External Surface Of Tubes Research Articles

Oxygen corrosion of reboiler tube served in production of dilution steam from heat exchanger of petrochemical refinery

This paper reports the oxygen corrosion of heat exchanger tubes serviced in dilute steam generation system. The floating head heat exchanger employed in a petrochemical industry underwent remnant corrosion failures particularly at lower passes of tube bundle. Initially remote field eddy current inspection technique was employed for tube bundle conditional assessment and further, root cause of degradation of tube had been analyzed. Comprehensive experimental techniques including visual examination, tube chemistry analysis, external tube surface morphological analysis, elemental mapping, scanning electron microscopy, light microscopy and x-ray diffraction studies were employed. The tube surface at the degraded position with its cross sectional examination was rigorously evaluated as well. Moreover, actual steam parameters such as pH, conductivity and iron & chloride concentration have been monitored in the petrochemical facility. The detailed analysis concluded that dissolved oxygen in boiler feed water (BFW) was the root cause for corrosion failure. Several other foreign elements were detected and source of each element was discussed in the context of actual heat exchanger operating conditions. Simulated conditions of BFW service have electrochemically been evaluated in two different dissolved oxygen concentrations at 7.5 ppm and 12.7 ppm, respectively. Even small increase of dissolved oxygen in BFW could lead to promotion of oxygen reduction reaction. Theoretically SA179 tubes operated at pH of 9.75 could lead to formation of bicarbonate and it could protect the tube in absence of dissolved oxygen as evaluated by Molecular Dynamics simulation. However, according to obtained results, tube has been degraded due to presence of dissolved oxygen in BFW. Various contributing factors such as external paint/coat removal, absence of de-aerator system in BFW treatment and improper corrosion inhibitor programme were discussed. Several practical countermeasures have been suggested to mitigate oxygen corrosion failure in petrochemical refineries.

Innovative experimental approach for the dynamic Multi-Variable investigation of Pulsating heat Pipes

Pulsating Heat Pipes (PHP) are passive two-phase heat transfer devices characterized by a simple structure and high heat transfer capabilities. Despite this, their large-scale application is still hindered by the actual unpredictability of their dynamic behavior during the startup and the thermal crisis. An innovative experimental apparatus is designed to systematically investigate the above-mentioned phenomena. It consists in a square loop made of four borosilicate transparent glass tubes joined at corners by means of brass connectors. The external tube surface is coated with several transparent Indium Tin Oxide heaters. The device is used to topologically reproduce 5, 7, and 11 evaporator/condenser sections PHPs with a 2 mm inner diameter tube, filled with pure ethanol and tested in horizontal position. First a steady-state characterization is performed. The condenser temperature is varied from 10 °C to 40 °C; the input power goes from 10 W to 40 W. Results show that both the increase of number of evaporator sections and condenser temperature enhance the PHP performance in terms of equivalent thermal resistance. Then, the effect of the condenser temperature on the initial liquid phase distribution is analyzed. It is observed that at low condenser temperatures (10 °C, 20 °C), the liquid phase tends to gather in the condenser sections rather than the evaporator sections and it is arranged in short liquid plugs; conversely, for high condenser temperatures (30 °C, 40 °C), the amount of liquid phase in evaporators is higher and it is arranged in long liquid plugs. Finally, the transient behavior is studied in terms of startup time. It is observed that the parameter influencing most the startup time is the initial fluid distribution; the startup time increases with the increase of evaporator sections volume occupied by liquid phase and with the length of liquid plugs; on the other hand, the startup time decreases with the increase of number of menisci in the evaporator sections.

Modelling of the Rolling Process of Titanium Alloy Tube Billets in Laboratory Conditions on a RSP 14-40 Rolling Mill

The development of screw rolling technology for the production of hot-deformed tubes over Ø250 mm on a SVP-500 rolling mill faces a number of challenges that influence the quality of tubes, such as: the screw trace formed on the external surface of tubes and bending of tubes that makes impossible subsequent manufacturing operations. The following experimental and laboratory research was performed to solve these problems: a number of experimental tube billets with and without mandrels were rolled to various strains on a RSP 14-40 laboratory rolling mill to obtain the best ratio of wall thickness to the external diameter. The temperature field distribution and the strain intensity in the deformation area of tube billets rolling were analyzed. A quality assessment of the influence of the section of the calibrating rolls used in industrial technology on the external surface of hot-deformed tubes was also carried out at the modeling stage. The angle changing options of the calibrating rollers section of the screw rolling mill were analyzed for the subsequent tool improvement.

