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Compressed Air System Efficiency Factors: Heat Recovery

Poor air compressor performance can be caused by two factors: the first is wasted air from leaks, artificial demand, and inappropriate usage, while the second is compression heat. By knowing how to properly address these issues, businesses can improve operation of their compressed air systems and in turn lower plant-wide energy costs, reduce downtimes, increase system productivity and product quality, and secure longer equipment service life.

Air compressors are excellent sources for heat recovery. This is because air compression produces heat energy that is concentrated in the decreasing volume of air. To maintain proper operating temperatures, the compressor must first transfer excess heat to a cooling media before air can spread out into the pipe system. When done right, businesses can recover up to 90% of this heat and reuse it during operations. Energy recovered through closed loop cooling systems within water-cooled compressors also help improve a compressor’s operating conditions, reliability, and service life thanks to the equal temperature level and high cooling water quality.

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Recovering Heat Using Air-Cooled Oil Injected Rotary Screw Compressors

Air-Cooled Lubricant Injected Rotary Compressors can recover heat which can be reused in space heating, industrial drying, preheating aspirated air for oil burners, or other applications in need of warm air. Ambient atmospheric air is heated by flowing it throughout a system’s lubricant cooler and aftercooler, where it obtains heat from both the compressed air as well as the oil used for compressor lubrication and cooling. The heat extracted is relatively low in temperature (under 100 F) and can be utilized for limited applications only.

Usually enclosed in cabinets and with heat exchangers and fans, packaged compressors can be modified by adding ducting or possibly another fan to handle duct loading and to remove back pressure on a compressor’s cooling fan. A thermostatically controlled hinged vent can modulate these heat recovery systems. This vent can also be thermostatically regulated to deliver constant temperature to the heated area.

It must be noted though that energy recovered from air-cooled compressors may not always give off sufficient heat when needed. The amount of heat recovery may vary and depend on a compressor’s load. For recovery to be possible, a corresponding energy requirement, usually achieved with an ordinary system supply, is needed. Energy recovered is recommended for use as supplementary energy to the system – with the available energy always utilized during compressor operation.

Recovering Heat Using Water-Cooled Oil Injected Compressors

Due to the extra heat exchange stage required and the lower heat temperature available, recovering heat from water-cooled compressors for space heating is uncommon but not impossible. Since most water-cooled compressors are fairly large, they can recover heat at efficiency rates of 50-60%. Water-cooled oil injected rotary screw compressors with heat exchangers allow potential extraction of wasted heat from lubricant coolers, producing hot water. Heat exchangers can heat non-potable or potable water depending on its design. If hot water is not needed, the oil is sent to the standard lubricant cooler.

The hot water produced can be used for central heating or boiler systems, plating operations, industrial cleaning processes, heat pumps, or other applications where hot water is needed. With heat exchangers, production of hot air and hot water is possible. Operators are allowed to vary hot air/hot water ratio to some extent as well.

The thermal match between recoverable heat and heat needed, an hourly match between when heat is produced and when it is needed, and the installation costs are all vital to heat recovery value. System conditions must be examined and evaluated first to ensure if heat available is still economical to recover. A heat bypass system should still be set-up for times when an air compressor is still operating and heat is not needed.

Recovering Heat Using Water-Cooled Oil-Free Compressors

Greater heat recovery is possible with oil-free screw compressors. Like in all compressors, electrical energy flowed in is converted into heat. Low-pressure and high-pressure elements can produce discharge temperatures of more than 300F. This heat is usually found on low-pressure and high-pressure compression elements, the intercooler, the aftercooler, and the oil cooler. Some compressor manufacturers present built-in recovery systems that allow circulation of water across the four components resulting to heat transfer producing hot water up to 194F.

Recovery Heat Using Engine-Driven Compressors

Compressors that are engine-driven offer the same kind and volumes of low-grade heat existing from the air end. These compressors also present a higher-temperature option off the engine. Generating low-pressure heat from the exhaust is possible depending on an engine’s size. As a general rule, 30% of energy from gas input is available as high-temperature heat. With low-enough application temperature, as much as 90% of input energy can be recovered.

SPECO offers cost-effective parts and services for better compressor maintenance as well as management of compressor heat recovery. We carry a wide selection of best-priced Original Equipment Manufacturer (OEM) and replacement parts to meet all your air compressor needs. Easily shop for compressor parts and lubricants by visiting www.soparts.com/products today!