How To Improve The Thermal Performance Of The Finned Heater Exchanger?
At present, the dividing wall type is the most common and most widely used at home and abroad. The design and calculation of other types of heat exchangers often refer to dividing wall heat exchangers. As a general equipment, heat exchangers are classified according to their working principles and can be divided into three types: partition type, hybrid type and heat storage type.
1. Variable pitch finned heater exchanger
First, the design principle
When the evaporator used in the low-temperature refrigeration system works under low-temperature conditions (below 0℃), the surface of the evaporator generally has the problem of frosting. The effect of frosting on the performance of the heat exchanger is manifested in two aspects: reducing its heat transfer coefficient and increasing air resistance. A reasonable heat exchanger structure should reduce these two effects at the same time. When frost begins to form, the surface roughness of the evaporator increases, causing the heat transfer area to increase, and the gas flow rate also increases, so the heat transfer coefficient K increases at the beginning of frosting, but as the frost layer continues to thicken, the heat transfer The thermal resistance of heat increases, and the final heat transfer coefficient K decreases.
When the airflow passes through the evaporator, due to the continuous deposition of water vapor in the air on the surface of the fin tube, the relative humidity of the air decreases, and the amount of frost on the surface of the fin coil gradually decreases along the direction of the airflow, so the first few rows of tubes in the evaporator The frosting is more serious, and the frosting of the last few rows of pipes is relatively light. If the variable-spacing fin structure is adopted to make the fin spacing downstream along the wind direction smaller and smaller, the higher heat transfer efficiency can be maintained under frost conditions and the frosting time can be prolonged. After the evaporator adopts the variable-pitch fin structure, when the air sweeps the staggered fins, the staggered distribution of the fins makes the upstream fins have a flow around the downstream fins, which strengthens the heat exchange capacity of the front half of the fins. The distribution of the rear fins makes the flow channel narrower and the flow velocity increases, so that the heat transfer of the rear half of the fins is also strengthened.
2. Thermal performance of variable pitch fin heat exchanger
The improved heat exchanger adopts a variable-pitch fin structure, which can be approximated as staggered fins in theory. Therefore, the theory of staggered fins can be borrowed in the analysis. Figure 1 is a two-dimensional model of the fluid longitudinally swept staggered fins, the fin spacing is H, and the thickness is h. Figure 2 is a schematic diagram of the improved variable-pitch fin heat exchanger. The structure is actually staggered fins. The heat exchange capacity of the fins is strengthened; when the airflow flows through the next few rows of tubes, a set of fins added after the improvement makes the flow cross section rapidly narrowed, the flow velocity increases, and the fluid is further squeezed on the original basis. The disturbance is more severe, so that its heat exchange capacity has been strengthened.
