US2075511A - Heat exchanger - Google Patents

Description

March 30, 1937. w, L. DE 'BAUFRE HEAT EXCHANGER Filed Jan. 17, 1936 l M F /NVENT0R 9142 6. A e $au%u.

Patented Mar. 30, 1937 v UNITED STATES PATENT OFFI CE 10 Claims.

This invention relates to the art of exchanging heat between fluids of different temperatures. It is particularly applicable to interchangers for air separation and gas liquefaction plants below atmospheric temperature where the shell must withstand an internal fluid pressure and is subjected internally to corrosive action, either during normal operation, while cooling down, or while defrosting.

Objects of the invention are to reduce the time of cooling from room to operating temperatures, to reduce the loss of stored up refrigeration in warming from operating temperatures to room temperature for defrosting, to reduce the fixed charges against the products of air separation plants by reducing the initial cost of such interchangers, and facilitating their manufacture and erection.

These objects are attained by using steel for the shells of such interchangers instead of copper heretofore used. Steel has a safe working strength about double that of rolled or drawn copper, so that a shell of steel need be only about one-half the thickness of a copper shell to safely withstand the same internal fluid pressure. Large seamless steel shells are more readily produced commercially than copper shells of twice their weight, where the maximum size of copper ingot limits the length of a tube that can be drawn of given diameter and thickness. A single piece of seamless steel tubing would there- 'fore be available for making a long interchanger shell where two or more lengths of copper tubing would be required. The manufacture of such interchangers would thus be facilitated by utilizing steel in place of copper for the shells. In erection, lighter interchangers with steel shells would be more readily handled than interchangers with copper shells.

Due to greater ease of manufacture and erection, the initial cost of such interchangers would be reduced. This reduction in cost is an addition to the great difference between the price of steel and the price of copper and the less cost of transporting the lighter 'in'terchanger. Hence, the initial cost of such interchangers would be appreciably reduced with a corresponding reduction in flxed charges against the product of a plant employing such interchangers.

of copper while the specific heat is somewhat greater, so that about the same amount of heat must be abstracted in cooling a given volume of each metal through the same temperature range.

' a copper shell and copper tubes.

The density of steel is somewhat less than that copper shell, about one-half as much heat would therefore be abstracted to cool the shell to operating temperatures and about one-half the refrigeration would be wasted upon warming the interchanger from operating temperatures to room temperature to defrost the same.

The substitution of a steel shell for a copper shell of the same strength would thus accomplish all the above stated objects of the invention. But such substitution has not heretofore been made in plants which must operate for long periods of time because steel corrodes under the action of oxygen in a moist atmosphere. During normal operation of interchangers below atmospheric temperature, water vapor in the gases being cooled is condensed to water or deposited as frost on the inside surface of the shell. The internal surface of the shell of such interchangers is therefore subjected to moisture and to any free oxygen in the gases being cooled or used for defrosting.

A further object of the invention is to protect a steel shell internally against corrosion due to oxygen in a moist atmosphere. This object is accomplished by means of a thin copper shell within the steel shell. The thin copper shell would be unable to resist by itself the internal fluid pressure. Under the internal pressure, therefore, the thin copper inner shell would give way and, enlarge (or creep) until it would rest against the steel outer shell. The steel outer shellwouldthen support the internal fluid pressure but would be protected from corrosive action by the fluid within the interchanger by the thin copper inner shell between it and the fluid. This protection must be extended to the ends of the steel outer shell.

A further object of the invention is to provide a construction and a method of assembling which would protect the steel outer shell throughout its length and at thev ends from the corrosive influence of the fluid withinthe in terchanger.

In using a steel shell with copper tubes extending between tube sheets and heads of noncorrosive bronze, relative expansion of shell and tubes would be accentuated as compared with Even with a copper shell and copper tubes, there is relative expansion of shell and tubes due to temperature changes taking place more rapidly andto a greater extent in the metal of the tubes than in the metal of the shell. With a steel shell and copper tubes, such differences would be accentuated because the coefficient of linear expansion of steel is about two-thirds that of copper.

A further object of the invention is to pro vide means for permitting relativeexpansion of shell and tubes and yet protect the steel outer shell from internal corrosion.

The copper inner shell would tend to expand and contract at a different rate from that of the steel outer shell. But if the copper inner shell were made very thin, it would creep and enlarge under the internal fluid pressure and be held firmly against the steel outer shell with which it would be forced to expand and contract with changes in temperature. This is another object of the invention.

The foregoing, together with such other advantages as may hereinafter appear or are incident to the invention, are realized by the construction illustrated in preferred form in the drawing, where Fig. 1 is a sectional view of the proposed interchanger and Fig. 2 is an enlarged detail thereof.

