PRACTICAL ELECTRICAL ENERGY CONSERVATION ON INDUSTRIAL SITES: ARTICLE 4 - COMPRESSED AIR SYSTEMS

Air supplied to a cylinder/actuator on a machine.
Note the exhaust port, from which the air is lost
when the cylinder is depressurised.

Compressed air is used widely in industry for applications such as the actuation of automatic valves and slides, to power tools such as hand-held drills and grinders and also for a wide range of open-blowing applications. Various compressor designs exist, each with their own energy consumption characteristics. Depending on the size of the plant concerned and the applications to be assessed, energy consumption associated with compressed air can be daunting to analyse and optimise. There are however a few basic considerations that, if kept in mind, can help to rapidly reduce the energy usage associated with compressed air on industrial sites.

Before getting into the many different aspects regarding energy efficiency in compressed air systems, I would like to make an important point regarding the quantum of energy used for compressed air production. The energy used to produce compressed air is supplied to the induction motors driving your compressors. If you know the average power consumption of these motors (you will need to measure this with a power logger), and the running hours of your compressors, you can quite easily calculate the energy consumption (and energy costs) associated with compressed air production. This immediately allows you to prioritise energy efficiency opportunities in this area relative to other energy-consuming activities on your site.  The effort invested into analysis for a site operating a small 5.5 kW piston (also called “reciprocating”) compressor would generally be much less than for a site operating  a 90 kW oil-filled screw compressor, for example.

As with utilities such as steam, a systems view is essential when assessing compressed air systems. This means looking at the production, distribution and usage of compressed air, on the understanding that savings are achievable through improvements in efficiency in all of these areas. It is also important to appreciate that elements of the system interact with each other, and any changes made must be assessed in terms of their impact on the system. Halving your compressed air usage would have implications for the loading of your compressor, and may necessitate a re-think in terms of its operating philosophy, for example. Let’s look now at some of the individual components of a compressed air system and the types of efficiency opportunities typically presented.

Compressors

Compressors take in ambient air and increase the pressure of this air from atmospheric conditions to a level determined by the capabilities and settings of the compressor. Compressors should be located in cool, dust-free areas, to ensure that the air being compressed is as dense as possible to begin with, and to prevent clogging of the intake filters. Blocked filters operate with increased pressure drop and therefore increase the amount of energy required to produce a unit of compressed air. The drive system of your compressor should be as efficient as possible, and this entails:

• Using motors with high efficiency levels
• Using direct drives or cogged/synchronous belts wherever possible (instead of v-belts), and ensuring that these belts are aligned and tensioned correctly
• Using variable speed drives where appropriate (more on this below)

Depending on the type of compressor concerned and its duty, control systems can vary. "Start/stop”  philosophies are commonly employed on small piston compressors which are lightly loaded. Screw compressors are often operated using a “loaded/unloaded” philosophy. In some circumstances, screw compressors are operated using variable speed drives. It is necessary to understand each individual compressed air system as a whole in order to determine whether the method employed is appropriate. It is not always optimal to install a VSD on an existing screw compressor, since VSD’s in themselves incur small energy losses and both compressor efficiency and motor efficiency vary with speed. VSD’s add most value for screw compressors that are sometimes under-loaded or which supply processes with variable air demand, since the amount of energy used when “off-load” can be significant (up to 50% of the energy required when “on-load” in my experience).  Where multiple compressors are in service, significant opportunities for energy conservation are often available in the optimisation of the control schemes used to control them, and here again system considerations are paramount.

It is generally necessary to log compressors over a period of time to assess power consumption and system pressure variations. This provides necessary information on loading and also provides insights into demand (for compressed air) behaviour and the suitability of downstream equipment, particularly as regards items such as air receivers. When doing such logging, recognise that it will be necessary to note what is physically happening in the field, and to relate this back to the data acquired through logging. It is never a case of simply setting up a logger and coming back the next day to analyse compressor data. You will need to take note of which users are drawing air and at which times, allowing you to relate this back to the recorded compressor behaviour.

In terms of the air pressure delivered, it is best to operate the system at as low an average pressure as possible. Producing air at higher pressures than necessary not only wastes energy at the compressor itself, but also exacerbates losses due to leaks and “artificial” uses. A simple example of an artificial use would be air usage incurred for actuating valves, slides and other such equipment at a higher pressure than necessary. Such pistons travel over a given stroke distance, regardless of the air pressure applied. Higher pressures simply increase the speed of the stroke, but this speed may not be necessary, and because the mass of air in the cylinder volume is higher at higher pressures, losses result. A final point regarding air pressure is that energy efficiency and the minimum pressures demanded by individual processes are not the only considerations when determining the optimal pressure of the compressed air produced. System capacity is a further very important consideration. Generally, the pressure of the air produced has to be high enough to overcome line losses while still delivering the air at the pressure and rate required by users. Producing air at a further pressure premium allows a given compressed air system more capacity to respond to sudden demands. The installation of additional receiver capacity is one way to get around this, but of course this has a cost, not only in terms of capital, but also in terms of losses for systems that are regularly depressurised.

