US2094923A - Heating of fluids - Google Patents

Description

Oct. 5, 1937. MEKLER 2,994,923

HEATING OF FLUIDS Filed July 8, 1935 2 Sheets-Sheet 1 OQQO'Q QQQQQQQOQQQQOQ OQOQQOQQQQQQQQQ Q .192 z. 572 07".- [e a" I. 72? aye/wen Oct. 5, 1937. A,MEK| ER- 2,094,923

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Patented Oct. 5, 1937 1 UNITED STATES PATENT OFFICE HEATING F FLUIDS Application July 8,

3 Claims.

This invention particularly refers to an im-v proved process and apparatus for improving the distribution of heat in furnaces of the type wherein a major portion of the heat imparted to 5 one portion of the fluid conduit is supplied by radiation from the materials undergoing combustion and the hot refractory walls of the furnace while the major component of the heat imparted to another portion of the fluid conduit is supplied by convection, i. e. directcontact between the fluid conduit and'the hot combustion products.

Furnaces of the generaltype above referred to are commonly termed radiant and convection furnaces and this general method of heating is now commonly employed in most modern furnaces for the continuous heating of fluids including steam boilers as well as oil conversion and distillation furnaces. The advantages of this type of furnace are too well known to require discussion here. One of its chief disadvantages commonly encountered, however, results from the excessive and uncontrolled superimposing of radiant and convection heat at some point in the fluid conduit which, inmost cases, is adjacent a point in the furnace where the general direction of flow of the hot combustion gases or the materials undergoing combustion is changed. The high rates of heating at this point in the fluid conduit, resulting from the absorption of heat transmitted by radiation from the materials undergoing combustion and the hot refractories as well as heat transmitted by washing of the fluid conduit with high temperature combustion gases, is particularly detrimental in cracking operations wherein excessive localized heating usually results in the formation and deposition of coke in the fluid conduit which limits both the efliciency of the furnace and the length of time which it can be operated without shutting down for cleaning.

The primary purpose of the present invention' 50 jected to excessive additional heating by convec- .tion resulting from washing of thesurfaces with hot combustion gases. This is accomplished by the introduction of controlled amounts of steam into the stream of combustion gases at a suitable 55. point in their path of flow ahead of their point 1935, Serial No. 30,222

of contact with the fluid conduit. This not only serves to partially blanket that portion of the fluid conduit washed by the combustion gases but, since steam has a high specific heat, it will act as a carrier, absorbing heat in that portion of the furnace adjacent its point of introduction and imparting it to the fluid conduit in a zone farther along the path of flow of the combustion gases wherein heating by convection predominates; Preferably saturated or exhaust steam is employed for the purpose of the present invention since more heat will, of course, be absorbed by low temperature steam than by superheated steam' or, when desired, it may be supplied to the furnace in the form of water or water-vapor. Normally, however, the temperature of the furnace and the quantity of water em'- ployed is such that it will be in the form of steam by the time it is commingle d with the combustion gases. I

Figure l of the accompanying diagrammatic drawings illustrates one specific form of furnace in which the features of the present invention may be utilized to advantage. It will, however, be understood that the invention is not limited to use in this or in any other specific form of structure since it is applicable to any heater I wherein heat absorbing surfaces exposed to radiation are also subjected to heating by the wash of hot combustion gases.

Figure 2 of the drawings illustrates one specific flow arrangement for the oil in the same furnace shown in Figure 1, and Figure 3 is illustrative I of the'type of heating curves obtained with and without employing the features of the present invention in a furnace of the type shown in Figures 1 and 2.

Referring to the drawings, the main furnace structure comprises end walls I and 2, a roof 3, a floor l and side walls not-shown in the particular view of the furnace here illustrated. The interior of the furnace is divided by means of bridge wall 5 into two zones comprising combustion and heating zone/6 and fluid heating zone I.

In the particular case here illustrated two parallel rows of horizontal tubes 8 are'located adjacent the roof 3 of the furnace and receive heat at a relatively high rate by radiation from the materials undergoing combustion in combustion andheating zone 6 and the hot refractory walls of the furnace. Another tube bankcomprising a plurality of rows of horizontallydisposed tubes 9 is located within fluid heatingzone I and this portion of the fluid conduit receives fluid heat predominantly by contact with the hot combustion gases from combustion and heating zone 6.

The upper two or three rows of tubes in fluid heating zone I are also, in many cases, subjected to appreciable heating by radiation from the combustion gases as well as from the adjacent hot refractory furnace walls.

