The recovery of compressor heat can substantially reduce the total cost of energy in most commercial or industrial facilities. Thanks to the laws of physics, the most plentiful useful output of an air compressor is heat.
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Less than 10% of the air compressor energy input is actually turned into useful work at compressed air powered machines or tools. The rule of thumb used for fully loaded air compressors is that 2,545 btu per hour of heat is produced for each nameplate hp. Of this, about 80% is recoverable for other uses. If you can’t remember your conversion factors, this means that for each kilowatt of compressor power input, about 0.8 kilowatt can be recovered for other uses.
The easiest and most common use of the heat of compression is to supplement building heat. In cooler months, the hot air produced by air cooled compressors can be redirected inside the building. Hot water produced by liquid-cooled compressors can also be used in this way, but it is more complicated.
The most effective use of this heat takes already heated filtered plant air (not outside air) and passes it through the compressor cooler. This hot air can be redirected to cooler plant areas like warehouses or loading docks and will reduce the amount of electric or gas fired heat required in those locations. Some more modern compressors have been designed so they are quite quiet—in a fairly clean plant, compressors can be installed in open areas, so 100% of the heat they produce makes it to plant areas.
But unless you live in Antarctica, this use of heat is seasonal—therefore, only a fraction of the annual heat energy can be effectively used. The best recovery value comes when the heat is directed to a process that runs continuously. Some examples of other uses are listed:
• Boiler make up water
• Producing hot water for clean up
• Process drying applications
• Process heating
• Input to reverse osmosis systems to make them more efficient
• Regeneration heat in desiccant dryers, saving purge air.
An example of the savings potential for heat recovery displacing natural gas, a fully loaded 100 hp compressor running 5 days per week for 4,160 hours per year would produce recoverable heat worth about $4,300 per year based on natural gas priced at $4.50 per 1000 cubic feet.
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Many manufacturers now offer heat recovery options for their compressors, available for both air-cooled and liquid-cooled types.
Heat recovery can be implemented in plants both large and small. An example of the use of heat recovery is at a small fiberglass products manufacturer located in a northern location that is subject to very cold winter temperatures. Because of dust contamination concerns, this company brought in 100% outside air to cool their compressors. After cooling the compressors, the heat of compression was directed outdoors, requiring supplemental electric heat to run in the compressor room to prevent the compressor from freezing in winter.
An energy analysis showed that the air compressors were producing, on average, about the same required heat as the electric heater. The ducting was modified by installing temperature controlled dampers to spill heat of compression into the room to temper the air. This allowed a 20 kW electric heater to be turned off during operating hours. This heater was consuming an estimated $20,000 per year of electricity per year. The ducting modifications cost less than $5,000.
Heat recovery can also be implemented in larger sites. At a solar cell manufacturer in a tropical environment, the cutting-edge plant design recovers 140° F cooling water discharge from their large centrifugal compressors and redirects it to be used in heating of chemical vats for product treating. The use of this waste heat for this purpose, rather than electric heaters, saves the plant over $600,000 per year—or about 20% of their compressor energy input.
For more information about energy calculations for heat recovery please see the document at this link: https://www.compressedairchallenge.org/library/factsheets/factsheet10.pdf
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