Visualization of flow patterns during condensation in dimpled surface tubes of different materials

Flow patterns and their transitions during condensation of R410A inside two three-dimensional enhanced tubes and a plain tube were visualized and analyzed. Two three-dimensional enhanced tubes are made of stainless steel and copper, respectively, and their inner surfaces have an identical structure consisted of two different enhancement units. The primary enhancement unit refers to the helical dimples, while the background one is an array of petal-shaped dents on the external surface of tubes. The flow pattern visualizations were performed in the following test conditions: mass velocities 100–200 kg m−2s−1, and vapor quality 0.1–0.9. Stratified, stratified-wavy, and annular flows were observed for the smooth tube, while another two flow patterns, including intermittent and semi-annular flow was seen for the enhanced tubes. The results indicated that the transition between annular flow to intermittent flow was shifted to a lower vapor quality for two novel enhanced tubes compared to the equivalent smooth tube. Besides, the void fraction under stratified and stratified-wavy flow was deduced by image processing and compared to some general correlations. A new void fraction model for enhanced tubes was proposed based on the present experimental results.

Comparison of the Evaporation and Condensation Heat Transfer Coefficients on the External Surface of Tubes in the Annulus of a Tube-in-Tube Heat Exchanger

An experimental study was carried out to explore the heat transfer characteristics on the outside of smooth and enhanced tubes, during evaporation and condensation of R134A in the annulus of a tube-in-tube heat exchanger. The three-dimensional enhanced surface tube consisted of primary enhancement patterns and secondary patterns; results were compared to the performance of an equivalent smooth tube. The equivalent external diameter of the inside horizontal copper tubes used in this study was 19.05 mm, while the outer tube varied in size, allowing a comparison of heat transfer for different annulus dimensions. Tests were conducted with a fixed inlet/outlet vapor quality and a constant saturation temperature for varied mass velocities in the range of 30 to 100 kg/(m2∙s). For condensation, the ratio of heat transfer coefficient enhancement (enhanced tube/ smooth tube) was up to 1.78; this can be attributed to the turbulence increase, as well as liquid film re-distribution, produced from the dimples. Furthermore, the condensation heat transfer coefficient increased rapidly with increasing mass flux. For flow boiling in the annulus between the 1EHT tube and outer tube, the heat transfer coefficient during boiling was 11–36% higher when compared to the smooth tube at xave = 0.35, while the performance of the 1EHT tube was not as good as the smooth tube at xave = 0.5. The heat transfer deterioration can be explained by decreased effective nucleate flow boiling heat transfer area and the flow pattern transition between a slug/wavy-stratified flow to wavy-stratified flow.

Internal oxide thickness measurement by ultrasonic method

ABSTRACTLong-term exposition of re-heater and super-heater tubes in fossil-fired steam boilers at high temperature causes the growth of the iron oxide scale (magnetite) on the both internal and external tube surfaces. The presence of the internal oxide scale on the water side of the tube acts as a thermal insulator reduces heat transfer through the tube wall. Persistent overheating of the tube wall is a crucial issue, which leads to a metallurgical failure of the tube. The oxide scales react as a thermal insulator on the water side and reduce the heat transfer through the tube wall. Relatively thin oxide scale contributes to a reduction of boiler efficiency and causes persistent overheating of the tube wall. The growth of internal oxide scale is a time- and temperature-dependent process with a negative impact on the remaining creep-to-rupture life.

