US3028855A - Heat exchanger - Google Patents

Description

April 10, 1962 J. w. BROWN, JR

HEAT EXCHANGER Filed Oct. 7, 1960 IN VEN TOR. Jax/d h/ dean/M JR BY Emu/016A, imam w Wan L- 8. Km

,arrayMf/S Unite States 3,628,855 Patented Apr. 10, 1962 3,028,855 HEAT EXCHANGER John W. Brown, Jr., Lakewood, Ohio, as'signor to Brown Fintube Company, Elyria, Ohio, a corporation of Ohio Filed Oct. 7, 1960, Ser. No. 61,303 3 Claims. (Cl. 126-116) This invention relates to heat exchangers, and more particularly to heat exchangers adapted for service at high temperatures in which a fluid to be heated flows in an annular space between an outer or shell tube and an inner tube having heat transfer elements in the form of longitudinally extending external fins.

Heat exchangers of this general type are used for heating air or other gases and various liquids to relatively high temperatures. The fluid to be heated flows through the annular space between the shell tube and the inner tube while the heating fluid flows through the inner tube; such heating fluid may be supplied by a gas or oil burner discharging its products of combustion directly into the interior of the inner tube. Preferably, the fluid to be heated enters and leaves the annular space through an inlet conduit and an outlet conduit which extend laterally through the wall of the shell tube at points spaced longitudinally of such tube. The fluid passing to or from these conduits must flow circumferentially around the inner tube in the zones adjacent the conduits. .In the inlet zone, the fluid to be heated entering the shell tube through the inlet conduit flows circumferentially around the inner tube and is distributed to the spaces between the longitudinally extending heat transfer fins on the inner tube, through which spaces it flows toward the outlet conduit; in the outlet zone, the fluid which is discharged from the spaces between the fii1s flows circumferentially around the inner tube to the outlet conduit in the shell tube. It is impossible, however, for the fluid to flow circumferentially in suflicient quantity around a portion of the tube carrying imperforate heat transfer fins when such fins extend into close proximity to the inner surface of the shell tube as is usually the case.

In conventional heat exchangers zones of circumferential flow are provided by making the fins on the inner tube so they stop short of the inlet and the outlet conduits in the shell tube. In such heat exchangers considerable ditficulties are caused when, as has been heretofore the practice, the heating fluid at its highest temperature is initially introduced into such a bare portion of the inner tube located in such a zone of circumferential flow of the fluid to be heated. The absence of heat transfer fins on such a portion of the inner tube causes a low rate of heat transfer from the inner tube to the fluid to be heated, which results in a localized substantial increase in temperature of the inner tube. In some services, the bare inner tube in such zone may reach such a high temperature that the life of the tube is considerably shortened due to overheating, thermal stresses of corrosion; this may occur even if the tube should be made of expensive heat-resistant alloys rather than of ordinary steels. These problems are particularly serious in conventional heat exchangers in which a gas or liquid fuel burner is connected at one end of the exchanger, so that the inner tube in effect acts as a combustion tube for the burner and the hottest flame and combustion gas and resulting radiant heat impinge on a bare portion of the inner tube which extends through a zone of circumferential flow and has no heat transfer elements which can rapidly transfer heat away from the tube.

The above problems are further intensified in conventional counterflow heat exchangers embodying such an arrangement and having the outlet from the shell tube located at the burner end of the inner tube. In this case, the fluid to be heated is at its highest temperature as it passes to the outlet conduit through the adjacent zone of circumferential flow, while in this same zone the inner surface of the bare portion of the inner tube is also exposed to the heating fluid at its highest temperature. As a result, the temperature differential between the heating fluid and the fluid to be heated is the least in the zone in which the inner tube has the lowest heat transfer characteristics. This portion of the inner tube, therefore, is not subjected to the cooling effects that would result from either the provision of heat transfer elements or from a large temperature differential between the heating fluid and the fluid to be heated. Consequently, in counterflow heat exchangers this portion of the inner tube often deteriorates particularly rapidly because of harmful effects of excessive heat.

A general object of the present invention is the provision of heat exchangers of the general type described which overcome the problems and disadvantages indicated above. Another object is the provision of such a heat exchanger which can be manufactured at a reasonable cost.

