Conductor for heat exchange apparatus

Application filed December 24, 1926. Serial No. 156.902.

The present invention relates to conductors for heat exchange apparatus, and more particularly, to conductors which are required to be subjected to the corrosive action of .water C and heated gases.

In heat exchange apparatus such as economizers and super-heaters, the tubes are preferably of steel, in order to afford sufficient mechanical strength to resist the usual 10 stresses to which they are subjected. However, steel tubes are open to the serious objection that they are quickly-corroded, not only by the action of the heated gases, but also in the case of economizers, by the feed 15 water at certain temperatures. In order to resist corrosion, it has beenproposed to cover the tubes with cast iron jackets which provide a measure of protection against the action of gases on the exterior. Aside from 20 the disadvantage of unduly increasing the weight of the structure, the jackets afford no adequate solution of the problem because they cannot protect against internal corrosion by the water, and the best remedy for this latter 25 defect which has been devised for economizers consists in regulating the temperatures throughout the apparatus so that internal corrosion is localized in a portion of the tubes. Moreover, in the construction of such 30 apparatus,. another serious problem is encountered, namely, the erosion or pitting of the surface by mechanical impact therewith of swiftly moving solid particles which are inevitably present-in the gas stream. In 35 this respect, a tube with cast iron jackets offers no advantages over a plain steel tube, except such as may be provided by the greater thickness of its walls.

The object of the present invention is to ⁴⁰ provide a strong and relatively light conductor for a heat exchange apparatus which will satisfactorily resist both external and internal corrosion under all conditions and which affords adequate protection against ⁴⁵ erosion by impact of solid particles.

With this object in view, the principal feature of the present invention ^consists in a heat exchange conductor, / preferably of wrought steel, which is provided both internally and externally with a thin, tough, con tinuous. and adherent coating of corrosion resisting soft metal. This coating is conveniently of lead which has high resistance to corrosion and which may be readily applied to the tube, but may be of any alloy of soft gg metals which has the desired effect of preventing attack on the tube by the water or gases. This coating completely covers all portions of the tube, even though the tube may be formed with an extended surface to eo increase the heat transmission. The coating is extremely thin, being of a thickness of approximately one one-thousandth of an inch or less, so that its presence does not add materially to the weight of the structure or 65 increase the stresses which the tube must withstand.

The accompanying drawing is an elevation partly in section of the preferred form of tube for heat exchange apparatus acccofding 70 to the present invention.

The illustrated embodiment of the invention consists of an economizer conductor adapted for the heating of feed water for boilers. The water is passed internally 75 through the tube and the exterior of the tube is swept by heating gases passing from the boiler to the stack. Referring to the drawing, the conductor consists of a tube 2 of wrought steel of sufficient thickness to with- 80 stand the internal pressure of the water. The conductor is formed with an extended surface consisting of a series of regularly spaced flanges or,discs 4 which are mounted on the tube. In order to secure the discs 85 firmly to the tube and to insure a high quality of heat transmission between them, the tube is internally expanded by passing therethrough a tool of slightly larger diameter than the interior of the tube. This action 90 serves to expand the metal of the tube wall so that the flanges are slightly embedded in the wall, thus securing a firm contact. The action of passing the expanding tool through the tube results in forming slight bulges on 05 the tube wall between adjacent flanges whicti action also enhances the security of attachment of the tube and discs. The bulges are exaggerated in the drawing for purposes of illustration, but in practice, are of only suffi- Joo

1,821,702 cient magnitude to be barely perceptible upon looking through the tube.

The conductor, after attachment of the discs, is next subjected to the operation of coating it with soft metal. Preparatory to the coating operation, the conductor is pickled in a suitable bath which is conveniently sulphuric acid. From the pickling bath the conductor is passed to an alkaline bath, preferably a solution of ammonium chloride, which neutralizes the acid. It is then immersed in the. molten soft metal. This bath is preferably commercially pure lead which has been found to be most suitable for the purpose of resisting the corrosive and erosive actions of the gas and water in actual use. In order to cause firm adherence of the lead to the conductor, the surface of the bath is provided with a suitable flux, such as zinc chloride. The conductor floats on the bath and may be occasionally forced below the surface and rotated manually in order to afford contact of the lead with all parts of the conductor. During this operation, it is advisable to dust ammonium chloride occasionally over such portions of external surface as appear to resist the deposit of the lead film thereon. The action of the flux on the surface of the bath has been found to be sufficient for causing adequate coating the interior of the tube. After the tube has been completely coated, as will be evidenced by the appearance of the shiny continuous coating of lead on the exterior, the conductor is removed from the bath and is quickly immersed in a quenching bath consisting of kerosene or other oil. The quenching bath improves the appearance of the tube, imparting the eifect of a smooth, continuous, integral structure, and at the same time enhances the bond between the lead and the steel. After cooling, the tube is ready for use and may be employed for the construction of any heat exchange apparatus such as an economizer, super-heater or condenser.

The lead coating is illustrated in the drawing by the heavy black line 6. The thickness of the coating is necessarily exaggerated for purposes of illustration and as previously stated, is not of an average thickness greater than one one-thousandth of an inch. There is, however, a somewhat greater deposit ol lead at the junction of the tube and discs, appearing in the form of fillets indicated at 8. This increased thickness at the point of attachment of the discs to the tube is of importance, because it provides additional protection at the points where corrosion is most likely to occur, and where, by virtue of its tendency to impair the contact of the discs with the tube, it would prove to be most serious.

The coating of lead or other corrosionresisting soft metal, has been found not only to provide all necessary protection against external corrosion by heating gases or internal corrosion by the feed water, but also to afford protection against destruction by impact of solid particles therewith. These solid particles which are present in all flue gases, 70 have a detrimental effect on the usual heat exchange tubes, but peculiarly the soft metal film is not affected by them to any perceptible degree. In addition to its long life and high durability, the conductor of the present in- 75 vention possesses a high efficiency of heat transfer because of the intimate connection of the flanges with the tube.