Poly(phenylenesulfide)-based coatings for carbon steel heat exchanger tubes in geothermal environments

To inhibit corrosion and fouling by calcium silicate and silica scales of the carbon steel heat exchanger tubes in geothermal power plants operating at brine temperature of 160°C, their internal surfaces were lined with high-temperature performance poly(phenylenesulfide) (PPS)-based coating systems. The systems included the PPS containing polytetrafluoroethylene (PTFE) as an anti-oxidant additive, silicon carbide (SiC) as a thermally conductive filler, and aluminum oxide-rich calcium aluminate (ACA) as an abrasive wear resistant filler. Then, the lined tubes underwent an eleven-month-long field exposure at the site of power plant. The results from the post-test analyses of the exposed liners revealed that these PPS coating systems had an excellent thermal stability, and satisfyingly withstood this brine temperature, and also greatly resisted the permeation of brine, demonstrating that they adequately protect the tubes against corrosion in a wet, harsh geothermal environment. Furthermore, modifying the surfaces of the PPS top layer with PTFE, retarded the hydrothermal oxidation of the liner, thereby reducing significantly the rate of scale deposition and creating surfaces unsusceptible to reactions with scales. Thus, all the scales deposited on the liner's surfaces were readily scoured off by hydroblasting at a very low pressure. By contrast, although the stainless steel tubes had a great protection against corrosion, the formation of passive oxide layers at their outermost surface sites was detrimental in that they became more susceptible to scale deposition and developed strong adherence to the scales. As a result, the high-pressure hydroblasting was required to remove this scale adhering to the tubes' surfaces.     

Failure analysis of heat exchanger tubes

Primary waste heat exchanger tubes of material ASTM A213 grade T11 failed after operation of only three and a half months. The heat exchanger was of the bayonet type with boiler water inside the tubes and secondary reformer outlet process gas at the shell side. The heat exchanger environment was rich in hydrogen, carbon monoxide and nitrogen. The temperature of the process gases was 960 °C and the heat exchanger was producing steam at a temperature of 306 °C and a pressure of 1500 psig. The failed, used and new heat exchanger tubes were subjected to stereo/optical microscopy, chemical analysis and hardness testing. The cause of the failure was thoroughly investigated using optical/scanning electron microscope equipped with energy dispersive spectrometer. The study revealed that the material was exposed to thermal cycling and excessive local heating. The same was also confirmed by simulated experimentation. These conditions lead to thermal fatigue of the material with consequent failure.     

Erosion–corrosion of heat exchanger tubes

The shell and tube heat exchanger failed before 5 years of operation. The failure was caused by pits on the tube outside surface developing until its perforation. Inside the Cu-DHP tubes in soft temper a cooling agent circulated to cool down industrial water at the shell side from 16 to 4 °C. The cooled water was hard, rich in chloride ions with relatively low sulphate ions concentration and pH ranging from 4 to 7. The leaking tubes were subject to standard metallographic examination, hardness measurement, scanning electron microscopy and X-ray energy dispersion analysis. It was found that the tube damage was caused by erosion–corrosion induced by two factors: disturbed flow of water containing suspended solid particles and chemical composition of water rich in chlorides that resulted in loss of stability of protective cuprous oxide layer.     

Failure analysis of AISI 321 tubes of heat exchanger

The present study is about the failures of tubes from a tube shell heat exchange in one of the compression stages used to cool and purify the gas produced in an off-shore platform. Conventional hydrostatic tests did not detect leaking in the heat exchanger, but these tests were conducted at room temperature, i.e., very distant to the service temperature (163 °C). However, tightness tests with helium gas have detected leaking. Some tubes were taken off for analysis and the liquid penetrating test detected cracks in crevices formed between the tube and the tube sheet. The cracks were found to be typical stress corrosion cracks with branched pattern. A non-destructive test with Eddy current probes was developed to give high percentage of detection. The work was conclusive about the main cause of failures. Due to the increase of chloride content of the water processed to concentrations higher than 1000 mg/L over the years, the substitution of AISI 321 tubes by superduplex steel is recommended.     

Assessment of operation of an underground closed-loop geothermal heat exchanger

The results of calculations that allow one to obtain the characteristics of a closed-loop geothermal heat exchanger (C-LGHE) on the basis of our own approximate mathematical model are presented. These results are given in the form of appropriate tables containing the reduced quantities characterizing operation of such a geothermal heat exchanger. On this basis, C-LGHE operation is analyzed and conclusions on the possibilities of utilization of geothermal energy in binary power stations are drawn.__________Published in Inzhenerno-Fizicheskii Zhurnal, Vol. 78, No. 1, pp. 131–137, January–February, 2005.     