According to the present invention, these problems are solved and the above disadvantages are overcome by the provision of a heat exchanger comprising an inner tube having external radial fins which extend longitudinally for a major portion of the length of the tube and substantially throughout the zone of circumferential flow of fiuid to be heated located at the end of the inner tube at which the heating fluid is introduced; and a shell tube so formed that its inner surface closely approaches the fins of the inner tube throughout the major portion of the length of the inner tube and the major portion of the length of the shell tube, but is spaced substantially away from the inner tube for a minor portion of the length of the shell tube to provide a zone of circumferential flow adjacent a lateral conduit opening into the shell tube. The hottest heating fluid is introduced into the end portion of the inner tube carrying the heat transfer fins and located in the zone of circumferential flow, and not into a bare or unfinned section of the tube. The heat transfer fins provide a high rate of heat transfer in the circumferential flow zone; thus overheating, corrosion, thermal stresses or other deterioration of the inner tube are reduced, the life of the inner tube is increased without the necessity of using expensive alloys, and the efficiency of the heat exchanger as a whole is augmented.

Further objects and advantages of the invention will become apparent from the following description of a preferred form thereof, reference being made to the accompanying drawings in which:

FIGURE 1 is a longitudinal view, partly in section, illustrating a fired counterflow heat exchanger embodying my invention;

FIGURE 2 is a section taken along line 2-2 of FIG- URE 1, showing the fins on the inner tube and how they extend through the zone of circumferential flow at the burner end of the heat exchanger; and

FIGURE 3 is a detail to an enlarged scale showing a portion of the inner tube and the fin members welded to it, and how the fins closely approach the inner surface of the shell tube throughout the major portions of the lengths of the inner tube and the shell tube.

In the drawings, the invention is illustrated as embodied in a horizontal type heat exchanger comprising a longitudinally extending shell tube It having an inlet conduit 11 and an outlet conduit 12 extending laterally throughthe wall of the shell tube 10 near its opposite ends. The shell tube is also preferably provided with an expansion joint 13 and is supported by any convenient means such as the pedestal supports 14, 15, Hand 17,

:3 supports 15, 16 and 17 being provided with rollers as shown to permit movement of the shell tube due to expansion or contraction.

An inner tube 29 is disposed concentrically within the shell tube 10, to define an annular space between such tube for the major portion of their lengths. Inner tube 20 has on its surface a plurality of heat transfer elements taking the form of external fin members 21 which extend through such annular space and longitudinally of the inner tube for substantially its entire length from the inlet 11 and to and past the outlet 12 of the shell tube 36.

Preferably, the external fin members 231 are in the form of channels, as shown in FIGURES 2 and 3, each fin member comprising a pair of imperforate fin portions 22 and a base portion 23. The fin members are formed into such channels and welded to the exterior of the inner tube, preferably in accordance with the method and apparatus disclosed in my Patents Nos. 2,298,249 and 2,298,250 issued October 6, 1942.

The fin portions 22 extend substantially radially into close proximity to the inner surface of the major portion 24 of the shell tube 10, which portion extends throughout the length of the tube 10 except in the zone of circumferential flow 25 adjacent the outlet conduit 12 of the shell tube and surrounding the end of inner tube 20 in which heating fluid is introduced, as will be described in more detail later.

In the embodiment shown, the other zone of circumferential flow 26 is formed by omission of the fins from the portion of the inner tube 10 opposite the inlet conduit 11.

Each end of the inner tube 20 is connected to the shell tube 10 to prevent escape of fluid to be heated at these points. These connections may take any suitable form, such as a conventional packing gland arrangement as indicated at 27 at the inlet end of the shell tube, and a flanged connection at the other end, shown as comprising a plate 28 on the inner tube bolted to an internally extending flange 29 welded to the end of the shell tube; or connections such as shown in Patent No. 2,745,683 may be used.

In the usual conventional counterflow fired heater having a longitudinally finned inner tube, there is opposite the outlet conduit a zone of circumferential flow which is similar to the zone 26 shown opposite the inlet conduit 11 in FIGURE 1 of the illustrated embodiment, in that it is formed by omission of fins from the portion of the inner tube in the circumferential flow zone, and the interior of the shell tube is the same size in this zone as in the major portion of the shell tube. A fuel burner is mounted at the end of the inner tube so as to discharge flame and the hottest combustion gas directly into the bare portion of the inner tube extending through such a zone of circumferential flow located opposite and adjacent the lateral outlet conduit from the shell tube. Consequently, the high rate of heat input to the inside of this portion of the inner tube, and the low rate of heat transfer from the outside of this portion of the tube due to the absence of heat transfer fins, often cause this portion of the inner tube to be seriously overheated and damaged in such a conventional heat exchanger.

