Background
The heat exchanger is widely applied to industries such as chemical industry, petroleum industry, refrigeration industry, nuclear energy industry and power industry, and due to the worldwide energy crisis, the demand of the heat exchanger in industrial production is more and more, and the quality requirement of the heat exchanger is higher and more. In recent decades, although compact heat exchangers (plate type, plate fin type, pressure welded plate type, etc.), heat pipe type heat exchangers, direct contact type heat exchangers, etc. have been rapidly developed, because the shell and tube type heat exchangers have high reliability and wide adaptability, they still occupy the domination of yield and usage, and according to relevant statistics, the usage of the shell and tube type heat exchangers in the current industrial devices still accounts for about 70% of the usage of all heat exchangers.
The supercritical carbon dioxide thermodynamic cycle technology is a novel thermodynamic cycle technology formed by using supercritical carbon dioxide as a working medium, has the advantages of high efficiency, small occupied area, simple system and the like in a medium-high heat source temperature range compared with the current steam Rankine cycle technology, is a novel power generation technology capable of replacing the current steam Rankine cycle on a large scale in the future, and has wide application prospects.
The outlet temperature of the heat source of the supercritical carbon dioxide thermodynamic system exceeds 500 ℃, and even reaches 700 ℃ in certain design requirements. In the temperature regulation process of the load control of the thermodynamic system, low-temperature carbon dioxide and high-temperature carbon dioxide are required to be mixed, so that the aim of reducing the temperature of the carbon dioxide working medium is fulfilled. Because the temperature of the carbon dioxide is very high, the temperature is not easy to be too high to realize the rapid adjustment of the temperature; therefore, it is necessary to adjust the temperature under a large temperature difference. Based on the above background requirements, the invention provides a built-in supercritical carbon dioxide large-temperature-difference mixed heat exchanger.
Disclosure of Invention
The invention aims to provide a built-in supercritical carbon dioxide large-temperature-difference mixer and a control and adjustment method, which solve the technical problem of how to realize temperature adjustment under the condition of large temperature difference, reduce thermal expansion stress and thermal fatigue and improve the reliability of a temperature control process.
A large-temperature-difference hybrid heat exchanger with built-in supercritical carbon dioxide comprises a hot pipeline, a divergent pipe, a mixing pipe and a convergent pipe which are sequentially connected, wherein a heat adjusting valve is arranged between the hot pipeline and the divergent pipe, a cold pipeline is connected to the outer side of the mixing pipe, a spiral pipe is arranged in the mixing pipe, the cold pipeline penetrating through the mixing pipe is connected with the spiral pipe, a cold adjusting valve is arranged on the cold pipeline, and the cold pipeline is connected with the mixing pipe through a flange.
The reducing pipe and the reducing pipe are used for connecting the mixer and the peripheral pipeline, so that the connection between the peripheral pipeline and the mixer is smoothly transited, and the resistance and the pressure drop loss are reduced; the internal flow area of the mixing tube is comparable to the internal flow area of the hot line to reduce additional form resistance pressure drop due to flow area changes.
The connecting point of the cold pipeline and the mixing pipe is connected by a flange, sealed by a filler and then compressed by a flange cover. The connection mode can allow the cold and hot pipelines to generate certain sliding under the action of thermal expansion under the condition of large temperature difference, and reduce local thermal stress. Under the long-term operation condition, the accumulated damage caused by thermal fatigue can be reduced, and the service life of the equipment can be prolonged.
The flow area inside the mixing tube is comparable to the flow area inside the hot line.
To reduce the additional pressure drop caused by the change of the flow area.
And a filler for sealing is arranged in the flange.
The connection mode can allow the cold and hot pipelines to generate certain sliding under the action of thermal expansion under the condition of large temperature difference, so that the local thermal stress is reduced; under the long-term operation condition, the accumulated damage caused by thermal fatigue can be reduced, and the service life of the equipment can be prolonged.
The spiral pipe is a three-section periodic spiral, a plurality of small holes are formed in the pipe wall along the way, the sizes of the small holes are distributed in three sections in different spiral sections, and the spiral pipe comprises an I section, an II section and an III section.
From the stage I to the stage III, the sizes of the small holes are gradually increased, and the number of the small holes is gradually increased.
The spiral pipe is made of stainless steel with high linear expansion coefficient not less than 14 multiplied by 10-6℃-1。
The length of the high linear expansion coefficient material may vary with temperature.
The spiral pipe has certain temperature control capability, and when the temperature is higher than the set temperature, the spiral pipe expands to enhance heat transfer exchange of cold and hot fluids, thereby being beneficial to inhibiting temperature rise; otherwise, the temperature drop is suppressed.
