CN111811315A - A heat exchange tube and heat exchanger of a high-efficiency liquid nitrogen heat exchanger - Google Patents

Abstract

The invention relates to a heat exchange tube of a high-efficiency liquid nitrogen heat exchanger and the heat exchanger, wherein the outer surface of a heat exchange tube body is provided with a hydrophobic layer, and the inner surface of the heat exchange tube body is provided with a thin heat resistance layer. The invention has the following advantages and effects: the heat exchange effect is good, quick cooling can be realized, the use efficiency of the liquid nitrogen is high, and a large amount of cold energy of the cold nitrogen after the liquid nitrogen is vaporized can be recycled.

Description

A heat exchange tube and heat exchanger of a high-efficiency liquid nitrogen heat exchanger

technical field

The invention relates to the technical field of heat exchangers, in particular to a heat exchange tube and a heat exchanger of a high-efficiency liquid nitrogen heat exchanger.

Background technique

The application of cryogenic gases and cryogenic liquids in engineering applications is becoming more and more extensive. In high temperature areas or seasons, the density of the air decreases, which brings many adverse effects on the operation performance of the gas turbine. Therefore, it is often necessary to cool the intake air of the unit. In addition, when the air contains a small amount of low boiling point solvent, such as methanol, etc., it needs to be recovered by condensation during recovery. Common cooling units can achieve a small cooling interval and a long start-up time, and the unit is also relatively large, making it difficult to achieve rapid pre-cooling and lower-temperature cooling.

As the preparation technology of liquid nitrogen becomes more and more mature, the source is wide and the cost is low, it is used more and more in engineering. Under normal pressure, the saturation temperature of liquid nitrogen is -196°C, which can absorb a lot of heat during vaporization. The liquid nitrogen phase change process can achieve low-temperature rapid cooling. However, there are two problems in using liquid nitrogen as the cooling medium. Since the temperature of the heat exchange tube is lower than the freezing point of common liquids such as water, frost and ice are easily formed on the surface of the heat exchange tube, resulting in additional thermal resistance that weakens the heat transfer performance. Inside the pipeline, since the characteristic temperature of liquid nitrogen on stainless steel is about -148.15°C, the liquid nitrogen in the pipeline undergoes film boiling for a long period of time, which makes the temperature of the tube wall. The decline is slow, it is difficult to achieve rapid cooling, and it also causes waste of liquid nitrogen.

SUMMARY OF THE INVENTION

The purpose of the present invention is to alleviate the problem of frost on the surface of the heat exchange tube and solve the problem of low boiling heat exchange efficiency inside the heat exchange tube, and to provide a heat exchange tube of a high-efficiency liquid nitrogen heat exchanger, a manufacturing method thereof and a heat exchange tube with the same The heat exchanger of the heat exchange tube to further improve the heat exchange capacity.

The object of the present invention is achieved through the following technical solutions:

A heat exchange tube of a high-efficiency liquid nitrogen heat exchanger comprises a heat exchange tube body, the outer surface of the heat exchange tube body is provided with a hydrophobic layer, and the inner surface of the heat exchange tube body is provided with a thin thermal resistance layer. The invention specially processes the inner and outer surfaces of the heat exchange tube to provide a heat exchange tube with a hydrophobic outer surface and an inner surface of a thin thermal resistance layer. It can significantly improve the internal boiling heat transfer efficiency.

Further, the thin thermal resistance layer is a Teflon coating, and the thickness of the thin thermal resistance layer is in the order of microns.

Further, the thin thermal resistance layer is coated on the inner surface of the heat exchange tube body by the following method:

(1) The inner surface of the heat exchange tube is polished, then ultrasonically cleaned with acetone, absolute ethanol and distilled water to remove oil stains, and dried with nitrogen for use;

(2) Pour Teflon into the heat exchange tube and rotate it until the coating is uniform;

(3) Baking is carried out, and then naturally cooled to room temperature, the preparation of the inner coating is completed.

