Design and Selection of Plate Heat Exchangers
Release time:
2021-04-14 17:10
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Plate heat exchangers are a new type of high-efficiency and compact heat exchanger that has been developed and widely used in recent decades. Their design and selection are crucial and directly affect their future use.
General Principles of Plate Heat Exchanger Design
Selection of Plate Corrugation Type
The main types of plate corrugation are herringbone corrugation and horizontal straight corrugation. The pressure bearing capacity of herringbone corrugated plates can be higher than 1.0 MPa, while that of horizontally straight corrugated plates is generally around 1.0 MPa; the heat transfer coefficient and fluid resistance of herringbone corrugated plates are both higher than those of horizontally straight corrugated plates. The selection of plate corrugation type mainly considers the operating pressure of the plate heat exchanger, the pressure drop of the fluid, and the heat transfer coefficient. If the operating pressure is above 1.6 MPa, herringbone corrugated plates must be used; if the operating pressure is not high and a low resistance is particularly required, horizontally straight corrugated plates are better; if the installation location is limited and high heat transfer efficiency is required to reduce the footprint of the heat exchanger, and the pressure drop is not restricted, herringbone corrugated plates should be used. The plate type or corrugation type should also be determined according to the actual needs of the heat exchange occasion. For situations with large flow rates and small allowable pressure drops, plate types with low resistance should be selected, and vice versa.
Selection of Single Plate Area
If the single plate area is too small, the number of plates in the plate heat exchanger will be large, which means the footprint will increase, and the number of passes will increase (resulting in an increase in pressure drop); conversely, although the footprint and pressure drop are reduced, the necessary flow rate in the inter-plate channel cannot be guaranteed. It is not advisable to choose plates with a too small single plate area to avoid too many plates and too low inter-plate flow rate, resulting in a low heat transfer coefficient. This issue should be paid more attention to larger heat exchangers.
Process Selection
A process refers to a group of parallel flow channels in a plate heat exchanger in which a medium flows in the same direction, while a flow channel refers to the medium flow channel formed by two adjacent plates in a plate heat exchanger. Generally, several channels are connected in parallel or series to form different combinations of cold and hot medium channels. For general symmetrical flow path plate heat exchangers, when the volumetric flow rates of the two fluids are approximately equal, they should be arranged as equally-pass as much as possible. If the flow rates on both sides differ greatly, the side with the smaller flow rate can be arranged with multi-pass. In addition, multi-pass arrangement can also be adopted when the temperature rise or drop of a certain medium is large. The phase-change side of a phase-change plate heat exchanger is generally single-pass. For multi-pass heat exchangers, unless otherwise required, the same number of flow channels should be used for the same fluid in each pass.
The problem of whether the fluid flow distribution is uniform also manifests itself in the process combination method. Under a given total allowable pressure drop, multi-pass arrangement makes the allowable pressure drop corresponding to each pass smaller, forcing the flow rate to decrease, which is not conducive to heat exchange. In addition, unequal-pass multi-pass arrangement is one of the important reasons for the decrease in the average heat transfer temperature difference, which should be avoided as much as possible.
Flow Direction Selection
When there is no phase change heat transfer, counter-current flow has the largest average heat transfer temperature difference. In the engineering design of general heat exchangers, the fluid is arranged as counter-current flow as much as possible. For plate heat exchangers, to achieve this, both sides must be equal-pass. If unequal-pass is arranged, counter-current flow and co-current flow will alternate, and the average heat transfer temperature difference will be significantly smaller than that of pure counter-current flow.
Pressure Drop Requirement
In the design and selection of plate heat exchangers, there are generally certain requirements for the pressure drop, so it should be checked. If the checked pressure drop exceeds the allowable pressure drop, the design and selection calculation needs to be carried out again until the process requirements are met. Generally, manufacturers give the pressure drop calculation formula in the samples. From the perspective of technology and economy, the pressure drop of water-water heat exchangers should not be greater than 0.06 MPa.
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