The invention achieves the following remarkable effects: the invention provides a built-in supercritical carbon dioxide large-temperature-difference hybrid heat exchanger which can improve the heat exchange efficiency, is beneficial to realizing temperature adjustment under the condition of large temperature difference, reduces thermal expansion stress and thermal fatigue and improves the reliability of the temperature control process.
Detailed Description
In order to clearly illustrate the technical features of the present solution, the present solution is described below by way of specific embodiments.
Referring to fig. 1, the large temperature difference mixer with the built-in supercritical carbon dioxide mainly comprises a hot pipeline 1, a hot regulating valve 2, a divergent pipe 3, a mixing pipe 4, a convergent pipe 5, a spiral pipe 6, a cold pipeline 7, a cold regulating valve 8, a flange 9 and a mixing pipe outlet 10, and the system is shown in fig. 1.
The hot pipeline 1 is connected with a high-temperature supercritical carbon dioxide pipeline, and the upstream temperature exceeds 500 ℃; the thermal regulating valve 2 is connected to the thermal line 1, has the functions of stopping and regulating, and can control the flow on the thermal line 1 and the pressure behind the valve.
The divergent pipe 3 is used for connecting the heat pipeline 1 and the mixing pipe 4, so that the connection of the heat pipeline 1 to the mixing pipe 4 realizes smooth transition, and the resistance and the pressure drop loss are reduced. The mixing tube 4 is the main part of the mixer, and the inner diameter of the mixing tube 4 is larger than the inner diameter of the hot line 1 in order to accommodate the internal structure. Overall, the internal flow area of the mixing tube 4 is comparable to the internal flow area of the hot line 1 to reduce the additional pressure drop due to the change in flow area.
The reducer 5 is used for connecting the mixing pipe 4 and the mixing pipe outlet 10, so that the connection of the mixing pipe 4 to the mixing pipe outlet 10 is smoothly transited, and the resistance and the pressure drop loss are reduced. The cold line 7 is connected with high-pressure low-temperature carbon dioxide, and the injection pressure of the cold line is slightly greater than that of the hot line 1. The cold regulating valve 8 is connected to the cold pipeline 7 and has the functions of stopping and regulating, and can control the flow on the cold pipeline 7 and the pressure behind the valve. The flange 9 is the connecting position of the cold pipeline 7 and the mixing pipe 4, and the cold pipeline 7 is positioned at the inner side by adopting flange connection, sealed by using a filler and then compressed by using a flange cover.
The connection mode is static seal connection, and under the condition of large temperature difference, the cold and hot pipelines can be allowed to generate certain sliding under the action of thermal expansion, so that the local thermal stress is reduced. Under the long-term operation condition, the accumulated damage caused by thermal fatigue can be reduced, and the service life of the equipment can be prolonged. The flange is of a detachable structure and can be periodically maintained and replaced during overhaul.
Referring to fig. 2, 3 and 4, the spiral pipe 6 is positioned inside the mixing pipe 4, the spiral pipe 6 is a three-section periodic spiral, a certain number of small holes are formed in the pipe wall along the way, and the sizes of the small holes are distributed in different spiral sections with certain differences. In the I section 11 spiral tube, the small holes are smaller in size and fewer in number. In the section, because the temperature difference of the cold and hot fluid is the largest, the heat exchange area of the cold and hot fluid can be reasonably controlled by adopting the mode, and the phenomenon that the temperature is reduced too obviously due to too fast mixing is avoided. In section II 12, the size of the pores is constant and the number increases. In this section, since the temperature difference between the cold and hot fluids is significantly reduced compared to the section I11, the heat exchange area of the cold and hot fluids can be increased appropriately in this way. In section III 13, the size and number of apertures increases. In this section, since the temperature difference between the cold and hot fluids is already small, better mixing is achieved by increasing the heat exchange area of the cold and hot fluids.
The spiral pipe 6 adopts a coiling mode, and stainless steel with high linear expansion coefficient is selected as a material. The structure has certain temperature self-regulating capability. When the temperature of the spiral pipe 6 is higher than the set temperature, under the action of the temperature difference, the spiral pipe 6 with high linear expansion coefficient extends in the length direction, the on-way flowing distance of the fluid in the pipe is increased, the heat exchange between the high-temperature fluid and the low-temperature fluid is enhanced, and the effect of inhibiting the temperature rise of the spiral pipe 6 is further achieved; on the contrary, when the temperature of the spiral pipe 6 is lower than the set temperature, the spiral pipe 6 with high linear expansion coefficient can be shortened in the length direction, the on-way flowing distance of the fluid in the pipe is reduced, the heat exchange between the high-temperature fluid and the low-temperature fluid is weakened, and the effect of restraining the temperature reduction of the spiral pipe 6 is achieved.