Further, the baking process described in step (3) is specifically as follows: at a heating rate of 10° C./min, the temperature is first heated to 150° C. and then kept constant for 10 minutes, then heated to 200° C. and kept for 15 minutes, and finally kept at 340° C. Stop after 15 minutes.

Further, the hydrophobic layer is an anodic oxide film.

Further, the hydrophobic layer is coated on the outer surface of the heat exchange tube body by the following method:

(1) The mixed acid solution of sulfuric acid and phosphoric acid is configured to be used for electropolishing, and the electrolyte system composed of ammonium fluoride, water and ethylene glycol is used for anodization;

(2) the outer surface of the heat exchange tube is first mechanically polished and then ultrasonically cleaned with acetone, anhydrous ethanol and distilled water to remove oil stains, and dried with nitrogen for use;

(3) Vertically immerse the heat exchange tube into the acid solution configured in step (1), the heat exchange tube is used as the anode, and the carbon cylinder is used as the cathode, the carbon cylinder is sleeved outside the heat exchange tube, electropolished, and then rinsed with distilled water , blow dry with nitrogen;

(4) Insert the heat exchange tube vertically into the electrolyte system configured in step (1), the heat exchange tube is used as the anode, the carbon cylinder is the cathode, the carbon cylinder is sleeved outside the heat exchange tube, and the surface temperature of the sample is maintained by magnetic stirring Evenly, anodized, rinsed with distilled water, and dried with nitrogen, the anodized film was prepared.

Further, in the mixed acid solution of sulfuric acid and phosphoric acid in step (1), the mass fraction of phosphoric acid is 85%, the mass fraction of sulfuric acid is 98%, the ammonium fluoride in the electrolyte system is 0.125-0.250 mol/L, and the water concentration is 0.05 ~0.80mol/L.

Further, the working conditions of electropolishing in step (3) are: the temperature is kept at 50° C., the current is 0.25A/cm ² , and the polishing time is 20 minutes; the working conditions of anodizing in step (4) are: the temperature is kept at 20℃, oxidation voltage 50V, oxidation time 60min.

A high-efficiency liquid nitrogen heat exchanger, comprising a shell, a tube box flange, a nozzle flange, a heat exchange tube, a convex head, a head tube box, and a movable saddle, and the nozzle flange is arranged on the shell The upper or lower part of the body, the heat exchange tube is arranged inside the shell; the convex head is arranged at the end of the header box, and the header box adopts the box method The flanges are connected to both ends of the shell, and the heat exchange tubes are the above-mentioned heat exchange tubes.

Further, the heat exchange tube and the shell are made of 316L stainless steel;

The nozzle flange on the upper part of the casing is the liquid nitrogen inlet, the nozzle flange on the lower part is the nitrogen outlet, and the liquid nitrogen inlet and the nitrogen outlet are located on the same side of the casing;

A split-range partition is arranged in the middle of the shell for splitting the liquid nitrogen, the purpose is that the nitrogen vaporized in the first pass still has a large cooling capacity, and the shell-side gas can continue to be cooled in the second pass;

The casing is provided with a baffle plate, which is used to baffle the gas entering the casing. The gas enters from the shell side flange, is deflected by the baffle plate, and is discharged from the flange at the other end of the shell side. is the cooled gas;

The heat exchanger is also provided with a vent or a liquid drain for discharging the condensed liquid or excess gas in the shell.

In the actual use of the liquid nitrogen heat exchanger, it is often not necessary to cool the gas to the saturation temperature of liquid nitrogen, but to rapidly cool it above -148.15 °C. The film boiling in the tube and the frost outside the tube make this The process efficiency is lower, and the hydrophobic surface is more difficult to form a thick frost layer than the hydrophilic surface, and the heat transfer efficiency of transition boiling and nucleate boiling in the tube is also much greater than that of film boiling. In order to achieve different heat exchange purposes inside the tube and outside the tube, the present invention separately prepares the Teflon coating and the anodic oxidation hydrophobic film on the inner and outer surfaces of the heat exchange tube.

