Indian Journal of Pure & Applied Physics Vol. 49, January 2011, pp. 35-41
Condition monitoring of reciprocating air compressor using IR thermography S S Singh & A K Singh* Department of Aerospace Engineering, Defence Institute of Advanced Technology (Deemed University), Girinagar, Pune 411 025 *E-mail:
[email protected];
[email protected] Received 17 January 2010; revised 24 September 2010; accepted 9 November 2010 The usage of IR thermography as an effective condition monitoring tool on reciprocating air compressor has been studied. The technique allows monitoring of temperatures and thermal patterns while the equipments are running. The compressor cylinder fins tip temperatures are measured using infrared gun and thermocouple and have been further analysed by finite element method. Based on experiments and finite element analysis, equation is generated for internal surface temperature rise in the compressor cylinders due to running of the compressor. The analysis involved correlating the surface temperature with that obtained by finite element analysis. It is, further, highlighted in the paper that as the compressor under study does not cater for any kind of temperature measurement to analyse the performance of compressor, thermal imaging of such equipments can be beneficial in analysing the health of the equipments by acquiring thermal images over a specified interval of time which could be time specific. Keywords: Finite element analysis, Infrared, Thermograph, Thermocouple, Steady state
1 Introduction Infrared thermography is now being targeted as a versatile tool for condition monitoring of equipments1,2. Infrared imaging will help quickly and efficiently find the areas that are most in need of maintenance. The infrared thermal imaging method utilizes the radiant existence in the infrared spectral band from measured objects to measure temperature. It is non-intrusive, applicable remotely and suitable for measurement3 of a large area, and can also serve to record data for subsequent storage and processing with a PC. Ay et al.4 used an infrared thermal imaging camera to observe the surface temperature of a plate finned-tube heat exchanger and calculated the local heat transfer coefficient. IR thermography on equipments shows that the surface thermal patterns are a consequence of internal conditions. Basic aim of this study is to maintain the equipments by evaluating its performance at specified interval of time. Using finite element analysis5,6, we have shown that the internal conditions can be evaluated from the external surface conditions. The purpose of this study is to determine the applicability of IR thermography as a condition monitoring tool and also to evaluate inner surface temperature rise during compressor operation. 2 Instrumentation To perform experiments reciprocating air compressor ELGI THPC 500 unit consisting of two
sets of cylinders; one for high pressure and other for low pressure has been used. A portable Thermal Imaging camera (EEV make, Model P 4430) has been used for thermal imaging. It incorporates a video output which can be used for direct recording to a computer. The P4430 is a hand-held thermal imaging camera with pyroelectric vidicon detector having spectral response of 8-12 µm. It is a self-contained battery/ac power operated unit incorporating a miniature display monitor and can be fitted with interchangeable lens (Focal length 50/75 mm f 0.8) and angle of view 17/11 deg. The equipment is for general thermal imaging applications and gives monochrome thermal pictures. The external video signal is 1 volt 625 lines 50Hz (CCIR compatible) or 525 lines 60 Hz (optional), 30 frames / s and 200 lines per picture height for a temperature difference of 2°C (chopped) and 1°C (panned). A personal computer having Q motion card for image acquisition and WINVIDEO with Windows operating system has been used. An IR thermometer gun OPTEX THERMOHUNTER make (Model Q185) is used for direct temperature reading of the object surface having provision of emissivity adjustment. The temperatures have been measured using T type thermocouples. For the purpose, a data acquisition system using GENIE software comprising ADAM 4018 and ADAM 4520 data acquisition modules is used. The camera was calibrated with a hot plate (top surface cast iron)
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having built-in heater and temperature controller and surface temperature was measured with t type thermocouples, placed on the top surface and IR temperature gun for calibration and comparison purpose. The fin tip temperature is measured using infrared thermometer for all the fins on both cylinders. The thermocouple is placed at fin tip for fins 1, 3, 6 and 8 for low pressure cylinder and 1, 3, 6 and 9 for high pressure cylinder as numbered from top. The measured values as obtained using infrared thermometer and thermocouple for both the cylinders are presented in Tables 1-4. The temperature scaling of the thermographs has been done by interpolating the gray scale value of the portion of the image to the temperature range as obtained by the infrared thermometer.