As a modification, the spiral tube 6 is divided into a plurality of stages, and the linear expansion coefficient of the spiral tube 6 is gradually increased along the flow direction of the high-temperature supercritical carbon dioxide. Mainly along the flow direction of high temperature supercritical carbon dioxide, the difference in temperature is littleer and littleer, and heat transfer capacity is also littleer and more, consequently the nature needs increase heat transfer area and improves heat exchange efficiency, consequently does not need high linear expansion coefficient. Therefore, through the arrangement, on the one hand, the cost can be saved, meanwhile, the heat exchange is uniform on the whole, a heat exchange effect similar to countercurrent is formed, and meanwhile, the extension lengths of the sections are basically the same.
As an improvement, the linear expansion coefficient of the spiral tube 6 gradually increases by a larger and larger extent along the flow direction of the high-temperature supercritical carbon dioxide. The heat exchange efficiency can be further improved by the arrangement, which is in accordance with the temperature difference change rule and is the result of a great deal of research of the applicant.
The using process of the invention is as follows: the hot pipeline 1 is connected with a high-temperature supercritical carbon dioxide pipeline, and the cold pipeline 7 is connected with a high-pressure low-temperature supercritical carbon dioxide pipeline. The flow of the hot pipeline 1 is controlled by the hot adjusting valve 2, and the flow of the cold pipeline 7 is controlled by the cold adjusting valve 8, so that the flows on the cold and hot pipelines are matched with each other. The low-temperature carbon dioxide flowing in from the cold line 7 passes through the spiral pipe 6, and gradually realizes heat exchange with the high-temperature carbon dioxide from the hot line 1, and finally realizes complete mixing when the temperature difference is small. The fluid temperature at the outlet 10 of the mixing tube is controlled by the flow and temperature of the hot line 1 and the flow and temperature of the cold line 7.
The temperature regulation comprises the following specific steps:
(1) setting the temperature value of the fluid at the outlet of the mixing pipe 4 through a temperature controller;
(2) measuring the fluid pressure at the outlet of the mixing pipe 4 by using a fluid pressure gauge, measuring the fluid temperature at the outlet of the mixing pipe 4 by using a temperature sensor, and calculating to obtain the enthalpy of the fluid at the outlet of the mixing pipe 4 according to the measured temperature value and pressure value;
meanwhile, the fluid temperature of the hot pipeline 1 and the fluid temperature of the cold pipeline 7 are respectively measured by using thermocouples, and the fluid enthalpy of the hot pipeline 1 and the fluid enthalpy of the cold pipeline 7 are respectively obtained through calculation;
the flow and temperature of the heating line 1 are mi1And Ti1The flow and temperature of the cold line 7 are m respectivelyi2And Ti2The flow, temperature and pressure at the outlet 10 of the mixing tube 4 are m, respectively0、T0And P0. According to the physical property method of the carbon dioxide, the enthalpy of the fluid in the hot pipeline 1, the enthalpy of the fluid in the cold pipeline 7 and the enthalpy of the fluid in the outlet 10 of the mixing pipe are respectively hi1、hi2And h0。
hi1=h(p0,Ti1) (1)
hi2=h(p0,Ti2) (2)
ho=h(p0,To) (3)
According to the energy balance equation and the mass balance equation:
m0ho=mi1hi1+mi2hi2 (4)
m0=mi1+mi2 (5)
it follows that the flow ratio of the hot line 1 and the cold line 7 is:
the flow rate ratio is related to the length L of the spiral pipe, and the opening degrees of the hot adjustment valve 2 and the cold adjustment valve 8. When the temperature of the coil 6 is higher than the set temperature, the length L of the coil 6 is increased, the flow rate ratio is decreased, that is, the flow rate of the cold line 7 is increased, and the temperature rise of the coil 6 is suppressed. On the basis, the valve opening degree of each pipeline is adjusted by the hot adjusting valve 2 and the cold adjusting valve 8, so that the proportion of the hot pipeline 1 and the cold pipeline 7 meets the formula (6), and the preset outlet temperature can be achieved in a stable state. The control logic is shown in fig. 5.
And finally, according to the flow proportion of the cold pipe and the hot pipe, the opening degree of the cold pipe and the hot pipe valve is calculated by utilizing a valve opening degree curve and combining the pressure drop of the inlet and the outlet of the valve.
The optimal length value of the spiral pipe 6 is obtained by utilizing a comparison table of the flow rate ratio of the cold pipe and the hot pipe and the length of the spiral pipe 6, and the comparison table is obtained after a large number of tests and is not described in detail herein.
The technical features of the present invention which are not described in the above embodiments may be implemented by or using the prior art, and are not described herein again, of course, the above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and variations, modifications, additions or substitutions which may be made by those skilled in the art within the spirit and scope of the present invention should also fall within the protection scope of the present invention.