Compressed air, considered to be the “4th Utility,” is necessary in most manufacturing plants. The generation of compressed air requires large amounts of energy and can account for up to 10% of a plant’s total energy costs. Up to 93% of the energy required to compress air is converted to heat energy. By recovering and redirecting this heat, some of the operating costs associated with compressed air can be offset. Due to their operating principles and design, lubricated rotary screw air compressors are highly suited to the recovery of the heat of compression. In such units, this heat is removed by fl ooding the compression chamber with a lubricating fl uid. The fl uid is then separated from the compressed air and cooled by the use of an air-cooled heat exchanger. Additional heat can be recovered from the compressed air aftercooler. Cooling air fl ow is generated by the compressor package cooling fan. While water-cooled models are available, the recovery of this heat is more costly and complicated. The amount of heat recovered can vary with heat exchanger effectiveness but is typically 80-90%. Hot air recovered from the compressor can be from 35 F to 50 F higher than ambient. Heat recovery and—subsequently—energy savings are reduced when a compressor is operating at less than full capacity.
There are many different applications for recovered heat of compression. Each offers unique savings opportunities, as well as installation considerations and investment. Potential applications include the following:
Preheated make-up air… A preheated make-up air application (see Fig. 1) involves ducting the compressor package inlet outside the building. The outside air is heated as it is used for cooling and exhausted into the space surrounding the compressor. For every cubic foot of air pulled in by the heat recovery system, a cubic foot of air that would have infi ltrated the building at outside temperature is eliminated. Savings are realized because the plant’s primary heating system does not have to heat the air brought into the building by the heat recovery system.
This application can yield signifi cant savings and a short payback period since installation costs are minimized. No extensive ductwork or booster fans are required to connect to the plant’s primary heating system. Additionally, a higher fl ow fan may not be required in the compressor package. In colder environments, care must be taken so that the temperature of the outside air drawn into the compressor package is not so low that it will cause the air or moisture in the lines to freeze. Heat recovery systems are available that can automatically recirculate warm air to maintain a constant temperature inside a compressor package. This functionality also can provide a compressor with the ability to operate in unheated spaces, as well as maintain a more comfortable exhaust temperature.
Supplementary heating… A heat recovery system can also be used to supplement a plant’s primary heating system (see Fig. 2). In this instance, it is desirable for the air to be heated to a higher temperature than in a preheated make-up air application. This type of installation may require more extensive ductwork to distribute the heated air. Consideration must be given to the compressor package cooling fan. Extensive ductwork may necessitate a higher-fl ow fan or downstream booster fans.
As with a preheated make-up air installation, use of a thermostatically controlled heat recovery system drawing in outside air can increase savings by reducing infi ltration while still providing usable heat. A system without this level of control may not be able to heat the outside air to a temperature suffi cient for space heating.
Process heating… Recovered heat of compression may be used in process heating such as parts drying and boiler preheating. A benefi t to this application is the high rate of return due to the yearround heat recovery. This also can provide opportunity for heat recovery in warm climates. One installation consideration is proximity of the compressor to the point of usage. In addition to minimizing heat loss through ductwork, the cost of ductwork and booster fans will be minimized by placing the compressor close to the point of use.
In a heating or preheated make-up air system there are instances in which it is desirable to reject the recovered heat outdoors. This can be accomplished by use of additional ductwork and manually actuated dampers. It is possible to fabricate or purchase a system which will automatically utilize or reject the recovered heat of compression based on building and outside conditions.
Calculating potential savings and payback time from recovering heat of compression can vary depending on compressor size, operating conditions, local energy rates, use of the recovered heat, location and initial investment. The plot in Fig. 3 details savings potential based on energy cost and compressor size.
As shown in Fig. 3, a 300 HP compressor can generate 12,378 BTU/minute. This represents 7.42 therms/hour of usable heat that is worth $3710 per 1000 hours of compressor operation at $0.50/therm. Annual heating cost savings of up to $14,840 easily can be realized. These savings are achieved without negative impact to the compressor’s cooling effi ciency.
As energy costs continue to rise, utilizing recovered heat from the production of compressed air becomes more attractive—much more attractive! While systems can be fi tted to existing machines, the best time to confi gure a heat recovery system is upon the purchase and installation of new and replacement equipment.