Dew Point Calculations Research Articles

Reducing boiler maintenance costs with improved materials for air-heater tubes

Corrosion of boiler air-heater tubes costs the industry several million dollars each year in repairs, reduced boiler steam output and reduced boiler efficiency. There have been many cases where the reduced boiler steam output caused by leaking air-heater tubes has reduced factory crushing rates and electricity export. Corrosion of air-heater tubes can, in many cases, be minimised by improving the gas and air flow distributions with turning vanes and ductwork redesign, but in nearly all cases, there are some tubes on the cold air side of air-heaters that are still susceptible to dew-point corrosion. Using improved tube materials in parts of boiler air-heaters that are susceptible to dew-point corrosion will significantly extend air-heater life. Available materials and coatings were reviewed, and laboratory- and factory-scale trials, metal temperature measurements, dew-point calculations and a financial analysis-based ranking of commercially available tube materials were undertaken. S-TEN 1 had a similar corrosion performance to SS304 stainless steel. Both the S-TEN 1 and SS304 stainless steel tubes had significantly greater resistance to dew-point corrosion than the carbon-steel tubes typically used in the air-heaters of Australian sugar factory boilers. The good performance of S-TEN 1 in this project is not consistent with the poor performance of S-TEN 1 (no better than carbon-steel) in earlier trials carried out by Isis Mill. The conditions experienced by the trial tubes in this project were not as severe as those experienced by the tubes in the earlier Isis Mill trials, and this appears to be the main reason for the current improved performance of the S-TEN 1 tubes. This requires further investigation.

Analysis of Leakage and Corrosion of Air Preheater in Coal-Fired Power Plant

A unit of a coal-fired power plant (300MW) in North China was selected to study. The flue gas parameters (NH3, O2 and flue gas temperature) at the outlet of denitration system, oxygen content at the inlet and outlet of air preheater, and composition of combustion coal were tested. Then, the air leakage and corrosion of air preheater were evaluated from the operation of denitration system, air preheater leakage detection, and acid dew point calculation of flue gas. The influence of ammonium hydrogen sulfate from denitration system on the downstream air preheater is limited due to the low ammonia escaping. The air leakage rates of the air preheater at the side A and B were 8.45% and 3.01%, which are higher than the normal value (< 3%). Due to the air leakage of air preheater on side B, the cold end temperature of air preheater is significantly lower than the acid dew point of flue gas, resulting in the condensation of sulfuric acid mist in flue gas on the air preheater and the sulfuric acid corrosion on the air preheater.

Efficient and robust methods for direct saturation point calculations

New algorithms for the calculation of bubble and dew points of binary and multicomponent mixtures are presented. The proposed numerical methods are based on the derivation of the equations that govern phase equilibrium, by starting from the stability criterion of Gibbs and applying a modification, thus resulting in new sets of independent variables and iterative procedures. An additional change of variables is performed to obtain optimal scaling in the minimization problem which is nested in the proposed iterative schemes and thus obtain methods with improved speed and robustness. The algorithms can be applied to calculate at will the lower/upper pressure or low/high temperature bubble or dew point parts of phase diagrams, thus being robust tools when retrograde regions are considered. A simple and widely used initialization method is utilized at low and elevated pressures, while implementation guidelines are given to ensure robust iterative procedures. The proposed methodologies can be applied with or without the use of derivatives of the fugacity coefficients, although their use is strongly advised (if available), since the proposed methods become significantly faster. The new algorithms are tested by calculating saturation points of binary and multicomponent mixtures and prove to be efficient and robust, even in the proximity of critical points in some cases.

A hygrothermal green roof model to simulate moisture and energy performance of building components

Green roofs enjoy a good reputation concerning comfort in summertime, energy savings, rain water retention and durability. However, applied on moisture sensitive substrates like timber constructions, their high thermal inertia and limited drying potential may lead to failure, especially in case of insufficient design or workmanship. As the conditions below the greenery cannot be evaluated by dew point calculations, only a more detailed simulation allows for a realistic prediction of the hygrothermal performance of the components. This contribution introduces a new hygrothermal green roof model based on field and laboratory tests. In difference to previous models it focuses on the moisture and temperature conditions beneath the greenery using more simple approaches for the plant cover but more detailed approaches for the moisture balance in the drainage and growth media. Therefore material data for moisture storage, vapor diffusion and liquid transport were determined in the laboratory and adapted to the built in situation by the help of additional field tests. The model is validated by the help of measured temperature conditions beneath the green roof test fields, as well as concerning the moisture conditions in the critical layers of sensitive timber roofs! The validation shows that the performance of the green roof can be well reproduced throughout the whole year and under all climate conditions including rain water absorption and storage, evaporation and freezing influences. Thus it can be used for both, moisture and energy performance simulations of roofs with greenery under different climatic conditions.

Aerodynamic and Radiative Controls on the Snow Surface Temperature

Abstract The snow surface temperature (SST) is essential for estimating longwave radiation fluxes from snow. SST can be diagnosed using finescale multilayer snow physics models that track changes in snow properties and internal energy; however, these models are heavily parameterized, have high predictive uncertainty, and require continuous simulation to estimate prognostic state variables. Here, a relatively simple model to estimate SST that is not reliant on prognostic state variables is proposed. The model assumes that the snow surface is poorly connected thermally to the underlying snowpack and largely transparent for most of the shortwave radiation spectrum, such that a snow surface energy balance among only sensible heat, latent heat, longwave radiation, and near-infrared radiation is possible and is called the radiative psychrometric model (RPM). The RPM SST is sensitive to air temperature, humidity, ventilation, and longwave irradiance and is secondarily affected by absorption of near-infrared radiation at the snow surface that was higher where atmospheric deposition of particulates was more likely to be higher. The model was implemented with neutral stability, an implicit windless exchange coefficient, and constant shortwave absorption factors and aerodynamic roughness lengths. It was evaluated against radiative SST measurements from the Canadian Prairies and Rocky Mountains, French Alps, and Bolivian Andes. With optimized and global shortwave absorption and aerodynamic roughness length parameters, the model is shown to accurately predict SST under a wide range of conditions, providing superior predictions when compared to air temperature, dewpoint, or ice bulb calculation approaches.

