More compressed air, less energy consumption

Transcription

1 Innovative products and holistic solutions enhance efficiency More compressed air, less energy consumption Dipl.-Ing. (FH) Erwin Ruppelt und Michael Bahr In view of soaring energy prices and ambitious climate protection targets, modern industry is under unprecedented pressure to make production processes as efficient as possible. In addition to this, companies also have to maintain and increase their competitive edge, as well as strive to ensure sustained economic success. This latter objective can only be achieved, however, through significant energy savings and through drastic reductions in CO 2 emissions. It is here that compressed air, one of the main sources of energy for modern production, can help make a significant contribution. Thankfully, today s technology provides compressed air users with myriad ways to reduce energy consumption and to minimise CO 2 output. Innovative products, in addition to forward-thinking solutions, also play a key role. It has long been recognised that energy consumption accounts for the lion s share of compressed air costs, and this was already a subject of consider able discussion within the compressed air sector around the time of the first oil crisis in the early 1970s. During that period, rotary screw compressors were starting their ascendance as the technology of choice for industrial compressed air production. Spotting this trend early on, a renowned German manufacturer not only added rotary screw compressors to its extensive product range, but went on to develop proprietary energy-saving screw airend rotors. More air, more savings was already the company s slogan even back then. However, interest in high efficiency compressed air production waned in the following years and was, metaphorically, put on to the back burner. The topic of energy efficiency only came back to the fore at the end of the 90s with the signing of the Kyoto Protocol which obligated industrial nations to cut greenhouse gas emissions. At about the same time, the EU launched the Save II initiative 1. A study carried out within the framework of this initiative gave rise to the Druckluft effizient (Efficient Compressed Air) campaign, which was implemented from 2000 to This was a collaborative project between the VDMA (German Engineering Federation), Fraunhofer ISI, the German Energy Agency (Dena) and businesses from the compressed air sector. Over 30 % energy saving potential The study determined that compressed air systems in Europe had an energy saving potential, on average, of nearly 33 %. During the measurement program initiated by the Druckluft effizient campaign, the extensive knowhow of the compressed air sys tems provider proved highly valuable and insightful. The resulting compressed air audits revealed potential energy savings for the companies studied of between 18 and 70 % 2, depending on circumstance. This reveals that, in spite of some improvements, many companies compressed air installations are still more akin to those from the 1970s than to today s energy efficient systems. Additional potential for increased efficiency In addition to tried and tested methods, new technological developments also have an important role to play when it comes to increasing the efficiency of compressed air systems as a whole. In order to achieve best possible results therefore, detailed optimisation is required on the one hand, whilst improved matching of compressed air system components is necessary on the other. Electrical drives with enhanced performance In the last few years it has been possible to develop electric motors that operate at unprecedented levels of efficiency. This is attributable, in no small part, to the use of higher quality materials and improved production techniques. In order to unify the various energy efficiency standards for asynchronous motors from around the world as a globalised standard, the International Electrotechnical Commission (IEC) created the IEC international norm. It defines the efficiency classes IE1 to IE3, with IE3 being the highest class. In the EU, motors complying with this premium efficiency class will be obligatory for drives from kw as from January 1 st, With average industrial use, higher efficiency IE2 or IE3 motors usually pay for themselves within a year. The provider alluded to above already started installing IE3 motors in its new rotary screw compressors in 100 Pumps, Compressors and process Components 2011

2 2010 however, and the rest of the product range will follow suit in a gradual roll out. Energy saving compressors Modern compressors are prime examples of highly developed mechatronic systems. Consequently, their efficiency is not only determined by optimum interplay between mechanical and thermodynamic components, but also between electrical and electronic components. Power transmission too plays an important role: Modern direct drive systems (Fig. 1) eliminate the transmission losses associated with gear or belt driven systems as the drive motor and compressor airend rotate at the exactly the same speed. The energy efficiency of the airend itself can also be increased further through optimisation of the screw rotor profile and ancillary equipment for cooling, as well as through minimisation of internal pressure losses (Fig. 2). Fig. 2: Available with drive powers from 11 to 15 kw, this range of rotary screw compressors deliver up to between 11 and 14 percent more compressed air, depending on model, than their predecessors. This impressive performance boost has been achieved both through airend optimisation and the minimisation of internal pressure losses. Models are also available with an integrated refrigeration dryer or a refrigeration dryer and a sub-mounted compressed air receiver (See image). Flexible internal compressor control A further important efficiency enhancing component is the compressor s internal controller. In days gone by these systems often only had one control mode, yet modern industrial PCbased systems offer up to 5 pre-programmed options, thereby enabling compressor performance to be precisely matched to suit compressed air demand. A newly developed internal compressor controller (Fig. 3) also offers added advantages: This advanced system provides greater flexibility through its numerous interfaces and innovative plug-in communication modules. Therefore, connection to ener gy-saving master control systems, computer networks and/or remote diagnostics and monitoring systems couldn t be easier. A large display located on the control panel also simplifies on-site communication with the system, whilst the addition of an RFID reader ensures service continuity, increases security and significantly raises service quality. Fig. 1: With direct drive, the airend and electric motor are directly coupled to one another and run at precisely the same speed. This design completely eliminates the transmission losses associated with gear driven systems. Together with the coupling and stable coupling flange, the airend and motor form a highly efficient and durable unit that requires minimal maintenance. Fig. 3: These new industrial PC-based internal controllers not only enable demand-oriented performance matching and dependable monitoring, but ensure even greater flexibility through their numerous interfaces and innovative plug-in communication modules. Therefore, connection to energysaving master control systems, computer networks and/or remote diagnostics and monitoring systems couldn t be easier. Pumps, Compressors and process Components

