Heat exchanger

Abstract

A heat exchanger is disclosed having a first cylindrical tube and a lead screw which extends coaxially inside the first cylindrical tube; the inner surface of the first cylindrical tube has guiding grooves, and a cleaning element is secured to the lead screw in such a way that a rotating movement of the lead screw moves the cleaning element in the axial direction along the guiding grooves.

Claims

1. A heat exchanger comprising: a first cylindrical tube and a threaded spindle that runs coaxially in the first cylindrical tube, and a second cylindrical tube arranged coaxially to the first cylindrical tube, wherein the inner surface of the first cylindrical tube has guiding grooves, and wherein a cleaning element is secured to the threaded spindle in such a way that rotating the threaded spindle moves the cleaning element in an axial direction along the guiding grooves.

2. The heat exchanger according to claim 1, wherein the outer surface of the first cylindrical tube has a coil that runs spirally in an axially direction.

3. The heat exchanger according to claim 1, wherein the cleaning element is a hollow cylindrical cleaning element having a cylindrical circumference, wherein the inner surface of the cleaning element has a female thread corresponding to the thread of the threaded spindle, and wherein the outer surface of the cleaning element has outer grooves corresponding to the guiding grooves of the inner surface of the first cylindrical tube.

4. The heat exchanger according to claim 3, in which the cleaning element has recesses in the cylindrical circumference, and said recesses which extend parallel to the axial direction.

5. The heat exchanger according to claim 4, in which the recesses are equidistantly arranged in the cleaning element in the circumferential direction.

6. The heat exchanger according to claim 3, in which the female thread of the cleaning element has a diameter that increases in the axial direction.

7. The heat exchanger according to claim 1, wherein an inlet and outlet opening is present for a coolant, so as to admit and discharge coolant into or from a gap between the second cylindrical tube and the first cylindrical tube.

8. The heat exchanger according to claim 1, wherein an inlet and outlet opening is present for a working medium, so as to admit and discharge the working medium into or from a gap between the first cylindrical tube and the threaded spindle.

9. The heat exchanger according to claim 1, wherein a deposit store for contaminants washed away by the cleaning element is connected with the gap between the threaded spindle and the inner surface of the first cylindrical tube.

10. The heat exchanger according to claim 9, in which a heating element is present and arranged in such a way that contaminants present in the deposit store can be heated.

11. The heat exchanger according to claim 1, in which a position measuring device is present and arranged in such a way that a position of the cleaning element in the axial direction can be measured.

12. The heat exchanger according to claim 1, wherein a drive motor is present for driving the threaded spindle, and wherein a particle barrier is present between the drive motor and the gap between the threaded spindle and the inner surface of the first cylindrical tube.

13. The heat exchanger according to claim 1, wherein the threaded spindle has a trapezoidal profile as the thread profile.

14. The heat exchanger according to claim 1, wherein the threaded spindle has a cross thread as the thread.

15. The heat exchanger according to claim 14, in which a sliding block connected with the cleaning element is slide mounted in the threaded groove of the cross thread of the threaded spindle.

16. The heat exchanger according to claim 1 comprising more than one heat exchanger connected in series.

17. A method for liquefying a gas using a heat exchanger according to claim 1, said method comprising: flowing a coolant between the first and the second cylindrical tube, flowing a working medium between the first cylindrical tube and the threaded spindle wherein said working medium contains the gas to be liquefied, and flowing the coolant at a lower temperature than that of the working medium, and wherein the pressure and temperature of the coolant along with the pressure of the working medium are adjusted in such a way that the gas to be liquefied is liquefied in the working medium through heat exchange with the cooling medium.

18. The method according to claim 17, wherein the coolant is the same medium as the gas to be liquefied, and wherein the pressure selected for the coolant is lower than that of the working medium.

19. The method according to claim 17, wherein the gas to be liquefied is nitrogen, helium, oxygen or hydrogen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows a longitudinal section of an advantageous embodiment of a heat exchanger according to the invention;

(2) FIG. 2 shows a cooling coil as the first cylindrical tube of the heat exchanger depicted on FIG. 1;

(3) FIG. 3 shows a cleaning element of the kind used in the heat exchanger according to FIG. 1, and

(4) FIG. 4 schematically shows the cutout of a threaded spindle with a cross thread.

DETAILED DESCRIPTION OF THE INVENTION

(5) FIG. 1 presents a schematic, longitudinal section through an embodiment of a heat exchanger 13 of the kind that can be used in particular for cooling natural gas. In this simple configuration, the heat exchanger 13 has an outer cylindrical tube 1 that envelops a cooling coil 2. This cooling coil 2 is for its part designed as a cylindrical tube, and has at least one, preferably spiral channel 23 on its outer surface that serves to guide a coolant. As illustrated on FIG. 2, this channel 23 is generated by a corresponding coil 21 on the outer surface of the cooling coil 2. The inner surface of the hollow cylindrical cooling coil has guiding or profile grooves 22. This at least one guiding groove 22 serves to guide a cleaning element or reamer 12.

