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 electical 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.
The Impact Of Hardware
The type of compressor and the nature of its drive affect the amount of energy recoverable.
Oil-free screw compressors can recover more heat than any other type because they operate at much higher temperatures overall. Recovery of 90–95% of electrical energy input, and at some conditions even more, is possible.
Oil-lubricated screw units typically recover approximately 70% of the electrical input energy. Such compressors use oil as a coolant in the compression chamber to reduce the compression temperature. Therefore, less heat is recoverable than in oil-free units.
Centrifugal compressors operate at lower compression temperatures, limiting the amount of recoverable heat. For medium-pressure requirements, compression normally takes place in three stages; attempting to recover the energy from an interstage can create instability due to the dynamic compression principle. So, centrifugal compressors allow recovery of only about 30–35% of the energy.