3 High service quality Moreover, these controllers provide an excellent basis for plannable, demand-oriented maintenance resulting from continuous monitoring of compressor status and even the compressed air filters. This of course is essential for any comprehensive service concept that strives to ensure best possible dependability and availability, as well as optimised energy and maintenance costs. Needless to say, service should not be restricted solely to the compressors and other components, but should extend to cover the compressed air system as a whole. Energy efficient compressed air drying The impact of compressed air treatment on the energy efficiency of a compressed air system should also not be underestimated; this is especially true of drying. Significant strides have been made in recent years regarding the most efficient and widely used process (for pressure dew points to + 3 C) of refrigeration drying. New refrigerants and advanced refrigeration dryers equipped with energy saving cycling control, and with the ability to adapt to actual compressed air demand, have led to considerable energy savings. A highly efficient combination process involving refrigeration and desiccant drying is also now available for applications requiring considerably dryer air (pressure dew points to 40 C) (Fig. 4). Refrigeration-desiccant drying: Achieve low pressure dew points efficiently Refrigeration-desiccant drying is not a matter of simply installing a heat regenerating desiccant dryer downstream from a refrigeration dryer. Both components are combined in such a way that they ensure maximum reliability and energy efficiency. A supply of dry compressed air is essential for a wide range of applications and the use of a highly efficient refrigeration dryer is able to ensure a consistent pressure dew point (PDP) of +3 C at minimal cost: The compressed air that is to be dried flows as normal from the compressed air inlet, via the air/ air heat exchanger, to the air/refrigerant heat exchanger where it is cooled to +3 C. It then passes to a condensate separator where the resulting condensed water is removed and can then be fed back to the air/air heat exchanger for heat recovery purposes. The air then exits the system via the dryer outlet. The second stage is used to provide lower compressed air pressure dew points, e. g. 40 C. This is achieved by interrupting the airflow path between the condensate separator and the air/air heat exchanger. As a result, the compressed air passes to the desiccant dryer's air inlet instead. At this stage, the refrigeration dryer has already removed approximately 85 % of the moisture that was originally present in the compressed air. This means that the desiccant dryer only has to remove the remaining 15 % to achieve a PDP of 40 C. The compressed air to be dried flows alternately through two desiccant-filled chambers where it loses its residual moisture to the desiccant. The dried air then flows through the chamber outlet, via a particulate filter, to the refrigerant dryer s air/air heat exchanger in the first stage. From there, the air exits the system via the dryer outlet. In parallel to this process, a fan draws in ambient air via a filter and the air is then heated. From there, this hot air then flows through the second desiccant chamber. The desiccant in this chamber is charged with moisture from compressed air that was dried in the previous drying procedure which took place before the tanks were switched over. The ambient air that was drawn in via the fan and heated desorbs the water from the desiccant and is then discharged via the dryer s purge air outlet. As soon as most of the water load has been desorbed, the ambient air flow is stopped and a small volume of compressed air that has already been dried is passed through the chamber. This cools the desiccant bed so that the second chamber can then take over the drying process from the first chamber (once the first chamber reaches its adsorption capacity). The energy-saving effect of this process is huge: For a compressed air flow rate of 30 m³/min (at 7 bar), for example, and a pressure dew point of 40 C, the combination approach can achieve annual energy cost savings of over 14,000 compared to an equivalent heat regenerated desiccant dryer. Moreover, annual savings can amount to more than 25,000 when compared to an equivalent heatless regenerated desiccant dryer*. *) The energy cost saving is also based on the following general conditions: Maximum ambient temperature: 30 C, maximum compressed air inlet temperature: 40 C, 6000 operating hrs/yr, energy price: 0.15 /kwh Fig. 4: Designed for free air deliveries from 20 m 3 /min and pressure dew points to 40 C, refrigeration/desiccant dryer systems offer an energy-saving alternative to stand-alone desiccant drying. Refrigeration-desiccant dryer systems are already available for free air deliveries from 20 m³/min, which makes these energy-saving alternatives an attractive proposition for an even wider range of users. With combination dryers it is also possible to completely bypass the desiccant dryer section when ambient temperatures are higher, so that only the refrigeration dryer 102 Pumps, Compressors and process Components 2011

