US 3887002 A
Description (OCR text may contain errors)
United States Patent  Schoonman AIR-COOLED HEAT EXCHANGER WITH AFTER-CONDENSER  Inventor: Willem Schoonman, Wyckoff. NJ.
 Assignee: The Lummus Company, Bloomfield,
 Filed: Jan. 28, 1974 [2|] Appl. No.: 436,877
 U.S. Cl. 165/113; l65/l24; l65/DlG. l  Int. Cl. FZBb l/06  Field of Search l65/ll3. l24, 111, 122
 References Cited UNITED STATES PATENTS 686.432 l l/IQO] Wolt l65/l ll 3.424.235 H1969 Schoonman l65/l22 3.705.621 [2/1972 Schoonman l65/l24 FOREIGN PATENTS OR APPLICATIONS 900.407 12/l958 United Kingdom l65/l ll [4 1 June 3, 1975 Primary Examiner-Charles J. Myhre Assistant Examiner-James D. Liles Attorney. Agent. or FirmR. J. Holton; M. Klotz; A. L. Chen  ABSTRACT An air-cooled heat exchanger having U-tubes for condensing and/or cooling vapors such as steam. The main section of the exchanger comprises a plurality of U-tubes, each connected at one end to an inlet manifold and at the other end to a collection manifold for receiving liquid. Cooling air flows cocurrently with the condensing or cooling vapor, thus preventing subcooling or freezing.
A second section comprises additional U-tubes. for cooling and/or condensing uncondensed vapors from the main section. A U-tube of the second section is connected to each of the condensing collecting manifolds of the first section. ln a preferred embodiment, the second section of U-tubes is located in the same plane as the main section, and adjacent to the vapor inlet to the main section.
7 Claims, 6 Drawing Figures l 36 Ste um mpwmu: 3|s1s 3.887.002 SHEET 1 Steam Fig". l.
AIR-COOLED' HEAT EXCHANGER WITH AFTER-CONDENSER BACKGROUND AND PRIOR ART This invention is an improvement over the air-cooled heat exchanger disclosed in my U.S. Pat. No. 3,705,621, issued Dec. 12, 1972. This patent discloses an air-cooled heat exchanger for cooling and/or condensing gases or liquids, having two groups of U-shaped tubes. One end of each of the U-shaped tubes is connected to an inlet manifold. The other end of each U- shaped tube is connected to a manifold for collecting cooled liquid or condensate. Each group of U-tubes is connected to a separate collection manifold. The two groups of U-tubes are so disposed that the cooling air first flows over the upstream legs of the first group of tubes (connected to the inlet manifold), then over the upstream legs of the second group of tubes, then over the downstream legs of the second group of tubes (connected to one of the collection manifolds) and finally over the downstream legs of the first group of tubes (connected to the other collection manifold). Such an arrangement permits approximate equalization of the condensation in all tubes, although it does not equalize pressure in all the tubes.
When the apparatus is used for condensing vapors, sizeable amounts of vapor may pass through the tubes without being condensed, depending on the amount of vapor passing through the apparatus, and the heat ex change qualities thereof. In the apparatus shown in my U.S. Pat. No. 3,705,621, this uncondensed vapor is permitted to pass out of the collection manifolds through outlet 8, along with air or other non-condensible gas. However, such uncondensed vapor represents a loss in the overall system. When such a vapor is a desired product of a processing plant, the loss results in a lower yield of product. When the apparatus is used as a steam condenser, sizable amounts of steam may be lost, which requires large amounts of make-up water.
To prevent such losses, air-cooled vapor condensers are often equipped with additional sections, generally referred to as after-condensers or sub-cooling sections, for condensing and recovering this uncondensed vapor.
It is an object of the present invention to provide such an air-cooled vapor condenser having an additional section or sections for after-condensation or subcooling of uncondensed vapors.
It is another object of the present invention to provide such an air-cooled vapor condenser in which condensation in the various tubes of each section is substantially equalized.
