اولين سايت تخصصي آموزش و نگهداري و تعميرات ديگ بخار، ديگ آبگرم، ديگ روغن داغ و آموزش نگهداري و طراحي تاسيسات در ايران به صورت کاملا فارسي، و با استناد به آخرين دستاوردهي تکنولوژي ديگ هاي بخار و تاسيسات مربوطه، در جهان.

 

 

 


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HEAT TRANSFER IN THE RADIANT SECTION OF PETROLEUM HEATERS (part1): 1 - 2 - 3 - 4

 
 

ABSTRACT

A brief review of empirical equations for predicting heat transfer in the combustion chambers of steam boilers and petroleum heaters is followed by a study of eighty-five performance tests on nineteen furnaces differing widely in amount and arrangement of refractory cold surfaces. Operating conditions are available on furnaces with and without air preheat, with and without flue gas recirculation, fired with refinery cracked gas or oil fuel, and with a wide range of variation of excess air. The data are correlated by means of a theoretical equation and the deviations are no greater than the probable errors in the test data, and consistently less than those obtained by the empirical equation of Wilson, Lobo and Hottel. For simplicity of calculation the equation is presented in graphical form. An illustrative design problem has been included.

SUMMARY

In view of the trend of the petroleum industry toward ever-increasing radiant heat transmission rates, as well as higher tube skin temperatures and higher percentages of heat-receiving surface per unit of refractory surface, this investigation has been initiated to study the effect of these variables and to find a means of allowing for their effect in the design of tubular oil heaters.
Eighty-five tests of nineteen different furnaces have been analyzed in this study. The test data include furnaces with and without air preheat and recirculation of flue gas. Excess air varied from 6% to over 170%, and average radiant rates from 3,000 to 54,000 Btu per hour per sq, ft. of circumferential tube area. The furnaces themselves were square, rectangular, or cylindrical in shape and varied widely in arrangement of surfaces; the ratio of effective refractory surface to equivalent cold plane surface varied from 0.45 to 6.55. Refinery cracked gas was the most common fuel, but a number of tests were made using oil fuel.
In this report a general and simple theoretical treatment is presented which satisfactorily correlates all the data. The deviations from the observed radiant section duties are well within the probable accuracy of the data. The average deviations of the predicted heat to the oil in the radiant section from the observed are 5.3% as compared to 6.85% when using the Wilson, Lobo, and Hottel empirical equation. The maximum deviation has been reduced from 33% to 16%. The data indicate that the larger deviations occurring when using the empirical equation are partly due to break-down of the equation below average radiant rates of 5,000 and above 30,000 Btu per hour per sq. ft. of circumferential area. It is likely that the empirical equation is seriously in error when applied to furnaces operating tube skin temperatures above 1000° F., as well as in furnaces having a low percentage of refractory surface and low values of PL, the product of partial pressure of the radiating constituents of the flue gas and the mean beam length of the radiating beam. The data available do not indicate any restriction which should be placed on the use of the theoretical equation herein presented.

INTRODUCTION

In view of the trend of the petroleum industry toward ever-increasing radiant heat transmission rates, as well as higher tube skin temperatures and higher percentages of heat-receiving surface per unit of refractory surface, this investigation has been initiated to study the effect of these variables and to find a means of allowing for them in the design of tubular oil heaters.
Although the exact mechanism of heat transmission in the radiant section of furnaces is complicated by factors about which little is known, certain generalizations and fundamental principles are fairly well established and can be used to advantage in solving radiant heat transfer problems. Some of these factors and their bearing on heat transfer problems are discussed below.
The major transfer of heat in the radiant section of a furnace is due to radiation from the hot gas cloud to the ultimate heat-receiving surface and by heat re-radiated from from the hot refractory surfaces to the cold surface. Some of the heat is also transferred at the instant of chemical union of the molecules in the flame. Of the radiation from the gas cloud, the major part is due to radiation from the carbon dioxide and water molecules present in it. Incandescent soot particles are a source of some radiation, but with fuels and burners commonly used in the petroleum industry, combustion usually results in a practically non-luminous flame. Oil fuels tend to give a more luminous flame than refinery gas at the usual percentages of excess air because of the cracking of the oil particles to soot during the combustion period. Data are not available on the exact degree of luminosity of oil flames, but it is probably a function of burner design, the amount of steam used in atomization and the percent excess air used.
In modern furnaces increasing amounts of heat are transmitted directly from the gas mass and lesser amounts are transmitted by the way of the refractory because the current trend is to fill the radiant section with cold tube surface in the interest of economy. Since the radiating constituents in the flue gas are the H2O, CO2 and SO2 molecules present, the amount of heat radiated by them will be a function of their number and the temperature of the gas and cold surfaces. One measure of their number is their partial pressure. Another measure of their number is the mean length of the radiating beam in the gas mass. Hottel 1 has shown that the product PL, atmospheres-feet, expresses these two facts and permits the data on the radiation from gases to be correlated. For any given fuel, P is a function of the excess air used and L is a function of the furnace alone. An equation, to be valid for a wide variety of sizes and shapes of furnaces, must take into account the effect of PL on furnace performance.
Heat transfer by convection to the tubes in the radiant section of petroleum heaters accounts for only a small amount of the heat transferred, especially in high radiant rate furnaces. This convection transfer is more important in low rate furnacesbecause heat transfer by convection is proportional to the temperature difference Tg - Ts , between flue gas and cold surface, whereas the radiant heat transfer is proportional to the difference T4g - T4s where the temperatures are expressed as degrees Rankine.
In view of the complexity of the problem, numerous investigators have correlated furnace performance by means of empirical equations. To illustrate the basic approach several of these empirical treatments are briefly summarized and their outstanding limitations described. A more complete review of this earlier lititure has been made in a previous publication. page 2-->

 


 

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