Talking about the design of plate heat exchanger
1, the advantages of plate heat exchangers
The characteristics of plate heat exchangers are obvious compared with other heat exchangers: plate heat exchangers have high heat transfer coefficient, small footprint, compact structure and easy maintenance. Under the condition of equal heat transfer, the space occupied is only 1/2 to 1/3 of the shell-and-tube heat exchanger. And unlike the shell-and-tube heat exchanger, it is necessary to reserve a large space for the tube bundle to be repaired. The plate heat exchanger only needs to loosen the clamping screw, and can contact the surface of the heat exchanger plate 100% in the original space range, and is convenient to disassemble and facilitate cleaning. Small size, light weight and easy to install in a small space. Through the above analysis and comparison, it can be seen that the plate heat exchanger has many advantages over the shell-and-tube heat exchanger. In addition, the plate heat exchanger has the following advantages.
(1)The temperature difference is small
Because the plate heat exchanger has a high heat transfer coefficient and strong turbulence, after the flow heat transfer in the heat exchanger, the temperature of the primary and secondary hot water of the heat exchanger can be made. The difference is very small, and sometimes the temperature difference can approach 1 ° C ~ 3 ° C. This can greatly increase the thermal efficiency and improve the economics of the heat exchange equipment.
(2) Small heat loss
Because the edge of the plate and the surrounding gasket are exposed to the atmosphere, the heat loss is small, generally about 1%, no need to take insulation measures. In the case of the same heat exchange area, the heat exchange loss of the plate heat exchanger is only 1/5 of the shell-and-tube heat exchanger, and the weight is less than half of the shell-and-tube type.
On the one hand, when assembling the heat exchanger, it is convenient to increase or decrease the heat transfer plate according to the output and process requirements, and the plates can be rearranged and the process combination can be re-selected. On the other hand, the adaptability is also manifested in its use. Plate heat exchangers are widely used. At present, they are widely used in various engineering fields such as chemical industry, machinery, cement, petroleum, electric power, hot water heating, etc. In the process of heating, cooling, evaporation, condensation, waste heat recovery, etc., the purpose of application is achieved by performing heat exchange between the mediums in the plate heat exchanger.
(4)Flexible operation, easy maintenance
The heat transfer plate and the movable compression plate are hung on the beam of the machine. There is a rolling device above the pressure plate, which can easily open the device, clean it, and take out a plate. Check or replace the gasket.
By comparing the shell-and-plate heat exchanger, the following conclusions can be drawn: the heat exchanger of the plate heat exchanger has high heat transfer efficiency, small volume, light weight and easy disassembly and assembly, when cooling When the water quality is good, it is a relatively good heat exchanger equipment. Combined with the specific conditions of the heat exchange station of the district, the plate heat exchanger is suitable as the heat exchange equipment of the heat exchange station of the district.
2. Types of plate heat exchangers
Spiral plate heat exchanger
Advantages: The spiral plate heat exchanger is compact in structure and provides a large heat transfer surface per unit volume. The fluid allows a higher flow rate in the spiral plate, and the fluid flows in the spiral direction, and the stagnant layer is thin, so the heat transfer coefficient is large and the heat transfer efficiency is high. In addition, due to the large flow rate, dirt is not easily retained.
Disadvantages: spiral plate heat exchangers require high welding quality and difficult maintenance. Heavy weight, poor rigidity, special care should be taken when transporting and installing spiral plate heat exchangers.
Plate heat exchanger
Flat heat exchanger is a new type of high efficiency heat exchanger assembled from a series of metal sheets with a certain corrugated shape. A thin rectangular channel is formed between the various sheets, and heat exchange is performed through the half sheets. Compared with the conventional shell-and-tube heat exchanger, the heat transfer coefficient is much higher under the same flow resistance and pump power consumption, and there is a tendency to replace the shell-and-tube heat exchanger within the applicable range.
Shelf-fin heat exchanger
consists of a bundle of heat exchanger plates enclosed in a header tank with inlet and outlet for cold and hot fluids. The bundle of plates is alternately laminated by flat plates and corrugated fins and brazed. The cold and hot fluids flow through the sides of the plate to exchange heat, and the fins increase the heat transfer area, which promotes the turbulence of the fluid and enhances the equipment. The plate-fin heat exchanger has a very compact structure, good heat transfer effect, and a pressure of up to 15 MPa. However, its manufacturing process is complicated, the flow path is small, and the internal leakage is not easy to repair, so it is limited to clean non-corrosive fluids, such as heat exchangers for air separation.
