out to our full satisfaction. Therefore, we always ordered equipment from Aerzen with a good conscience when new units were needed. Due to its narrow, compact design, the “Delta Screw Generation 5” with sound absorbing hood is especially suitable for our station with limited space, not least due to the possibility to install it wall to wall because of its concept of front operation.”
 
In addition to the reliability, workshop manager Anton Konrad appreciates, above all, the ease of maintenance and the low maintenance expenditure for all units from Aerzen. Continuous checks and “usual” maintenance work are carried out by their own staff. Only general overhauls are carried out by the Aerzen staff. Even after operating times of 75 000 operating hours it was not necessary to replace essential and expensive components so far. “Due to the high portion of gypsum dust the blowers and compressors run under clearly more complicated conditions in our plant. Inspite of this, they have never failed,” said workshop manager Konrad. In special cases Aerzener
can also provide rented units as a reserve capacity via their ­subsidiary AIR for a limited time.
 
6    New installations
For reasons of safety, the new screw compressor VML 18  R-G5 of the “Delta Screw Generation 5” is always switched on by hand in coordination with the blending and the production plant. It continuously supplies the specified amount of compressed air with a maximum pressure of 3.0 bar. In the run-up to the purchasing decision the plant was tailored to the requirements of the conveying system in Sulzheim by means of a detailed design. Not only a varying bulk density but also the additional requirement for filter surface cleaning in the target silos had to be ­taken into account. The plant has been equipped with an overflow controller to maintain the constant maximum pressure of
3.0 bar (abs.). It opens as soon as the adjusted pressure in the connected system has been exceeded. Then a controlled blowing-off will prevent the final compression pressure from being exceeded, thus ensuring the required continuity of pneumatic conveying under optimum conditions. It means that the compressor will not be switched off after reaching the specified final pressure because the material being pneumatically conveyed in the piping system would immediately deposit and clog the lines. They could then only be cleaned with a considerable amount of manual work. Due to a diaphragm drive controlled by the admission pressure, the overflow controller runs with a very high control accuracy, a short response time and independent of external energy. An exit silencer can be installed downstream to reduce the sound-pressure level by up to 20 dB. The Sulzheim compressor is switched off fully automatically as soon as the corresponding silo has been filled and the associated pipeline been blown off. The compressor plant is monitored from a central control room.
 
“With our generation of conveying air we could achieve an almost ideal ratio of 1:1. We are convinced that the generated conveying air is used to convey our pulverulent material in our handling system in 99.5  % of the compressor operating time. Initially, we owe this to the plant builder who determined the required amount of air very precisely. Based on this specification, the specialists from Aerzener Maschinenfabrik supplied a tailored, energy-optimized compressor so that we can convey the material with an optimum economic efficiency. We achieved our goal, i.  e. to handle as much material as possible, with a “precision landing,” said works manager Weiss.
 
7    New compressor generation
The screw conveyor, type VML 18  R-G5, from Aerzener Maschi­nen­fabrik installed in Sulzheim is a machine of the new “Generation 5”. It is based on the successful generation 4, the features of which have been further developed systematically and combined with innovations. At present the new series of the “Generation 5” is available as belt-driven design for overpressure, underpressure and nitrogen applications. Due to the sound absorbing hood, which was particularly developed for the Delta Screw Generation 5, the main requirement, i.e. to reduce the sound level by approx. 5–6 dB (A), was also met for the rotary Delta Blower G5.
 
The new series consists of seven sizes for an intake volume flow from approx. 270 to 2600 m³/h with drive powers from 55 to 200 kW and overpressure of up to 3.5 bar. When operating the plant with low pressure, the VML compressors can be used up to a vacuum of 70  % (0.3 bar abs.), as special design up to a vacuum of 85  % (0.15 bar abs.). With these plants the cooling air fan is mounted on the extended motor shaft. Therefore, it runs fail-safe, without monitoring and without any additional external energy. The new “Generation 5” offers various essential advantages to the user:
 
7.1 High efficiency of up to 20 %
The maximum economic efficiency during the entire life (low lifecycle costs) was one of the most important goals when developing the new series. Up to 90  % of the costs of a compressor, related to its entire service life, are determined by the energy consumption. Therefore, the capital expenditure for a compressor for Aerzen will soon pay off. Due to various design measures, the units of the new Delta Screw Generation 5 could be optimized from an energetic point of view.
­–    The plants from Aerzen are supplied for low-pressure applications of up to 2 bar with a 3 + 4 profile and for pressures of up to 3.5 bar with a 4 + 6 profile in order to achieve a tailored adaptation of the compressor profile to the required final pressure and for a particularly energy-saving generation of compressed air.
­–    In addition, two different outlet openings per compressor profile will increase the energy efficiency, optimizing the inner compression depending on the pressure and the volume flow.
­–    Further savings of up to 3  % are due to the use of the especially energy-efficient motors of the class EFF1.
­–    The pressure loss could be reduced by up to 20  % due to special techniques in pressure sound damping.
 
7.2 Noise development reduced by up to 6 dB (A)
Compared to the previous generation, the sound level of the new “Generation 5” could be reduced by approx. 6 dB (A) on average, and in part even more, exclusively due to the deflection of air in the pressure silencer so that in many cases expensive special measures are superfluous, such as special sound-absorbing hoods. Adsorption material was deliberately not used because of the food purity requested by the customer.
 
7.3 ATEX certification as spark barrier
The compressor plants of the new “Generation 5” are particularly certified and tailored to the categories 2 and 3 for dust and gas areas of the European machine directive 94/9/EG for explosion-prone areas. The new system of sound damping could additionally be certified as spark barrier after comprehensive tests under conditions of practice. Thus, external spark barriers are no longer necessary, e.g. during pneumatic conveying of inflammable bulk materials. This new concept means a significant cost reduction as well as reduced pressure and energy losses in addition to a maximum plant safety.
 
7.4 Compact, space-saving and easy to maintain
Space costs money. Therefore, the new units of the “Genera­tion 5” have been designed in such a way that they can be mounted directly side-by-side without any gap and without any disadvantages for service and maintenance. All maintenance areas are accessible from the front with no hindrance to service and maintenance. Furthermore, now it is possible to check the oil level from outside without interrupting the process or production, respectively.
7.5 Patented belt drive      
The drive motor mounted on a rocker always ensures an optimum belt tension due to its own weight so that it is not necessary to retension even after a long operating time. The patented belt drive results in reduced maintenance and lifecycle costs. Furthermore, this drive principle makes it possible to adjust the performance data later by a simple and cost-effective replacement of the V-belt pulley and, if necessary, of the drive motor.
 
8    Conclusions
“We commissioned the new compressor plant in February 2008. Though the installed plant was a preproduction model of the new Generation 5, it immediately worked perfectly. During the first four weeks we regularly checked its function. It always ran trouble-free so that we do not need to pay any attention to it at all now. We can virtually ‘forget’ it. Reliable and robust air compressors with a long service life are indispensable in our branch for trouble-free and economic production. Therefore, there have never been any alternatives to the plants from Aerzener ­Maschinenfabrik. In all probability this will also be the case in the future,” predicted works manager Helmut Weiss.

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