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Chapter 4.8 - Compressors

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Selected questions Chapter 4.8: Compressors Short type questions 1. Define compression ratio for a compressor Ans 2. Compression ratio = Absolute discharge pressureof last stage Absoluteintake pressure What are the different components of a compressed air system network? Ans The different components that are available in a compressed air network are Intake air filters Inter-stage coolers After coolers Air dryers Moisture drain traps Receivers 3. List at least three different flow control methods adopted in a compressed air system? Ans Various flow control methods are Automatic On/Off control Load and Unload Multi-step control Throttling control 4. Define volumetric efficiency of a compressor? Ans 5. Volumetric efficiency = Free air delivery , m / min compressor displaceme nt What are the various instruments required for performance evaluation of compressed air systems by nozzle method? Ans Various instruments required are Thermometers or Thermocouple Pressure gauges or Manometers Differential pressure gauges or manometers Standard nozzle Psycho meter Tachometer/Stroboscope Electrical demand analyser 6. Define specific power requirement? Ans 7. The ratio of power consumption in kW to the volume delivered at ambient conditions. During the a leakage assessment test, compressor was loaded for 7 minutes and unloaded for 20 minutes. If the actual free air delivered is 200 CFM then evaluate quantity of leakage. 4.8 - Compressors (table format) 46 Selected questions = Leakage quantity 8. Actual FAD x % leakage = 200 x 26 = 51.85 cfm 100 Power consumption increases by 2% If the pressure setting of a compressor is reduced by 1 kg/cm2g, what impact it will have on power consumption? Ans 10. = What happens to power consumption when the temperature of the inlet air to the compressor rises by 8OC? Ans 9. 7 x 100 = 26% 7 + 20 = % leakage load time x 100 load time + unload time = Ans. The power consumption reduces by 6-10% What are the acceptable pressure drops in mains header and that in distribution system in a compressed air network? Ans Typical acceptable pressure drop in industrial practice is 0.3 bar in mains header at the farthest point and 0.5 bar in distribution system. Long type questions 1. What is the procedure of conducting free air delivery test of compressor by nozzle method? Ans 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: Receiver pressure Pressure and temperature before the nozzle Pressure drop across the nozzle Speed of the compressor KW, kWh and amps drawn by the compressor The above readings are taken for the 40%, 60%, 100% and 110% of discharge pressure values. The Free Air delivered (Qf) is given by: Free Air delivered Qf d T1 2(P3 P4) x (P3 x Ra) = kx x x x 4 P1 T3 k : Flow coefficient as per IS d : Nozzle diameter M T1 : Absolute inlet temperature K P1 : Absolute inlet pressure kg/cm2 P3 : 1/2 Absolute Pressure before nozzle kg/cm2 4.8 - Compressors (table format) 47 Selected questions T3 Absolute temperature before nozzle oK Ra : Gas constant for air 287.1 J/kg k P3-P4 2. : : Differential pressure across the nozzle kg/cm2 Explain the simple method of free air delivery assessment of compressor in shop floor. Ans. Simple method of conducting the Free Air Delivery of the compressor is by receiver filling method. Though this is less accurate, this can be adopted where the elaborate nozzle method is difficult to be deployed. Steps: Determine the volume of the connected receiver in m3. Determine the volume of the piping between the compressors and receiver in m3, if significant. Record the load/unload pressure settings of the compressors kg/cm2. Wait for the compressor to turn on, pressurize the receiver, and turn off or unload. When the compressor turns off or unloads, start timing (it is assumed that the compressor unloads completely). Note the receiver pressure. Note the time it takes for the receiver pressure to drop and for the compressor to turn on or load up. Repeat these steps over a reasonable period of time. The Free Air delivery is calculated as follows: FAD = Q x (PF P1 ) (Pa x t ) where, FAD - Free Air Delivery (m3/min) Q - Total volume of the receiver and pipeline connected from compressor to air