Compressed air systems are significant energy users in most industrial facilities, and understanding how to maximise their efficiency and minimise their operating costs is vital. The understanding of the basic drivers of efficiency in these systems is however fairly low, since as a utility/service, compressed air is not considered to be “core”. In this post I’ll discuss some of the common operating mistakes I see when assessing compressed air systems. Eliminating these mistakes will deliver savings for your operations and reduce emissions.
Mistake #1: Operating the system at too high a pressure
The higher the pressure at which compressed air is produced, the greater the amount of energy required to compress a given mass of air. To determine your ideal operating pressure, determine the minimum required pressure for each air user in your system, and set the compressor’s delivery pressure such that the user with the highest required pressure can be reliably supplied, taking system pressure drops into account. If the highest-pressure user uses only a small proportion of the total air supplied, consider a separate compressor for this user and reduce the pressure of the primary compressor.
Mistake #2: Operating without an air receiver
Air receivers are necessary to enable the compressed air system to cope adequately with sudden changes in air demand. They also act as useful separators, from which water and oil carryover can be periodically drained. In the case of screw compressors, lack of an air receiver, or use of an air receiver that is too small, can lead to short-cycling. In this scenario the compressor never unloads completely, and tends to therefore operate against a higher average back-pressure when in an unloaded state, increasing energy consumption. Short-cycling can also interfere with the lubrication cycle and increase risks of failure. As with anything, there are exceptions – for example a receiver may not be essential with use of a VSD compressor in combination with large distribution lines and within a system with a fairly steady air demand. But if in doubt, install a receiver.
Mistake #3: Operating with a compressor that is too large for the duty
Oversized fixed-speed screw compressors tend to waste energy, since energy is used when they are unloaded but they are not producing air in this state of operation. There are however also disadvantages to operating an oversized reciprocating compressor, the most obvious of these being the increased demand charges that come with periodic operation of a larger motor than necessary, and the lower efficiency levels associated with an oversized motor. It is true of most equipment that if too large, efficiency is compromised.
Mistake #4: Failure to regulate user pressures
Reducing the pressure at which air is used reduces the mass of air that is used, and hence reduces the load on the compressor. Unregulated systems therefore use more air than is necessary (so-called “artificial” air use) and hence require more energy to accomplish the same tasks. Air leakage rates also increase in systems that are not regulated, since leak rates are directly proportional to pressure.
Mistake #5: Ignoring air drying
Dry air is not necessarily an end-user requirement, and hence sometimes receives little focus. Moisture in the air reticulation system causes corrosion, blocking filters and roughening internal pipe surfaces. This increases pressure drops, and hence required input energy, since discharge pressures have to be increased to deliver air at the required flow and pressure to users. Fluids in distribution pipelines also increase pressure drop directly.
Mistake #6: Compromised suction conditions
Compressors are happiest when supplied with cool, dry air. This air is dense and the compressor has to work less hard to deliver the desired discharge pressure. Compressors also want air that is free of dust, since this blocks intake filters and increases pressure drop, leading to increased energy consumption. Take care then to locate your compressor away from activities that produce dust or generate heat. Remember also that most (up to 90%) of the input energy to a compressor is ultimately rejected as heat, and this heat must be diverted away from the compressor suction. Too often I come across compressors located in rooms from which the heat from the compressor is not removed, leading to high intake air temperatures and reduced efficiency.
One other important consideration is to ensure that the compressor intake is not close to an area containing toxic or explosive gases, as these could be discharged elsewhere in your facility when the air is used, compromising health and safety.
Mistake #7: Disregarding the impact of air leaks
Given the significant amount of effort that goes into producing compressed air, it should be regarded as an expensive commodity, and leaks literally represent cash flowing out of your compressed air system. It is however not uncommon to find users of compressed air who are blissfully unaware of the costs of producing the air, typically because compressed air is produced by “engineering” or “services” and is not really their problem. Air leaks should be managed through a comprehensive leak management system that includes tagging, and a works order should be raised for each individual leak in order to ensure its repair is followed through. Tracking the number of leaks, estimating their cost and making this information available to everyone using visual management tools is also good practice.
Mistake #8: Inappropriate uses of compressed air
Since it is easily accessible, compressed air becomes a convenient tool for practices such as blowing away dust and debris, facilitation of material flow on conveyors and in pipelines/ducts/chutes and I have even seen it used to keep employees cool in warm weather. These open blowing applications are an inappropriate use of compressed air, and very expensive. There are admittedly some applications for which alternatives are difficult to find, or for which use of an alternative could compromise overall productivity. For these, air pressures should be minimised through regulation, and some automation should be used to prevent open blowing from continuing when the air is not required e.g. use of solenoid valves.
Use of air driven machinery is a very inefficient way to use compressed air, and motorised alternatives are preferred wherever possible. There are however some advantages to use of air-driven equipment e.g. avoidance of explosion risks and prevention of damage to equipment such as pumps in the event of a valve being inadvertently closed downstream.
Mistake #9: Incorrectly specified ancillary components in the distribution system
Poorly specified filters and components can increase pressure drop significantly, leading to large unnecessary increases in operating costs. This is particularly true for the portions of the distribution system immediately downstream of the compressor, through which all of the air produced must pass. Incorrect choices in terms of drying equipment, receivers, automated drainage devices and the like can all lead to avoidable losses. Work with reputable OEM’s and always seek expert advice.
Mistake #10: Not taking a systems view of compressed air
Compressed air systems comprise compressors, distribution systems and users, and optimising energy use requires a considered, integrated approach. An efficient compressor supplying a large number of inappropriate air users does not make for an efficient system, as a simple example. It goes further than this when we look at compressed air as being part of site-wide network of systems. Heat recovery from compressors can lead to reduced consumption of boiler fuel, or reduced energy inputs for space heating. As with all matters of resource efficiency, a component-level approach can only take you so far.
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