[pullquote]Process plants use compressed air for several different purposes including conveying, process controls and actuators, process gas compressing, etc. Interestingly, compressed air is probably the most expensive utility stream in the industry on a per unit basis. In addition, it is the most misused, mismanaged and unaccounted (wasted) utility from an overall usage perspective. So, I compared the energy used in pumping systems (see, “Pump Out Energy Inefficiencies”) versus compressed air and was very surprised at the results. The chemicals sector (NAICS 325) uses a slightly higher amount of energy in producing compressed air (160 TBtu/yr) than pumping systems (151 TBtu/yr). More importantly, though, the compressed air system energy efficiency is only 11%, which means 89% of the energy supplied is wasted. No wonder compressed air is the most expensive utility in a plant!
The main subsystems of any compressed air system are generation, distribution and end use. All three subsystems work hand-in-hand to provide compressed air. A shortcoming or failure of any one of these subsystems leads to no available compressed air. Because production heavily depends upon the reliability of the system, most of us have built-in redundancies to enhance compressed air system reliability. Unfortunately, this redundancy, if not properly designed and used, can lead to significant loss in the system’s energy efficiency. Many avenues can improve energy efficiency in a compressed air system. Here, I will touch upon one of my favorites — the appropriate use of controls and storage.
The two main types of compressors are positive displacement (reciprocating, rotary) and dynamic (centrifugal, axial). The controls associated with each compressor type vary. Typically, all compressor systems are designed for peak demand with a margin of safety, whereas the actual operating load is much less than the peak demand. Every compressor has a full-load and design-point efficiency (being a turbomachine, part-load efficiency isn’t as good as the design point or full-load). Based on the demand, the compressor controls are set to provide the compressed air required by the plant at the highest efficiency and reliability. But remember, all control strategies have pluses and minuses. Ultimately, the optimum choice will depend on the system design, load profile and plant demand.
The simplest control strategy is the start-and-stop methodology. Being the simplest, it has its limitations in applications with frequent cycling. The second methodology — load/unload — eliminates the frequent cycling and keeps the compressor motor running (always providing air, which is unloaded when not needed), albeit at a cost in energy consumption. The third methodology has to do with modulation. The idea here is to introduce a flow restriction at the inlet, thereby restricting the air and meeting the system demand. Inlet guide vanes are one example of such a throttling device. Certain kinds of compressors can be designed with stepped controls — these are discrete load points (0%, 33%, 67% and 100%) or (0%, 50% and 100%). Sometimes, these different controls can be combined to offer a larger operating range and provide a tighter control on pressure. A more recent entry in the control scheme is the use of variable frequency drives. In principle, they offer a wide operational control with high energy efficiency; however, you may need to consider additional costs and site electrical specifications, especially for a retrofit application.
In the world of sophisticated networking and faster semiconductors, there also are some very simple and complex master controllers available. These can be implemented on the whole compressed air system to ensure that compressed air is produced most efficiently at all times. Finally, storage plays a very important role and goes hand-in-hand with any control strategy implemented. Storage is cheap; you always should look closely at the ability to implement storage in every compressed air system.
One final note: the Compressed Air Challenge is an excellent resource that provides valuable information, including fundamentals, sourcebooks, toolbox, best practices and training opportunities. I have personally used several of these resources as the need arises, and would recommend you bookmark it in your browser. May the “air” be with you!