ASH HANDLING
10. COMPRESSED AIR SYSTEM
10.3.5 Auxiliary sections/equipment
If the compressors are having dedicated cooling tower and cooling water systems, the audit should also cover these units. The procedure for conducting the energy audit of these systems is given separately.
• Cooling tower
• Cooling tower fans
• Water pumps
10.3.6 Instruments Required
The following instruments are required for conducting the compressed air energy audit
• Power Analyzer: Used for measuring electrical parameters such as kW, kVA, pf, V, A and Hz
• Temperature Indicator & Probe: To measure air & water inlet &
• outlet temperatures of inter-cooler and after-coolers
• Pressure Gauge: To measure operating pressure and pressure drop in the system
• Stroboscope: To measure the speed of the compressor and motor
• Stop watch: for conducting leak test, loading duration, unloading duration of compressors)
• Tensometer for belt tension check
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10.4 Measurements & Observation Be Made
While conducting the audit, the following measurements and observations are necessary
• Energy consumption pattern of compressors and associated equipment (complete system)
• Motor electrical parameters during load & unload (kW, kVA, Pf, A, V, Hz, THD) of individual loads
• Compressor loading pattern compressor pressure settings (load and unload pressure)
• Compressors operating hours and operating schedule
• Pressure drop across the filters
• Inter coolers and after cooler parameters ( Air & cooling media - inlet & outlet temperatures & pressures of inter coolers and after coolers)
• Cooling water flow rate to intercoolers and after coolers
• Compressor speed
• Motor speed
• Ambient air temperature and % RH
• Actual air pressure used for equipment operation
• Pressure drop in the system and in the suction filters, dryers, etc
• Air receiver volume at the compressor house & users
• Cooling tower performance (range and approach)
• Cooling tower fan and cooling water pump power consumption
While conducting the energy audit, the typical summary measurement sheet (Table 10-4) of compressed air system is given below. During the audit study, the auditor should measure/
collect the information pertaining to the parameters given below:
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Table 10-4: Compressors parameters
Location Compressor
Load (cut in) pressure, kg/cm2 Unload (cut off) pressure, kg/cm2 Motor rpm
Compressor power during load, kW Compressor power during unload, kW Inter cooler temperature, OC
Air in
After cooler temperature, OC Air in
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Ambient temperature, OC Ambient RH, %
Cooling water source
Cooling water flow to intercooler, lpm Cooling water flow to after cooler, lpm Power consumption during load, kW Power consumption during unload, kW Pressure drop across the filters, mmWC
10.4.1 System familiarization and operational details
Detailed interactions with the plant personnel have to be carried out to get familiarization for system detail and operational details. The entire system should be briefed in the report.
10.4.2 Energy consumption Pattern
If the plant is monitoring the energy consumption, it is suggested to record the data and monitor the daily and monthly consumption pattern. Collect the past energy consumption data, if available (month wise for at least 12 months, daily consumption for about a week for different seasons, daily consumption during the audit period).
Work out the total consumption of compressed air system to arrive at percentage to the total consumption of the auxiliary consumption. In case, energy meters are not installed to compressors & its auxiliary units of, instantaneous measurements can be carried out, based on the loading pattern the daily consumption can be worked out (as shown in Table 10-5).
Table 10-5: Energy consumption pattern (to be measured) Equipment Instantaneous
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The energy consumption of compressed air system and associated system : ... kWh
Total auxiliary power consumption : ... kWh
Contribution of compressed air system on total auxiliary power : ... % 10.4.3 Free air delivery test (Compressor output assessment test)
This test is to be conducted to assess the actual output of the compressor. The procedure of conducting the test is as follows:
• Isolate the compressor along with its individual receiver being taken for test from main compressed air system by tightly closing the isolation valve or blanking the receiver outlet. Ensure that the valves are closed completely.
• Check the pressure gauge and the temperature gauge installed for the air receiver for calibration and working status
• Open water trap drain outlet and empty the receiver and the pipeline.
• Make sure that water trap line is tightly closed once again to start the test.
• Start the compressor and activate the stopwatch.
• Note the time to attain the normal generated pressure P2 (in the receiver) from initial pressure P1
• Measure the temperature of ambient and compressed air (receiver) Calculate the capacity as per the formula given below.
Actual Free Air Discharge: P2 – P1 V
The above equation is relevant where the 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 t2OC 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).
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Once the actual output is assessed, then compare with rated output. If any deviation is found then look for the various factors causing deviation and arrive at the remedial measures to improve the performance. Efficiency evaluation Calculate the ideal kW as per formula given below.
Ideal Power kW = (NK/K - 1) x (QxPs/0.612) x [(Pd/PS) k - 1/NK- 1]
Measures actual kW during steady state load condition Calculate the actual efficiency
Ideal kW =
ActualkW Compare actual with the design efficiency.
The typical data sheet is given below (Table 10-6):
Table 10-6: Free air delivery test of compressors
Compressor Comp #1 Comp #2 Comp #3
Design output, m /min Time & Date
Atmospheric pressure, kg/cm2 a Initial pressure, kg/cm2 a Final pressure, kg/cm2 a
Receiver and connected pipe volume, m Ambient temperature, OC
Receiver temperature, OC Time taken to fill the receiver Actual output, m /min
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