Search

Steam is the most energy efficient, reliable and flexible way to transfer heat within most food processing operations. This chapter discusses the differences between plant, filtered and clean steam and the various issues that affect their quality and purity. It summarises the best practices in steam system design, operation and maintenance that will help prevent contamination problems affecting food quality in the future. Food manufacturers should take pains to identify and control potential steam system contamination.

Generation of air conditioners’ refrigerants suitable for countries with high ambient temperatures has become a necessity. Along with that, in this work, a non-azeotropic mixture (R-1270/R-600a), which is suitable for high ambient temperatures, composed of propylene (85%) and isobutane (15%) by mass was proposed. The heat transfer coefficient (HTC) and the frictional pressure gradient (FPG) of the boiling flow of this mixture were experimentally measured in a smooth horizontal tube and compared with that of R-22. Besides, the effect of mass flux (76.51, 95.64, and 114.76 kg/m2. s) and vapor quality (ranged from 0.1 to 0.7) on HTC were investigated. As a result, HTCs varied proportionally to both the vapor quality, and saturation pressure. Whereas, FPGs changed proportionally to vapor quality and inversely to saturation pressure. Additionally, at mass flux of 95.6 kg/m2 s, HTCs, and FPGs were enhanced by (19.04–67.32%), and (14.55–103.76%), respectively, w.r.t R-22. Moreover, the experimental results of HTCs and FPGs showed a good agreement with those of empirical correlations prediction.

The vertically inverted U-tube steam generator (UTSG) is widely used in the pressurized water reactor (PWR). The reverse flow behavior generally exists in some U-tubes of a steam generator (SG) under both single- and two-phase natural circulations (NCs). The behavior increases the flow resistance in the primary loop and reduces the heat transfer in the SG. As a consequence, the NC ability as well as the inherent safety of nuclear reactors is faced with severe challenges. The theoretical models for calculating single- and two-phase flow pressure drops in U-tubes are developed and validated in this paper. The two-phase reverse flow characteristics in two types of SGs are investigated base on the theoretical models, and the effects of the U-tube height, bending radius, inlet steam quality and primary side pressure on the behavior are analyzed. The conclusions may provide some promising references for SG optimization to reduce the disadvantageous behavior. It is also of significance to improve the NC ability and ensure the PWR safety during some accidents. Keywords: Two-phase natural circulation, Reverse flow, UTSG, U-tubes

Three-layer back propagation network (BPN) and genetic neural network (GNN) were developed in this study to predict the flow boiling heat transfer coefficient (HTC) in conventional and small-diameter channels. The GNN has higher precision than BPN (with root mean square errors of 17.16% and 20.50%, respectively) and other correlations. The inputs include vapor quality x, mass flux G, heat flux q, diameter D and physical parameter φ, and the predicted flow boiling HTC is set as the outputs. Influences of input parameters on the flow boiling HTC are discussed based on the trained GNN: nucleate boiling promoted by a larger saturated pressure, a larger heat flux and a smaller diameter is dominant in small channels; convective boiling improved by a larger mass flux and a larger vapor quality is more significant in conventional channels. The HTC increases with pressure both in conventional and small channels. The HTC in conventional channels rises when mass flux increases but remains almost unaffected in small channels. A larger heat flux leads to the HTC growth in small channels and an increase of HTC was observed in conventional channels at a higher vapor quality. HTC increases inversely with diameter before dry out. Keywords: Back propagation network (BPN), Genetic neural network (GNN), Flow boiling heat transfer coefficient, Conventional channel, Small channel

Accepted: 2019-05-09, 資料番号: SA1190071000

Experiments were carried out to investigate the boiling heat transfer characteristics of Al2O3-water nanofluids in swirl microchannels under terrestrial gravity and acceleration fields. A centrifuge with a two-meter long rotational arm was used to simulate the acceleration magnitude up to 9g and three various acceleration directions. Three test sections with different geometric parameters were applied. The volume concentration of Al2O3 nanoparticles with an average diameter of 13 nm was varied from 0.07% to 0.1%. The mass flow rate and vapor quality were in ranges of 3–6 kg/h and 0.4–1.0%, respectively. The effects of the mass flow rate, microchannel aspect ratio, vapor quality, nanoparticle volume concentration, and acceleration direction and magnitude were analyzed in a systematic manner. Experimental results showed that the acceleration direction and magnitude had significant influences on the boiling heat transfer. The heat transfer under configuration C was found to be superior to that under configurations A and B. Moreover, the heat transfer coefficient increased with increases of the mass flow rate and the volume concentration and decreased with the aspect ratio. Keywords: Acceleration, Boiling heat transfer, Microchannel, Nanofluid, Swirl microchannels

