Novel CO2 plate heat exchanger

Abstract

The invention discloses a novel structural form brazed plate heat exchanger for a transcritical CO2 heat pump water heater, which adopts an integrated brazed structure to replace the traditional sealing gasket design aiming at the requirements of high pressure (8-12MPa), supercritical state physical property mutation, pseudo-critical zone specific heat capacity peak value and high flow rate of CO2 transcritical cycle. The heat exchange efficiency of an existing herringbone corrugated plate type heat exchanger is improved by optimizing the corrugated form of the plate sheets and the sectional layout of the flow channels to adapt to the CO2 characteristic. The novel brazed plate type heat exchanger specifically comprises a plate sheet main body, fluid inlet and outlet corner holes and a heat exchange area are formed in the plate sheet main body, and high-pressure corrosion resistant metal plate sheets form a leakage-free plate bundle through braze welding. The surface of the plate sheet is provided with a sectional corrugated structure (the inclination angle of a high-temperature section is 40 degrees, the inclination angle of a medium-temperature section is 50 degrees, the inclination angle of a low-temperature section is 60 degrees, and a hollow turbulent flow column is additionally arranged), and each temperature section is matched with the physical property change in the CO2 supercritical condensation process; the reinforcing ribs and the brazing grooves are arranged on the edges of the sheets, so that the overall compression strength is improved. The structure solves the problems that a traditional heat exchanger is insufficient in pressure resistance, and the heat exchange efficiency is not matched with the CO2 physical property, and is suitable for a transcritical CO2 heat pump system.

Description

Technical Field

[0001] This invention relates to the technical field of heat exchange equipment for heat pump water heaters, and in particular to a new structural form of brazed plate heat exchanger adapted to the characteristics of transcritical CO2 working fluid, suitable for CO2 transcritical heat pump water heater circulation systems. Background Technology

[0002] With increasingly stringent environmental regulations and an urgent need for high-efficiency and energy-saving technologies, transcritical CO2 heat pump water heaters have become a core alternative to traditional Freon systems due to the environmental friendliness of CO2 (ODP=0, GWP=1). They offer significant advantages in producing high-temperature hot water, exhibiting high outlet water temperature and excellent co-operating efficiency (COP) under specific operating conditions. However, the characteristics of their working fluid place stringent requirements on the heat exchanger. High-pressure characteristics: In the transcritical CO2 cycle, the working pressure of the gas cooler reaches 8-12 MPa. The rubber gaskets of traditional plate heat exchangers are prone to aging and leakage, while ordinary brazed plate heat exchangers are prone to plate bundle deformation due to insufficient plate strength or unreasonable flow channel design under high pressure; Supercritical property mutation: In the supercritical state, the specific heat capacity of CO2 (c p A peak value appears near the quasi-critical point, followed by a sharp decline. Traditional constant-angle corrugated channels cannot match the strong heat transfer requirements in this region, especially in areas with drastic changes in physical properties, which can easily lead to local heat transfer deterioration and low local heat transfer efficiency. High flow rate and low resistance balance: CO2 has a low thermal conductivity, requiring high flow rates to enhance heat transfer, but excessively high flow rates will lead to a surge in pressure loss. The resistance characteristics of traditional channels do not match the flow rate requirements of CO2.

[0003] Existing brazed plate heat exchangers are not optimized for the aforementioned characteristics of CO2, resulting in problems such as insufficient pressure margin, mismatch between heat exchange efficiency and CO2 properties, and excessively high local pressure loss, which limit their application in transcritical CO2 heat pump water heater systems. There is an urgent need in this field for a brazed plate heat exchanger with in-depth optimization design for the properties of transcritical CO2 working fluid to solve the above problems and fully tap the performance potential of CO2 heat pump systems. Summary of the Invention

[0004] In response to the high pressure, supercritical property abrupt changes, high flow rate and low resistance requirements and potential corrosivity of transcritical CO2, this invention discloses a brazed plate heat exchanger with a new structural form. The pressure resistance is improved by integrated brazing seal, and the segmented flow channel design is matched to the changes in CO2 properties.