' preferably so regulated that combustion is substantially completed before the combustion products leave zone 6 of the furnace and the resulting hot combustion gases pass over bridge wall 5 and are directed downward through fluid heating zone 1 to flue II from which they may pass to a suitable stack, not illustrated.

It has been found in furnaces of the form here illustrated, which are employed extensively for the conversion of hydrocarbon oils, that when coking difliculties are encountered the greatest amount of coke is found in that portion of the tube bank adjacent the roof which is located above bridge wall 5 (the approximate location of this zone being indicated in the drawings at l2) and in some cases in the upper two or three rows of the fluid conduit in fluid heating zone I. This is attributed to the vfact that these particular zones, while subjected to relatively high rates of heating by radiation are also washed by the hot combustion gases and, therefore, subjected to appreciable and uncontrolled amounts of additional heating by convection. In this form 'of furnace therefore the present invention consists in providing for the introduction of steam into the combustion gases ahead of their pointof contact with the tubes in zone l2 and/or, when desired, above fluid heating zone 1. This may be accomplished by the use of suitable headers or spray lines indicated at I 3 and M to which water, water vapor or steam may be supplied and from which it may be introduced into the stream of combustion gases. The steam thus introduced serves to partially blanket the tubes of bank l2 and/or the upper two or three rows of tubes in fluid heating zone 1 to protect the same trated or to any particular sequence between the various portions of the fluid conduit. Preferably, however, the fluid undergoing heating passes in series through adjacent tubes in each section of the fluid conduit and the two sections are connected in series.

As a specific example of the operation and advantages of the present invention as applied to a furnace of the type above illustrated and described, employed for the conversion of hydrocarbon oil, the path of flow of the oil through the fluid conduit is indicated in Figure 2. The reference numbers in Figure 2 indicate portions of the furnace indicated by the same reference numbers in Figure 1. The charging stock is heated to. an outlet conversion temperature of approximately 910 F. The type of heating curve obtained without employing the features of the present invention is indicated in Figure 3 of the drawings as curve A. It will be noted that in that portion of the fluid conduit corresponding to zone l2 and to the upper rows of tubes in fluid heating zone I the rate of temperature rise is appreciably increased. In this operation the temperature of the combustion gases indicated by thermocouples located adjacent zone l2 of the fluid conduit approximates 15001550 F. while the combustion gas temperature measured by thermocouples located above fluid heating zone 1 approximate '1350" F. and the temperature of the gases leaving the fluid heating zone was approximately 975 F. This operation was terminated after a run of approximately ten days, due to hot tubes in zone I2 of the fluid conduit and a layer of hard coke approximately one-half inch thick was found in the tubes ofthis section. By introducing controlled quantities of exhaust steam into the stream of hot combustion gases at approximately the points indicated by headers l3 and M in Figure 1 of the drawings the temperature of the combustion gases adjacent zone 12 is reduced to approximately 1450 F; The type of heating curve obtained during this operation is indicated as curve B in Figure 3 of the drawings. The plant was operated continuously for a period of 22 days. At the end of this period 'the coke deposited in the tubes of section l2 of the fluid conduit averaged approximately onesixteenth inch in thickness; The capacity of the cracking unit was also increased from approximately 100 barrels per day to 1650 barrels per day by employing the features of the present invention.

I claim as my invention: g

1. In a process for the heating of fluids while passing the same in a continuous stream through a fluid conduit wherein a portion of the fluid conduit is subjected to relatively high rates of heating by radiation from combustion gases and hot refractory furnace walls, another portion of the fluid conduit is subjected to excessively high rates of heating by radiation and by direct contact with said combustion gases and another separate portion of the fluid conduit subjected to lower rates of heating primarily by contact with said combustion gases after their contact with the second mentioned portion of the fluid conduit, the improvement which comprises commingling regulated quantities of steam with the combustion gases, after their use in the heatingof the first-mentioned portion of the conduit and prior to their contact with the second mentioned portion of the fluid conduit, whereby to reduce the convection component of the heat supplied thereto and whereby to increase the heat supplied from the combustion gases to the succeeding portion of the fluid conduit.

2. In the heating of fluids in furnaces having fluid heating tubes subjected to relatively high rates of heating by radiation from combustion gases and hot refractory furnace walls, additionacesms 3 with the second-mentioned tubes, whereby to reduce the convection component of the heat supplied to the second-mentioned tubes and whereby to increase the heat supplied from the 5 combustion gases to the-third-mentioned tubes.

3. The improvement as defined in claim 2 further characterized in that additional quantities of steam are commingied with the combustion gases after their contact with the second-mentioned tubes and prior to their contact with the tiara-mentioned tubes.

LEV A. MEKLER.

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