In recent years, the plate heat exchanger technology has become more sophisticated, high heat transfer efficiency, small size, light weight, low coefficient of dirt, demolition, plate varieties, wide range of applications has been widely used in the heating industry. The plate heat exchanger assembly methods are divided into removable, welded, brazed plate-shell. Easy to disassemble and clean the detachable plate heat exchanger, the increase or decrease the area of ？？flexible heat exchanger, use more heating project. The detachable plate heat exchanger rubber gasket, heat-resistant temperature limit, applicable to water-water heat transfer. To improve the detachable plate heat exchanger performance optimization design.
Improve the performance of plate heat exchanger is a comprehensive economic benefits, should be determined by technical and economic comparison. The heat exchanger heat transfer efficiency and reduce the resistance of the heat exchanger should also be considered, and should be a reasonable choice of plate material and rubber gasket material and installation methods to ensure the safe operation of equipment, extended equipment life.
Two optimization design method
2.1 improve the efficiency of heat transfer
The plate heat exchanger is to ask the wall heat transfer heat exchanger, hot and cold fluid through the heat exchanger plate heat transfer fluid and the plate in direct contact heat transfer for heat conduction and convection heat transfer. The key is to improve heat transfer efficiency of the plate heat exchanger to improve the heat transfer coefficient and the logarithmic mean temperature difference.
① improve the heat exchanger heat transfer coefficient only while increasing the surface heat transfer coefficient of the plate of hot and cold on both sides, reducing the thermal resistance of fouling layer, selection of high thermal conductivity of the plate, reducing the thickness of the plate, in order to effectively improve the exchange Thermal heat transfer coefficient.
a. The surface of the plate heat transfer coefficient
Corrugated plate heat exchanger enables fluid turbulence (Reynolds number 150) in a smaller flow rate, so they can obtain a higher surface heat transfer coefficient, surface heat transfer coefficient and the geometric structure of the corrugated plate and The flow of the medium state. The waveform of the plate, including herringbone, straight-shaped, spherical. After years of research and experiment found that the corrugated cross sectional shape as a triangle (sine-shaped surface heat transfer coefficient, pressure drop smaller compression stress distribution, processing difficult-shaped plate has a high surface heat transfer coefficient , and the ripple of the angle the greater the medium velocity in the flow channel between the board the higher the greater the surface heat transfer coefficient.
and b. Reduce the dirt layer thermal resistance
Reduce heat exchanger fouling layer thermal resistance, the key is to prevent the fouling of the plate. Plate fouling thickness of 1 mm, the heat transfer coefficient is reduced by about 10%. Therefore, we must pay attention to monitoring the water quality of the heat exchanger hot and cold on both sides, to prevent the fouling of the plate, and to prevent debris in the water attached to the plate. Some heating units in order to prevent the Pirates of the water and steel corrosion in the heating medium, add the Pharmacy must pay attention to water quality and stickiness of pharmaceutical caused by debris contamination heat exchanger plates. If the water viscosity debris, using a dedicated filter for processing. Selection of Pharmacy, should choose the non-sticky Pharmacy.
(c) Selection of high thermal conductivity of the plate
The plate material to select the austenitic stainless steel, titanium, copper alloy. Stainless steel thermal conductivity, thermal conductivity of about 14.4 the W / (m · K), high strength, stamping performance, less susceptible to oxidation, the price is lower than the titanium alloy and copper alloy heating project in the most used, but its poor corrosion resistance to chloride ion.
(d) Reduce the thickness of the plate
The design thickness of the plate and its corrosion resistance has nothing to do, with the bearing capacity of the heat exchanger. Thicker plate can improve the ability of the pressure of the heat exchanger. Using the combinations of chevron plates, adjacent plates to each other upside down, the ripples touch with each other, forming a high density and uniform distribution fulcrum plate angle larvae L and the edge of the seal structure has been gradually improving, the heat exchanger has a good bearing capacity. Made detachable plate heat exchanger maximum pressure capacity has reached 2.5 MPa. Plate thickness on the heat transfer coefficient, the thickness is reduced to 0.1mm, symmetrical plate heat exchanger overall heat transfer coefficient increased by about 600W / (m · K), the asymmetrical increase of about 500 W / (m · K ) ¨. Meet the premise of the heat exchanger pressure capacity, we should try to use a smaller plate thickness.
② to improve the logarithmic mean temperature difference
The flow pattern of the plate heat exchanger countercurrent downstream and mixed-flow type (both countercurrent and downstream). In the same condition when the counter-current logarithmic mean temperature difference between the largest downstream minimum, mixed flow pattern between the two asked. Improve the heat exchanger logarithmic mean temperature difference method to maximize the use of countercurrent or close mixed countercurrent flow pattern, as much as possible to increase the temperature of the hot side fluid, reduce the temperature of the cold-side fluid.
③ import and export of pipe location
For single-flow arrangement of the plate heat exchanger for easy maintenance fluid import and export of pipe should be arranged in the side of the fixed end plate of the heat exchanger. Media the greater the temperature difference, the stronger the natural convection of the fluid, the formation of retention with the impact of the more obvious, so the media import and export location shall be under the progress of the hot fluid out of cold fluid into the layout to reduce the impact of stranded with improve heat transfer efficiency.
