Maintenance Resources

Thursday, November 5, 2009

Energy and Cost Savings in Air Compressor

Air is free but compressed air is too costly. And by its operation, We have a lot scope to save Energy and money from their operation. Common methods are

1. Arresting air leakage
2. Air inlet system should be cool and dirt
3. Pressure setting and compressor modulation
4. Installing VSD (variable speed drive) to optimized it’s power consumption on load and no load
5. Optimizing Air line to reduce pressure drop

But before to discuss its saving aspects, let’s see how costly it is

Cost of Compressed air = {(BHP) x (0.746) x (# of operating hours) x ($/kWh) x (% time) x (% full load BHP)}/Motor Efficiency

1. Where BHP—Compressor shaft horsepower (frequently higher than the motor nameplate
Horsepower—check equipment specification)
2. Percent time—percentage of time running at this operating level i.e full load % and no load %
3. Percent full-load BHP—BHP as percentage of full-load BHP at this operating Level
4. Motor efficiency—motor efficiency at this operating level

5. And Total cost = Full load cost + No load cost

A typical manufacturing facility has a 33.5 hp compressor (which requires 45.56 BHP)
That operates for 6800 hours annually (Assume compressor operated 2 shifts a day, 5 days a week) . It is fully loaded 85% of the time (motor
Efficiency = 95%) and unloaded the rest of the time (25% full-load BHP and motor efficiency = 90%). The aggregate electric rate is Rs. 7/kWh.

Cost when fully loaded =

{45.56 BHP) x (0.746) x (6800 hrs) x (Rs. 7/kWh) x(0.85) x(1.0)}/0.95

= Rs. 1447520

Cost when partially loaded =

{45.56 BHP)x(0.746) x(6800 hrs)x Rs. 7/kWh)x 0.15) x (0.25)}/0.90

= Rs. 60668

Yearly cost of Air generation
(Except Timely maintenance cost ) = Rs. 1447520 + Rs. 60668 = Rs. 1508188. + Std. Maintenance cost.

Now see bellow example for leakage cost
Stop all loads and check for load and unload pattern for 8 to ten cycles and take average for accuracy. at compressor end. Assume leakage test yields flowing results
Compressor capacity (m3/min) = 35
Cut in pressure = 6.8 KG/ cm2
Cut out pressure = 7.5 kg/cm2
Load KW drawn = 188 kw
Unload Kw drawn = 54 kw
Average load time = 1.5 min
Average unload time = 10.5 min

So, Leakage qty. = (1.5 / (1.5+10.5)) X 35 = 4.375 m3/min
Leakage per day = 6300 m3 / day
Power require for air generation = 188 kw /(35 m3/minX60) = 0.0895
Energy loss due to Leakage / day = 0.0895 X 6300 m3/day = 564 kwh
Assuming Rs. 7 /KWH in average cost of air leakage = Rs. 3948 / day , Rs 98700/ month.

SO..STOP Leakage!

2. Air Inlet System – Compressor room need to be well ventilated and need separate exhaust for hot air

3. Compressor modulation = Optimize sizing and scheduling of compressor
4. Installing VSD = A very sensible issue but before to install we need to check our requirement or else it will not give saving as desired fashion and still for many cases, motor with soft started and Pressure on/off control is cheap and efficient option. Flowing are hints
a) Due to efficiency factor of VSD, at full load, compressor with VSD drive takes more current then general compressor.
b) A compressor controlled by a variable speed drive draws power in almost direct proportion to the load, with energy consumption dropping to near zero at no-load. In contrast, conventional compressors are less efficient because at no-load they consume between 13 and 85 per cent of their full-load consumption, depending on the manufacturer and operating mode.
c) VSD system will give significant savings 0f 10 % to 30% of operating cost if and only if compressor is running with fluctuating load between 40 to 80 % and install with pressure sensor to modulate VSD outputs. Because No load loss will be almost near to zero and may run with friction of efficiency as per load demand.


  1. Thanks for sharing this useful information.

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  3. It seems you are mechanical engineer. I am an electronics/electrical enthusiast and I love mathematics as well. That's my fave subject when I was in college!