SUMMARY OF THE INVENTION In general, the invention comprises:
In an air-cooled heat exchanger for condensing vapors, comprising a first plurality of U-shaped tubes connected at one end to a vapor inlet manifold and at the other end to a first condensate manifold, a second plureality of U-shaped tubes connected at one end to a vapor inlet manifold and at the other end to a second condensate manifold, and means for directing air to flow over the external surfaces of the tubes, the improvemen comprising third and fourth condensate manifolds, .hird and fourth pluralities of substantially U-shaped tubes, each tube having an upstream leg and a downstream leg, the third plurality of the tubes connected at the end of the upstream leg to the second condensate manifold and at the end of the downstream leg to the third condensate manifold, the fourth plurality of substantially U-shaped tubes connected at the end of the upstream leg to the first condensate manifold and at the end of the downstream leg to the fourth condensate manifold, means for draining condensate from the third and fourth condensate manifolds, means for removing uncondensed vapor from the third and fourth pluralities of tubes, the tubes being so disposed that the same air stream passes over the external surfaces of the tubes of the first, second, third and fourth pluralities thereof.
DETAILED DESCRIPTION OF THE INVENTION The invention is described below, with reference to the drawings, in which:
FIG. 1 is a partial top view of one embodiment of the invention;
FIG. 2 represents a cutaway elevational view of another embodiment of the invention;
FIG. 3 represents a partial top view of a third embodiment of the invention;
FIG. 4 represents a side view of the embodiment of FIG. 3 taken along line 4-4;
FIG. 5 represents a cross-sectionai view of the main condenser shown in FIG. 4 taken along line 5-5; and
FIG. 6 represents a cross-sectional view of the aftercondenser shown in FIG. 4 taken along line 6-6.
As shown in the embodiments of FIGS. 1 and 2, the apparatus comprises a vapor inlet manifold 10, connected to a source of vapor (not shown) such as steam. The vapor flows into the upstream legs 12a, 14a of the U-tubes of the main condensing section. These are joined to the corresponding downstream legs 12b, 14b by U-shaped bends 13, resulting in continuous U- shaped tubes. The downstreamlegs 12b, 14b terminate respectively in condensate collecting manifolds l6 and I8. Condensate flows down through these manifolds by gravity, and is removed in condensate collecting pipe 36 and drain 38. The air flow is directed by fan 40 or other motive means, which may include baffles (not shown) such that the cooling air first contacts the hottest vapor in the upstream legs 12a of the outermost row of U-tubes. The partially heated air then comes into contact with the upstream legs 14a of an inner row of U-tubes, then with the downstream legs 14b of an inner row of U-tubes, and finally the warmed air contacts the downstream legs 12b of the outer row of U-tubes. The downstream legs 12b, 14b contain a mixture of uncondensed vapor and'condensate (e.g., steam and water). Thus, the air flows generally cocurrently with the flow of vapor and condensate.
The tubes 12a, 14a, etc., are preferably arranged in what is known as a triangular pitch, that is, the tubes are staggered with respect to tubes 124 so that a combination of two tubes in any row, for example tubes of the row 12a with the nearest tube in the other row (140), forms a triangle. Similarly, the tubes 12b and 14b are arranged in a triangular pitch. This permits maximum use of the cold air, so that any air which passes between the tubes 12a will be encountered by tubes 14a. As shown in the figures, the row of tubes 14a, 14b is preferably situated within the area enclosed by the U-shape of tubes 12a, 12b.
The tubes may be integral or composed of several sections connected together, as by welding. The figures 3 depict the apparatus as having two sets of l lkfil l 12. I4) joined by U-bends, with two condensate manifolds (16, I8). However. the exchanger may have three or more sets of U-tubes, each with its accompanying condensate manifolds.
Connected to condensate manifolds I8 and 16 respectively are the upstream legs 20a, 22a of the U- shaped tubes of the after-condenser. The upstream legs 20a, 220 are connected to the corresponding downstream legs 20b, 22b by Ubends 21. In this manner, the outer row of U-tubes 120, b in the main condensing section is connected through its condensate manifold 16 to an inner row of U-tubes 22a, b in the aftercondenser. Similarly the inner row of U-tubes 140, b of the main condensing section is connected through its condensate manifold 18 to the outer row of U-tubes 20a, b of the after-condenser. Such an arrangement contributes to the objectives of equalizing condensation and improving vapor distribution in the apparatus. The tubes of the after-condenser are, similarly to those in the main condenser section, either integral or composed of several sections connected together, and are also preferably arranged in a triangular pitch. Similarly to the tubes in the main condenser, the air is directed to flow first over the upstream legs 200, then over the upstream legs 22a, then over the downstream legs 22b, and finally over the downstream legs 20b, before exiting from the apparatus. The outer row of U-shaped tubes 20a, 20b terminates in condensate collecting manifold 24, and the inner U-shaped tubes 22a, 22b terminate in condensate collecting manifold 26. Condensate is drained from these manifolds by collector 36 and drainpipe 38.