The interchanger shown in the drawing is intended for use in air separation plants described in U. S. Patent 1,951,185, issued March 13, 1934. In such air rectification plants, compressed air is cooled in one or more interchangers from about room temperature to one hundred degrees or more below zero centigrade by products of rectification, more or less pure oxygen and nitrogen, returning from a rectifier at very low temperatures.

The interchanger consists of a number of small straight copper tubes l extending from tube sheet 2 at the upper end of the interchanger to tube sheet 3 at the lower end. Tube sheet 2 is part of a bronze casting or forging 4 and tube sheet 3 is part of a somewhat similar casting or forging 5. The multicompartment heads 6 and I are constructed as described and claimed in Patent No. 2,034,428, issued March 17, 1936. In air separation plants, cold oxygen would enter pipe 8, flow through a central group of the tubes I and leave through pipe 9. Cold nitrogen would enter pipe Ill, flow through the remainder of tubes l surrounding the central group, and leave through pipe II.

The returning cold oxygen and nitrogen-are brought into heat exchange relation with com? pressed air to be separated as the latter fiows around and across tubes l. The compressed air enters the interchanger through pipe l2 and leaves through pipe I 3. The compressed air is confined within the interchanger by shell l4 and flows back and forth across tubes l as determined by baffles [5.

Outer shell [4 is made of steel which has a high tensile strength and therefore may be com paratively thin to safely withstand the internal fluid pressure of the compressed air. To protect this steel outer shell from corrosion by oxygen -in the air in presence of moisture resulting from condensation of water vapor as the air is cooled, a copper inner shell I6 is provided. This copper inner shell completely covers the steel outer shell internally, being fastened at itsends as shown enlarged-in Fig. 2, to prevent any seepage of oxygen and moisture between the steel outer shell and the copper inner shell.

Heads 4 and 5 are of cast or forged bronze which isnot subject to corrosion by moist air. Tubes l are of copper by reason of high heat conductivity of this metal and lack of corrosion.

under the existing conditions. At the upper end, steel outer shell I4 is fastened into bronze head 4; but at the lower end, a packed stufllng box I! is provided as shown to permit relative motion of bronze head 5 and the lower end of steel outer shell l4. The construction shown at the ends of steel outer shell l4 not only protects the ends of outer shell 14 from any contact with the fluid flowing through the interchanger, but also provides the recess claimed in U. S. Patent No. 1,823,388, which reduces the dari'ger of obstructing outlet pipe l3 with snow and ice.

Referring to Fig. '2 as well as Fig. 1, this construction comprises a bronze cast or forged end piece l8 filling the gap between the outside diameter of outer shell 14 and the inside diameter of head 4, and a bronzegcast or.-forged ring IQ for clamping the end of copper inner shell l8 thereto. The end of steel outer shell I 4 is I threaded to fit internal threads in end piece I.

The threads on both 14 and I8 are tinned" before screwing outer shell l4 up to the shoulder shown in end piece l8. By heating, a fused metal joint is made between outer shell l4 and end piece I 8. This joint is made at both ends of steel outer shell l4 before proceeding further in assembling the interchanger.

Copper inner tube I6 is made, preferably by rolling and joining a thin copper sheet to fit within steel tube l4 as closely as practically possible. This copper cylinder is then slipped into place and the ends flared to fit the bevel on end piece l8. The surface of this bevel on end piece l8 and the external surface of copper inner shell l6 may first be tinned if desired. The threads on end piece l8 and ring l9 should be tinned as well as the bevel on ring I! and the surface of the flared portion of inner shell 16. Ring 19 is then screwed into place in end piece 18 hard against the flared end of inner shell l6. By heating, a fused metal joint is made, additional solder being added if desired to insure absence of leaks.

Steel outer shell l4 and copper inner shell l8 and ring l9 at each end. At the lower end, the

- end piece 18 may extend a longer distance over outer shell 14 than at the upper end, in order to provide a longer surface for operation of the expansion joint shown.

Upper head 4 may now be screwed into place, the external threads on end piece l8 and the internal threads in head 4 being first tinned in order to make a fused metal joint by subsequently heating.

Tubes l, assembled in baille plates l5 may now be slipped into inner shell [6 and the upper ends of tubes I soldered or otherwise fastened in tube sheet 2. Lower head 3 may now be put into place and the lower ends of tubes I soldered or otherwise fastened in tube sheet 3. Stuffing box I! may then be packed. 1

With fastening of heads 6 and 1 thereon, the

interchanger is ready for mounting with other parts of the air separation plant within a casing filled with heat insulating material to reduce heat leak into the apparatus from the surrounding atmosphere. Pipe 20 is provided for draining the interchanger of water resulting from condensation of water vapor while cooling air during normal operation and from melting ice and snow therein while defrosting.