Design and development of a 3D system for the measurement of tube eccentricity

We present a novel method for the 3D optical measurement of tube eccentricity. The prototype is based on two pairs of laser slits that illuminate the external and internal walls of the tube respectively. Each laser slit captures a 3D semi-profile in the zone close to the cut section of the tube. The laser slits are assembled following a suitably designed layout, which allows us to obtain the circumferential profiles of the internal and external tube surfaces. These profiles are fitted to two circles, and the eccentricity is measured as the distance between their respective centres. The system is suitable for monitoring the wall thickness in correspondence of tube cross sections characterized by scratches and chippings left by the cutting tool in a static way. In this paper, the method and the procedures developed to implement the measurement are described. The characterization of the laser slits, as well as the measurement performance of the system, is detailed. A number of experimental results highlighting the system performance in comparison with a 2D vision approach are discussed.

The system of carbon tetrachloride and closed carbon nanotubes analyzed by a combination of molecular simulations, analytical modeling, and adsorption calorimetry

Using the combined techniques of molecular simulation, simple analytical modeling, and adsorption calorimetry, we propose new models describing adsorption onto closed carbon nanotubes. The models are capable of describing the adsorption isotherms and calorimetric enthalpy of carbon tetrachloride adsorption measured on three different closed carbon nanotubes. It is shown that the assumption of the presence of two types of surface centers (high- and low-energy centers) on external tube surfaces is sufficient to describe experimental adsorption and calorimetric enthalpy data.

Thermal performance analysis of heat exchanger for closed wet cooling tower using heat and mass transfer analogy

In closed wet cooling towers, the heat transfer between the air and external tube surfaces can be composed of the sensible heat transfer and the latent heat transfer. The heat transfer coefficient can be obtained from the equation for external heat transfer of tube banks. According to experimental data, the mass transfer coefficient was affected by the air velocity and spray water flow rate. This study provides the correlation equation for mass transfer coefficient based on the analogy of the heat and mass transfer and the experimental data. The results from this correlation equation showed fairly good agreement with experimental data. The cooling capacity and thermal efficiency of the closed wet cooling tower were calculated from the correlation equation to analyze the performance of heat exchanger for the tower.

Experimental determination of shell side heat transfer coefficient and pressure drop for an oil cooler shell-and-tube heat exchanger with three different tube bundles

In this paper, the heat transfer coefficient and pressure drop on the shell side of a shell-and-tube heat exchanger have been experimentally obtained for three different types of copper tubes (smooth, corrugated and with micro-fins). Also, experimental data has been compared with theoretical data available. Correlations have been suggested for both pressure drop and Nusselt number for the three tube types. A shell-and-tube heat exchanger of an oil cooler used in a power transformer has been modeled and built for this experimental work in order to investigate the effect of surface configuration on the shell side heat transfer as well as the pressure drop of the three types of tube bundles. The bundles with the same geometry, configuration, number of baffles and length, but with different external tube surfaces inside the same shell were used for the experiment. Corrugated and micro-fin tubes have shown degradation of performance at a Reynolds number below a certain value ( Re < 400). At a higher Reynolds number the performance of the heat exchanger greatly improved for micro-finned tubes.

Mineralogical gradients associated with alvinellids at deep-sea hydrothermal vents

Alvinella pompejana and Alvinella caudata live in organic tubes on active sulphide chimney walls at deep-sea hydrothermal vents. These polychaete annelids are exposed to extreme thermal and chemical gradients and to intense mineral precipitation. This work points out that mineral particles associated with Pompeii worm ( A. pompejana and A. caudata) tubes constitute useful markers for evaluating the chemical characteristics of their micro-environment. The minerals associated with these worm tubes were analysed on samples recovered from an experimental alvinellid colony, at different locations in the vent fluid–seawater interface. Inhabited tubes from the most upper and lower parts of the colony were analysed by light and electron microscopies, X-ray microanalysis and X-ray diffraction. A change was observed from a Fe–Zn–S mineral assemblage to a Zn–S assemblage at the millimeter scale from the outer to the inner face of a tube. A similar gradient in proportions of minerals was observed at a decimeter scale from the lower to the upper part of the colony. The marcasite/pyrite ratio of iron disulphides also displays a steep decrease along the few millimeters adjacent to the external tube surface. The occurrence of these gradients indicates that the micro-environment within the tube differs from that outside the tube, and suggests that the tube wall acts as an efficient barrier to the external environment.