(6) Located inside of the cooling coil 2 coaxially thereto is a threaded spindle 3. The threaded spindle 3 is driven by a drive motor 4, and mounted in a bearing point preferably designed as an axial/radial mixed bearing 5. At the other end of the threaded spindle 3, the latter is mounted in a bearing point preferably designed as a plain bearing bushing 8. Also present at this end of the heat exchanger 13 is a thermally decoupled condensate reservoir 7, along with a heating element 9 for heating condensate in the condensate reservoir 7.

(7) At the other end of the heat exchanger 13, a particle barrier 11 separates the drive motor 4 from the work area for the working medium. The particle barrier 11 also serves to protect the drive motor 4 and bearing 5 against coarse particles, but does not act as a gas seal.

(8) In the embodiment according to FIG. 1 illustrated here, several outer cylindrical tubes 1 are connected by a clamping device 10. The clamping device 10 is structured in such a way that two union nuts with a female thread are screwed onto the outer cylindrical tube 1, which in turn is provided with a male thread. The union nuts are drawn together by means of screws, and the individual segments are pressed together and sealed by a gasket. Several such outer cylindrical tubes can also be understood and referred to as an outer cylindrical tube.

(9) A cleaning element or reamer 12 is arranged next to the particle barrier 11 in its resting position. When starting up the drive motor 4, the threaded spindle 3 is made to rotate, so that the reamer 12 is shifted on the threaded spindle along the guiding or profile grooves 22 of the cooling coil 2 in an axial direction. In the present example, a threaded spindle 3 with a trapezoidal profile is used, for example. Reversing the direction of movement of the reamer 12 presupposes a reversal in the rotational direction of the threaded spindle 3. Another type of design for the threaded spindle 3 is described further below in conjunction with FIG. 4.

(10) For example, moist, dirty working medium is guided via a working medium inlet opening 1 into the gap between the threaded spindle 3 and cooling coil 2 during the operation of the heat exchanger 13, and flows in an axial direction to the working medium outlet opening 15 at the end of the heat exchanger 13. The working medium here flows in the profile grooves 22 on the inner surface of the hollow cylindrical cooling coil 2 (see FIG. 2) along the rotational axis of the threaded spindle 3. Coolant is supplied to the space between the cooling coil 2 and outer cylindrical tube 1 via a coolant inlet opening 16, flows to the other end of the heat exchanger 13, and exits the latter through the coolant outlet opening 17. The coolant here flows spirally in an axial direction in the channel 23 formed between the outer cylindrical tube 1 and cooling coil 2. The coolant withdraws heat from the cooling coil 2, so that heat is in turn withdrawn from the working medium.

(11) In a special application, natural gas at a pressure of 4 to at most 220 bar from an underground cavern is heated to a temperature of approx. 20 C. In a first heat exchanger, the working medium is cooled preferably to 1 C. In a second heat exchanger connected in series with the first heat exchanger, the working medium is preferably cooled to 40 C. to 60 C. In a third stage, the working medium is preferably cooled to 80 C. to 150 C., and in a last stage, the working medium is liquefied via a heat exchanger once again connected in series. The temperature of the natural gas is here lowered down to 196 C., wherein the natural gas is supercooled. The first stage here precipitates a majority of the water portion, the next stage predominantly precipitates the higher hydrocarbons, CO.sub.2 and other accompanying substances. The reamers 12 present in the respective stages of the heat exchangers 13 make it possible to clean condensed constituents from the respective heat transferring surfaces.

(12) In this concrete interconnection case, the first two heat exchanger stages are cooled by refrigerators, and the other two by liquid nitrogen, cryogenic, liquid CNG or cryogenic, gaseous nitrogen. The maximum operating pressure of the heat exchanger is 300 bar, while the permissible operating temperatures measure 100 C. to 200 C.

(13) The different pressure correlations between the cooling medium, for example nitrogen at a maximum of 10 bar, and the working medium, here CNG with accompanying substances to include nitrogen of 4 to 220 bar, nitrogen at a high pressure (e.g., at 10 bar) can be made to liquefy and precipitate by liquid nitrogen at a low pressure (e.g., at 1 bar), owing to the different, pressure-dependent phase transitions. The heat exchanger 13 proposed here can thus also be used for liquefying nitrogen.