The heat exchange tube proposed by the present invention can be used for a liquid nitrogen heat exchanger, and the heat exchange tube has a hydrophobic outer surface and an inner surface of a thin thermal resistance layer. The hydrophobicity of the outer surface makes the surface more difficult to frost than the general surface, and the thermal resistance layer on the inner surface shortens the film boiling time and improves the overall heat exchange capacity of the heat exchange tube. Therefore, the heat exchange tube proposed by the present invention has a larger The application prospect of , its advantages are as follows:

Compared with the conventional heat exchange tube, the outer surface of the heat exchange tube can form bead-like condensation due to the hydrophobicity when cooling the gas, and it is difficult for the droplets to form a liquid film on the outer surface, which is easy to fall off under the action of gravity, so it is difficult to frost, It provides a cooling surface for the gas behind and improves the heat exchange capacity of the outer surface; compared with the conventional heat exchange tube, when liquid nitrogen is passed into the inside of the heat exchange tube, the surface temperature drops quickly due to the effect of the internal thermal resistance layer Below the characteristic temperature of Leidenfloss, the internal boiling has advanced from the film boiling stage to the transition and nucleate boiling stage. Because the heat transfer capacity of the transition boiling and nucleate boiling stages is much stronger than that of the film boiling stage, the heat transfer capacity of the inner surface is further improved.

Compared with the prior art, the present invention has the following beneficial effects:

1. A hydrophobic surface is prepared on the outer surface of the heat exchange tube to slow down frost formation and improve the cooling and heat transfer capacity of the outer surface;

2. A thin thermal resistance layer is prepared on the inner surface of the heat exchange tube, which shortens the internal film boiling time and significantly improves the boiling heat transfer capacity of the inner surface;

3. The overall heat exchange capacity of the heat exchange tube is enhanced, and at the same time, it has better durability.

Description of drawings

1 is a structural diagram of a shell and tube heat exchanger of a preferred embodiment of the present invention;

Fig. 2 is the structure schematic diagram of the coating prepared on the inner and outer surfaces of the heat exchange tube according to the preferred embodiment of the present invention;

Fig. 3 is the pipeline prepared as above and the surface cooling comparison diagram of ordinary stainless steel pipeline;

Figure 4 is the test results of 3 repeated experiments.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

As shown in FIG. 1, the shell and tube heat exchanger provided by the present invention includes: a shell 1, a tube box flange 2, a nozzle flange 3, a heat exchange tube 4, a baffle 5, a convex head 6, a sealing The head tube box 7, the movable saddle part 8, the air discharge port/drain port 9, the split-range partition 10, the tube side is connected to liquid nitrogen, and the shell side is connected to the cooling medium. Among them, the inner surface of the heat exchange tube 4 has a thin thermal resistance layer, and the outer surface has a hydrophobic anodic oxide film, as shown in FIG. 2 .

The characteristic of the hydrophobic surface of the heat exchange tube in this embodiment is that the droplets are difficult to spread on the surface, but form bead-like condensation and fall off quickly, and it is not easy to form a thick frost layer; the characteristic of the thin thermal resistance layer on the inner surface is that the surface temperature can be rapidly When the temperature is lowered to below the characteristic temperature of Leidenfloss, the liquid nitrogen can advance from film boiling to transition boiling or nucleate boiling, thereby improving the overall heat exchange capacity of the heat exchange tube.

The outer surface can be prepared by other methods, as long as the hydrophobic layer is formed, and the inner surface can also be made of other materials, as long as the thickness is thin and the thermal conductivity is small.

The invention has good heat exchange effect, can realize rapid cooling, and has high use efficiency of liquid nitrogen, and a large amount of cold energy of cold nitrogen after vaporization of liquid nitrogen can also be recycled.

The heat exchange tube of the present invention can be made of stainless steel, and a thin thermal resistance layer and a hydrophobic surface are respectively prepared on the inner and outer surfaces of the stainless steel tube.