receiver (m3) PI - Initial Pressure of the receiver (kg/cm2) PF - Final Pressure of the receiver (kg/cm2) Pa - Atmospheric Pressure, (kg/cm a) t - Average filling time (min) Numerical type questions 1. In an engineering industry located in Southern India, there are 4 numbers of compressors installed for compressed air requirement. All the four will be in operation at any point of time. The rated capacities of compressors are 100 cfm each, whereas the actual output of each are given below: Compressor #1 : 80 cfm Compressor #2 : 75 cfm Compressor #3 : 90 cfm Compressor #4 : 95 cfm The power consumption of these compressors is 25 kW, 27 kW, 23 kW and 28 kW respectively. The compressor is continuously operated at 7 kg/cm2g pressure. During the measurements it was observed that the leakage losses constitute around 45% of the actual output of the compressors. 4.8 - Compressors (table format) 48 Selected questions Estimate the following a) Power loss due to leakage b) Estimate the power savings if 50% of the leakages are arrested Note: The compressors have on and off control Ans. a) Output of all four compressors : 80+ 75++90+95 : 340 cfm : 25+27+23+28 : 103 kW Specific power consumption : 0.303 kW/cfm Leakage quantity : 45% of actual output : 0.45 x 340 : 153 cfm : 0.303 x 153 : 46.4 kW Power consumed by compressors Equivalent power loss b) If 50% leakages are arrested i.e. 76.5 cfm, the power loss is 23.2 kW Hence power savings 2 : 23.2 kW One Plant has one compressor of capacity 1680 m3/h. Free air delivery of the compressor is carried out by filling the receiver. The test data is as follows: Receiver capacity Interconnecting pipe Initial pressure in receiver Inlet air pressure to compressor Final pressure Time taken to fill the receiver Inlet air temperature Air temperature in the receiver Motor rpm (D1) Motor pulley diameter (N1) Compressor rpm (D2) Compressor Pulley diameter (N2) Average duration of loading Average duration of unloading Power consumption during loading Power consumption during unloading Cost of energy A: B: : 10 m3 : 1 m3 : 1.0 kg/cm2a 1.0 kg/cm2a : 8.25 kg/cm2a : 3 minutes (180 seconds) : 30 oC : 40 oC : 1400 : 300mm : 700 rpm : 600 mm : 40 minutes in an hour : 20 minutes in an hour : 150 kW : 25 kW : Rs. 5.00 per kWh Find the Free air delivery of the compressor and % output when compared to rated capacity. Evaluate hourly consumption of air. C: How much energy (kWh) that the compressor will consume in an hour and in a day. Also evaluate the cost of energy per day(24 hours operation). D: Evaluate the specific power (cfm/kW) during loading period and overall specific power after considering the loading & unloading of compressor. E: Plant was interested to reduce the unloading time of the compressor by reducing the pulley diameter of the motor. Evaluate the speed of the compressor required for 10 minutes 4.8 - Compressors (table format) 49 Selected questions unloading and 50 minutes and accordingly evaluate the diameter of the pulley of the motor. Equation: D1 x N1 = D2 x N2 F: Ans. Estimate the hourly power consumption, energy savings after replacement of the pulley and payback period. Consider the cost of pulley and belts is Rs 40000 and operating hours of the compressor is 8000 in a year. (consider during loading the power consumption is 120 kW and during unloading the power consumption was 20 kW) Free Air Delivery in m3/min is given by: Volume in m3 x (Final pressure Initial pressure) in kg/cm2a x Inlet temp in K = ----------------------------------------------------------------------------------------------Time in minutes x Inlet air pressure in in kg/cm2a x Outlet air temp in K = : 11 x (8.25-1) x (30+273) ------------------------------------------3 x 1 x (40+273) : 3.67 x 7.25 : 25.75 x 0.968 m3/min : 1545 m3/h : 914 cfm % output is given by : Actual FAD x 100 / Rated output : 545 x 100 / 1680 : 92% B: Hourly air consumption Hourly consumption of air: Average duration of loading : 40 minutes in an hour Average duration of unloading : 20 minutes in an hour % loading of the compressor : 40 x 100/ (40+20) : 66.7% loading Hourly consumption of air is given by : % loading x actual out put : 0.667 x 1545 : 1030 m3/h : 609 cfm C: Energy consumption: % of loading of the compressor : 66.7 % % of unloading of the compressor : 33.3 % Hourly energy