Fin no No1 No2 No3 No4 No5 No6 No7 No8 No9
Fin no No1 No3 No6 No8
Fin no No1 No2 No3 No4 No5 No6 No7 No8 No9 No10 No11
1min 22.0 22.0 21.5 21.5 21.4 21.4 21.3 21.3 21.0
3 Experiments on Compressor The reciprocating air compressor ELGI THPC 500 unit consists of two sets of cylinders; one for high pressure (Fig. 1) and other for low pressure (Fig. 2). These are fitted in V-configuration on a crankcase. The first stage compression end temperature amounts to 70°C while second stage amounts to 140°C. The heat dissipation is through circular fins of reducing areas. The set-up for the thermal imaging of compressor cylinders is shown in Fig. 3. The thermal images have been acquired in two phases. The first phase incorporated acquisition of thermal images of low pressure cylinder for seven minutes of compressor running with images acquired for every minute of compressor running. The second phase incorporated acquisition of thermal images of high pressure cylinder for seven minutes of compressor
Table 1 — IR Temperature at fin tips of LP cylinder IR Temperature (oC ) 2min 3min 4min 5min 24.0 28.0 30.5 32.5 24.0 26.5 30.0 32.0 23.8 26.5 29.0 31.5 23.8 25.5 28.5 31.0 23.7 25.5 28.5 31.0 22.6 24.5 28.0 30.8 22.4 24.0 27.0 30.5 22.2 24.0 26.0 30.0 22.0 23.0 25.0 29.0
6min 34.5 34.0 34.0 33.8 33.8 33.6 33.2 33.0 31.0
Table 2 — Thermocouple temperature at fin tips of LP cylinder Thermocouple Temperature (oC ) 1min 2min 3min 4min 5min 6min 21.26 23.55 27.13 29.03 31.907 33.686 21.26 23.52 25.57 28.48 31.44 33.596 21.23 22.46 25.22 27.51 30.637 33.426 21.23 21.94 23.844 25.69 30.00 32.906 Table 3 — IR temperature at fin tips of HP cylinder IR Temperature (oC ) 1min 2min 3min 4min 5min 28.0 29.0 31.0 36.0 42.0 28.0 29.0 31.0 34.5 40.0 27.8 28.5 30.5 34.5 37.5 27.8 28.5 30.5 33.5 36.5 27.6 28.3 30.5 33.0 36.0 27.6 28.2 30.2 31.0 34.0 27.6 28.2 30.2 30.5 34.0 27.4 28.0 30.0 30.5 33.5 27.4 27.5 30.0 30.5 33.0 27.2 27.5 29.5 30.0 31.0 27.0 27.5 29.0 29.5 30.0
6min 48.0 44.0 39.0 37.5 36.5 35.0 35.0 34.0 33.5 33.0 32.0
7min 37.5 37.0 36.5 36.3 36.0 36.0 35.5 35.0 33.5
7min 37.107 36.387 35.657 34.576
7min 55.0 50.0 40.0 39.5 39.0 38.5 38.0 38.0 37.5 36.0 35.5
SINGH & SINGH: CONDITION MONITORING OF RECIPROCATING AIR COMPRESSOR
Fin no No1 No3 No6 No9
Table 4 — Thermocouple temperature at fin tips of HP cylinder Thermocouple Temperature ( oC ) 1min 2min 3min 4min 5min 6min 27.86 28.65 30.92 35.57 41.24 47.42 27.74 27.92 30.72 33.53 35.72 38.36 27.48 26.98 28.62 30.89 33.16 33.62 27.33 26.72 28.12 30.55 32.99 33.36
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7min 53.65 39.77 37.64 37.56
Fig. 3 — Experimental set-up on compressor Fig. 1 — High pressure cylinder
Fig. 2 — Low pressure cylinder
running with images acquired for every minute of compressor running. The thermal images of high pressure and low pressure cylinders for alternate minutes are shown in Fig. 4.
4 Modelling and Analysis in ANSYS The cross-section of cylinders is modeled as per dimensions taken from instructional manual of the compressor. The model development and analysis has been carried out in Ansys software Version 8.0 environment. The material of the cylinders is cast iron of thermal conductivity 50 W/mK, density 7.272 kg/m3 and specific heat7 420 J/kg K, taken in the analysis. The cylinders have been meshed using PLANE 77 element. A transient conduction and convection analysis has been carried out to generate the fin tip temperature. A convection coefficient7,8 of 45W/m2 K has been used on the outer surface of the cylinders for analysis. The point of application of the thermal load has been evaluated by selecting the point of stroke length at which the delivery valve opens and to the point at which end of piston stroke occurs. The maximum temperature rise occurs within this portion of the cylinder and the inner cylinder surface temperature rises over a period of time due to hot compressed air and piston friction. At the outer surface, heat is dissipated to the environment via circular fins of varying sizes from top to bottom. The analysis is based on the evaluation that the outer surface temperature is a function of inner surface temperature and higher the outer surface temperature,
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INDIAN J PURE & APPL PHYS, VOL 49, JANUARY 2011
Fig. 4 — Thermographs of HP and LP cylinders
it will be an indication of higher inner temperature rise. In accordance with the compressor manual, the first stage compression end temperature reaches 70 deg and second stage reaches 140 deg. The rise in temperature happens over a span of compressor operation. The IR gun and TC which give the fins temperature at different timings of operation is taken
