Energy recovery offers ways to reduce compressor heat

Poorly designed compressed air systems waste vast amounts of energy—and waste money in the process. Energy recovery can be an effective way to utilize all energy created for the compressed air system.

04/29/2014


Energy efficiency will always be a hot topic in the plant engineering world. When you find ways to effectively increase energy efficiency, you save money and resources. 

Estimates indicate that poorly designed and maintained compressed air systems in the U.S. account for up to $3.2 billion in wasted energy costs annually. If you are searching for ways to increase energy efficiency, and save money and resources in the process, one of the first places you should look is your compressed air system.

Air compressors are a critical component in a wide variety of manufacturing and processing applications across all industries. While continuous advancement in design and packaging have helped substantially increase energy efficiency in today’s air compressors compared to previous generations, the fact remains that only a small fraction of electrical energy input, around 10% to 15%, is actually delivered as compressed air energy. Consider the cost of electricity to power air compressors, which can be as high as 40% of a plant’s electricity bill, and finding new ways to increase energy efficiency has never seemed more imperative.  

Consider the following scenarios and determine if each statement applies to your compressed air system:  

  • The system receives regular and ongoing maintenance from professional service engineers.
  • The system was recently upgraded to include the latest compressor technology.
  • The system recovers 50% to 94% of electrical input energy in the form of hot air or water.  

If you answered no to one or more questions, there may be energy efficiency opportunities to be found within your compressed air system. It seems like every day a new product or technology promises to be faster, better, and more efficient than ever. If the goal is to make your compressed air system more energy efficient by reducing the amount of energy required to produce the end product, then it might be time to take the next step from energy efficiency to energy recovery. 

What is energy recovery?

When air is compressed, the energy transforms from electrical energy to potential energy in the form of compressed air and thermal energy, or heat. After compression, air is often at a temperature of 325-400 F. Before compressed air can be distributed into the piping system and delivered to the end process, the heat has to be extracted. Unless heat is recovered, it becomes waste.

In theory, the total amount of recoverable energy from compressed air is 94%; depending on the design of the energy recovery unit, most manufacturers can recover anywhere from 50% to 94% of the available thermal energy. The following example of heat production illustrates potential energy recovery. A total of 6% of heat is unrecoverable due to the small amount of heat that remains in the compressed air or is dissipated into ambient air.

Instead of becoming waste, the heat energy produced by compressing air can be recovered and reused in other manufacturing processes. A large number of industries can benefit from energy recovery, though industries with higher running hours and continuous duty applications, such as the use of process water, are able to reach a higher level of savings in a shorter amount of time. This is because energy recovery is most effective when the heat extraction process involves a water-cooled after-cooler.

Extracting heat from systems

Recall that after compression, the air is often at a temperature of 325-400 F. To lower the temperature of the air, most compressor installations feature a two-stage design and come standard with an intercooler between the first and second stage, and an after-cooler, which is located after the second stage. The intercooler and after-cooler can either be air- or water-cooled. As the air passes through either of these types of after-coolers, the heat is extracted and can be redirected to other uses.

Air-cooled systems are more common in small- to medium-sized compressors, and heat recovery is often limited to the cool seasons, when the recovered energy can be used as heated air and is rerouted to nearby buildings for supplemental heating. This type of heat recovery installation offsets the energy required to perform the same function and can help facilities save thousands of dollars on annual heating bills.

Water-cooled systems operate through one of three main principles: open systems without circulating water, open systems with circulating water, and closed systems with circulating water. The most recommended system is the closed system with circulating water, where water continually circulates between the compressor and some form of external heat exchanger, which then transfers the recovered heat to the intended process. 

There are many benefits to a correctly implemented closed-water energy recovery system. A closed-water system requires little supervision and has low maintenance costs. Unlike an open-water system, where water is supplied by an external source such as a municipal water main, lake, stream, or well, the closed-water system uses treated water. Therefore, there will be little to no mineral deposit buildup in the cooling media components, which enhances the compressor operating conditions, reliability, and service life.

Once the compressed air has passed through a water-cooled system, the water now contains the energy that was first used to compress the air. This heated water can then be used throughout the facility to preheat process water, and the energy that would normally be used in these processes is offset. Imagine your costs to heat process water disappearing. With energy recovery, the savings quickly add up, resulting in a short return on investment and ongoing cost savings from year to year.  

As the worldwide demand for energy continues to grow and resources continue to dwindle and subsequently rise in price, manufacturers need to actively seek ways to increase energy efficiency to remain competitive in the global marketplace. Upgrading to more advanced technologies, implementing a regular and ongoing maintenance schedule, and installing an energy recovery system are all important steps on the path to an energy and profit-conscious future. 

Brian Blum is responsible for CTS optimization marketing for Atlas Copco Compressors. Edited by Jessica DuBois-Maahs, associate content manager, CFE Media, jdmaahs(at)cfemedia.com.

 



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