consumption is given by: = (% loading x load power) + (% unloading x unload power) : (0.667 x 150) + (0.333 x 25) : 108.375 kWh 4.8 - Compressors (table format) 50 Selected questions Daily energy consumption : 108.375 x 24 : 2601 kWh Cost of energy per day : 2601 x 5 : Rs. 13005 /day D: Specific energy (cfm/kW): During loading : Actual FAD in cfm / Load power in kW : 914/150 : 6.09 cfm/kW Over all specific energy is given by = Actual hourly air consumption in cfm / Actual energy per hour : 609/108.37 : 5.67 cfm/kW E: Speed of the compressor, Pulley size of the motor At present 609 cfm is obtained in 40 minutes when compressor is operating at 700 rpm. The same 609 cfm should be obtained in 50 minutes after changing of the pulley. The speed of the compressor (proposed)N2 : 700 x 40 / 50 : 560 rpm Proposed motor pulley diameter is D1 : N2 x D2/N1 : 560 x 600 /1400 : 240 rpm Caution: The proposed speed of the compressor is 560 cfm. Please refer the instruction manual of the compressor for the lowest speed that the compressor will operate with sufficient lubrication since the lubrication pump is directly connected to the compressor shaft. Other wise external pump for lubrication can be provided which may consume about 0.3 -0.4 kW. This should considered during the techno-economics evaluation F. Power consumption & Payback period Hourly consumption of air is given by : (120 kW x (50/60)) + (20 kW x (10/60)) : 103.33 kWh Energy savings : 108.375 103.33 kW : 5 kW Annual energy savings : 5 kW x 8000 h : 40 000 kWh Annual cost savings : Rs. 40000 x 5 = Rs. 200, 000 i.e Rs 2.00 lakh Investment required : Rs. 40, 000 Payback period : 0.2 years (2.4 months) 4.8 - Compressors (table format) 51 Selected questions 2. A reciprocating two stage air compressor of 492 cfm capacity is operating in a manufacturing industry. The FAD test was conducted by Pump up Method. The following parameters were measured during the test. Receiver diameter : 1.78 m Receiver height : 0.8 m Volume of interconnecting pipelines : 0.09 m3 Time taken to fill up the receiver : 1.48 minutes Initial pressure : Atmospheric Receiver pressure : 7.75 kg/cm2 (g) Ambient temperature : 32 oC Compressed air temperature : 32 oC a. Calculate the actual FAD delivered by the compressor and express it in terms of percentage of design FAD. b. Also calculate the Isothermal efficiency and specific power consumption if the actual power consumption by the compressor is 89.7 kW. Assume motor efficiency as 90%. a. Calculation of actual FAD of compressor Volume of the receiver : x (1.78)2 / 4 x 0.8 : 2.0 m3 Volume of interconnecting pipelines : 0.09 m3 Total volume : 2.09 m3 Time taken to fill the receiver : 1.48 minutes Initial pressure : 1.0 kg/cm2 a Receiver pressure : 8.75 kg/cm2 a FAD : (8.75 1.0) x 2.09/ (1.0 x 1.48) : 10.94 m3/min : 492 cfm : 13.85 m3/min Rated FAD of the compressor Actual FAD as percentage of rated FAD : b. 79 % Isothermal Efficiency Compression ratio, r : 8.75 / 1.0 : 8.75 : 1.0 x 10.94 x 60 x ln (8.75)/36.7 : 38.8 Actual Power consumption, kW : 89.7 Isothermal Efficiency : 38.8 / (89.7 x 0.9) : 48.06 % Isothermal Power, kW Specific Power Consumption, 3. kW/m3/min: 89.7 / 10.94 = 8.2 The following data are available for a single stage reciprocating air compressor. Receiver volume Time taken to fill up the receiver 4.8 - Compressors (table format) : : 8.5 m3 4 minutes 52 Selected questions Initial pressure Final pressure Cylinder bore Cylinder stroke Speed of motor Speed of compressor No. of cylinders Compressor input power No. of acting Calculate: a. Free air delivery b. Volumetric Efficiency c. Isothermal Efficiency d. Specific power consumption : : : : : : : : : 1 kg/cm , a 3.5 kg/cm2 a 0.3 m 0.15 m 1445 rpm 650 rpm 1 18.5 kW single acting : (3.5 1.0) x 8.5 / 4 : 5.31 m3/min : x (0.3)2 / 4 x 0.15 x 650 : 6.89 m3/min : FAD x 100 / Compressor Displacement : 5.31 x 100 / 6.89 : 77.1 % Compression Ratio : 3.5 Isothermal Power : 1.0 x (5.31 x 60) x loge (3.5 /36.7) : 10.88 kW : 10.88 x 100 / 18.5 : 58.8 % : 18.5 / 5.31 : 3.48 kW / m3/min a. Free air delivery FAD b. Volumetric Efficiency Compressor Displacement Volumetric Efficiency c. Isothermal Efficiency Isothermal Efficiency d. Specific Power Consumption 4.8 - Compressors (table format) 53

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