Almost all industrial sites are equipped with air compressors. Among many other applications, compressed air is used as a motive fluid in instrumentation equipment (controls and actuators), packaging, conveyors, pneumatic tools, process machinery, etc.
Compressed air can also be used as a feedstock, such as in fermentation reactors to provide oxygen for bacteria growth.
In general, compressed air represents the third highest user of energy. In the world, about 10% of all electricity and circa 15% of all motor system energy in the manufacturing industry.
Compressed air systems energy costs accounts for 75% of the overall lifetime costs (ADEME, France).
The following figure shows the main components of a compressed air installation. When identifying energy saving actions from compressed air systems, three areas should be considered:
- production (i.e.: compression and drying) – estimate of 35% of average potential savings
- distribution (i.e.: whole network) – estimate of 15%
- demand (consuming processes) – estimate of 50%
Common Specific Energy Consumption factors (“SEC”) is around 120 W.h per normal cubic meter at a discharge pressure of 7.5 bar. This SEC does not include air drying.
Example: Typically, a compressor with a power of 75 kW will deliver circa 625 Nm3/hr.
The compressor power is :
- proportional to absolute temperature (1% increase in work for each 3°C temperature increase)
- increases with an increase in discharge pressure (7.5% power increase per additional bar).
Cost of compressed air leaks
Leaks are the main cause of energy waste in compressed air systems. Worldwide, air leak rates average circa 20% of total power consumption. Leakage occurs anywhere: connection points, couplings, fittings, pressure regulators, condensate traps, processing machines, pipe joints, etc.
Leakage is proportional to the square of the leak point diameter and to the air pressure. The annual energy costs for a leak can be computed using the following formula:
Annual cost of a leak € = leakage rate (m3/hr) x kW/m3/hr x operating hours x €/kWh
The figure below was taken from real site energy monitoring. It shows the measurement of total site electrical power and compressor room power. The compressor is estimated to consume a power of 40 kW (data log from monitoring system on maintenance week-end of Oct. 29 – year hidden). In production mode, the compressor power is 90-130 kW
The leak rate cannot be accurately determined from this graph, but it is estimated to be in the range 20-25%. A leak detect/repair campaign can save at least 10% of the energy consumed (conservative estimate). This figure shows it is essential to constantly monitor power consumption in order to track overconsumption.
Effect of network pressure
The following table shows the savings which can be made when the network pressure is decreased. For example, decreasing pressure from 8 (to read vertically on the left side) bars to 6.5 bars (to read horizontally in the yellow row) reduces the compressor power consumption by 10.6%. In general, air pressure required by most equipment is less than 6 bars.
Air drying power consumption
In order to prevent moisture condensation in pipes and control elements, compressed air is dried. In most systems, drying is carried out by cooling the air (refrigeration) or by adsorption. The Table below shows that adsorption dyring requires 3 to 5 times more energy than refrigeration drying. Therefore, adsorption should only be reserved to processes with low water dew point requirements.
Refrigeration air drying | Adsorption air drying |
Condenses air moisture by air cooling (continuous) Output Air Tdew-point = 3°C (5°C is more common) Power = about 4 W per m3/hr | Dries by adsorption of water, then dryer regeneration (batch ) Output Air Tdew-point = -40 °C à -80°C Needs 9 to 20 W per m3/hr |
Heat recovery from air compressors
More than 75% of the compressor electric power is rejected as heat in air compression systems. This heat can be recovered to be used in the processes, to produce hot water or to heat adjacent building zones, as illustrated in this figure.
Listing of potential energy savings actions from compressed air systems
Energy savings can be generated in compressed air systems thru:
- decrease in flow demand (reducing consumption and leaks);
- use refrigeration drying instead of adsorption drying (depending on required water dew point);
- with adsorption systems, use dew-point controllers to reduce regeneration with hot compressed air; use heated ambient air for adsorbent regeneration;
- decrease network pressure: proper pipe & network sizing, filter regular cleaning; the “10%” empiric rule states that total pressure drop should be less than 10% of discharge pressure (for a discharge pressure of 7.5 bars, expect at most 0.8 bar pressure bar from the compressor to the farthest consuming item in the floor shop; installing air storage tank near the process machinery;
- close the network loop;
- increase efficiencies in electric motor and compressor;
- install optimized controls;
- for multistage compression: provide better intercooling to decrease suction temperatures at intermediate stages;
- monitoring using proper instrumentation: pressure & temperature gauges, flowmeters, dew point temperature gauges, power meters;
- eliminating inefficient compressed air usages (e.g., cleaning, blowing);
- replacing compressed air by other means (electric tools, vacuum pump…);
- AVOID THE USE OF COMPRESSED AIR WHENEVER POSSIBLE
