By adjusting the fin spacing of the heat exchanger, the fin spacing is designed to be variable, and the structure optimization is realized. The heat transfer performance is 
compared with the heat exchanger before the improvement, and the heat transfer coefficient of the heat exchanger is improved. This method is suitable for the heat exchanger 
used in the evaporator of low temperature refrigeration system. When the airflow passes through the evaporator, due to the continuous deposition of water vapor in the air on 
the surface of the finned tube, the relative humidity of the air is reduced due to the dehumidification effect, and the frosting amount on the surface of the finned coil along the 
airflow direction is decreasing. If the variable pitch structure is adopted, it can maintain its high heat transfer efficiency under frosting conditions and prolong its defrosting time. 
When the evaporator adopts the structure of variable fin spacing, the staggered distribution of fins is actually formed. When the air flows across the staggered fins, the staggered 
distribution of fins makes the upstream fins flow around the downstream fins. Due to the flow around the front fins, the heat transfer of the first half of the fins is strengthened, 
and the distribution of the rear fins narrows the flow channel, increases the flow velocity, and the heat transfer of the second half of the fins is also strengthened.

Through the structural improvement of the variable fin spacing, the air cooler can still maintain a high heat transfer coefficient under the frosting condition without changing the 
overall dimensions, namely height, width and total length of the tube. The heat transfer coefficient of the air cooler with variable fin spacing structure is 9.8 % higher than that of 
the air cooler with equal fin spacing structure, and the heat transfer area is improved. The purpose of heat transfer enhancement is achieved by increasing the heat transfer coefficient 
and heat transfer area. Strengthen the fluid flow in the pipe, pipe wall processing variable pitch internal threads.

Without increasing the overall size of the equipment under the premise of increasing the inner surface heat transfer area, strengthen the disturbance of the fluid in the tube, in the original 
heat exchanger tube wall processing variable pitch internal threads. When the heat transfer coefficient inside the tube is large and the heat transfer coefficient outside the tube is small, the 
convective heat transfer resistance outside the tube will become the main resistance of heat transfer. The use of extended surface plays an important role in reducing the volume of heat 
exchanger and improving the efficiency of heat exchanger. At present, needle fins, corrugated fins, shutter fins, triangular fins, single-sided slotted strip fins, split-tooth rectangular fins 
and so on have been developed. The increase of the inner surface area is mainly concentrated in the development of special-shaped tubes. Looking at various shapes of enhanced tubes, 
the common feature is that the heat transfer area increases to varying degrees while taking into account the pressure drop, and the heat transfer coefficient is improved through two 
mechanisms to enhance heat transfer.

The heat transfer boundary layer is the most important factor limiting the increase of heat transfer coefficient. It is produced in the laminar bottom layer near the tube wall and has a gradual 
thickening process. The roughness of the tube wall and the regular grooves and ribs will destroy the laminar flow state and inhibit the development of the boundary layer. At the same time, 
the flow limiting effect of grooves and ribs on the fluid contributes to the thinning of the boundary layer, and the flow around the fluid produces an axial vortex, which can cause the boundary 
layer to separate, and the radial temperature gradient of the fluid body is reduced, which is helpful for heat transfer.