In the illustrated embodiment of the invention, and as shown in FIGURES l and 2, a liquid or gaseous fuel burner 30 constituting the source of heating fluid is mounted at the end of the inner tube 20 nearest the outlet conduit 12 of the shell tube, so that the flame and hottest combustion gas emitted by the burner first pass into the portion of the inner tube 20 positioned adjacent to and opposite the outlet conduit 12. The burner is mounted in any suitable manner; for example, it may be mounted on the end of a supply pipe 32 which is supported by bracket 33 mounted by bolts 34 and spacers 35 on a flange 36 fixed to the end of the inner tube projecting from the shell tube near the outlet conduit 12.

In distinction to conventional practice, however, the

longitudinal fins 22 on the exterior of the inner tube 20 continue imperforate and undiminished in radial dimensions, throughout the portion of the inner tube which lies adjacent the outlet conduit 12, and into which the hottest products of combustion are emitted. Because of the high heat transfer capacity provided by these fins, this finned portion of the inner tube is protected against overheating as compared to the correspondingly located bare portion of the inner tube of a conventional counterflow heater. In order to permit the fluid to be heated to flow circumferentially around this finned portion of the inner tube as the fluid passes from the spaces between the fins to the outlet conduit 12, the shell tube 10 is so constructed that in the vicinity of the outlet conduit 12 there extends for a minor portion of the length of the shell tube a sub stantially enlarged annular space extending around the fins 22 and constituting a zone 25 of circumferential flow. This is accomplished in the illustrative embodiment by offsetting the minor wall portion 37 of the shell tube 10 outwardly from the major wall portion 24 of the shell tube, and fixing it to annular end plates 28 and 38 to form the zone 25. The outlet conduit 12 communicates laterally with this enlarged portion of the shell tube, preferably through a passage 39 fixed to wall portion 37 and generally helically shaped to reduce fluid flow resistance, as shown in FIGURE 2.

In operation of the heat exchanger shown, the heating fluid, comprising the flame and other products resulting from combustion of liquid or gaseous fuel by burner 30, is at its highest temperature when it is introduced into the inner tube within the finned portion thereof located in the zone of circumferential flow 25. The heating fluid initially radiates heat at a high rate to the inner surface of inner tube 29. As the heating fluid travels along the length of the inner tube, heat transfer by radiation becomes of lesser importance while transfer of heat to the inner tube by convection becomes of increasing importrance, the temperature of the heating fluid being gradually lowered. At the end of the inner tube near the inlet conduit 11 the gas discharges into a suitably connected stack 40.

Meanwhile, air or other fluid to be heated enters through inlet conduit 11, flows circumferentially through zone 26 around the bare portion of inner tube 20, and distributes itself into the spaces between the fins 22 on such tube. It then passes through the annular space defined by major portion 24 of the shell tube 10 and the finned inner tube within such shell tube portion; the surfaces of this portion of the inner tube 20 and the fins 22 on it heat the fluid being heated by transferring to it at a high rate heat transmitted to tube 20 as described above. The thus heated fluid then discharges from the spaces between the fins into the enlarged wall portion 37 of the shell tube 10 providing the zone of circumferential flow 25. It is further heated as it passes through this zone, by the heat transferred to it by the surfaces of the portion of the inner tube 10 and the fins on it in this zone, which portion is subjected to the heating fluid emitted by the burner. The fluid being heated then passes from the zone of circumferential flow to the outlet conduit 12.

Thus, even though the burner 30 is located so that heating fluid which is at its hottest temperature is initially introduced into the portion of the inner tube 20 which lies in a zone of circumferential flow, and this portion of the tube is therefore subjected to intense and largely radiant heat, the fins 22 on this portion of the tube prevent overheating since they rapidly transfer the heat to the fluid to be heated.

On the other hand, the portion of the inner tube 20 in zone 26, which has low heat transfer characteristics because it has no heat transfer fins which can impede circumferential flow of fluid passing to the outlet conduit 12, is not exposed to high temperature heating fluid so it cannot become overheated. Therefore, no portion of the inner tube is subjected to the excessively high localized temperatures occurring in conventional practice; the inner tube as a Whole thus is free of overheating, thermal stresses, corrosion, or other adverse conditions Which could damage it and reduce its life. Furthermore, the heat transfer efiiciency of the inner tube 26 and of the heat exchanger as a whole is increased, since the hottest heating fluid first contacts the inner tube where it has high heat transfer characteristics.

