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ENERGY_PERFORMANCE_ASSESSMENT_FOR_EQUIPMENT_AND_UTILITY_SYSTEMS_(CHAPTER-8:ENERGY PERFORMANCE ASSESSMENT OF COMPRESSORS)

 

ENERGY_PERFORMANCE_ASSESSMENT_FOR_EQUIPMENT AND UTILITY SYSTEMS

(CHAPTER-8:ENERGY PERFORMANCE ASSESSMENT OF COMPRESSORS)

Introduction

The compressed air system is not only an energy intensive utility but also one of the least energy efficient. Over a period of time, both performance of compressors and compressed air  system reduces drastically. The causes are many such as poor maintenance, wear and tear etc. All these lead to additional compressors installations leading to more inefficiency. A periodic performance assessment is essential to minimize the cost of compressed air.

Purpose of the Performance Test

To find out:

1.Actual Free Air Delivery (FAD) of the compressor

2.Isothermal power required

3.Volumetric efficiency

4.Specific power requirement

The actual performance of the plant is to be compared with design / standard values for assessing the plant energy efficiency.

Performance Terms and Definitions


Field Testing

Measurement of Free Air Delivery (FAD) by Nozzle method

Principle:If specially shaped nozzle discharge air to the atmosphere from a receiver getting its supply from a compressor, sonic flow conditions sets in at the nozzle throat for a particular ratio of upstream pressure (receiver) to the downstream pressure (atmospheric) i.e. Mach number equals one.

When the pressure in the receiver is kept constant for reasonable intervals of time, the airflow output of the compressor is equal to that of the nozzle and can be calculated from the known characteristic of the nozzle.

Arrangement of test equipment

The arrangement of test equipment and measuring device shall confirm to Figure 8.1.

Nozzle Sizes

The following sizes of nozzles are recommended for the range of capacities indicated below:

Flow Nozzle: Flow nozzle with profile as desired in IS 10431:1994 and dimensions

Measurements and duration of the test.
The compressor is started with the air from the receiver discharging to the atmosphere through the flow nozzle. It should be ensured that the pressure drop through the throttle valve should be equal to or twice the pressure beyond the throttle. After the system is stabilized the following measurements are carried out:
1.Receiver pressure
2.Pressure and temperature before the nozzle
3.Pressure drop across the nozzle
4.Speed of the compressor
5.kW, kWh and amps drawn by the compressor
The above readings are taken for the 40%, 60%, 100% and 110% of discharge pressure values.

Measuring instruments required for test
1.Thermometers or Thermocouple
2.Pressure gauges or Manometers
3.Differential pressure gauges or Manometers
4.Standard Nozzle
5.Psychrometer
6.Tachometer/stroboscope
7.Electrical demand analyser

Calculation Procedure for Nozzle Method

Example
Calculation of Isothermal Efficiency for a Reciprocating Air Compressor.

Assessment of Specific Power requirement
Measurement of FAD by Pump Up Method
(Note: The following section is a repeat of material provided in the chapter-3 on Compressed Air
System in Book-3.)
Another way of determining the Free Air Delivery of the compressor is by Pump Up Method - also known as receiver filling method. Although this is less accurate, this can be adopted where the elaborate nozzle method is difficult to be deployed.

Simple method of Capacity Assessment in Shop floor
1. 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 it, thus closing the receiver outlet.
2.Open water drain valve and drain out water fully and empty the receiver and the pipeline. Make sure that water trap line is tightly closed once again to start the test.
3.Start the compressor and activate the stopwatch.
4. Note the time taken to attain the normal operational pressure P1 (in the receiver) from initial pressure P2.
5.Calculate the capacity as per the formulae given below:

Actual Free air discharge
The above equation is relevant where the compressed air temperature is same as the ambient air
temperature, 1.e., perfect isothermal compression. In case the actual compressed air temperature at
discharge, say t2°C is higher than ambient air temperature say t1°C (as is usual case), the FAD is to be
corrected by a factor (273 + t1) / (273 +t2).

Example
An instrument air compressor capacity test gave the following results (assume the final compressed
air temperature is same as the ambient temperature) - Comment?

Capacity shortfall with respect to 14.75 m3/minute rating is 1.577 m3/minute i.e., 10.69 %, which
indicates compressor performance needs to be investigated further.

Solved Example:
In a medium sized engineering industry a 340 m3/hr reciprocating compressor is operated to meet
compressed air requirement at 7 bar. The compressor is in loaded condition for 80% of the time. The
compressor draws 32 kW during load and 7 kW during unload cycle.
After arresting the system leakages the loading time of the compressor came down to 60%.
Calculate the annual energy savings at 6000 hours of operation per year.
Solution:
Average power consumption with 80% loading
= [0.8 x 32+0.2 x 7] = 27kW
Average power consumption with 60% loading after leakage reduction
                                                           = [0.6 x 32+0.4x 7]
Saving in electrical power =5 KW
Yearly savings =5 x 6000
                                                           = 30,000 kWH


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