Drainpipe 38 is preferably connected to a loop-seal construction (not shown). The loop-seal results in the maintenance of a water level in the lower portion of the condensate manifolds I6, 18, 24 and 26. Uncondensed and uncondensiblc gases are trapped in the manifolds and are drawn through the after-condenser U-tubes 20 and 22 and the steam/air takeoff (as described below).
FIGS. I and 2 depict two embodiments of apparatus for removing condensate and uncondensed vapors from the after-condenser section. In FIG. 1, the con densate is drained from collection manifolds 24 and 26 via collector 36 and drainpipe 38, together with the condensate from manifolds l6 and 18 while the uncondensed gases are removed via tubes 30 and 32 respectively, and outlet 34. In this embodiment, the uncondensed gas (air and some steam) takeoff section com prises tubes which are arranged perpendicularly to the flow of steam and condensate in the U-tubes, and are cooled by the same air stream, subsequent to its passage over the U-tubes. In FIG. 2, however, the conden sate and uncondensed gases are removed via a common outlet 42 connected to manifolds 24 and 26, and the condensate and uncondensed gases are separated in another unit. Other methods of removing the condensate and uncondensed gases will be apparent to those skilled in the art.
For improved flow distribution. the U-tubes of the after-condenser section are located at a point in the apparatus adjacent to the vapor source. In the embodiment of FlG I. for instance. vapor steaml is introduced into the lower portion of \apor inlet 10. consequently the after-condenser U-tubcs 20, 22 are located near the bottom of the heat exchanger apparatus. Conversely, in FIG. 2, the vapor (steam) is introduced into are upper portion of vapor inlet 10, and the tubes 27},
22 of the alter-condenser section are located substantially above the main condensing tubes. In the embodiments of FIGS. l and 2, the tubes 20, 22 of the aftercondenser are situutcd so 3.1: to partially overlap the plane of the tubes 12, I4 of the main condensing sec tion. In the embodiment of FIG. 2. the air distribution to the after-condenser section is assisted by the use of baffles 28. These may also be used in the other embodiments of the invention.
FIGS. 36 depict another embodiment of the invention, in which the tubes of the after-condenser are located in the same plane as those of the main condenser. As shown in FIG. 3, the condensate collecting manil5 folds I6 and 18 are connected to auxiliary manifolds l7 and 19 respectively, to which are connected the upstream legs 22a, a, respectively of the U-tubes of the after-condenser. The remaining aspects of this embodi ment are the same as those in FIGS. 1 and 2, and condensate and uncondensed gases may be removed from manifolds 24 and 26 by any of the means shown in FIGS. 1 and 2.
For simplicity, FIGS. I and 3 depict only one pair of U-tubes in each of the main and after-condenser sections. In practice, however, each section will be comprised of numerous pairs of U-tubes, as shown in FIGS. 2 and 4. The tubes may be arranged horizontally, but are preferably sloped, as shown in FIGS. 2 and 5, to permit drainage of condensate by gravity.
A common factor in a design of equipment of this type is the necessity of preventing sub-cooling and consequent freezing of condensate in the tubes of the exchanger. Few of the designs devised to overcome or prevent this problem have proven suitable for use in extreme weather conditions, e.g., at low ambient temperatures, Nor are most designs sufficiently versatile for use in areas of the world in which the temperature may vary widely from season to season. By utilization of the design of the present invention, sub cooling of the condensate may be prevented, even at low ambient temperatures. The invention is not limited, however, to use at such temperatures; it can be used in any situation in which vapor is to be condensed by indirect heat exchange with cooling air. In fact. sub-cooling does not only occur at low ambient temperatures but may also take place at higher temperatures in operational situa tions in which the vapor flows at a low rate, and condensate accumulates in the tubes. Such may be the case, for example, with varying flow rates of vapor, such as may occur from a turbine or other equipment having variable usage or loading.
Similarly, the danger of freezing or sub-cooling of condensate produced in the after-condenser sections of air-cooled heat exchangers is also a problem. This danger is particularly present if the after-condenser is cooled by a stream of air in a separate unit from the main condenser. as the flow rate of steam passing through the tubes of the after-condenser will be less than that of the main condenser since most of this steam has been condensed. Since the steam is now more diluted with air than in the main condenser, this diluted steam will be heat exchanged against compara tively cold cooling air.
