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Technology, Service, Parts & Maintenance

Recover Heat from Air Compression


The following is an excerpt from the article Recover Heat from Air Compression by Deepak Vetal, product marketing manager, oil-free screw, centrifugal, and high-pressure air, Atlas Copco Compressors. The full version can be read online or in the September issue of Chemical Processing.

Manufacturers face more-stringent mandates to restrict carbon dioxide emissions. Recovering the heat byproduct of compressed air is one step process plants can take to help reduce CO2 emissions and the risk of penalties for failing to meet emerging environmental standards. Doing so also will decrease total lifecycle cost.

In air compression, the electrical energy used to compress air is transformed into heat. However, it isn’t the only source of energy during air compression. Another important source is the humidity contained in the air entering the compressor inlet. Following compression, this humidity is condensed into liquid as the air is cooled in the intercooler and aftercooler. In the process of cooling this air, the latent heat of condensation is released into the cooling water. (The amount of latent heat of condensation depends upon the temperature and relative humidity of the intake air.)

The process of compressing the air includes several instances where heat is generated and transferred to the cooling water system. Some of the heat of compression passes to the cooling water around the compression elements via the element cooling jackets; a significant amount of heat is released to the coolers’ cooling water system. Additionally, the heat removed from the hot oil in the oil cooler is discharged into the cooling water system.

The percentage of recoverable electrical energy depends upon the compressor’s operating conditions. Intake air temperature, relative humidity, water temperature and pressure all play a role.

Some energy is lost through motor inefficiencies, via radiation of components such as the element and the cooler, and in residual heat and heat of condensation remaining in the outlet air.

If we combine all these factors in an overall view (Figure 1), the net result is that, for typical industrial conditions, we can recover 90–95% of the electrical input energy. And in specific conditions, recovering more than 95% is possible. An energy recovery system uses this energy to produce hot water.

Energy Recovery Basics

Compressors fitted with energy recovery technology feature a modified cooling water circuit to guarantee recovery of the largest possible amount of energy at the highest possible temperature. The cooling loop flows through the oil cooler, then through the compression elements’ jackets, and finally through coolers. The water that leaves a compressor with energy recovery can be as hot as 194?F; the temperature can be regulated to meet process requirements.

A control unit specifically designed for energy recovery systems (Figure 2) can transfer the energy recovered while offering optimal protection of the compressor. The control unit is installed between the air compressor and the separate cooling water circuit, creating a completely independent and closed energy recovery circuit.

The energy recovery control unit can manage the recoverable energy and water flow of multiple compressors — up to its maximum limit for supplied energy. The unit often comes with several safety features, a de-aeration system, a pressure relief valve, and an expansion valve that controls the pressure in the water system.

Keep Reading…

You can read the full article on the process and benefits of recovering heat from compressors here. To learn more about Atlas Copco’s line of compressors and the energy savings they offer, contact our experts today or leave a comment below.

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