Current research focuses on the performance of helically coiled tubes as a passive heat transfer enhancement technique for R134a flow condensation from an entropy generation perspective. Similar to other enhancement techniques, helical coils, are accompanied by pressure drop as a penalty, diminishing their performance, so that these coils are of lower performance compared to straight tubes where the increase in entropy generation due to pressure drop overcomes the decrease in entropy generation due to enhanced heat transfer. Unlike previous studies that have largely investigated heat transfer and pressure drop characteristics of helical coils regardless of their performance, this study employs entropy generation analysis as an effective method to distinguish flow and geometrical conditions at which helical coils are of lower entropy, i.e. higher performance, compared to straight tubes. The findings reveal that, for both helical and straight tubes, entropy generation is enhanced as tube diameter, mass velocity, vapor quality, and wall heat flux increase and saturation temperature decreases. Additionally, applying helical coils within wider ranges of mass velocities can be justified at lower values of tube (≤8.3 mm) and coil diameters (≤200 mm), saturation temperatures (≤40 °C), and vapor quality (≤0.6), and at higher values of wall heat flux (≥15 kW/m2). These results substantiate that employing helical coils in lieu of straight tubes is not justifiable always (for all flow and geometrical conditions) although they are of superior heat transfer performance compared to straight tubes.

For thermal recovery processes in heavy oil reservoirs, wellbore heat loss is an important issue because high steam quality is a prerequisite for a successful operation. It usually dominates the phase state of steam in reservoirs. This loss also has an important interaction with reservoir injectivity. This chapter will introduce mathematical models for calculating wellbore heat loss with different wellbore configurations, including onshore and offshore wells and vertical and horizontal wells. The heat transfer behaviors of pure steam injection processes and hybrid enhanced oil recovery (EOR) processes are also comparatively studied. Based on these models, optimal operation parameters for hybrid EOR processes are provided.

Tri-Generation Systems (TGS) are designed for combined cooling, heating, and power production within a single process. In this paper, the integration of Concentrated Solar Power (CSP) together with TGS is studied. The tri-generation setup consists of a CSP facility that is integrated onto a desalination plant, producing three different levels of steam (very high pressure, low pressure and very low pressure), in addition to power and freshwater, simultaneously. Due to the complexity of the process, it was found imperative to investigate different factors that would improve the overall performance of the system, and ultimately decrease associated costs and thermal losses. Moreover, the use of molten salts offers many advantages, compared to more conventional fluid options, such as mineral and synthetic oils. Hence, a Mixed Integer Nonlinear Program (MINLP) that allows the selection of optimal heat transfer media, in TGS, has been formulated, and the performance of three different molten salts (solar salt, Hitec, and Hitec XL) has been analysed. The operating temperature of molten salts was found to have a significant effect on the steam quality generated in TGS-CSP systems. Moreover, solar salt resulted in the lowest energy production cost (at 28 cents/kWh) amongst all options. On the other hand, Hitec and Hitec XL were found to yield higher energy costs, estimated at 45 and 47 cents/kWh, respectively.

To address the concurrent water and energy shortage issues regions where geothermal sources are abundant, three geothermally sourced combined power and freshwater generation technologies are investigated. Two of them are based on traditional power generation systems, including a steam system (SS) and a single flash system (SFS). The other one is a proposed trilateral flash system (TFS). Instead of focusing solely on their power generation potentials as previous geothermal exploitations did, the condensation process which produces desalinized freshwater is particularly investigated. To obtain a comprehensive evaluation, system performance under various geothermal wellhead conditions have been considered and compared. Results indicate that, for a typical liquid-dominated well, SS has absolute power generating advantage over SFS and TFS under low wellhead pressure and high wellhead vapor quality conditions. However, the TFS shows more stable power-generating and freshwater-generating performance when the wellhead condition varies, especially when the vapor quality gets lower. Efficiency of the total-flow turbine of TFS determines its system power potential. A turbine efficiency of 50% enables TFS to obtain comparable specific power with traditional power systems. Moreover, fresh water generation is a distinct advantage of the TFS, qualifying it a promising choice in remote arid geothermal terrains for both power and freshwater generation.

Power plants with a number of rotating equipment, condition monitoring and vibration analysis is important. For turbines other Turbovisory instruments are also important. Gas analyzers are important for pollution control and combustion optimization. Both boiler and turbine performance depends on Feed water and steam quality especially for super critical plants. Water quality maintained by monitoring the same and by various treatments and blow down. SWAS has been discussed at length covering various sampling methods. Under special instruments all these condition monitoring instruments, gas and liquid analytical instruments working principles, standard data sheets have been covered. For special instruments, accessories play important role so same has also been covered. This also includes selection procedure, application & installation notes for various special instruments.

This article presents a nonlinear modeling approach describing the enthalpy dynamics between the inlet of the economizer and the outlet of the evaporator in a once-through boiler. The model is used to design an LQG controller for the steam quality at the evaporator outlet and to estimate the firing power, which constitutes a disturbance to the process. Robustness properties of the overall controller design are evaluated and the results are presented in simulations utilizing a high-fidelity model acting as the real plant.