(14) For purposes of cleaning the heat transferring surfaces, for example to remove water or ice in the first stage or higher hydrocarbons, CO.sub.2 and other accompanying substances in the second and additional stage, the threaded spindle 3 of one stage is made to rotate by the drive motor 4. As a result, a translational movement is imparted to the reamer 12, which engages into the thread of the threaded spindle 3 on the one hand and into the profile grooves 22 of the cooling coil 2 on the other. On its way toward the condensate reservoir 7, the reamer 12 entrains the mentioned condensed accompanying substances. The latter are pushed into the condensate reservoir 7 once having reached it. Due to the defined thread pitch of the threaded spindle 3, the position measuring device 6 can determine the position of the reamer 12 from the number of measured revolutions of the drive motor 4. As soon as the position of the condensate reservoir 7 has been reached, the rotational direction of the drive motor 4 is reversed, so that the reamer 12 wanders back to its resting position. It makes sense for the resting position to be the upper end position and the position of the condensate reservoir 7 to be the lower end position of the reamer 12 in the vertical position of the heat exchanger.

(15) The accumulated condensate is heated via the heating element 9, and thereby made to melt. The accompanying substances can be discharged through a condensate drain 18 by opening a downstream valve.

(16) For example, the heat exchanging surfaces of the heat exchanger 13 are cleaned after empirically determined period durations or upon reaching an externally measured maximum permissible differential pressure, which makes it possible to infer a reduction in the free flow cross section in the work area caused by deposited accompanying substances. Cleaning yields the highest and most constant possible heat transfer value. The heat exchanger 13 requires a smaller construction volume by comparison to systems in prior art.

(17) The segmented structure of the heat exchanger 13 enables a modular structure. As a consequence, the heat transfer capacity can be varied by enlarging or reducing the heat transfer surfaces.

(18) By using the mentioned position determining device 6, the actual position of the reamer 12 is always monitored. Any seizing can be detected early by measuring slippage.

(19) Let it be noted that the heat exchanger 13 described here can be adapted and used not just for natural gas liquefaction, but also for a plurality of industrial applications with corresponding working media. As a fairly simple replacement part, the reamer 12 can be tailored to the requirements of the respective areas of application, and quickly replaced in the event of damage.

(20) FIG. 3 shows a reamer 12 or cleaning element 12 of the kind that can be used in the heat exchanger 13. Depicted are the outer grooves 122 of the reamer 12, which correspond to the guiding grooves 22 of the cooling coil 2. The female thread 121 of the reamer 12 corresponds to the thread of the threaded spindle 3. The reamer 12 has recesses or milled grooves 123. The latter provide the reamer 12 with teeth or claws, which prevent deposits from accumulating in the thread and ending up blocking the reamer 12. Specifically, the deposits can enter into the gap through the recesses and milled grooves 123, and with the heat exchanger in a vertical position drop downwardly toward the condensate reservoir 7. Furthermore, the inner diameter of the reamer 12 that enlarges in the direction of cleaning movement enables an easier introduction into the contaminated threaded spindle as the cleaning process begins.

(21) Finally, FIG. 4 shows an alternative configuration of a threaded spindle 3, which involves a cross threaded spindle 3. The shaft with cross thread is marked 31. The sliding block running therein is marked 32. In this configuration, the reamer 12 is connected with the sliding block 32, and moves in an axial direction as the threaded spindle 3 rotates.

(22) As already explained above, the advantage here is that the sliding block 32 that slides into the threaded groove is moved from a first direction of movement into a second, opposite direction of movement while the threaded spindle 3 is rotated in a single rotational direction, without changing the rotational direction of the shaft 31.

(23) Overlapping the left-hand and right-hand thread results in a typically deltoid-shaped pattern on the shaft 32.

(24) As also described above, the threaded spindle 3 permits an energy-economizing process, since the electric motor does not have to be slowed down and restarted. In addition, the position of the reamer 12 does not have to be measured, thereby eliminating the need for the position measuring device 6. The cleaning process of the heat exchanger 13 is shortened yet again by eliminating the directional reversal.

REFERENCE LIST

(25) 1 Outer cylindrical tube, second cylindrical tube 2 Cooling coil, first cylindrical tube 3, 3 Threaded spindle 4 Drive motor 5 Axial/radial bearing 6 Position measuring device 7 Condensate reservoir, deposit store 8 Plain bearing bushing 9 Heating element 10 Clamping device 11 Particle barrier 12 Reamer, cleaning element 13 Heat exchanger 14 Working medium inlet opening 15 Working medium outlet opening 16 Coolant inlet opening 17 Coolant outlet opening 18 Condensate drain 21 Coil 22 Guiding groove, profile groove 23 Channel 121 Female thread of cleaning element 122 Outer groove 123 Recess, milled groove 31 Shaft of threaded spindle 3 32 Sliding block