Local heat transfer in the intertube space of a heat exchanger with spiral tubes

The article explains the methods and presents the results of the experimental investigation of local heat transfer in bundles of spiral tubes with heat supply to the heat carrier that is nonuniform over the cross section.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 41, No. 2, pp. 197–202, August, 1981.     

High-performance coatings for micromechanical mirrors

High-performance coatings for micromechanical mirrors were developed. The high-reflective metal systems can be integrated into the technology of MOEMS, such as spatial light modulators and microscanning mirrors from the near-infrared down to the vacuum-ultraviolet spectral regions. The reported metal designs permit high optical performances to be merged with suitable mechanical properties and fitting complementary metal-oxide semiconductor compatibility.     

Performance of thermal barrier coatings in high heat flux environments

Thermal barrier coatings were exposed to the high temperature and high heat flux produced by a 30 kW plasma torch. Analysis of the specimen heating rates indicates that the temperature drop across the thickness of the 0.038 cm ceramic layer was about 1100°C after 0.5 s in the flame. An as-sprayed ZrO₂?8wt.%Y₂O₃ specimen survived 3000 of the 0.5 s cycles without failing. Surface spalling was observed when 2.5 s cycles were employed but this was attributed to uneven heating caused by surface roughness. This surface spalling was prevented by smoothing the surface with silicon carbide paper or by laser glazing. A coated specimen with no surface modification but which was heat treated in argon also did not surface spall. Heat treatment in air led to spalling in as early as one cycle from heating stresses. Failures at edges were investigated and shown to be a minor source of concern. Ceramic coatings formed from ZrO₂?12wt.%Y₂O₃ or ZrO₂-20wt.%Y₂O₃ were shown to be unsuited for use under the high heat flux conditions of this study.     

Failure analysis of stress corrosion cracking in heat exchanger tubes during start-up operation

The cracking failure of a new heat exchanger during first start-up operation has been analyzed. Through the investigation of the operating history of the equipment, analysis of the chemical compositions of tube material and corrosion products, metallographic test of specimens with cracks, the cracking mode can be described as the Stress Corrosion Cracking (SCC) of austenitic stainless steel. This kind of cracking was induced by the chloride in high temperature steam and tensile stress. The residual tensile stress due to seal expansion has been proved by numerical calculation. The pre-heating steam which was polluted by the catalyst with chloride is the main reason for the tube cracking in this case.     

Bamboo-like,oxygen-doped carbon tubes with hierarchical pore structure derived from polymer tubes for supercapacitor applications

In this paper, bamboo-like, O-doped carbon tubes with hierarchical pore structure have been fabricated by the direct pyrolysis of dual cross-linked polydivinylbenzene (PDVB) tubes. The bamboo-like, cross-linked PDVB tubes are firstly synthesized by cationic polymerization of divinylbenzene in cyclohexane using BF₃/Et₂O complex as the initiator. After a secondary cross-linking being imposed by Friedel–Crafts reaction in CCl₄ using anhydrous AlCl₃ as the catalyst, the obtained dual cross-linked, carboxylic acid functionalized PDVB tubes are directly subjected to pyrolysis, yielding bamboo-like, O-doped porous carbons. The resultant O-doped porous carbon tubes (BCTF-900, pyrolyzed at 900 °C) exhibit a trimodal pore structure (micro-, meso-, and macropores) with a relatively high specific surface area of 595 m² g^?1 and a low total pore volume of 0.37 cm³ g^?1. Such bamboo-like carbon tubes display good volumetric capacitive performance (254 F cm^?3 at 0.5 A g^?1), moderate volumetric energy density (12.9 Wh L^?1 at 428 W L^?1), and excellent cycling stability (the capacitance retention has remained at 96.9% after 10000 cycles at 2 A g^?1). Due to their unique bamboo-like architecture and trimodal pore structure, the PDVB-derived carbon tubes should have widely application prospect.     

Nonstationary heat and mass transfer with a reduction of the heat load in a heat exchanger with twisted tubes

The results of an investigation of the nonstationary mixing of the heat-transfer agent accompanying the reduction of the heat load in a bundle of twisted tubes are presented, and a generalizing dependence for calculating the effective coefficient of diffusion is derived.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 54, No. 4, pp. 533–539, April, 1988.     

