ENERGY EFFICIENCY IN COMPRESSED AIR SYSTEMS

Compressed air is a common utility and  often a significant energy user in factories and commercial installations.  Compressed air systems comprise the compressed air source (the compressor), the distribution system and the users of compressed air. To save energy, review all 3 areas.

 

Why the system and not just the compressor?

A systems approach is necessary because the compressor interacts with the rest of the system, and changes made downstream affect compressor operation and energy efficiency. As a simple example, if the amount of air used is reduced, the loading of the compressor would reduce, with potentially significant implications for the approaches required to increase its efficiency.  It is commonly advised that when evaluating and optimising systems, one should start with users and then work back to the energy source. I will take the same approach with this post.

 

Air Users

Minimising compressed air use reduces the amount of air that has to be compressed and is a powerful way to reduce the energy required by a compressed air system. I am always amazed by the low level of awareness in factories with respect to the cost of compressed air, and how it is almost treated as being "free". In generic terms, one can reduce air use by reducing the time for which the air is used and/or the flow rate it is used at. What this means in practical terms varies widely between different facilities.

 

Reductions in flowrate are typically achieved through local pressure regulation or restrictions in flow at point of use (e.g. through use of a modulating valve), and this should be considered for larger users in particular. This effectively reduces the mass of air used, but clearly this can only be done if the process objectives are still met. In some instances, compressed air use can be eliminated and replaced with low-pressure blowers. Where more air is used than is required to perform a task, this is known in the industry as "artificial demand".

Reducing the time for which air is used is really about stopping the air flow when it is not required. This can either be done manually, in which case training and work practice optimisation is needed, or via some form of automation. A common example is an unscrambler, as found on packaging lines, which uses compressed air to move/orientate items (e.g. caps for aerosol cans) in a hopper prior to them being fed to the point of use. Solenoid valves fitted onto the air supply and hardwired to the conveyor switch would ensure than when the line is not producing, air supply is terminated. Be aware that such "open blowing" applications are often considered to be inappropriate air users, and should be minimised as far as possible.

Another inappropriate use for compressed air is the use of air-driven pumps (compressed air is very inefficient as an energy source, since most of the energy that goes into the production of compressed air is dissipated as heat). Since compressed air is easily distributed around factory sites, it is not surprising that some creative approaches are applied in exploiting its convenience. I once arrived at a factory for an energy assessment and was greeted by an employee cleaning the walkway with a custom-made "air broom" - a shaft and handle with an air supply used to blow dust away.

 

Distribution System

Compressed air distribution systems have a significant impact on energy use. It is important to ensure that pressure drops are minimised, otherwise higher source pressures are required to deliver the air at the pressure required by users. Line sizes are hence important (if lines are too small, pressure drops are higher), as is the minimisation of bends in pipework. It is also important to ensure that the condition of the pipework is maintained in good order. Corrosion roughens up pipe surfaces, increasing frictional losses. While I have seen some facilities with very large (in diameter) air distribution lines, and this is also good from a storage perspective, there are implications in terms of installation costs. I recommend welded pipework or even better, screw joints rather than flanged pipework for air distribution systems. It is not uncommon to find defunct equipment that is still in the compressed air network, and through which air is still passed, contributing to pressure drops and energy losses. Equipment such as air filters should be sized correctly for the pressures and flows required. I have seen a few plants in which incorrect equipment was specified with astronomical impacts on costs. Each item in the distribution network represents an additional pressure drop, so use correctly specified equipment and only use the equipment that is absolutely necessary to do the job.

Compressed air leaks can add hugely to operating costs, and should be minimised through an ongoing leak detection and repair programme. This should be integrated into routine maintenance practices rather than be a stand-alone initiative. Take care in selecting equipment when seeking to minimise leaks. Quick-release couplings are notorious sources of air leaks, for example. Look out also for "intentional" air leaks, such as drain ports that are left open in order to allow for the constant removal of moisture, and which leak air continuously as a consequence. Most leaks are audible and the best time to detect them is during breaks, when equipment is not running but the compressed air network is still pressurised. These days there is some very advanced condition monitoring equipment available (such as ultrasonic detectors) that can not only detect leaks but also quantify them.

Incoming air has a humidity level, and hence has to be dried, since compression concentrates this moisture. Driers are a topic all of their own, each with their own energy use implications. Most larger screw compressors come equipped with integrated refrigerated driers these days, and most sites provide further backup with stand-alone drying systems, which can themselves be refrigerated or be of the desiccant type (other types exist but these two are the most common). Be aware of the consequences of not drying air effectively. Liquid in the distribution pipelines not only accelerates corrosion, it also contributes directly to pressure drop. Pay attention to automatic drains, which can fail and become leak points, or can also be cycled too frequently, leading to excessive amounts of air being lost with the water removed.

 

Compressors

Efficiency in compressed air production is about minimising the amount of energy required to produce a given quantity of air, and different compressors have different inherent efficiency levels. Various compressor-specific factors impact on efficiency, including the compressor motor, drive and the air-end design. Loading is an important driver of efficiency, and like with most equipment, low loading levels lead to inefficient operation.  The pressure of the air produced is related to the efficiency with which it can be produced, and it is best to produce compressed air at as low a pressure as possible. The location of the compressor is important, and cool (and therefore dense) incoming air is better than warm air. This is not only about atmospheric conditions, but also about keeping the compressor location away from heat sources in your facility. This includes the heat generated by the compressor itself, which should be removed from the immediate vicinity of the compressor intake. Dusty areas should also be avoided, as dust blocks intake filters/screens and increases pressure drop.

 

Poorly loaded screw compressors are inefficient, and VSD's can assist in reducing energy consumption where air demands are variable. A further important consideration is that of heat recovery from oil-flooded screw compressors. Much of the input energy to a compressor is rejected as heat from the oil and the air produced, with some OEM's reporting recovery levels as high as 90%. The common practice is to reject this heat to the environment, either in an air stream or via cooling towers in the case of water-cooled compressors. This heat can be recovered and used to produce hot water (temperatures in excess of 70 deg.C are possible) or hot air. The hot air can be used for space heating (not so attractive for warmer/temperate climates as in my home country of South Africa, where this application is only needed for a few months of the year) or for processes requiring hot air. The idea is to choose a heat sink that requires more input energy than is rejected by the compressor, in order to maximise recovery levels. A good application would be to supply combustion air for a boiler or furnace. 

 

The reason I am mentioning heat recovery here is because it has a marked impact on whether to go with a VSD replacement, or whether to rather keep your existing fixed speed screw compressor and employ heat recovery. While a poorly loaded screw compressor is inefficient, a VSD replacement compressor is typically very expensive, and it may be more attractive to employ heat recovery, since this is cheaper to implement. The point is that while your poorly loaded screw compressor will be inefficient, most of these losses would be recovered with a heat recovery system. A case-by-case approach is however required. The economics of heat recovery depend on electricity and fuel prices (assuming you are not using an electrode boiler or electrical heating system), and an analysis specific to your circumstances is essential.

The above is but an introduction to the energy efficiency considerations associated with compressed air. Where multiple compressors are employed, system optimisation becomes more complex. Remember also to back up each opportunity with a quantitative assessment of savings when making decisions regarding implementation.

Copyright © 2015, Craig van Wyk, all rights reserved