An experimental investigation was performed to evaluate the heat transfer and pressure drop characteristics of the hydrocarbon refrigerant R-600a during flow boiling inside a horizontal smooth tube with an inside diameter of 8.25 mm and a newly developed dimpled tube. The inner surface of the helically dimpled tube is enhanced by a newly modified pattern consists of both shallow and deep protrusions. The experimental tests were carried out varying: the refrigerant mass fluxes within the range of 155–470 kg/m2 s; the vapor qualities up to 0.8; the constant heat flux of 15.8 and saturation temperature of 56.5 °C. Observations clearly indicate that the heat transfer performance is improved as tube’s inner surface enhanced by this new pattern of protrusions. The experimental results show that the heat transfer coefficients of the dimpled tube are 1.29–2 times larger than a smooth tube with a pressure drop just ranging between 7% and 103% larger than the smooth tube. The highest enhancement in the heat transfer coefficient occurs at vapor quality of 0.2 and mass flow rate of 155 kg/m2 s. On the other hand, the maximum increase of pressure drop takes place at vapor quality of 0.8 and mass flow rate of 305 kg/m2 s.

This work presents the experimental evaluation of the energy performance of a modified single-stage CO2 transcritical refrigeration plant with an internal heat exchanger (IHX) based on vapour injection in suction line. This modification, which is only applicable to refrigeration plants with an expansion process divided in two stages with a liquid receiver between them, consists of extracting saturated vapour from the liquid receiver in order to: decrease the vapour quality at the evaporator inlet, and reduce the superheating degree at the compressor suction by means of the expansion and following injection of the extracted refrigerant. Three different injection points have been evaluated experimentally: before the IHX, after the IHX and just before the suction chamber of the compressor. The experimental measurements show that the cooling capacity and COP can be enhanced in 9.81% and 7.01%, respectively. Furthermore, a reduction in the discharge temperature of the compressor up to 14.7 C has been measured inside the evaluation range., The authors are indebted to the Spanish Ministry of Education and Science (CTM2008-06468-C02-02/TECNO) and to the Spanish Ministry of the Environment and Rural and Marine Affairs (200800050084716) for the economic support given to the present work.

Three-dimensional simulations of condensation of refrigerant R134a in a horizontal minichannel are presented. Mass fluxes ranging from 50 kg m−2 s−1 up to 1000 kg m−2 s−1 are considered in a circular minichannel of 1 mm diameter, and uniform wall and vapour–liquid interface temperatures are imposed as boundary conditions. The Volume of Fluid (VOF) method is used to track the vapour–liquid interface; the effects of interfacial shear stress, gravity and surface tension are taken into account. The influence of turbulence in the condensate film is analysed and compared against the assumption of laminar condensate flow by employing different computational approaches and validating the results against experimental data. Under the assumption of laminar condensate flow, experimental heat transfer coefficient values at low mass fluxes can be predicted, but the computed heat transfer coefficient is found to be almost independent of mass flux and vapour quality. Only when turbulence in the condensate film is taken into account does the numerical model capture the influence of mass flux that is observed in the experimental measurements.

Publisher Summary Steam-supply systems commonly are used in industrial facilities as a general heat source as well as a heat source in pipe and vessel tracing lines used to prevent freeze-up in nonflow situations. Steam traps must be used for automatically purging condensable and noncondensable gases, such as air, from the steam system. Five major types of steam traps are commonly used in industrial applications: inverted bucket, float and thermostatic, thermodynamic, bimetallic, and thermostatic. Each type of steam trap uses a different method to determine when and how to purge the system. As a result, each has a different configuration. When properly selected, installed, and maintained, steam traps are relatively trouble free and highly efficient. The critical factors that affect the efficiency of steam traps include capacity and pressure ratings, steam quality, mechanical damage, and calibration. Steam traps are designed for a relatively constant volume, pressure, and condensate load. Operating practices should attempt to maintain these parameters as much as possible.

The effect of a noncondensable gas on forced-convective condensation inside a vertical tube was experimentally studied. This work was focused on the study of the heat transfer in the presence of a noncondensable gas under conditions of low steam quality (in values lower than 0.3 in this paper), knowledge of which is essential for understanding turbulent film condensation. The results show that, unlike previous investigations which were for the high-quality regime (in values higher than 0.7 in this paper), noncondensable gas volume fraction has little influence on condensing heat transfer. Although the noncondensable gas fraction still affected the interfacial heat flux, much more heat was convected downward by turbulent transport. Therefore the wall heat flux was not nearly as affected by the noncondensable gas fraction.