into account as reference during analysis in ANSYS. The thermal load is applied to modeled cross-section at an assumed value which would generate the fins tip temperature to the nearest values as obtained by IR gun and TC. The point of application of thermal load has been considered by selecting the point of stroke length at which the delivery valve opens and to the
SINGH & SINGH: CONDITION MONITORING OF RECIPROCATING AIR COMPRESSOR
Fin no No1 No2 No3 No4 No5 No6 No7 No8 No9
1min 22.227 22.227 22.182 22.137 22.091 22.091 22.091 22.091 22.091
Table 5 — FEA temperature (°C ) at fin tips of LP cylinder 2min 3min 4min 5min 6min 23.365 26.549 29.505 32.463 35.648 23.365 26.549 29.505 32.463 35.648 23.092 25.641 28.007 30.374 32.923 22.82 24.732 26.508 28.284 30.197 22.547 23.824 25.009 26.195 27.472 22.547 23.824 25.009 26.195 27.472 22.547 23.824 25.009 26.195 27.472 22.547 23.824 25.009 26.195 27.472 22.547 23.824 25.009 26.195 27.472
7min 39.287 39.287 35.835 32.383 28.931 28.931 28.931 28.931 28.931
Fin no No1 No2 No3 No4 No5 No6 No7 No8 No9 No10 No11
1min 28.076 28.076 26.923 26.923 25.769 24.165 24.165 24.165 24.165 24.165 24.165
Table 6 — FEA temperature (°C ) at fin tips of HP cylinder 2min 3min 4min 5min 6min 29.712 31.115 36.023 41.164 47.708 29.712 31.115 36.023 41.164 47.708 28.248 29.384 33.36 37.525 42.826 28.248 29.384 33.36 37.525 42.826 26.783 27.653 30.697 33.886 37.944 25.319 25.922 28.034 30.246 33.062 25.319 25.922 28.034 30.246 33.062 25.319 25.922 28.034 30.246 33.062 25.319 25.922 28.034 30.246 33.062 25.319 25.922 28.034 30.246 33.062 25.319 25.922 28.034 30.246 33.062
7min 54.719 54.719 48.506 48.506 42.293 36.079 36.079 36.079 36.079 36.079 36.079
Table 7 — Comparison of temperature (°C ) at Fin No 1 of LP cylinder 1min 2min 3min 4min 5min 6min IR 22.0 24.0 28.0 30.5 32.5 34.5 TC 21.26 23.55 27.13 29.03 31.907 33.686 FEA 22.227 23.365 26.549 29.505 32.463 35.648 Assumed Inner Temp 22.5 25.0 32.0 38.5 45.0 52.0
7min 37.5 37.107 39.287 60.0
Table 8 — Comparison of temperature (°C ) at Fin No 1 of HP cylinder 1min 2min 3min 4min 5min 6min IR 28.0 29.0 31.0 36.0 42.0 48.0 TC 27.86 28.65 30.92 35.57 41.24 47.42 FEA 28.076 29.712 31.115 36.023 41.164 47.708 Assumed Inner Temp 35.0 38.5 41.5 52.0 63.0 77.0
7min 55.0 53.65 54.719 92.0
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point at which end of piston stroke occurs. The inner surface thermal load and subsequent fin tip temperature to the nearest value with the ones as obtained by IR temperature gun and thermocouple, are analyzed for given duration of compressor running. The various analytical results as evaluated have been presented in Tables 5 and 6 and the FEM analysis of both the cylinders for alternate minutes have been shown in Fig. 5.
temperatures as obtained from IR thermometer, thermocouple and FEA is presented in Tables 7 and 8 and the results are plotted in Figs 6 and 7. The Inner Surface Temperature for LP Cylinder can be evaluated as a function of time (t) in minutes of operation by using the following relation:
5 Results and Discussion The maximum accuracy is analysed for topmost fin. The comparison drawn for maximum fin
The Inner Surface Temperature for HP Cylinder can be evaluated as a function of time (t) in minutes of operation by using the following relation:
Inner temperature (°C) = −0.0694 (t)3 + 1.1607 (t)2 + 0.9444 (t) + 20.071
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INDIAN J PURE & APPL PHYS, VOL 49, JANUARY 2011
Fig. 5 — FEM analysis of HP and LP cylinders
SINGH & SINGH: CONDITION MONITORING OF RECIPROCATING AIR COMPRESSOR
Fig. 6 — Comparison of maximum fin temperature of LP cylinder
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equipment can be beneficial in analysing the health of the equipment by showing similar thermal images over a specified interval of time which could be as per schedule may be quarterly or half-yearly or depending on the usage of the equipment. In case, the thermal images show a different thermal profile then further investigations can be initiated to identify the possible causes for different thermal pattern. For the cases where thermal patterns are found to be same then certain maintenance routines can be rescheduled which will be cost-effective in terms of manpower and spares replacements. Acknowledgement Authors are thankful to Vice-Chancellor, Defence Institute of Advanced Technology (Deemed University), Girinagar, Pune 411025 for granting permission to publish this work. References
Fig. 7 — Comparison of maximum fin temperature of HP cylinder
Inner temperature (°C) = −0.0833 (t)3 + 2.3512 (t)2 − 4.6012 (t) + 37.714 6 Conclusions The thermal images of both the cylinders show variation in intensity due to rising surface temperature over a span of compressor operation. The compressor under study does not cater for any kind of temperature measurement to analyse the performance of compressor and hence, thermal imaging of the
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