4 which is equipped with an energy-saving control system does the compressed air drying. In some instances this method can completely replace the far more energy intensive process of desiccant drying and, in other cases, it s even possible to use recyclable heat from the compressor to regenerate the desiccant material. Analysis of actual compressed air demand Of course it s necessary to achieve best possible efficiency from the individual components within the compressed air installation, but this requirement is not in it self sufficient to ensure optimised efficiency of the system as a whole. The key to success lies in the optimised integration of these separate components into the sys tem as a whole. This is where advanced computer-aided demand analyses prove invaluable: With their help, it is possible to determine actual compressed air demand over time, establish the efficiency of a compressed air sys tem and to identify how availability and performance may be improved. Special planning software can then subsequently be used to simulate and compare various sys tem options, as well as accurately calculate potential energy savings. Optimised compressed air management There are also many areas for potential cost savings when it comes to compressed air system monitoring and control. Advanced master control and management systems provide users with the transparency and performance required to tap into these savings. Stateof-the-art adaptive 3-D-Control technology (Fig. 5) further enhances efficiency. Efficiency in compressed air production depends on how well the components within the system work together. In essence, the main factors to consider are operational reliability, compressed air availability and energy efficiency. This is where compressed air management systems make a real difference. With a powerful industrial PC at their core, they not only control and Fig. 5: This advanced master control and management system not only provides users with exceptional transparency of operations processes, energy consumption and costs. Its adaptive 3-D-Control also takes compressor switching losses into account, thereby enhancing energy efficiency even further.

5 monitor the system as a whole, but also provide the user with exceptional transparency of operations processes, energy consumption and costs. The latest generation of master controllers feature adaptive 3-D-Control (patentpending) which considers the three crucial dimensions that affect energy-efficient compressor control within a compressed air station: 1. Switching losses associated with compressor start-up and shutdown. 2. Additional energy consumption for pressure increases above the required pressure. 3. Control losses resulting from idling and FC losses. In order to ensure optimum performance, the system constantly analyses the relationship between these factors, calculates the best possible result and controls the compressors accordingly. This strategy therefore achieves impressive energy efficiency even with widely fluctuating compressed air demand. An added advantage is that the number of switching operations i. e. the number of startups, load/idling switching sequences, and shutdowns is also considerably reduced. Using this new adaptive 3-D-Control technology, the compressed air management system also strives to deliver best possible pressure performance values. The decisive factor of course is that the system pressure, in so far as possible, should not fall below the prescribed pressure required by the application. One of the main problems in maintaining the necessary pressure using control processes up until now stems from the systeminherent lag in the reaction of compressors and blowers; this is particularly evident in regard to the dead time between a start signal and the commencement of air delivery. Adaptive 3-D-Control however makes allowances for this delay through anticipatory switching operations and it is this adap tive optimisation which enables the required pressure to be maintained better than ever before. Large increa ses above required pressure and temporary spikes in particular there fore no longer occur, and any moderate increases aren t a cause for concern in most applications. In other words, the 3-D-Control optimises the compressed air station s performance within this pressure tolerance range, taking into account the three dimensions of ener gy efficiency mentioned above. A visualisation tool is also provided as standard: Information includes real-time compressor status, current control panel status, and a network pressure log for the previous operating phase, as well as maintenance and alarm messages. Furthermore, data from the management system s long-term memory can be displayed in graphical format using optionally available visualisation software. A password protected function enables information dating back up to one year to be displayed and evaluated from various aspects; detailed costs reports can also be generated. This capa bility therefore provides the basis for detailed compressed air audits and allows the user to keep a constant eye on compressed air costs and system performance. In addition, the specific power of all connected compressors or blowers within the system can be displayed in comparison to a reference value. This allows the user to determine at a glance if the compressor system is performing as efficiently as it should be. Also able to control, analyse and monitor performance of older third party compressors and components within the compressed air station, the described management system additionally provides remote diagnostics capability. Using heat recovery Heat recovery is another area that provides huge potential for signifi- Fig. 6: 100 percent of the electrical energy input to a compressor is converted into heat. With fluid-cooled rotary screw compressors, up to 96 percent of this energy can be recovered for reuse. Perhaps surprisingly though, the usable energy for work applications comes from the ambient surroundings: During the compression process and conversion of the electrical drive energy into heat, the compressor charges the air it draws in with energy potential. This corresponds to approximately 25 percent of the compressor's electrical power consumption. This energy is only usable however once the compressed air expands at its point of use and in so doing absorbs heat energy from the ambient surroundings. 104 Pumps, Compressors and process Components 2011

