Evaluation of Heat Losses in Fire Tube Boiler

Transcription

1 Evaluation of Heat Losses in Fire Tube Boiler S. Krishnanunni 1, Josephkunju Paul C 2, Mathu Potti 3, Ernest Markose Mathew 4 1 Asst.Professor, Dept of Mechanical Engineering, Axis College of Engineering and Technology, Thrissur , Kerala, India 2 Professors, 4 Associate Professor, Dept of Mechanical Engineering, M. A. College of Engineering, Kothamangalam , Kerala, India 3 Assistant Executive Engineer (Boiler) Travancore Titanium Products Limited, Thiruvananthapuram , Kerala, India Abstract The efficiency of oil fired fire tube boiler was calculated by evaluating the heat losses. Investigation on the performance of the boiler was conducted by examining the heat losses, identifying the reasons for losses, measuring the individual loss and developing a strategy for loss reduction. This study was carried out in Texmaco package horizontal fire tube boiler at Travancore Titanium Products Ltd (TTPL), Trivandrum, Kerala. The boiler efficiency was measured by indirect method. Heat losses in dry flue gas and due to unburned fuel were found to be the major problems. Since they were interrelated, installation of Zirconium oxygen sensor was recommended as a common remedy. Keywords Boiler, Fire tube boiler, Boiler efficiency, In direct method I. INTRODUCTION Boiler is a closed pressure vessel in to which water can be fed and by applying heat continuously, evaporated in to steam [2]. The function of the boiler is to generate steam at the desired conditions efficiently and with low operating costs. Low pressure steam is used in process an application whereas high pressure superheated steam is used for generating power via steam turbines. Boilers consist of a number of tubes for maximum heat transfer. These tubes runs between steam distribution drum at the top of the boiler and water collecting drums at the bottom of the boiler. Steam flows from the steam drum to the super heater before entering the steam distribution system. The manufacturers of boiler often cite various numbers, like the boiler have a thermal efficiency of 85%, combustion efficiency of 87%, a boiler efficiency of 80%, and a fuel-to-steam efficiency of 83% [4]. These efficiencies depend on certain factors. Thermal efficiency reflects how effectively the boiler vessel transfers heat; combustion efficiency indicates the ability of the burner to use fuel completely without generating carbon monoxide or leaving hydrocarbons unburned; boiler efficiency indicates the ratio of energy output of the boiler to the fuel energy supplied to the boiler. FIG 1 OIL FIRED FIRE TUBE BOILER A fire-tube boiler is a type of boiler in which hot gases from fire pass through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam. The fire-tube boiler developed as the third of the four major historical types of boilers: low-pressure tank or "haystack" boilers, flued boilers with one or two large flues, fire-tube boilers with many small tubes, and highpressure water-tube boilers. Fire tube boilers are often characterized by their number of passes, referring to the number of times the combustion (or flue) gases flow the length of the pressure vessel as they transfer heat to the water. Each pass sends the flue gases through the tubes in the opposite direction. The advantage of fluid boilers over a single large flue boiler is that the many small tubes offer, far greater heating surface area for the same overall boiler volume. The general construction is as a tank of water penetrated by tubes that carry the hot flue gases from the fire. Since the strongest practical shape for a pressurized container is cylindrical majority of the parts are cylindrical. 301

2 This cylindrical tank may be either horizontal or vertical. Boilers mix air with fuel to provide oxygen in the combustion process. For safety reasons, a small amount of additional or excess air is always provided to assure that all fuel is burned inside the boiler [8]. By operating a boiler with a minimum amount of excess air, stack heat losses can be decreased and the combustion efficiency can be increased. But, unnecessary amount of excess air can lead to [3]: Burner/control system imperfections Variations in boiler room temperature, pressure, and relative humidity Need for burner maintenance Changes in fuel composition To achieve optimum combustion efficiency, heat losses must be minimized. Combustion losses are either fuel losses (that increase toward the fuel-rich end) or air losses where heat energy is wasted by heating excess air that is lost through the flue. Excess air results in the flue gases containing free oxygen. The percentage of oxygen in flue gas will increase with increasing excess air, and the proportion of carbon dioxide will correspondingly fall [4]. II. METHODOLOGY OF THE STUDY This study was carried out in Texmaco package horizontal fire tube boiler maintained in the boiler house at Travancore Titanium Products Ltd (TTPL), Trivandrum, Kerala, as a part of industrial training. This is a three pass, low pressure, stationary boiler. The fuel is used in the boiler is grade-6 heavy oil. The circulation of water is done by a centrifugal pump and saturated steam of C is produced. The specification of boiler is given below:- Texmaco Package Boiler Horizontal oil fired boiler Size :- 9 6 and Length : Capacity :- 9.1 tone/hr Working Pressure : Kg/cm2 Design pressure : Kg/cm2 Heating surface : Sq.ft area Chimney :- 3 6 Diameter 50 Height Three pass boiler First Pass :- 3 4 Diameter 2 9 Length 302 Second Pass (164 tubes):- Stay tubes (inner 2 and outer 2.5 ) Flue tubes (inner 2.25 and outer 2.5 ) Three Pass (66 tube each side) Forced draft fan :- 25 HP Feed Pump :- 15 HP Oil pump :- 5 HP The boiler efficiency was evaluated by indirect method. Daily observations on mass of fuel consumed and mass of steam produced were recorded for a period of eight months (Jan-Aug 2011). The data on the various constituents of fuel and flue gas were also recorded at monthly intervals. By analysing these data, heat losses due to eight different aspects were observed. The boiler efficiency was calculated by using the following formulas [1]. A. Theoretical Air required B. Excess Air supplied (EA) C. Actual mass of Air Supplied (AAS) D. Mass of dry flue gas E. Percentage of heat loss in dry flue gas Flue gas temperature should be 100C or 200C more than the temperature inside the boiler. It should not exceed this limit, if so then it will be lost through dry flue gas. F. Percentage of heat loss by evaporation of water due to the presence of H 2 in fuel Oxygen combine with extra hydrogen present in the fuel to form water and gets evaporated. Thus heat is lost by evaporation of water. G. Percentage of Unaccounted loss Unaccounted losses are the losses that occurr due to the observation error, convection and conduction losses. These are fixed losses and are assumed to be 2% as per BS 845.

