8. ENERGY PERFORMANCE ASSESSMENT OF COMPRESSORS 8.1 Introduction

Transcription

1 8. ENERGY PERFORMANCE ASSESSMENT OF COMPRESSORS 8.1 Introduction The compressed air system is not only an energy intensive utility but also one o the least energy eicient. Over a period o time, both perormance o 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 ineiciencies. A periodic perormance assessment is essential to minimize the cost o compressed air. 8.2 Purpose o the Perormance Test To ind out: Actual Free Air Delivery (FAD) o the compressor Isothermal power required Volumetric eiciency Speciic power requirement The actual perormance o the plant is to be compared with design / standard values or assessing the plant energy eiciency. 8. Perormance Terms and Deinitions Compression ratio Isothermal Power Isothermal Eiciency Volumetric eiciency : Absolute discharge pressure o last stage Absolute intake pressure : It is the least power required to compress the air assuming isothermal conditions. : The ratio o Isothermal power to shat power : The ratio o Free air delivered to compressor swept volume Speciic power requirement: The ratio o power consumption (in kw ) to the volume delivered at ambient conditions. 8.4 Field Testing Measurement o Free Air Delivery (FAD) by Nozzle method Principle: I specially shaped nozzle discharge air to the atmosphere rom a receiver getting its supply rom a compressor, sonic low conditions sets in at the nozzle throat Bureau o Energy Eiciency 107

2 or a particular ratio o upstream pressure (receiver) to the downstream pressure (atmospheric) i.e. Mach number equals one. When the pressure in the receiver is kept constant or a reasonable intervals o time, the airlow output o the compressor is equal to that o the nozzle and can be calculated rom the known characteristic o the nozzle Arrangement o test equipment The arrangement o test equipment and measuring device shall conirm to Figure Nozzle Sizes The ollowing sizes o nozzles are recommended or the range o capacities indicated below: Nozzle size (mm) Capacity (m /hr) Flow Nozzle: Flow nozzle with proile as desired in IS 1041:1994 and dimensions Measurements and duration o the test. The compressor is started with the air rom the receiver discharging to the atmosphere through the low nozzle. It should be ensured that the pressure drop through the throttle valve should be equal to or twice the pressure beyond the throttle. Ater the system is stabilized the ollowing measurements are carried out: Receiver pressure Pressure and temperature beore the nozzle Pressure drop across the nozzle Speed o the compressor kw, kwh and amps drawn by the compressor The above readings are taken or the 40%, 60%, 100% and 110% o discharge pressure values. Measuring instruments required or test Thermometers or Thermocouple Pressure gauges or Manometers Dierential pressure gauges or Manometers Standard Nozzle Bureau o Energy Eiciency 108

3 Psychrometer Tachometer/stroboscope Electrical demand analyser P 1 T 1 P 2 FILTER AIR COMPRESSOR RECEIVER P 4 P T Nozzle THROTTLE VALVE DISCHARGE TO ATMOSPHERE P P 4 FLOW STRAIGHTENER BY-PASS Figure 8.1: Test Arrangement or Measurement o Compressed Air Flow 8.5 Calculation Procedure or Nozzle Method I. Free air delivered, Q ( m s π / sec) = k x x d 4 x x P1 2 T1 2( P P4 )( P x Ra ) T 1 / 2 k : Flow coeicient as per IS d : Nozzle diameter M T 1 : Absolute inlet temperature o K P 1 : Absolute inlet pressure kg/cm 2 P : Absolute Pressure beore nozzle kg/cm 2 T : Absolute temperature beore nozzle o K R a : Gas constant or air J/kg k P -P 4 : Dierential pressure across the nozzle kg/cm 2 Bureau o Energy Eiciency 109

4 II. Isothermal Eiciency = Isothermal power / Input power P1 xq xloge r Isothermal power ( kw ) = 6.7 P 1 = Absolute intake pressure kg/ cm 2 Q = Free air delivered m /hr. r = Pressure ratio P 2 /P 1 III. Power consumption, kw Speciic power consumption at rated discharge pressure= Free air delivered, m / hr Freeair delivered inm / min IV. Volumetric eiciency = x 100 Compressor displacement in m / min Compressor displacement π 2 = x D 4 x L x S x χ x n D = Cylinder bore, metre L = Cylinder stroke, metre S = Compressor speed rpm χ = 1 or single acting and 2 or double acting cylinders n = No. o cylinders 8.6 Example Calculation o Isothermal Eiciency or a Reciprocating Air Compressor. Step 1 : Calculate Volumetric Flow Rate k : Flow coeicient (Assumed as 1) d : Nozzle diameter : 0.08 metre P 2 : Receiver Pressure -.5 kg / cm 2 (a) P 1 : Inlet Pressure kg / cm 2 (a) T 1 : Inlet air temperature 0 o C or 0 o K P : Pressure beore nozzle 1.08 kg / cm 2 T : Temperature beore the nozzle 40 o C or 1 o K P P 4 : Pressure drop across the nozzle = 0.06 kg / cm 2 R a : Gas constant : 287 Joules / kg K Free air delivered, Q ( m s π / sec) = k x x d 4 x x P1 2 T1 2( P P4 )( P x Ra ) T 1 / 2 Free air delivered 1 / 2 s π x0.06 x1.08 x 287, Q ( m / sec) = 1x x(0.08) x x = 0.91 m /sec = m / h Bureau o Energy Eiciency 110

