Energy Efficiency of Dry Tank Technology

In a recent study it was estimated that the EUs production and treatment of compressed air for use in industrial processes accounts for around 10% of the sectors total electricity consumption. It is estimated that a third of this energy can reasonably be expected to be saved through the implementation of energy efficiency measures.

The  Dry Tank Technology (DTT) system is an innovative approach which provides a minimalist, low-cost system for the energy efficient production of dry compressed air. This counter intuitive and potentially disruptive innovation provides a system capable of controlling and maintaining a supply of liquid-water-free, unsaturated compressed air to the point of use under varying environmental conditions. With the exception of large scale processes which operate under well characterised load conditions the majority of compressed air systems have to accommodate large variations in demand and, particularly in the case of the smaller systems, these variations are indeterminate. Running compressed air systems intermittently and/or under partial load is the main cause of inefficient generation. The fundamental approach of the DTT system addresses this issue allowing electric motors to be consistently run at peak efficiency, integrates air drying with production and provides a simple means of reducing energy loss through heat recovery.  Further energy savings can be realised from the removal of unnecessary component pressure drops and their associated leakage pathways and through the improved energy efficiency when used in combination with existing drying technologies.

Impact on Energy Efficiency

Estimates of energy usage attributable to the compression and treatment of compressed air generally rely on market studies, individual user assessments and trade body industrial intelligence.  Whilst reasonably accurate usage figures are obtainable for the numerically smaller, high-energy usage market segments e.g. process gas, low pressure compressors, it is much more difficult to estimate these figures for standard industrial users of compressed air.  In addition, recent studies[1] have indicated that even when reasonable assumptions have been made, errors in total compressor energy usage may be as large as 30-40%.  Given also that internal energy auditing practices within the majority of companies in this segment may, in most instances, be less than rigorous it is not surprising that costs for compression and gas processing are not well documented.  Whilst further EU based studies[2] have indicated EU usage at 60 TWh/yr (10 to 110kW compressors) and 20 TWh/yr (110 to 300kW compressors) these figures fail to take into account the most populous and very significant smaller compressor market segment (<10kW) which accounts for 65% of the total units sales where gaps in the studies point to an energy consumption in the region of ~25 TWh/yr.  It is in this most populous market segment where the diversity of end users and end use consumption result in extreme variations in usage patterns and load profiles thus precluding the production and drying of compressed air from efficient modes of operation.

In fact studies 2,[3] have indicated that by improving motor efficiencies and control systems, recovering waste heat, improving the cooling and drying of compressed air, optimising device and frictional losses and reducing leaks that system energy efficiency improvements of between 5 and 50% are possible.
The Dry Tank Technology system[4] by virtue of its innovative approach to the production and drying of compressed air fundamentally addresses the causes that give rise to excess energy consumption.  Unlike existing compressed air drying methods which can only achieve their maximum efficiency under full load conditions, the DTT system is designed to provide maximum efficiency even for intermittent operation and for partial load conditions. The DTT system condenses and removes the majority of water vapour at source, prior to storage, under controlled conditions independent of the demand for supply. This water removal methodology has a number of key and associated benefits.  The table below highlights these benefits and the likely energy and resource savings for the <10kW market segment alone:

Table showing the energy efficiency benefits possible when using the Dry Air Supply technology. Defined and qualified by benefit type.
©AGISEN limited 2017

No attempt has been made to address or quantify energy saving which may be accessible for  compressor systems <10kW.  However, the technology is equally applicable to compressors above 10kW where savings of the order of up to 30% (15TWh/yr) on energy demand of >50TWh/yr is reported to be possible over the next  30 years.

Innovation and Benefits

A traditional air drying arrangement consists of a compressor which compresses air which is then stored and partially cooled in a receiver. The saturated vapour pressure (SVP) of the air drops with temperature and water is condensed in the receiver. If no dryer is used then this saturated air is passed through the distribution system to the point of use. During this process all the pipework and ancillary equipment becomes wet. If the system pressure drops, most usually due to load demand, then even more water is liberated into the distribution pipework and travels as vapour in the compressed air stream. If a refrigerant dryer is used then the saturated air from the receiver is cooled and water removed; it is now unsaturated. The delivered air is dry but its dew point will vary depending upon load.

The DTT system uses a controlled pressure and temperature environment isolated from the storage receiver and other system components to provide a source of water-free, unsaturated compressed air. Pressure dew point differentials of up to 6 degrees celsius below ambient temperature independent of load is possible. This system has the following benefits in the arrangements described.

Replacement of Refrigerant Dryers

  • As the DTT system is only operational when the compressor is loaded whilst a refrigerant dryer needs to be in continuous operation throughout the working day a significant energy saving (~75%) is possible under indeterminate and intermittent compressor load conditions.
  • The DTT system compressor is optimally loaded at all times. Normally system pressure drops, due to load demand, would result in inefficient pumping and motor operation. Motor energy efficiency improvements of the order of 10% is possible.
  • Although low-cost refrigerant dryers are available, they have very short operational lifetimes circa (18 months) and a very limited (none) spare parts provision.  The DTT system will provide a water-free drying solution with an operational lifetime that is likely to exceed that of the compressor.

Used in Combination with Desiccant Dryers

  • The DTT system will typically remove between 75% and 86% of  the water  from the compressed gas. This significantly reducing the drying demand placed upon a desiccant dryer and the amount of dry bypass air required for regeneration. Typical savings of the order of 35% are anticipated.

Energy Efficiency Benefits

As no saturated or water-laded air is present within the whole compressed air distribution system:

  • The need for distribution pipe drains loops, automatic and manual water drains/traps and air filtering equipment is reduced hence reducing pipe joints and reducing system leak losses.  The reduction in leak losses leads to substantial improvements in energy efficiency.
  • A reduction in filtering and water removal equipment from the distribution system reduces friction losses and the amount of pressure required by the compressor to deliver a required system pressure.
  • The amount of corrosion and pipe scale within the compressed air distribution system is reduced.  This result in lower frictional losses (across filters and at the pipe wall) and more efficient operation of valve and regulator.

Further benefits include:

  • Less use of  high global warming potential (GWP) refrigerant gases.
  • Improved efficiency of heat recovery systems due to localisation at the point of highest gas temperature, the compressor.
  • Fewer system inspections (GWP and general); less water.
  • Fewer maintenance issues and filter changes. Lower service and replacement costs.
  • Improved power tool life.
  • No need to dry system pipework when changing between applications e.g. power work and grit blasting/spraying. This usually requires blowing water out of the pipework.
  • Greatly reduces the need to rework jobs e.g. spraying, the costs saving here are unspecified but can be the most significant saving depending upon the application.

See Also

References

[1] Van Elburg.M. and Van Den Boorn. R., Ecodesign Preparatory Study on Electric motor systems / Compressors,2014

[2] Radgen, P. & Blaustein, E. (2001). Compressed Air Systems in the European Union, Energy, Emissions, Saving Potentials and Policy Actions. LOG-X Verlag GmbH, ISBN 3-932298-16-0, Stuttgart, Germany

[3] The Carbon Trust, 2012. How companies can save money from thin air

[4] Litt, T.J., and Murray, S.C., Agisen Limited., 2016., Compressed Gas Drying System. United Kingdom Patent Application 1618232.1

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