[0005] This new type of brazed plate heat exchanger is formed by integral brazing of multiple metal plates, eliminating the need for traditional gaskets and avoiding leakage risks under high pressure. The plates are made of 316L stainless steel. The segmented flow channel design means that the CO2 flow side (condensation side) is divided into three sections along the medium flow direction, corresponding to the CO2 temperature variation range: High-temperature range (CO2 temperature range: 80-100℃, supercritical high-temperature state): Corrugation tilt angle 40°. In this range, CO2 properties are far from the critical point, with lower density, higher viscosity, and relatively lower heat transfer coefficient. Steep corrugations can strongly disrupt the boundary layer, effectively enhancing the heat transfer of CO2 gas properties and overcoming the adverse effects of its low thermal conductivity and high viscosity. The lower tilt angle can reduce resistance at high flow rates, adapting to the lower specific heat capacity requirement of CO2 at this time. Mid-temperature range (CO2 temperature range: 50-80℃, supercritical high temperature state and pseudocritical high specific heat capacity region): corrugation tilt angle 50° to increase turbulence intensity and match the strong heat transfer demand of increased CO2 specific heat capacity; Low-temperature section (CO2 temperature range: 30-50℃, peak specific heat capacity region and low-temperature supercritical state): A turbulence column is added to the existing 60° corrugated inclination angle. In this section, the isobaric specific heat capacity of CO2 reaches its peak, exhibiting extremely strong heat transfer capacity, but the viscosity changes significantly, leading to a substantial increase in flow resistance. By using a high inclination angle and modifying the structure to enhance turbulence, the influence of CO2 property changes during cooling on heat transfer is addressed, matching the strong heat transfer capacity in the peak specific heat capacity region while controlling pressure loss.

[0006] Brazing and frame structure: Copper-based brazing alloy is used for overall brazing to ensure seamless connection between plates and withstand pressure of over 15MPa. Attached Figure Description

[0007] Figures 1 to 4 are schematic diagrams of the internal plate structure of the novel plate heat exchanger applicable to transcritical CO2 heat pump water heater systems according to the present invention. Figure 1 shows a plate heat exchanger plate with a corrugation angle of 40°, Figure 2 shows a plate heat exchanger plate with a corrugation angle of 50°, and Figure 3 shows a plate heat exchanger plate with a corrugation angle of 60° and equipped with baffles. Figure 4 This is a schematic diagram of the plate arrangement. In the diagram: 1—CO2 fluid inlet hole, 2—heat exchange zone, 3—CO2 fluid outlet hole, 4—cooling water outlet hole, 5—cooling water inlet hole, the fluid inlet and outlet holes of the plates shown in Figures 1 to 4 are arranged the same, 6—corrugation angle is 40°, 7—corrugation angle is 50°, 8—corrugation angle is 60°, 9—hollow turbulence column. Detailed Implementation

[0008] To further disclose the invention's content, features, and functions, a detailed description is provided below in conjunction with the accompanying drawings.

[0009] The following description and figures fully illustrate the specific embodiments described herein to enable those skilled in the art to practice them. The novel brazed plate heat exchanger of this invention is constructed by stacking and brazing multiple 316L stainless steel plates using a vacuum brazing process. Specific corrugated structures are pressed onto the surface of the plates in each temperature range to form flow channels. The plates in the high-temperature and medium-temperature ranges are stacked by rotating them 180°, while the plates in the low-temperature range are stacked in the same direction, thus forming an overall interwoven refrigerant and water channel. Four interface flanges are brazed onto the exterior of the heat exchanger body, serving as the refrigerant CO2 inlet (1), refrigerant CO2 outlet (3), cooling water inlet (5), and cooling water outlet (4), respectively. Its core innovation lies in: based on the expected changes in the physical properties of CO2 in the flow direction within the heat exchanger, combined with… Figure 1-3 The corrugated structure on the heat exchange plate is divided into three regions with different geometric characteristics, and the geometric parameters and structure of the heat exchange plate in each region are dynamically optimized based on the physical properties of CO2.