5 Step 2 : Calculate Isothermal Power Requirement P xq xloge Isothermal power ( kw ) = r P 1 - Absolute intake pressure = 1.04 kg / cm 2 (a) Q -Free Air Delivered = m / h..51 Compression ratio, r = = x xloge.6 Isothermal power ( kw ) = = 48. 4kW 6.7 Step : Calculate Isothermal Eiciency Motor input power = 100 kw Motor and drive eiciency = 86 % Compressor input power = 86 kw Isothermal eiciency = Isothermal Power x 100 Compressor input Power = 48.4 x 100 = 56% Assessment o Speciic Power requirement Speciic power consumption = Actual power consumed by the compressor Measured Free Air Delivery In the above example the measured low is m /hr and actual power consumption is 100 kw. Speciic power requirement = = kw/m /hr 8.8 Measurement o FAD by Pump Up Method (Note: The ollowing section is a repeat o material provided in the chapter- on Compressed Air System in Book-.) Another way o determining the Free Air Delivery o the compressor is by Pump Up Method - also known as receiver illing method. Although this is less accurate, this can be adopted where the elaborate nozzle method is diicult to be deployed. Bureau o Energy Eiciency 111

6 Simple method o Capacity Assessment in Shop loor Isolate the compressor along with its individual receiver being taken or test rom main compressed air system by tightly closing the isolation valve or blanking it, thus closing the receiver outlet. Open water drain valve and drain out water ully 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 taken to attain the normal operational pressure P 2 (in the receiver) rom initial pressure P 1. Calculate the capacity as per the ormulae given below: Actual Free air discharge Q P P V P T 2 1 = 0 Nm / Minute Where P 2 = Final pressure ater illing (kg/cm 2 a) P 1 = Initial pressure (kg/cm 2 a) ater bleeding P 0 = Atmospheric Pressure (kg/cm 2 a) V = Storage volume in m which includes receiver, ater cooler, and delivery piping T = Time take to build up pressure to P 2 in minutes The above equation is relevant where the compressed air temperature is same as the ambient air temperature, i.e., perect isothermal compression. In case the actual compressed air temperature at discharge, say t 2 0 C is higher than ambient air temperature say t 1 0 C (as is usual case), the FAD is to be corrected by a actor (27 + t 1 ) / (27 + t 2 ). EXAMPLE An instrument air compressor capacity test gave the ollowing results (assume the inal compressed air temperature is same as the ambient temperature) Comment? Piston displacement : m /minute Theoretical compressor capacity : m 7 kg/cm 2 Compressor rated rpm 750 : Motor rated rpm : 1445 Receiver Volume : 7.79 m Additional hold up volume, i.e., pipe / water cooler, etc., is : m Total volume : 8.22 m Initial pressure P 1 : 0.5 kg/cm 2 Final pressure P 2 : 7.0 kg/cm 2 Bureau o Energy Eiciency 112

7 Atmospheric pressure P 0 : kg/cm 2,a Compressor output m /minute : ( P ) P2 1 Total Volume Atm. Pressure Pumpup time ( ) 8.22 : = 1.17 m /minute Capacity shortall with respect to m /minute rating is m /minute i.e., %, which indicates compressor perormance needs to be investigated urther. Bureau o Energy Eiciency 11

8 QUESTIONS 1) What is meant by Free Air Delivery? 2) Describe the method o estimating low by nozzle method. ) Describe the method o estimating low by pump up method. 4) Deine the term isothermal eiciency and explain its signiicance. 5) Deine the term volumetric eiciency and explain its signiicance. 6) How is speciic power requirement calculated? REFERENCES 1. IS 1041:1994: Measurement o airlow o compressors and exhausters by nozzles. 2. IS 5456:1985 code o practice or testing o positive displacement type air compressors and exhausters. Compressor perormance Aerodynamics or the user by M Theodore Gresh- Butterworth Heinemann. Bureau o Energy Eiciency 114