Heat exchangers (HEs) are equipment commonly used in the industrial sector, which allow the transfer of thermal energy between two or more fluids. There are different HE types that are commonly classified in regenerators and open or closed HEs. Specifically, regenerators are devices in which fluids flow alternately in the same space without coming into contact. Herein, a convective heat exchange between fluids and solid surfaces has to be taken into account while a conduction phenomenon characterises the solid components. In the open-type HEs, the fluids come into intimate contact. Indeed, the incoming streams of hot and cold fluids mix generating a single outgoing fluid stream. In closed-type exchangers, instead, the fluids are not mixed being separated by a solid, generally the wall of a tube resulting, as the regenerators, in a heat exchange due to convection between the solid and fluids and conduction within the solid itself [1–4].
In any case, a HE has to be designed considering specific equations [5]. In detail, the exchanged thermal power is quantified looking at the thermal capacity of both hot and cold fluid and the inlet and outlet temperatures of these fluids. This thermal power is transferred relying on the heat exchange of the equipment related to its surface transmission providing an indication of HE efficiency. The last parameter is an important index that is influenced by the type of HE architecture. In the air-cooled HEs, finned tubes have been proposed to increase the energy efficiency [6]. In detail, cold air is blown into bundles of these tubes inside which the hot processed fluid flows [7]. By doing so, the exchange surfaces increase substantially at a HE’s fixed volume. However, the global heat transfer is affected by several variables studied in various scientific works. For example, Mon and Gross[8] examined the effect of the fin pitch on the heat transfer coefficient (HTC) considering an in-line and unevenly spaced arrangement of the tubes. They analysed the effects of fin spacing on four-row annular-finned tube bundles in staggered and in-line arrangements by a 3D numerical study. Bilirgen et al.[9] executed a numerical study by using computational fluid dynamics achieving a relationship between the HTC and specific finned tubes’ geometrical variables such as material, pitch, height and thickness of the fins. They stated that the ratio between the pitch and the height of the fins has to be around 1.5 to achieve an optimum compromise between pressure drop and HE efficiency. Finally, Wong and Lee[10] numerically investigates the thermal performance of horizontal tubes with circular fins taking into account parameters such as fin diameter, fin spacing, fin material, and tube diameter and considering temperature difference of 50 K between the tube and the environment.
The finned tubes can be obtained by different manufacturing routes [11–13]. Indeed, they can be processed by plastic deformation or by joining of different parts using mechanical fastening and welding techniques [14–16]. Furthermore, bimetal finned tubes have also been proposed [12, 17]. The plastic deformation, executed by the rolling-extrusion process, is the most used technique that can be performed for both the single and the multi-material HE solution [18, 19]. The rolling-extrusion can be divided in three phases, named biting, finishing rolling and shaping [12]. Its main feature is the use of three rotating heads, composed of a series of disks. The number of disks present on each head and their diameters are selected based on the required fins’ geometry. Specifically, these heads, which, at first, are properly placed with respect to the tube’s surface, start gripping and rotating the tube, ensuring a simultaneous roto-translation movement (biting phase). The rotation speed of the heads, together with the applied pressure and positioning, is fundamental for a proper shaping of the helical profile. The tube is gradually deformed in the biting phase using disks with diameters that increase progressively. The second process phase, i.e. the finishing rolling, is executed by using disks characterised essentially by constant diameters because the fin’s formation is almost completed in the biting phase. The aim of this phase is mainly to reduce the effects of material elastic springback improving, at the same time, the profile quality. The process is completed by the last process phase, i.e. the shaping phase, where disks with a constant diameter further improve the fin’s profile moving the tube through the exit of the rolling-extrusion machine. The process dynamics are deeply influenced by the size and properties of the tubes to be processed. According to that, the tubes are generally made of two different materials: the base tube, which can be made of low carbon steel or stainless steel and the liner tube, made of 1xxx or 6xxx aluminium alloys [20]. These two tubes are coupled coaxially before the process starts. The angle of the helical profile, the pitch and the height of the fins are the geometrical variables that have to be, instead, considered for the process investigation. Finally, friction between tube and disks has to be taken into account to guarantee a stable roto-translation movement of the tube, connecting the geometrical variables to a process condition that can be controlled by the type of lubricant to be employed.
The rolling-extrusion process can be characterised by non-conformities and part ruptures that affect both the tubes and rotating heads. These problems have a significant impact on both product quality and costs, resulting in a drop of production capacity. Defects that can be most found in industrial practice are ruptures at the tip and/or twisting of the fins and pitch inhomogeneity that worst affect the HEs’ efficiency. Furthermore, these defects lead to head damages and, for severe cases, to breakages resulting in production stoppage.
In the proposed research activity, an industrial case was analysed, where the rolling-extrusion is employed to produce finned tubes for air-cooled HEs. In detail, a 2D numerical model was set taking into account specific experimental conditions. The process variables, considered in the analysis, are the working temperature and the forming movement and the dimension of each disk of the heads. The stress-strain trends were investigated to understand the process settings responsible for observed production defects. The numerical model was, subsequently, employed changing the input values of the highlighted variables. The working conditions that result in lowering the risks arising from non-conformities and part ruptures were pointed out.