The preparation method of the Teflon coating on the inner surface of the heat exchange tube is as follows:

(1) The surface of the heat exchange tube is polished and then ultrasonically cleaned with acetone, absolute ethanol and distilled water for 15 minutes to remove oil stains, and dried with nitrogen for use;

(2) pour Teflon into the tube and rotate to be evenly coated;

(3) Bake the sample, at a heating rate of 10°C/min, first heat up to 150°C, then keep the temperature constant for 10 minutes, then heat up to 200°C, hold for 15 minutes, and finally stop at 340°C, hold for 15 minutes, and then cool down naturally When it reaches room temperature, the inner coating is prepared.

The preparation method of the anodic oxide film on the outer surface of the heat exchange tube is as follows:

(1) A mixed acid solution of sulfuric acid and phosphoric acid is prepared for electropolishing, wherein the mass fraction of phosphoric acid is 85%, and the mass fraction of sulfuric acid is 98% (Sinopharm Group), and the volume ratio of the two is 2:3. In the electrolyte system, the ammonium fluoride is 0.125-0.250 mol/L, and the water concentration is 0.05-0.80 mol/L. An electrolyte system composed of ammonium fluoride, water and ethylene glycol is prepared for anodization, wherein ammonium fluoride is 0.125-0.250 mol/L, and water concentration is 0.05-0.80 mol/L;

(2) Immerse the heat exchange tube vertically in the electropolishing acid solution, the heat exchange tube is used as the anode, and the carbon cylinder is used as the cathode. The size is 0.25A/cm ² for 20 minutes, then rinsed with distilled water and dried with nitrogen;

(3) Immerse the heat exchange tube vertically into the electrolyte system, the heat exchange tube is used as the anode, the carbon cylinder is the cathode, the carbon cylinder is sleeved outside the heat exchange tube, and the surface temperature of the sample is maintained at about 20 ℃ by magnetic stirring, The voltage was maintained at 50V for 60min, then rinsed with distilled water and blown dry with nitrogen to complete the preparation of the anodic oxide film.

The above-mentioned preparation process of the inner and outer surfaces cannot be reversed. The inner surface thermal resistance layer should be prepared first. The Teflon coating has good chemical stability and insulation, so it can protect the inner surface when the outer surface is anodized.

As shown in Figure 3, the cooling rate of the surface of the stainless steel tube prepared by the present invention is much higher than that of the ordinary stainless steel tube, and the heat exchange capacity is greatly improved; The inlet pressure of liquid nitrogen in each experiment is exactly the same, so there is an inevitable error), and the agreement of the experimental data is within an acceptable range, indicating that it has good durability.

Table 1 shows the contact angle results of the hydrophobic surface. The larger the contact angle, the better the hydrophobicity. It can be seen that the hydrophobic surface of the present invention has better hydrophobicity.

Table 1 Hydrophobic surface contact angle results

group Contact angle/° hydrophobic surface 135～140 polished stainless steel surface 80～84

The foregoing description of the embodiments is provided to facilitate understanding and use of the invention by those of ordinary skill in the art. It will be apparent to those skilled in the art that various modifications to these embodiments can be readily made, and the generic principles described herein can be applied to other embodiments without inventive step. Therefore, the present invention is not limited to the above-mentioned embodiments, and improvements and modifications made by those skilled in the art according to the disclosure of the present invention without departing from the scope of the present invention should all fall within the protection scope of the present invention.

Claims (10)

