Energy Conservation In Compressed Air Systems

-Vishnu RC VijayanEnergy Conservation In Compressed Air Systems

ContextIntroductionCapacity/Pump-Up TestMethods To Improve PerformanceReferences

IntroductionEvery manufacturer specifies the FAD of the Compressor as the Motor rating

For Eg a 1000 Scfm (Standard cubic feet per minute) Compressor requires a 220 KWH MotorSo, 1000 x 60 Std. cu. ft of air per hour requires 220 KWHi.e 220 KW generates 60000 std cubic feet

So, 1 KW generates 273 std cubic feetCost of 1 KWH is @ Rs. 5.00.So, power cost of 1 cu. Ft. is @ 2 paise, or 0.02 Rupees

Thus, the running cost for a 1000 Scfm Compressor, at 80% load, becomes: 1000 x 60 x 0.8 x 0.02 x 24 = Rs. 23K per day, or ~ @ 75 Lacs p.a.And we have not taken the Capital cost of the Compressor into account

Why Energy Conservation is NeededStudy Conducted by Forbes MarshallAir compressors account for significant amount of electricity used in Indian industries. The average for all Industries is around 10%

Statistical DataBetween 10% and 20% of electricity consumed is for generating compressed air itself

Of the total cost, 76% is attributed to electricity, 12% to Maintenance and 12% to Equipment and Installation

Only 50% of a compressor's air supply is used for productionThe rest is lost through waste and air leaks

Pump-Up TestTo find out: Actual Free Air Delivery (FAD) of the compressorSpecific power requirement

Methodology of Capacity Test

This test needs less skill and simple arithmetic calculation and any skilled person can conduct the test under the supervision of a qualified engineer

Instruments needed for the test are: (1) stopwatch (2) Clip on power meter which measures KW

Use of conventional multi-meter to measure line current to calculate the power should be avoided because the value of power factor cannot be assumed and this may lead to wrong conclusion

Therefore, only clip on power meter should be used to measure the actual power drawn by the air compressor panel meter installed can be used

The steps involved in conducting the test are- Isolate the air receiver from the air network by closing the delivery valve after the receiver

Measure the approximate volume of piping network and filters like oil filter, intercooler and after cooler etc, between compressor and receiver in m3

Hold the clip on power meter in any one of the phase in the switch panel Empty the air receiver and set the pressure gauge at 0 bar (gauge)

Drain water from the receiver and do not forget to close the drain valve Start the compressor and start the stop watch once the compressor comes to loading mode

Simultaneously measure the KW in the clip on power meter for each pressure till the compressor reaches the normal working pressure Note down the time in seconds taken to fill the receiver to the normal operating pressure, i.e. compressor reaches unloading mode

The compressor capacity is estimated using the equation. Free air delivered using the following formulae: Actual Free Air Delivered (Q) = [(P2-P1)/P0]*[V/T] P2 = Final pressure after filling [kg/ cm2] P1 = Initial pressure [kg/cm2 ] after bleeding P0 = Atmospheric Pressure [kg/cm2] V = Storage volume in m3 which includes receiver, after cooler, and delivery piping T = Time take to build up pressure to P2 in minutes.

The above equation is relevant where compressed air temperature is same as the ambient air temperature, i.e., perfect isothermal compression

In case the actual compressed air temperature at discharge, say t2O C is higher than ambient air temperature say t1O C (as is usual case), the FAD is to be corrected by a factor (273 + t1) / (273 + t2) Calculations

Specific Energy Consumption(SEC) = KW/FAD

Isothermal Efficiency = Isothermal power / Input powerIsothermal power kW= P xFAD x log(r)/ (36.7)

Volumetric efficiency= FAD/ (Compressor displacement)Compressor displacement = (/4 )X D2 x L x S x n

ISO 1217:2009/Amd 1:2016

Methods to Improve PerformanceLocation and Size of FiltersLow inlet tempFilter to reduce Wear

ElevationLess to avoid Volumetric Efficiency

Inter-cooling in between StagesEconomy in size and Cost of ReceiverTo avoid Condensation in Receiver

Optimizing Pressures

LeakagesEstimated as major drawback

Use of Blowers

Pneumatic Tools

References

Forbes MarshallSurveyon Energy Conservation

National Productivity CouncilWebsite: http://www.em-ea.org/

Bureau of Energy EfficiencyWebsite: www.beeindia.gov.in