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.

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.

Figure 3. Warm cooling air can be exhausted directly into the space to be heated or added to the main plant heating system.

Energy saving applications

Space heating: Warm cooling air can be exhausted directly into the space to be heated or added to the main plant heating system. If the compressor cooling air exhaust temperature drops below an acceptable level or below the level of the main plant heating system (part load compressor operation results in less heat generated), a thermostat-activated control valve may be required. A booster fan will ensure it is warm enough before the air is bled in the area to be heated.

Additional heat can be added to the system from many sources.

Figure 4. Check the performance of existing ductwork by tracing the temperature with a surface pyrometer.

Heat barrier: When mobile equipment or people move frequently into and out of a building, the temperature may fall to the discomfort of the occupant. A heat barrier can prevent this (Figure 4). The compressor cooling air exhaust is ducted between the two sets of doors, forming a continuous “lock” of warm air, inhibiting the outflow of warm air from the building. It is very easy to check the performance of existing ductwork by tracing the temperature with a surface pyrometer (Figure 5).

Figure 5. Ensure ductwork is working properly.

Water-cooled compressors: Heat recovery energy saving and projects are often much easier to implement with the heat well trapped in the controlled water flow. Regardless of the type of air compressor, the maximum expected heated discharge cooling water temperature is about 130 ˚F. The system layout is for two 220-bhp rotary screw compressors with a 2,500-scfm-sized water-cooled refrigerated air dryer (Figure 6). The expected energy recovery was almost $43,000/year based on heating the river process water up from the average temperature of 60 ˚F to a nominal 94 ˚F. This takes an energy load off the current natural gas heater system.

Figure 6. The system layout is for two 220-bhp rotary screw compressors with a 2,500-scfm-sized water-cooled refrigerated air dryer.

The total savings from this project was measured at $60,000/year due to the pre-cooling effect of the heat-reclamation heat exchanger loop on the primary 10-fan closed 60/40 polyglycol air-cooled cooling system. Only two fans ran most of the summer, and only one fan or less during colder weather.

Converting energy recovery to savings

Often, when considering the heat recovery in Btu/hr from compressed air systems in the form of heated air or water, it is difficult to understand this in the form of energy dollars saved. These formulas and procedures successfully convert Btu/hr to recoverable energy dollars when the cost per unit and Btu content of the unit of alternative fuel being replaced in energy dollars are available.

Formulas

(Total Btu / hr saved)/(Btu / unit fuel) = units of fuel/hr

Unit fuel / hr x hours =  total units
                                           of fuel saved

Total units saved annually x rate / unit = dollar savings / year

Example I: Natural gas

Cost of natural gas = $7/MCF or $0.007/CFT (cubic ft)
Total Btu/hr recovered = 250,000 Btu/hr
Total annual hours of operation = 6,240 hours

(Btu/hr saved)/(1,000 Btu/Cubic ft) = cubic ft/hour saved

250,000/1,000 = 250 cubic ft natural gas per hour

Units fuel/hr x hours = 250 cubic ft/hr x 6,240
1,560,000 cubic ft of natural gas savings
1,560,000 cubic ft x $0.007/cubic ft = $10,920 saved annually

Example II: Electricity

Cost of electricity = $0.07/kWh
Total Btu/hr recovered = 250,000 Btu/hr
Total annual hours of operation = 6,240 hours

250,000 ÷ 3413 Btu/kWh saved = 73.25 kW saved/hr
73.25 x 6,240 = 457,080 kWh/year
457,080 kWh x $0.07 = $31,995 annual energy savings

Example III: Kerosene

Cost of kerosene = $1.10/gal
Total Btu/hr recovered = 250,000 Btu/hr
Total annual hours of operation = 6,240 hours

(250,000 Btu/hr saved)/(135,000 Btu/gal) = 1.86 gal/hr

1.86 gal/hr x 6,240 = 11,606 gal
11,606 gal x $2.50 = $29,015 per year