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Welding
Figure 2 : Increase in area and turbulent flow induced by dual enhanced tubes
Surface modifications can be performed to increase the heat duty calculated from the equation [ 1 ]. Low fins are created by a cold deformation of the outside surface of the tube . One of the consequences is that it implies an increase of the outside tube area ( A in equation [ 1 ]). Internal ribs are created by a cold deformation of the inside surface of the tube . It generates local flow turbulences which increases heat transfer coefficient ( U ), in equation [ 1 ], with minimal impact on pressure drop . Figure 1 shows the design of enhanced tubes whereas Figure 2 illustrates the performances modifications . Low fins and internal ribs modify the surface area , pressure drop and heat transfer coefficient by a factor that depends on fin / rib density and shape , on tube diameter and thickness , on the fluid properties , and on the process conditions . Typical range of coefficients are summarized in Table 1 . Selection of tubes and the related impact on heat exchanger performances can be assessed using a thermal process design and simulation software such as HTRI that includes Neotiss tubes references and performances , under its brand HPT ( High Performance Tubes ). Even in the case where the cost of enhanced tubes is higher than the cost of smooth tubes , the total price is always balanced by the fact that less material is necessary ( decrease of required tube length and / or number of tubes ), shell cost decreases because the size is reduced , and assembly costs also decrease when less tubes are necessary . Significant gain can also be realized by debottlenecking cases where heat duty is limited in an existing shell size .
Figure 3 : Example of process from the strip to ready-to-ship tubes
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