While threadedand soldered joints are preferred as shown, other types of fused metal joints may be used, such as formed by brazing or'welding. Also, while steel and copper and bronze are preferred for the outer shell, inner shell, end

pieces and heads as previously described, other materials may be used.

A nickel lining would have the advantage over a copper one in that the coeificient of thermal expansion of nickel is nearerto that of steel than the coeflicient of expansion of copper. To provide for the fact that the thermal expansion of copper is different from that of steel, the copper inner shell is made thin, so that if subjected alone to the internal fluid pressure, the stress therein would be above the creep limit of copper at the existing temperature. Then, with any tendency for the outside diameter of the copper inner shell to become different from the inside diameter of the steel outer shell, creep of the copper occurs until equality again results. As soon as the interchanger is subjected to the operating compressed air pressure, the copper inner shell becomes firmly pressed against the steel outer shell even though made a loose fit, and this condition continues with expansion and contraction due to temperature changes. V

The outside of the steel outer shell would be effectively protected against the slight corrosive action to which it is subjected by coating with an enamel which will not crack at low temperatures, before applying the insulating material for reducing heat leak into the interchanger from the surrounding atmosphere. The steel outer tube may be tinned or galvanized on the outside if desired, as such a protective coating would be satisfactory on the outside of the interchanger, although it would not be satisfactory inside.

I claim:

1. Interchanger for cooling compressed air below atmospheric temperature by returning products of rectification, including a steel outer shell, bronze end pieces attached to said outer shell, a copper inner shell fastened to said end pieces,

a bronze head and tube sheet fastened to one of said bronze end pieces, a bronze head and tube sheet with gland forming a stufling box surrounding the other of said bronze end pieces, packing material in said stuffingbox, and copper tubes 4 extending through said shells and having ends fastened in said tube sheets, whereby said steel shell is protected internally against corrosion and can expand or contract relative to said copper tubes with changes in temperature.

-)0 2. Interchanger for cooling compressed air below. atmospheric temperature by returning products of rectification, including a steel outer shell of sufficient strength to safely withstand the fluid pressure of thecompressed air, bronze end pieces attached to said outer shell, and a copper inner shell fastened to said bronze end pieces, whereby said steel outer shell is protected by said copper inner shell and said bronze end pieces from corrosion internally due to free oxygen in presence 0 of moisture during normal operation or while defrosting.

3. Interchanger for cooling a fluid under pressure by heat exchange with one or more other fluids, including a metal outer shell of sufilcient strength to safely withstand said pressure but of material which would be corroded by direct con- ,tact with the fluid cooled,,metal end pieces atend pieces and of material which is not corroded by direct contact with the fluid cooled, whereby said outer shell is protected from corrosion by the fluid cooled.

4. Interchanger for cooling a fluid under pressure by heat exchange with one or more other fluids as in claim 3, wherein the stress in said inner shell due to pressure of the fluid cooled is above the creep limit of the material so that said inner shell is kept firmly pressed against said outer shell as expansion and contraction occur with changes in temperature.

5. Interchangeri'or cooling a fluid under pres-. sure by heat exchange with one or more other fluids as in claim 3, wherein said end pieces are attached to said outer shell by screw threads and said end pieces extend over said outer shell beyond. said threads.

6. Interchanger for cooling a fluid under pressure by heat exchange with one or more other fluids as in claim 3, wherein the ends of said inner shell are clamped to said end pieces.

7. Interchanger for cooling a fluid under pressure by heat exchange with one or more other fluids as in claim 3, including fused metal joints between said inner shell and said end pieces whereby leakage is prevented of the fluid cooled into the space between said inner and outer shells.

8. Interchanger for cooling a fluid under pressure by heat exchange with one or more other fluids as in claim 3, including a head at one end into which one of said end pieces is fastened, and a fused metal joint between said head and said end piece, whereby leakage is prevented of the fluid cooled.

9. Interchanger for cooling a fluid under pressure by heat exchange with one or more other fluids as in claim 3, including a head at one end with gland forming a stufiing box with one of said end pieces, and packing material in' said stufing box, whereby relative motion of head'and end piece can occur without leakage of the fluid cooled.

10. Interchanger for cooling a fluid under pressure by heat exchange with one or more other fluids as in claim 3, including a head having an internal diameter greater than the external diameter of said outer shell and one of said end pieces fills the gap between said head and said outer shell.

WILLIAM LANE DE BAU'FRE.

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