2.2 to reduce heat transfer resistance
Improve the board asked the average velocity of flow channel media can improve the heat transfer coefficient and reduce the heat exchanger area. And the increase of flow rate, will increase the resistance of the heat exchanger, to improve the circulation pump's power consumption and equipment cost. Circulating pump power is proportional to the third power of the medium flow rate by increasing the flow rate slightly higher heat transfer coefficient is not economic. When hot and cold media flow is relatively large, the following methods can be used to reduce the resistance of the heat exchanger, and higher heat transfer coefficient.
① using thermal mixing board
Thermal mixing board both sides of the plate corrugation geometric structure is the same plate according to the angle of the chevron corrugated divided into hard board (H) and soft board (L), angle (usually 120 or so) is greater than 90. Hard board, angle (usually 70 or so) less than 90. FPC. High surface heat transfer coefficient of thermal mixing board hard board, fluid resistance, the FPC is the opposite. The combination of hard board and soft board may be composed of high (HH), (HL), low (LL) 3 kinds of features of the flow channel, to meet the needs of different working conditions.
Hot and cold media flow is relatively large, the heat can reduce the mixing board than the symmetric single-flow heat exchanger plate area. The thermal mixing board hot and cold on both sides of the corner hole diameter is usually equal to the hot and cold media flow ratio is too large, the angle of the side of the cold medium [J] pressure loss. In addition, the thermal mixing board design technology is difficult to achieve an exact match, and often resulted in savings of plate area is limited. Therefore, the hot and cold media flow ratio is too large should not be used thermal mixing board.
2 non-symmetric plate heat exchanger
Symmetric plate heat exchanger, which formed from the same plates of both sides of the plate corrugation geometric structure of the flow of cold hot runner cross-sectional area equal to the plate heat exchanger. Asymmetric type (ranging from cross-sectional area of ？？type) plate heat exchanger according to the hot and cold fluid heat transfer characteristics and pressure drop requirements, changing the geometric structure of both sides of the plate waveform, the formation of the flow of cold hot runner cross-sectional area ranging from the plate heat exchanger, wide flow channel side of the corner of larvae L larger diameter. Asymmetric plate heat exchanger heat transfer coefficient decreased small, and the pressure drop significantly reduce. Hot and cold media flow is relatively large, asymmetric single-process than the use of symmetrical single-flow heat exchanger can reduce the plate area of ？？15% 3O%.
③ The combination of multi-process
When hot and cold media flow when the combination of multi-process layout, using one side of the small flow more process, in order to improve the flow, higher heat transfer coefficient. One side of the large flow with fewer processes, to reduce heat transfer resistance. The combination of multi-process mixed-flow type, the average temperature difference was slightly lower. The fixed end plate of the plate heat exchanger using a combination of multi-process and activities of the end plates were taken over by the heavy workload of maintenance.
The ④ based heat exchanger bypass pipe
The bypass pipe when the hot and cold media flow can be asked of one side of the large flow of import and export of heat exchanger located, reducing the heat exchanger flow and reduce drag. In order to facilitate adjustment, the control valve should be installed on the bypass pipe. The approach should be adopted to counter flow cold medium the higher the temperature of the heat exchanger to ensure that the cold medium temperature after the confluence of the heat exchanger outlet to meet the design requirements. Set the heat exchanger bypass pipe heat exchanger has a higher heat transfer coefficient, reducing the resistance of heat exchanger, but the adjustment is a little complicated.
The choice of form of the ⑤ plate heat exchanger
Tract medium average flow rate of 0.3 ~ 0.6m / s is appropriate, the resistance is not greater than 100 kPa is appropriate flow between the heat exchanger plates. Can refer to the table different hot and cold media flow, a choice of different forms of plate heat exchanger, the table of non-symmetrical flow channel cross-sectional area of ？？plate heat exchanger 2. Symmetric or asymmetric type, single-process or multi-flow plate heat exchanger, heat exchanger may be configured to bypass pipe, but shall be subject to detailed thermodynamic calculation.
2.3 rubber gasket material and installation
① the choice of material
Water-water heat exchanger, the hot and cold media on the rubber gasket no corrosion. The selection of rubber gasket material the key is temperature and sealing, rubber gasket material according to the literature selection.
② The mode selection
The rubber gasket common installation method for the bonding type, snap. Adhesive type heat exchanger assembly, rubber gasket glue adhesive plate seal tank. The snap-buckle structure, the use of rubber gaskets and the edge of the plate heat exchanger assembly, the rubber gasket to the plate sealed tank. The snap-in installation work is very small, and the demolition of the heat exchanger rubber gasket damage is low and there is no glue may contain chloride ions cause corrosion of the plates, so use more.
2.4 a reasonable selection of plate material
Stainless steel plate may produce corrosion failure in a little corrosion, crevice corrosion, stress corrosion, intergranular corrosion, uniform corrosion, a higher incidence of stress corrosion. Stress corrosion caused by chloride ion up to the plate material can be a reasonable selection of the mass concentration of chloride ions in the medium.