[0010] Region 1: Supercritical high-temperature region, corresponding to the supercritical high-temperature state of CO2 physical properties, such as... Figure 1 As shown, taking CO2 fluid as an example, the fluid flows through the entire plate via inlet 1 and then enters heat exchange zone 2. Here, the corrugation angle of the heat exchange zone is 40°. The heat from the CO2 fluid is transferred to the surface of the metal plate in direct contact with it via convection. The plate is heated, and the heat is rapidly transferred from one side of the heated plate to the other side of the plate via thermal conduction through the solid metal itself. After the temperature on the other side of the plate rises, the cold water in direct contact with the plate absorbs heat from the plate surface through convection, thus raising its own temperature. The core function of this heat exchange zone is to reduce the flow resistance in the high-velocity zone.

[0011] Region 2: Supercritical mesotemperature region and pseudocritical high specific heat capacity region, such as Figure 2 As shown, the corrugation angle is 50°. The core function of this heat exchange zone is to reduce the flow pressure drop, reduce compressor power consumption, and improve equipment safety by increasing the corrugation angle.

[0012] Region 3: High-efficiency zone with peak specific heat capacity, i.e., the outlet zone of the plate heat exchanger, such as... Figure 3 As shown, in this section, the specific heat capacity of CO2 at constant pressure reaches its peak. The corrugation angle is set to 60°, and the height of the hollow turbulence column is the same as the corrugation depth, which enhances the degree of fluid turbulence and gives full play to the advantage of high specific heat capacity. The core function of this heat exchange zone is to enhance heat exchange.

Claims

1. A novel brazed plate heat exchanger structure suitable for transcritical CO2 heat pump water heaters, characterized in that: It includes a plate group consisting of multiple heat exchange plates stacked and brazed together. The plate group forms a refrigerant CO2 channel and a water channel that are isolated from each other and exchange heat through the plates. The heat exchanger has a refrigerant CO2 inlet, a refrigerant CO2 outlet, a cooling water inlet, and a cooling water outlet. The metal plates are made of a high-pressure corrosion resistant material. The plate surface is provided with a segmented corrugated structure distributed along the CO2 flow direction. The segmented corrugated structure divides the CO2 flow channel into: a high-temperature section (CO2 inlet region, 80-100℃): the corrugation inclination angle is 40° to meet the turbulence enhancement requirements in the low specific heat capacity region; a medium-temperature section (pseudo-critical region, 50-80℃): the corrugation inclination angle is 50°; and a low-temperature section (high-efficiency heat exchange region, 30-50℃): the corrugation inclination angle is 60° and hollow turbulence columns are added to enhance fluid turbulence and mitigate the local heat transfer deterioration caused by drastic changes in the physical properties of CO2 working fluid during cooling. While enhancing heat exchange, it also meets the resistance balance requirements after the specific heat capacity decreases. The plates are integrated and sealed by a brazing layer to form a leak-free flow channel that can withstand high pressure.

2. The brazed plate heat exchanger according to claim 1, characterized in that, Different corrugated tilt angles were set for CO2 working fluids at different temperature ranges to match the changes in the physical properties of CO2 working fluids.

3. The brazed plate heat exchanger according to claim 1, characterized in that, Hollow turbulence columns of the same height as the corrugation depth are added to the plates in the low-temperature section to enhance the turbulence intensity of the CO2 working fluid on the plates in this area, thereby enhancing the heat transfer effect.

4. The brazed plate heat exchanger according to claim 1, characterized in that, The high-pressure corrosion resistant material is 316L stainless steel to resist potential acid corrosion from CO2 under high pressure.

5. The brazed plate heat exchanger according to claim 1, characterized in that, The heat exchange area in the low-temperature section accounts for a large proportion of the total heat exchange area of ​​the plate heat exchanger, corresponding to the strong heat exchange demand in the CO2 critical temperature zone.

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