1. a heat exchange tube of a high-efficiency liquid nitrogen heat exchanger, comprising a heat exchange tube body, characterized in that the outer surface of the heat exchange tube body is provided with a hydrophobic layer, and the inner surface of the heat exchange tube body is provided with a thin heat barrier layer. 2. The heat exchange tube of a high-efficiency liquid nitrogen heat exchanger according to claim 1, wherein the thin thermal resistance layer is a Teflon coating, and the thickness of the thin thermal resistance layer is on the order of microns . 3. The heat exchange tube of a high-efficiency liquid nitrogen heat exchanger according to claim 2, wherein the thin thermal resistance layer is coated on the inner surface of the heat exchange tube body by the following method: (1) The inner surface of the heat exchange tube is polished, then ultrasonically cleaned with acetone, absolute ethanol and distilled water to remove oil stains, and dried with nitrogen for use; (2) Pour Teflon into the heat exchange tube and rotate it until the coating is uniform; (3) Baking is carried out, and then naturally cooled to room temperature, the preparation of the inner coating is completed. 4. the heat exchange tube of a kind of high-efficiency liquid nitrogen heat exchanger according to claim 3, is characterized in that, described in step (3), the baking flow process is specifically: be first warmed up to 10 ℃/min heating rate After 150°C, the temperature was kept constant for 10min, then the temperature was raised to 200°C and then kept for 15min, and finally at 340°C, the temperature was kept for 15min and then stopped. 5 . The heat exchange tube of a high-efficiency liquid nitrogen heat exchanger according to claim 1 , wherein the hydrophobic layer is an anodic oxide film. 6 . 6. The heat exchange tube of a high-efficiency liquid nitrogen heat exchanger according to claim 5, wherein the hydrophobic layer is coated on the outer surface of the heat exchange tube body by the following method: (1) The mixed acid solution of sulfuric acid and phosphoric acid is configured to be used for electropolishing, and the electrolyte system composed of ammonium fluoride, water and ethylene glycol is used for anodization; (2) the outer surface of the heat exchange tube is first mechanically polished and then ultrasonically cleaned with acetone, anhydrous ethanol and distilled water to remove oil stains, and dried with nitrogen for use; (3) Vertically immerse the heat exchange tube into the acid solution configured in step (1), the heat exchange tube is used as the anode, and the carbon cylinder is used as the cathode, the carbon cylinder is sleeved outside the heat exchange tube, electropolished, and then rinsed with distilled water , blow dry with nitrogen; (4) Insert the heat exchange tube vertically into the electrolyte system configured in step (1), the heat exchange tube is used as the anode, the carbon cylinder is the cathode, the carbon cylinder is sleeved outside the heat exchange tube, and the surface temperature of the sample is maintained by magnetic stirring Evenly, anodized, rinsed with distilled water, and dried with nitrogen, the anodized film was prepared. 7. the heat exchange tube of a kind of high-efficiency liquid nitrogen heat exchanger according to claim 6, is characterized in that, in the sulfuric acid and phosphoric acid mixed acid solution in step (1), phosphoric acid mass fraction is 85%, sulfuric acid mass fraction is 98%, the ammonium fluoride in the electrolyte system is 0.125-0.250 mol/L, and the water concentration is 0.05-0.80 mol/L. 8 . The heat exchange tube of a high-efficiency liquid nitrogen heat exchanger according to claim 6 , wherein the working condition of the electropolishing in step (3) is: the temperature is kept at 50° C., and the current is 0.25A/ cm ² , the polishing time is 20 minutes; the anodizing conditions in step (4) are: the temperature is kept at 20° C., the oxidation voltage is 50 V, and the oxidation time is 60 minutes. 9. A high-efficiency liquid nitrogen heat exchanger, comprising a shell (1), a tube box flange (2), a nozzle flange (3), a heat exchange tube (4), a convex head (6), a head A tube box (7), a movable saddle (8), the connecting flange (3) is arranged on the upper or lower part of the casing (1), and the heat exchange tube (4) is arranged on the casing (1). 1); the convex head (6) is arranged at the end of the head box (7), and the head box (7) is connected to the The two ends of the casing (1) are characterized in that the heat exchange tube (4) adopts the heat exchange tube according to any one of claims 1-8. 10. a kind of high-efficiency liquid nitrogen heat exchanger according to claim 9, is characterized in that, The heat exchange tube (4) and the shell (1) are made of 316L stainless steel; The nozzle flange on the upper part of the casing (1) is a liquid nitrogen inlet, and the nozzle flange on the lower part is a nitrogen outlet, and the liquid nitrogen inlet and the nitrogen outlet are located on the same side of the casing; A split-range partition plate (10) is arranged in the middle of the casing (1) for splitting the liquid nitrogen; A baffle plate (5) is arranged in the casing to deflect the gas entering the casing; The heat exchanger is also provided with a vent or liquid drain (9) for discharging the condensed liquid or excess gas in the shell.

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