In brief:

• Every compressed air system has excess heat of compression that’s probably recoverable.
• Specific compressed air supply equipment can utilize heat of compression to eliminate other energy uses.
• Installation of air or water lubricant-cooled rotary compressors will allow potential recovery of 85% to 90% of the motor horsepower in the form of heated air or water, depending on the type of cooling.

Every compressed air system has excess heat of compression that’s probably recoverable. You’ve already paid for it. Look for places to use it.

The basic inefficiency of compressed air as a power source (8 hp of electric power to produce 1 hp of work with compressed air) dictates a substantial rise in temperature called heat of compression (8 hp of work in – 1 hp of work accomplished = 7 hp of heat). Recapturing this heat can be a very effective project for energy conservation.

Installation of air or water lubricant-cooled rotary compressors will allow potential recovery of 85% to 90% of the motor horsepower in the form of heated air or water, depending on the type of cooling. Water-cooled compressors will discharge heated cooling water at a maximum temperature of up to 130 ˚F. Air-cooled units will usually discharge cooling air from 5 ˚F to 20 ˚F above the inlet cooling air. To be effective, this cooling air may need thermostatic controls.

Eliminate other energy uses

Specific compressed air supply equipment can utilize heat of compression to eliminate other energy uses.

Hank van Ormer is president of Air Power USA. Contact him at hank@airpowerusainc.com and (740) 862-4112.

Air reheater: Hot cooling oil (190 ˚F to 200 ˚F) or cooling water (130 ˚F) can be used to reheat saturated or high relative humidity compressed air to lower the relative humidity in the pipe. In some applications, this will not only eliminate outside pipe sweating, but also, under proper operating circumstances, deliver hot, dry air to the process.

This contains more usable energy because the heat raises the pressure with less air volume required, and the water vapor is still in the airstream in the form of a usable gas as long as it does not cool below the pressure dewpoint and the vapor then falls out as water.

Heat of compression dryer: Heat of compression from an oil-free compressor is used to deliver hot air to a heat of compression (twin tower or rotary drum type split stream) with a low relative humidity (due to the temperature of  more than 200 ˚F) to the regenerating tower. The lower relative humidity hot air strips the moisture from the desiccant beads and then goes through the aftercooler to remove the water vapor, in the form of condensed water, to a 100 ˚F or lower pressure dewpoint. The air then travels through the drying tower. These dryers can be run with virtually no energy use or compressed air loss and, depending on the application, still deliver a very acceptable pressure dewpoint. Calculating the energy savings would be accomplished by calculating the energy operating cost of the alternative dryer or dryers (Figure 1).

Figure 1. Heat of compression dryers can be run with virtually no energy use or compressed air loss and, depending on the application, still deliver a very acceptable pressure dewpoint.

How much heat (Btu/hr) is available?: Figure 2 shows some values for lubricant-cooled rotary screw compressors. Oil-free and other lubricated units usually use water-cooled in the larger sizes, and the retained heat of compression in the cooling water or air media would be similar.

Figure 2. Air-cooled units usually try to collect all the air in one airstream and then duct the heated air as required to a desired location. Heat recovery energy saving and projects are often much easier to implement with the heat well trapped in the controlled water flow.

Methods of heat recovery: Air-cooled units usually try to collect all the air in one airstream and then duct the heated air as required to a desired location. This would be similar to the space heating and heat barrier examples shown (Figure 3). Note on the space heating sketch a booster fan has been added to ensure there is enough fan to move the cooling air without restricting the cooling air flow. When installing this ductwork, both intake and discharge, work closely with the service provider or installer or risk damaging the compressor from running too hot.