Daniel Stewart, ERS, for Zondits
Compressed air is a notoriously expensive energy source but one that is widely used in commercial and industrial settings because of its ability to be easily stored, the high output-to-weight (and output-to-size) ratio of pneumatic actuators and tools, and the low maintenance requirements of end-use devices. As a general rule of thumb, one can assume that in a typical compressed air system only about 5%–8% of the electricity that is powering an air compressor is eventually translated into useful energy for the end-use devices. For this reason, compressed air systems are regularly targeted during energy audits looking to lower cost and deepen energy savings.
[mks_pullquote align=”left” width=”300″ size=”18″ bg_color=”#1e73be” txt_color=”#ffffff”]As a general rule of thumb, one can assume that in a typical compressed air system only about 5%–8% of the electricity that is powering an air compressor is eventually translated into useful energy for the end-use devices.[/mks_pullquote]As a best practice, the first place to start while optimizing a compressed air system is reducing the demand-side loads. Reducing distribution system leaks and inappropriate uses of compressed air throughout a facility can significantly reduce energy consumption and most often pay back within 1 year. Further energy savings can be achieved by implementing better controls, optimizing distribution system configurations, or installing a more efficient compressor, such as one driven by a variable frequency drive (VFD). But the inherent losses associated with the heat that is generated by the viscous friction of air on itself during compression limits the total system efficiency to approximately 10% in most scenarios. Recovering as much of this waste heat as possible is also essential to maximizing the overall efficiency of a compressed air system. In addition, keeping an air compressor cool is important in maintaining the equipment’s internal tolerances and operating efficiency.
For air-cooled compressors, it is most common to install a bypass damper system that can direct waste heat back into the facility or to the outdoors, depending on space-heating needs. These systems can be implemented in a cost-effective manner if there are heating loads in neighboring spaces. However, the higher cost of installing the duct work and booster fans necessary to transport the reclaimed heat to more remote locations often results in a prohibitively long payback. For example, a 250 hp air compressor will generate approximately 600 MBh of waste heat and require approximately 15,000 cfm of airflow.
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Water-cooled compressors produce waste heat in the form of cooling water that is pumped through an integrated heat exchanger within the compressor. These water loops can be opened or closed. Open-loop systems may require extensive water treatment to avoid scaling or introducing dissolved solids and potential sewer disposal fees, all of which increase operations and maintenance (O&M) costs. Open-loop systems are somewhat limited in the temperature rise that the compressor’s heat exchanger can impose on the cooling water; thus, the waste heat is considered low grade because it typically exits the unit at no higher than 90°F. This heat can be used to preheat boiler feed water or process water but is unsuitable for most heating, ventilating, and air conditioning (HVAC) applications. Closed-loop systems do not require the same level of water treatment as their open-loop counterparts, and they are capable of producing waste heat that can reach as high as 100°F and often require less pumping energy. Because the system is closed loop, the cooling water can be a glycol mixture and therefore be suitable for many HVAC applications.
As with many energy efficiency measures, a balance must be struck between optimizing a system’s operation and limiting implementation costs. Each facility has unique operational requirements that must be considered when installing a heat recovery system, and there is no “one-size-fits-all” solution.
Image credit: Scot Industrial Air