3, general principles of plate heat exchanger design
Select the corrugated pattern of the board
The corrugated pattern of the plate is mainly herringbone corrugated and horizontal straight corrugated. The pressure bearing capacity of the herringbone corrugated sheet can be higher than 1.0 MPa, and the pressure bearing capacity of the horizontal flat corrugated sheet is generally about 1.0 MPa; the heat transfer coefficient and fluid resistance of the herringbone corrugated sheet are higher than the horizontal straight corrugation. Plate. Select the corrugated pattern of the plate, mainly considering the working pressure of the plate heat exchanger, the pressure drop of the fluid and the heat transfer coefficient. If the working pressure is above 1.6MPa, there is no choice to use herringbone corrugated sheets; if the working pressure is not high and the resistance is required to be reduced, it is better to use horizontal flat corrugated sheets; if it is limited by the installation position A high heat exchange efficiency is required to reduce the heat exchanger footprint, and the resistance drop can be unrestricted, and a herringbone corrugated sheet should be used. The plate type or corrugated type should also be determined according to the actual needs of the heat exchange occasion. For the case where the allowable pressure drop is large, the plate type with low resistance should be selected, and the plate with large resistance should be selected.
Selection of board area
If the area of the board is too small, the number of plates of the plate heat exchanger is large, and even if the floor space is increased, the number of passes increases (resulting in increased resistance); And the resistance drop is reduced, but it is difficult to ensure the necessary flow rate between the plates. It is not advisable to select a plate with too small a single plate area, so as to avoid too many plates, the flow velocity between the plates is too small, and the heat transfer coefficient is too low. This problem should be paid attention to for a large heat exchange station.
Selection of flow rate
The flow rate of the fluid between the plates affects the heat transfer performance and the pressure drop of the fluid. The high flow rate is high, but the heat transfer coefficient is also increased; otherwise, the opposite is true. Generally, the average flow rate between plates is 0.2-0.8 m/s. When the flow rate is lower than 0.2 m/s, the fluid will not reach the turbulent state and a large dead zone will be formed. If the flow rate is too high, the resistance drop will increase sharply. In the specific design, a flow rate can be determined first to calculate whether the resistance drop is within a given range; or the initial value of the flow rate can be obtained by a given pressure drop.
Selection of process
The process refers to a set of parallel flow channels in the same flow direction of a medium in a plate heat exchanger, and the flow channel refers to the medium flow in the plate heat exchanger, adjacent to two plates aisle. In general, several flow channels are connected in parallel or in series to form different combinations of cold and heat medium channels. For a plate heat exchanger with a generally symmetrical flow path, when the volume flow of the two fluids is roughly equivalent, it should be arranged in an equal process as much as possible. If the flow rates on both sides are very different, the side with the small flow rate can be arranged in multiple steps. In addition, when a medium temperature rise or temperature drop is large, a multi-pass arrangement can also be adopted. The phase change side of the phase change plate heat exchanger is generally one-way. Multi-pass heat exchangers, unless otherwise required, generally the same number of runners should be used for the same fluid in each pass.
The problem of uniform fluid flow distribution is also reflected in the process combination method. Under the given total allowable pressure drop, the multi-pass arrangement makes the corresponding allowable pressure drop of each pass smaller, forcing the flow rate. Lower, which is not good for heat transfer. In addition, the multi-pass arrangement of the out-of-range is one of the important reasons for the reduction of the average heat transfer temperature difference and should be avoided as much as possible.
Selection of flow
When there is no phase change heat, the countercurrent has a large average heat transfer temperature difference. In the engineering design of general heat exchangers, the fluid is arranged to be countercurrent. For plate heat exchangers, to do this, the sides must be equal. If the arrangement is not equal, the forward and reverse flows will alternate. At this time, the average heat transfer temperature difference will be significantly less than the pure reverse flow.
Pressure drop requirements
In the design and selection of plate heat exchangers, there is a general requirement for pressure drop, so it should be checked. If the check pressure drop exceeds the allowable pressure drop, the design selection calculation must be repeated to know that the process requirements are met. The general manufacturer gives the pressure drop calculation formula in the sample. From the technical and economic point of view, the pressure drop of the water-heat exchanger should not exceed 0.06 MPa.