Compressor sizing has a significant impact on energy efficiency. Excess compressed air production capacity can waste a significant amount of energy in the case of screw compressors, since they will spend a lot of time off load. For piston compressors, excess capacity means increased stoppage and start-up frequencies, increasing wear and tear and power demand peaks. Where there is a large baseload air demand, consider supplying this with a screw compressor, with fluctuations above this baseline supplied through a smaller piston compressor, or through the use of appropriately sized air receivers. Many of the sites I investigate have far too much air capacity, generally because sites anticipate future increases in air demand with expansion of the site when purchasing the compressor. This is a wasteful practice.

Distribution

The most obvious energy conservation opportunity here is in reducing air leaks. These leaks are not always due to poor maintenance, but could also be a result of poor operational practices, e.g. bleed valves that are left open on air receivers to remove accumulated water and oil. The quantification of the energy losses associated with air leaks is tricky, and in most cases is an estimate. You can find various charts which correlate the size of the aperture (which is where the estimation bit comes in, since it’s often too small to measure accurately) and the air pressure to quantify the rate of air loss. This can then be correlated back to the compressor capacity and energy consumption to convert the leak to an electrical energy value in order to calculate its cost. In many systems it is possible to log the compressor to determine the extent of system air leaks – all that is required is a period during which there is zero demand for air, in which case the compressor is running only to compensate for leaks. For compressors that go on and off load, care should be taken when interpreting results however, since energy consumption profiles will be different at higher compressor loadings i.e. when there are demands for air.

From a design point of view, air distribution lines clearly need to be sized such that pressure drop is minimised as far as possible. For a given flow rate, this is a function of their length, the number of bends in the pipe work, the materials used and of course the diameter of the lines. Distribution systems need to allow for the periodic removal of water and oil, both of which affect air quality (with potentially serious consequences for product quality where air comes into contact with product) and also increase pressure drop. Of course, it is necessary to have the requisite equipment to deal with these contaminants at source, through filters, refrigerant driers and automatic bleed-off systems. Such systems will never operate at 100% efficiency, and hence the installation of additional bleed-off points throughout the distribution network is still advisable. The various fittings in a compressed air distribution system can all contribute to system pressure drop, and hence filters, driers and other equipment should be regularly serviced.

Where areas of a plant are taken out of service, it is good practice to isolate the air supply lines to these areas, since leaks can often arise and go unnoticed. It is also a good idea to install distribution lines in areas that are easy to access. This not only makes maintenance easier, it also allows for the rapid detection of air leaks. I have audited plants in which air lines are run below ground, in conduits, under equipment and also very high up. This makes leak detection nearly impossible.

Inspection of the air distribution system should be practiced regularly, and leaks should be promptly addressed. Such inspections can easily be scheduled as part of your site’s preventive maintenance programme. The point here is that air leak management is an ongoing process, rather than a one-off activity. For large plants, you may want to invest in specialist leak detection equipment such as ultrasonic detectors, which can also be used for various other condition monitoring purposes. I suggest that you address the obvious leaks first before making such an investment.

Usage of compressed air

Each process that uses compressed air should be investigated to assess whether that usage can be reduced, and if so, the potential reduction should be quantified in order to compare the benefits of reduction to any potential risks. Reduction can imply:

• Using the air for a reduced period of time;
• Using the air at a lower pressure than currently used – here regulators may be needed;
• Using the air more efficiently through modifications to equipment;
• Eliminating the use of compressed air – in this case a more-efficient alternative has to be found e.g. use of an electrical device instead of an air-powered one or use of a low-pressure blower to dry or cool product rather than a compressed air stream. In some cases air can be exchanged for another lower-cost medium, such as cooling water for example.

The usage aspects of compressed air efficiency require intimate knowledge of your processes, and cannot be left to an outsider, not even a compressed air expert. The issue here is that the consequences of overzealous actions taken in the name of energy efficiency could be out-of-specification product, rework, product recalls or even hazards to consumers. These cost far more than the compressed air savings. Hence it is best for compressed air experts and process owners to work together on this aspect of efficiency.

Inappropriate uses of compressed air should be stopped. Examples of inappropriate uses are things like open blowing to remove dust from equipment and clothing, for example. I come across many sites which have numerous air tap-off points installed across the plant exclusively for such uses. Of course it is not good enough to simply say that such uses should be stopped without providing an alternative. The reasons for their existence need to be investigated and root causes need to be eliminated for cessation to be sustainable.

The regulation of air pressure at point of use is vital to ensuring that only as much air as is needed is actually used. Note however that since there is a pressure drop across a regulator, these devices are not devoid of energy losses.Unregulated air usage can lead to significant artificial air usage, and can also lead to poor process performance in some applications. For example, air-powered spray painting machines operating with variable air pressure will produce inconsistent paint application rates. 

In summary then, assess the scale of your energy usage for compressed air production within the context of your total energy consumption to determine the level of effort required to optimise energy use in this area. Begin with simple best practices first, such as reducing system pressure, fixing leaks and reducing/eliminating inappropriate and artificial uses. Remember that as an integrated system, compressed air production, distribution and usage all interact with each other, and also interact with the processes to which the air is being supplied. Design issues such as compressor location, use of VSD’s, piping layout, receiver configuration and others will require some financial outlay to correct, where necessary. In many cases however, the costs can be justified by the efficiency gains. Proceed in an incremental manner, taking care to review system impacts as you go along.