3 H. Percentage of heat loss due to Blow down Water evaporates continuously leaving behind the salts in the boiler. This leads to continuous increase in the concentration of these salts in the boiler drum which requires regular blow down and results in wastage of fuel. I. Percentage of heat loss due to radiation Radiation losses occur due to the transfer of heat from the side walls of the boiler to outside. These are fixed losses and are assumed to be 1% as per BS 845. J. Percentage of heat loss due to unburned fuel During incomplete combustion, carbon present in the fuel converted in to carbon monoxide which results in the liberation of only 52% of the total heat in the fuel. For every kg of unburned carbon, 8084 Kcal of heat is lost. K. Percentage of heat loss due to moisture in air Certain amount of heat is lost due the presence of moisture in air. The percentage of this lost is normally negligible. L. Percentage of heat loss due to moisture in fuel Certain amount of heat is lost due the presence of moisture in a fuel. The percentage of this lost is normally negligible. Table I Average value of mass of fuel consumed and mass of steam produced Month Mass of Fuel (m f ) consumed [ton] Mass of steam (ms) produced [ton] January February March April May June July August Table II shows the data on various constituents of fuel and flue gas. There was no variation in the various constituents of the fuel used during the observation period. The flue gas analysis showed that less amount (20ppm) of carbon monoxide was formed in the month April where the oxygen content in the flue gas was minimum (4%). Percentage of heat loss in dry flue gas, heat loss by evaporation of water due to the presence of H 2 in fuel, unaccounted losses, heat loss due to blow down, radiation, unburned fuel, moisture in air and moisture in fuel were calculated in every month based on the data given Table I and Table II and the results are given in Table III. Table II Data on various constituents of fuel and flue gas M. Net efficiency of the boiler III. RESULTS AND DISCUSSION The average values of daily observation on mass of fuel consumed and mass of steam produced are given in TableI [5]. 303

4 Table III Boiler efficiency by indirect method From Table 3, it is evident that maximum amount of heat (7.38%) was lost along with flue gas. It was followed by heat loss due to evaporation of water due to the presence of hydrogen in fuel (6.88%) which is neglected as it is not coming under the operational side of the boiler. Third loss was unaccounted which occur due to the observation an error, convection and conduction loss which is assumed as 2% as per BS 845. Fourth loss was due to blow down (1.4%) which can only be rectified by installing Automatic blow down controller. Its life span is short as its sensor requires high maintenance. So the durability and efficiency of the instruments can ensure by maintenance. Fifth loss was due to radiation occurs due to the transfer of heat from the boiler side- walls to outside. These are fixed losses and are assumed to be 1% as per BS 845. Sixth loss was due to unburned fuel (0.83%). If heat loss in dry flue gas can be controlled, then heat loss due to unburned fuel can be automatically reduced to zero. Heat loss due to the presence of moisture in fuel (0.01%) and air (0.4%) are negligible. Among them, heat lost along with the flue gas and due to unburned fuel was considered as important reasons for the declining boiler efficiency. The percentage of heat losses is represented in a pie chart given in figure II. 304 FIG 2 PERCENTAGE OF HEAT LOSSES IV. CONCLUSION The heat losses in dry flue gas and due to unburned fuel are interrelated. So they can be controlled by the same method. Installation of Zirconium oxygen sensor is suggested as the remedy for both the problems. This instrument can reduce the stack loss approximately 3.5% and boiler efficiency can be improved from 80% to 83%.

5 REFERENCES [1 ] Energy performance assessment of boilers, Bureau of Energy Efficiency., pp [2 ] Analysis of boiler efficiency- Case study of thermal power stations, pp [3 ] Energy efficiency- Boiler combustion monitoring and oxygen trim system, Washington state university energy program., (2003), pp [4 ] Bhatia, A., Improving energy efficiency of boiler systems, Continuing education and development engineering, pp [5 ] Efficient generation of steam, Petroleum Conservation Research Association (PCRA) [6 ] Quality certificate of Furnace oil., Mangalore Refinery and Petrochemicals Limited [7 ] Robert, L. L., Energy hand book, Von Nostrand Reinhold Company (Second Edition) [8 ] Combustion trim for boiler, Sustainable energy authority Victoria, pp [9 ] Industrial boilers, Longman Scientific Technical, (1999). [10 ] [11 ] [12 ] [13 ] 305