6 cant energy savings. 100 percent of the drive energy fed to a compressor is converted into heat. Air- and watercooled rotary screw compressors are the best-suited compressor technologies for efficient heat recovery. The greatest part of the energy consumed by these compressors, some 76 percent, is transferred to the cooling medium and then removed in the fluid after cooler. A further 15 percent of the energy can be recovered as heat via the compressed air aftercooler, whilst heat losses from the electric motor account for up to 5 percent. Modern, fully-encapsulated rotary screw compressors can use targeted cooling to recover this energy. In total therefore, 96 percent of the electrical drive ener gy fed to the compressor can be reused for heating purposes (Fig. 6). Only 2 percent of the energy is lost as radiant heat and 2 percent remains as heat in the compressed air. The most efficient and also the simplest method of heat recovery with rotary screw compressors is to directly use the cooling air that has been heated by the compressor. Air ducts feed the warmed cooling air into nearby store rooms or workshops. If there is no heat demand then the surplus heat is simply released to atmosphere via a damper or louvres. Thermostatically controlled dampers control the flow of warm air to maintain consistent room temperatures. In addition to providing heating or heating support for operating or storage areas, the recyclable heat can also be used for example to support drying processes, to create hot air curtains, or to preheat burner combustion air for heating systems (to increase efficiency). The associated investment costs are often amortised within a year. Even if only one rotary screw compressor is being operated, the implementation of a heat recovery sys tem can considerably reduce energy consumption and costs. By replacing the original energy source with reusable heat from the compressor, a 15 kw machine (operating for 1000 hours) can save around 790 in fuel oil or around 740 in natural gas each year. Furthermore, purely as a sideeffect, the environment is spared some 4.8/3.8 tonnes of CO 2 emissions. Reusable compressor heat can also be fed into the hot water heating and service water systems. The most cost effective way of doing so is with plate heat exchangers. The heat exchanger is connected to the compressor s cooling circulation system and transfers energy from the warm coolant to the water which is to be heated. Depending on whether the hot water is to be used for heating purposes, as showering or washing water, or for sensitive production and cleaning processes, fail-safe heat exchangers may also be used in conjunction with plate heat exchangers. Using these heat exchangers, some 70 to 80 percent of the installed compressor power can be used for heating purposes without the need for any additional energy consumption. This method of heat recovery can also be used with primarily water-cooled rotary screw compressors. Minimisation of leakages The energy losses incurred as a result of leakages in the compressed air distribution pipe network can be equated, in biological terms, to severe blood loss: leakage rates of 10 to 25 % are common. Over the course of a year such losses add up to appreciable additional energy consumption and, as a consequence, costs. With help from modern detection equipment however, leakages can be quickly located and rectified. This enables users to keep losses to an absolute minimum, although sadly they can never be completely eliminated. There are also other causes of ener gy losses and large pressure differences within the compressed air distribution network. These causes include contamination in the piping, inadequate pipe diameters and unfavourable pipe layout that adversely affects flow performance. These shortcomings should therefore also be corrected or avoided. Summary Through systematic implementation of the savings measures described above, compressed air system operators are now able to tap into the near 33 percent average efficiency potential identified back in 2000 better than ever before. Projected on a European scale, these savings amount to approximately 25 billion kwh per year. More over, a further 32 billion kwh would be saved if only 50 % of all operators in Europe equipped their compressed air installations with a heat recovery sys tem. References 1: Blaustein, Edgar; Radgen, Peter (Hrsg.): Compressed Air Systems in the European Union. Energy, Emissions, Savings Potential and Policy Actions. Stuttgart : See: Seitz, Anja: Ergebnis analyse der von Kaeser Kompressoren durchgeführten Air-Audits für die Kampagne Druckluft-effizient. Diplomarbeit Fachhochschule Coburg, Fachbereich Maschinenbau (2004) Authors: Dipl.-Ing. (FH) Erwin Ruppelt, Executive Project Engineer, Kaeser Kompressoren GmbH; Michael Bahr, Press Officer, Kaeser Kompressoren GmbH Pumps, Compressors and process Components