The present design possesses safeguards against the freezing or sub-cooling of condensate in both the main and after condenser sections. Because of the cocurrent flow of air and steam, the danger of sub cooling the condensate in the downstream legs of the tubes 12b, 14b, b. and 22b is eliminated, since the air which is heat exchanged against the condensate (which is present in greater amounts in these sections than the corresponding upstream sections of the tubes) has been warmed by previous contact with hot steam. Similarly, though condensate may accumulate in the bottom of manifolds 16, 18, 24, and 26, there is no danger of subcooling here, since air passing over the manifolds has been sufficiently warmed by previous contact with the U-tubes.
An additional advantage of the construction of the present invention is that it permits substantial equalization of the condensation of vapor in the various tubes. In a conventional air-cooled heat exchanger having straight tubes, the greatest amount of condensation will take place in the row of tubes closest to the source of cooling air, since these tubes will be contacted by the air at its coldest temperature. However, in the present invention, the condensation in U-tubes 12 and 14, for example, will be approximately the same. This is because while the upstream leg 12a of the U-tube 12 is contacted by the coldest stream of air, the downstream leg of the same tube 12b, is contacted by the air which has been substantially warmed. while both legs 14a and 14b of the inner U-tube are contacted by air at intermediate temperatures. A similar situation occurs with the U-tubes 20 and 22 of the after-condenser. In the same fashion, the temperature of the air passing over each U-tube of each pair becomes substantially equalized.
Similarly, when the air/vapor takeoff system as shown in FIG. 1 is used. an additional advantage is found because the vapor outlet tubes 30 and 32 are contacted by the same cooling air stream used for heat exchange of the main apparatus, the stream having been warmed by previous passage across the surfaces of one or more sets of U-tubes. This again prevents any sub cooling in the air/vapor takeoff (due to its low vapor content) and results in a small additional amount of condensate being recovered.
While this invention has been described in terms of certain preferred embodiments and illustrated by way of particular drawings, these are only illustrative, as
many equivalents and alternatives will present themselves to those skilled in the art as within the spirit and proper scope of the invention. The invention is therefore not to be construed as limited, except as set forth in the following claims.
I. In an air-cooled heat exchanger for condensing vapors, having a main condensing section comprising a first outer plurality of U-shaped tubes having upstream legs connected to a vapor inlet manifold and downstream legs connected to a first condensate manifold,
a first inner plurality of U-shaped tubes connected at one end to a vapor inlet manifold and at the other end to a second condensate manifold, and means for directing air to flow over the external surfaces of the tubes. the improvement comprising an after-condensing section comprising third and fourth condensate manifolds, a second outer plurality of U-shaped tubes connected at one end to the second condensate manifold and at the other end to the third condensate manifold, a second inner plurality of U-shaped tubes having upstream legs connected to the first condensate manifold and downstream legs connected to the fourth manifold, the second inner plurality of U-shaped tubes being located within the space enclosed within the second outer plurality of U-shaped tubes, means for draining conden sate from the third and fourth condensate manifolds, means for condensate vapor from the second outer and inner pluralities of U-shaped tubes, the tubes being so disposed that the same air stream passes over the external surfaces of the tubes of the first and second inner and outer pluralities.
2. The apparatus of claim 1, wherein the second outer and inner pluralities of U tubes are disposed in a triangular pitch arrangement.
3. The apparatus of claim 1, further comprising a means for gravity drainage of liquid from each of the pluralities of tubes.
4. The apparatus of claim 1, wherein the second inner and outer pluralities of U-tubes are located adjacent to point of introduction of vapor into the vapor inlet manifold.
5. The apparatus of claim 1, wherein the up-stream and down-stream legs respectively of the first inner and outer pluralities of U-shaped tubes are located in the same plane as the up-stream and down-stream legs respectively of the second outer and inner pluralities of U-shaped tubes.
6. The apparatus of claim 1, wherein the means for removing uncondensed vapors from the second inner and outer pluralities of U-shaped tubes comprises a first substantially straight tube connected to the third condensate manifold and a second substantially straight tube connected to the fourth condensate manifold, said straight tubes overlaying and being substantially perpendicular to the U-shaped tubes and being so disposed as to be contacted by the air stream subsequent to its passing over the external surfaces of the U- shaped tubes.
7. The apparatus of claim 1, comprising common means for removing condensate and uncondensed vapor from the second inner and outer pluralities of U- shaped tubes.