土壤耦合热泵系统垂直埋管换热器性能影响因素的模拟分析

采用对钻孔内、外2个传热空间分别讨论的传热分析方法,建立土壤耦合热泵系统垂直埋管换热器传热的数学模型。模拟分析冬季换热过程中埋管周围土壤温度的变化,得出取热工况下地埋管换热器换热性能的影响因素。模拟结果将有助于提高热泵系统的循环经济性能。     

Effects of external operating environments on fatigue of heat‐exchanger copper tubes

The purpose of this study is to analyze the effects of surface defects (eg, notches) and external environment conditions (eg, operating temperature, the number of re‐weldings) on the static strength and fatigue of C1220T‐O copper tubes used in the heat exchangers of air conditioners. Instead of using standardized specimens, as is done in general rotary bending fatigue tests, special specimens were fabricated in this study by inserting metal plugs on both ends of the copper tubes to perform fatigue tests on the actual tube product, and then the fatigue characteristics were evaluated using stress‐life (S‐N) curves. Regarding the welding conditions (maximum 1000°C and 10 seconds), the grain size grew (grain size number decreased), and the hardness decreased as the number of re‐weldings increased. The effects of the operating temperatures on the fatigue life were examined at a room temperature of 25°C and a heat exchanger operating temperature of 125°C, resulting in the same fatigue limit (70.21 MPa) at both room and operating temperatures. However, the fatigue limit of 37.46 MPa measured in the notched specimens (radius of 3 mm, depth of 0.2 mm) was lower than that obtained from those without notches. The material constant (1.07) used in the Peterson equation was then computed from the fatigue notch factor (1.87 = 70.21/37.46), and the stress concentration factor (2.18) of the notched tube specimens was obtained from the structural analysis. This material constant can be used to predict a decrease in the fatigue limit over varying notch sizes in copper tubes (C1220T‐O).     

Failure of carbon steel tubes in a fluidized bed combustor

Plain carbon steels are used in power plants for transfer of heat from the flue gases/fluidized bed to water. In the present study, the failure of a few super heater tubes at localized regions in an atmospheric fluidized bed combustor was studied. The main reason of the failure was attributed to high oxidation at these localized regions and spalling off of the oxides due to high operating temperature and high velocity of the alumina and silica particles in the fluidized bed. Obvious design changes and/or bandaging with a stainless steel sleeve at the affected regions coming in direct path of the fuel nozzle were recommended in this case.     

Mathematical modeling of nonstationary separation flow around a grid of heat exchanger tubes

To estimate the intensification of the surface heat- and mass-transfer process in tubular heat-exchange apparatus, a mathematical model is proposed for the analysis of the frequency and scalar characteristics of the force action of the flow of a single-phase heat carrier in the tube grid.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 53, No. 1, pp. 42–47, July, 1987.     

Investigation of the transfer properties of a stream in a heat exchanger with spiral tubes

The article demonstrates the possibility of measuring the effective diffusion coefficients and intensities of turbulence in bundles of spiral tubes by the method of diffusion from a point source; relationships for calculating these magnitudes were obtained.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 38, No. 6, pp. 965–971, June, 1980.     

Failure of hydrocracker heat exchanger tubes in an oil refinery by polythionic acid-stress corrosion cracking

Heat exchanger tubes of a hydrocracker unit in an oil refinery developed cracks at the bent sections after about 48 h of operation at 400 °C following a period of downtime. The tubes were fabricated from 321 stainless steel. Detailed analysis by various electron-optical techniques and X-ray diffraction showed that the tubes were cracked by stress corrosion cracking induced by polythionic acid. Due to the presence of H₂S in the environment, the inner surface of the tubes was converted into sulfur-bearing scale during operation. This could promote the formation of polythionic acid by aqueous condensates during downtime leading to cracking in the presence of residual internal stresses.     

A heat exchanger model for air-to-refrigerant fin-and-tube heat exchanger with arbitrary fin sheet

A new model for simulating air-to-refrigerant fin-and-tube heat exchangers, with arbitrary fin sheet which encompasses variable tube diameters, variable tube locations, variable tube pitches, internal as well as external jagged edges, variable number of tubes per bank and variable location of fin cuts, is introduced. This model is based on a segment-by-segment approach and is developed to be a general purpose and flexible simulation tool. To account for fin conduction and air propagation through the heat exchanger, it is spatially modeled on a Cartesian grid. A new methodology for air side propagation, required for arbitrary fin sheets, is introduced. The model prediction is validated against experimental data for a condenser using R410A as the working fluid. The predicted results agree within ±5% of overall heat load, and ±25% for total refrigerant pressure drop.     

Impact of a vertical geothermal heat exchanger on the solar fraction of a solar cooling system

This paper presents the influence on the solar fraction of coupling a Solar Cooling System (SCS) with a Geothermal Heat Exchanger (GHX). The SCS analysis was made using a mathematical model and was supported by experimental data of a vertical GHX. First, the cooling capacity of the SCS was calculated, considering the cooling load required for a building of 420 m³. The results show that a 12.30 kW cooling capacity SCS would be necessary to satisfy the maximum cooling load requirement during the summer. A 1 kW PV array was considered to assist the SCS. With the constructed GHX a soil temperature model was validated to estimate the temperature of the output water at different GHX depths. The solar fraction meets between 10 and 23% of the energy demand when the condenser is air cooled. However, when a GHX of 1, 2, 4 and 10 m is implemented the solar fraction increases.