C to BN conversion in multi-walled nanotubes as revealed by energy-filtering transmission electron microscopy

We used a field emission high-resolution 300 kV electron microscope (Omega filter) to study the nanostructures appearing during consecutive stages of chemical reaction between multi-walled chemical-vapor-deposited (CVD) C nanotubes (NTs) and boron oxide in flowing nitrogen at >1573 K. We found that C→BN conversion in the tubular layers occurs through inhomogeneous crystallization of B/N domains onto and within undulating defective graphitic C shells opened by oxidation. The crystallization preferentially takes place at open edges, internal and external tube surfaces. Enrichment of shells with B and N at the expense of outflowing C gradually occurs. In the course of the transformation unique longitudinal nanotubular heterojunctions composed of C-rich and BN-rich domains form.

Non-flat surfaces analysis by glow discharge spectrometryApplication to Zircaloy 4 assembly cladding tube.

A glow discharge system is developed which allows to evaluate concentration profiles directly on the external surface of tubes while keeping a good resolution in depth, avoiding the flattening of tubes which can lead to cracks and deformation of the surface oxide layer. The geometry and the operating parameters of the device have been optimized for the characterization of Zircaloy 4 tubes used to manufacture a pressurized-water reactor fuel assembly. The principle of that modification can be extended to tubes with other geometries by adjusting the semicircular slit to the diameter of the tube to be analyzed.

Theoretical analysis of the thermal resistance of non-uniform fouling on cross-flow heat exchanger tubes

A theoretical analysis of steady-state heat conduction through homogenous fouling on the external surface of tubes in cross-flow has been made. Modelling the fouling geometry as an eccentric annulus, a finite difference procedure in bipolar coordinates has been developed to predict the effect of fouling on the thermal performance. Theoretical predictions for a range of geometry, thermal conductivity and variable convection boundary conditions are presented. It is shown how, for a range of foulant thickness, eccentricity and thermal conductivity, the rate of heat transfer to a given size tube can be reduced or increased. The case of non-homogeneity of the fouling is explored with a view to extending the analysis towards varying thermal conductivity.

Intensification of heat transfer in the condensation of water vapor on horizontal tubes with annular grooves

Results are presented from an experimental study of the mean heat-transfer coefficient in the film condensation of water vapor on the external surface of tubes with transverse periodic grooves of smooth configuration.

Texture inhomogeneity and the strain distribution in shear-spun steel tubes

The through-thickness distribution of texture and its relation to the distribution of plastic strain were studied in shear-spun steel tubes. The effects of heat treatment on the texture and on its distribution were also determined. At the external tube surface the deformation approximated to a uniform elongation with a large superimposed tangential shear. At the internal surface only the uniform elongation, analogous to ideal rolling conditions, was present. In the mid-thickness region extensive periodic shear parallel to the radial direction occurred, leading to a wavy flow-line pattern and extremely non-homogeneous textures. This local tilting of the pattern of metal flow drastically reduced the apparent texture in the mid-thickness region. Only solution treatment affected the texture, acting to reduce some components preferentially with respect to others. Aging treatments alone had no effect. The average texture, as determined from composite texture specimens, did not respond to solution heat treatment in the same way as the surface textures did. Changes in the processing parameters (in this case from forward to backward shear spinning) had a pronounced effect both on the pattern of metal flow and on the resultant textures.

High Temperature Furnace Corrosion Of Type 309 Alloy Steel

Abstract Severe corrosion occurred on the external surface of tubes and tube supports of 25 Cr-12 Ni alloy steel in a furnace operating at metal temperatures from 1600 to 2100 F. Corrosion to a depth of ½ inch occured in six weeks' time. This furnace was operating under normal firing conditions with little sulfur in the gas. An investigation of the problem has revealed that the metal loss of furnace parts was accelerated by contaminants fluxing the hot metal surfaces. 4.2.3