In the above described heat exchanger, the zone of circumferential flow 26 at the inlet end of the heat exchanger may be similar to zone 25 by being formed of an enlarged portion of the shell tube 1d through which passes a finned portion of inner tube 10. Since, however, this portion of the finned tube is subjected to heating fiuid which has been substantially cooled, there is usually no need for such construction at the inlet end.

The arrangement described above provides counterflow heat exchangers where the discharge temperature of the fluid to be heated must be high. It is evident that a heater embodying the invention can also be made for concurrent flow by merely reversing the inlet and outlet conduits in the shell tube, in which case the burner initially discharges its hottest products of combustion within the portion of the inner tube covered by fins and contained in an enlarged zone of circumferential flow communicating with the inlet conduit.

The present invention thus makes possible the circumferential flow necessary for efiicient entrance or egress of fluid to be heated, while overcoming the problems outlined above as arising in conventional practice from exposure of a bare or unfinned portion of the inner tube to high temperature heating fluid. While the present invention has been disclosed in connection with a horizontal counterflow type of heat exchanger embodying a gas or liquid burner, it is apparent that it may be used in vertical heat exchangers, those of the concurrent flow type, and those embodying other sources of heating fluid than that illustrated. Furthermore, while the heat exchanger disclosed as embodying the invention has inner and shell tubes of circular cross section, the invention may also be used in heat exchangers in which either or both of the inner or shell tubes has other than a circular cross section, such as an oval, square, polygonal, or rectangular cross section.

It will thus be appreciated that the invention may be adapted to heat exchangers of different types than that specifically disclosed herein, which is given by Way of example only. The essential characteristics of the invention are set forth in the appended claims.

I claim:

1. A heat exchanger comprising an elongated inner tube having a plurality of generally radially projecting imperforate continuous external heat transfer fins extending longitudinally of a major portion of the length of said tube and on an end portion thereof; means for initially supplying heating fluid into said finned end portion of said inner tube; an elongated shell tube surrounding said inner tube and having an inner surface extending in close proximity to the fins on said inner tube throughout a major portion of the length of said shell tube, and having an enlarged inner surface extending for a minor portion of the length of said shell tube and surrounding and spaced a substantial distance from the fins on said finned end portion of said inner tube to define a space through which fluid in said shell tube may flow circumferentially around said finned end portion of said inner tube; and a fluid conduit opening laterally into said space defined by said enlarged inner surface of said shell tube.

2. A heat exchanger comprising an elongated inner tube having a plurality of generally radially projecting imperforate continuous external heat transfer fins extending longitudinally of a major portion of the length of the tube and on an end portion thereof; a fluid fuel burner disposed to initially introduce heating gas into said finned end portion of said inner tube; an elongated shell tube surrounding said inner tube and having an inner surface extending in close proximity to the fins on said inner tube throughout a major portion of the length of said shell tube, and having an enlarged inner surface extending for a minor portion of the length of said shell tube and surrounding and spaced a substantial distance from the fins on said finned end portion of said inner tube to define a space through which fluid in said shell may flow circumferentially around said finned end portion of said inner tube; and a fluid conduit opening laterally into said space defined by said enlarged inner surface of said shell tube.

3. A heat exchanger comprising an elongated inner tube having a plurality of generally radially projecting imperforate continuous external heat transfer fins extending longitudinally of a major portion of the length of said tube and on an end portion thereof; means for initially supplying heating fluid into said finned end portion of said inner tube; an elongated shell tube surrounding said inner tube and having an inner surface extending in close proximity to the fins on said inner tube throughout a major portion of the length of said shell tube, and having an enlarged inner surface extending for a minor portion of the length of said shell tube and surrounding and spaced a substantial distance from the fins on said finned end portion of said inner tube to define a space through which fluid in said shell tube may flow circumferentially around said finned end portion of said inner tube; a fluid outlet conduit opening laterally into said space defined by said enlarged inner surface of said shell tube; and, spaced longitudinally of said shell tube at a location remote from said outlet conduit in said shell tube, an inlet into which fluid enters said shell tube to pass through the spaces between said heat transfer fins and circumferentially through said enlarged space out through said outlet conduit.

References Cited in the file of this patent UNITED STATES PATENTS 1,558,848 Doble Oct. 27, 1925 2,483,489 DeLancy Oct. 4, 1949 2,637,314 Wallis et al. May 5, 1953 2,742,269 Schefels et al. Apr. 17, 1956

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