Publisher Summary This chapter reviews the literature on the critical heat flux (CHF) in forced flow boiling of Helium. This work is motivated by the need to cool superconducting magnets. Superconducting magnets are used in several advanced technology applications, e.g., magnetic fusion energy, magnetic energy storage, and high-energy physics. Helium is a reliable method for cooling superconducting magnets. There are substantial amounts of data available in the pertinent range of physical parameters, i.e., mass, velocity, and pressure. However, a unified effort to study the effects of all the parameters is still lacking. There is no single correlation available which can predict CHF under different physical conditions and geometrically different test sections. Most of the correlations have been tested for a very limited amount of data. A correlation to predict the boundary vapor quality is also needed. Two separate correlations may have to be developed for burnout of the first and second kind. Since the Soviet data show that the burnout phenomenon in helium is similar to that in steam-water flow, a correlation based upon phenomenological approach can be developed.

This chapter discusses the properties of water and steam. Fluids consist of a very large number of molecules moving in random directions within the fluid. When the fluid is heated, the speeds of the molecules are increased, increasing the kinetic energy of the molecules. There is an increase in volume because of an increase in the average distance among molecules, causing the potential energy of the fluid to increase. The sum of the internal energy and the pressure energy of a fluid is called the enthalpy of the fluid, denoted by the symbol H and measured in joules. When water is heated at a uniform rate, a stage is reached where the addition of more heat does not result in a corresponding increase in temperature. The temperature at which this occurs is called the saturation temperature, and the water is called saturated water. As heat is added to saturated water, it is turned into saturated steam. The amount of heat required to turn one kg of saturated water into saturated steam is called the specific latent heat of vaporization.

The equation of state, vapor pressure, saturated liquid density and constant volume heat capacity for the refrigerant dichlorotetrafluoroethane have been reported in the literature. This information has been used to compute the following thermodynamic properties as functions of temperature and. pressure: specific volume, heat capacity, enthalpy and entropy. These properties are computed for the superheated vapor, both above the critical temperature and below it down to saturation temperature. In addition, the latent heat content of the saturated vapor and the saturated liquid density were calculated for given temperatures. These computations were performed on both IBM 704 and 7090 computers. Approximately 1800 digital storage locations were required. The equations used, method of solution, and computer set-up are described herein. Mollier, Cp – t, and compressibility curves are presented for this fluid.

From international symposium on two-phase systems; Haifa, Israel (29 Aug 1971). See CONF-710836-. Experimental results are reported on the propagation of dryout in vertical up-flow of steam and water in an electrically heated tube at a pressure of 6.9 x 10/sup 6/ N/m/sup 2/. The 1.27-cm bore, 3.66-m long tube was machined from Nimonic 75 to withstand the high post burnout temperatures and also to provide a material with a very small change of electrical resistivity with temperature. An axi-symmetrical heat flux distribution of approximately cosine form was obtained by machining of the tube wall. Measurements were made of local wall temperature as the tube was driven into steady dryout by gradual increase of input power. At mass velocities up to approximately 2.7 x 10/sup 3/ kg/s/m/sup 2/, the behavior of the system was well ordered, it being possible by small power increments (approximately 1%) to drive the steady state dryout front gradually upstream. Each power increment gave rise to a well-defined wall temperature versus length locus which showed no subsequent rewetting of the heated surface once dryout had occurred. At high mass velocity (> 2.7 x 10/sup 3/ kg/s/m/sup 2/), the dryout front became very sensitive to small powermore » changes and first dryout zones appeared suddenly but stably more than 1 m upstream of the tube outlet. Dryout zones were often short and rewetting occurred after only a few centimeters with no apparent dryout elsewhere. In some cases, however, two simultaneous dryout zones were observed. Measured dryout positions and steam quality were compared with those predicted by a local conditions hypothesis and by a hydrodynamic or boiling length concept. It was found that a hydrodynamic correlation of burnout was superior to the local conditions hypothesis. The burnout position was not predicted satisfactorily by either theory, particularly for the high mass velocities. The exact position depends on the increment of power applied which takes the tube into flrst dryout. (auth)« less

Burnout conditions for flow of boiling water in vertical tubes have been studied experimentally in the following ranges of variables: Diameter, d = 10 mm Heated length, L = 2000, 3000 and 5000 mm Pressure, p = 100, 120, 140, 160, 180 and 200 bar Inlet subcooling, δti = 8-272°C Mass velocity, G = 156-7560 kg/m2s Heat flux, q/A = 13-353 W/cm2 Burnout steam quality, xBO = —0.3-1.0 The measurements were compared with the Tong, CISE and Macbeth burnout correlations, and it was found that for the pressure range studied in the present paper, none of these correlations yield satisfactory predictions. However, the measurements were in very good agreement with the correlation by Peskov. Finally, a new burnout correlation has been derived for the high-pressure range. This correlation is in good agreement with the present results and with data from other sources.