Characterisation of a microwave re-entrant cavity resonator for phase-equilibrium measurements and new dew-point data for a (0.25 argon + 0.75 carbon dioxide) mixture

An apparatus based on a microwave re-entrant cavity resonator, originally built for accurate measurements of the dielectric permittivity of natural gas mixtures, was refurbished and extensively characterised. This was done to enable the future investigation of phase equilibria and (p, ρ, T, x) behaviour of fluid mixtures utilizing the present experimental technique. Vacuum resonance frequencies and Q-factors of the resonator’s modes were modelled using both analytic and finite element methods, and found to compare well with experimental values. The finite element models helped to identify two whispering gallery-type modes not previously reported for such cavity geometries. The models also predict distributions of the electric and magnetic fields in the re-entrant cavity resonator useful for identifying regions in the cavity more sensitive to the presence of a liquid. Following the resonator’s characterisation, its ability to measure dew points was tested using a gravimetrically prepared (0.2501 argon+0.7499 carbon dioxide) mixture over the temperature range from (252 to 280)K at pressures from (2.8 to 6.9)MPa. The combined expanded uncertainty with a level of confidence of approximately 95% (k=2) in dew-point temperature and pressure ranged between (0.025 and 0.044)K and from (0.009 to 0.015)MPa, respectively. We compared the experimental dew-point pressures with the recently developed multi-parameter equation of state optimised for combustion gases (EOS-CG), showing relative deviations in the range of (0.044 to 0.479)%. In contrast, relative deviations of up to −4% from the GERG-2008 equation of state and two different implementations of the Peng-Robinson equation were observed, although these models are not optimised for dew point calculations of carbon dioxide rich mixtures.

A consistent method for simultaneous calculation of upper and lower dew point pressures of gas condensate fluids

Gas condensate reservoir fluids exhibit unique thermodynamic behavior which is considered as the controlling factor to develop these types of reservoirs. In the region between critical and cricondentherm temperature, two dew points are observed called the upper and lower dew pressures. In general, estimating the dew points are performed applying separate algorithms based on initial guesses for upper and lower dew point pressures. In this paper, an efficient and accurate algorithm is developed to simultaneously calculate the upper and lower dew pressures. The new algorithm is based on approximating the objective function for dew points on Chebyshev polynomials to get a closed form expression at a specified temperature which enables one to find all physically meaning roots, i.e., upper and lower dew point pressures. Examining the accuracy of the new technique shows that Chebyshev polynomials approximate the objective function with a high degree of accuracy which in turn is requisite to accurately calculate the dew points. Estimating dew point pressures of different gas condensates exhibits the capability of the new algorithm to accurately predict the upper and lower dew point pressures. A significant feature of the new method is that it is free of estimating an initial guess for dew point pressures. An efficient and adaptive procedure is developed to simultaneously predict the upper and lower dew points curve. The procedure systematically finds the next temperature for dew point calculation. Increasing the temperature, in a specified region of temperature, depending on the system under study, the vertical distance between upper and lower dew points decreases until they become identical at cricondentherm above which no roots are found. The new method successfully predicts the dew points, specially, in the aforementioned region. Applying the newly developed method to predict dew points curve of synthetic gases and gas condensates allows concluding that the new method successfully predicts the dew points boundary. The results also exhibit good agreements when compared to the experimental dew points.

Investigations into the air heater ash deposit formation in large scale pulverised coal fired boiler

A mineralogical study was undertaken of air heater deposits in a 300MWe pf boiler located in Western Australia in order to understand deposit formation in the air heater sections of the boiler as an aid to implement possible remediation actions. Several air heater deposit samples were collected in the selected regions of the air heater along with samples of the feed coal, bottom ash and fly ash for comparison of ash chemistry and mineralogy. The deposit samples were examined using a combination of ash chemistry and quantitative X-ray diffraction analysis; the ash samples by bulk chemical analysis to determine the major element chemistry and mineralogy by quantitative X-ray diffraction.Chemical and mineralogical analysis showed that the deposits are unusual in containing high amounts of sulphate, particularly of aluminium and, to a lesser extent, iron. From the analyses it appears that the formation of the deposits is due to the high sulphate content which is acting as a cementing agent. There is an indication that temperature could be another factor in formation of the deposits, with a decrease in temperature leading to the formation of sulphurous acid which then reacts with the reactive glassy amorphous fly ash phase to form the aluminium and iron sulphates. Dew point calculations indicated that this is a possible deposit formation mechanism based upon the air heater temperature data obtained from the utility. There was no evidence that unburnt carbon has played a significant role in deposit formation. Large temperature fluctuations resulting from the inherent nature of the operation of the air heater are a significant factor in deposit formation and a practical solution to consider would be the use of an SO2 absorbent placed prior to the air heater.

Molecular Simulation for Thermodynamic Properties and Process Modeling of Refrigerants

The calculation of mixture dew and bubble points and the simulation of fluid processes occurring under specified enthalpy or entropy changes are important in the refrigeration and other industries. These calculations are typically carried out using an empirically based multiparameter equation of state approach, such as that incorporated in the industry-standard REFPROP software package. We consider the capabilities of more fundamentally based molecular simulation methodology for performing these calculations. This approach requires a much smaller parameter set, can reliably extrapolate beyond the thermodynamic conditions used to determine the parameters, and can be used for the prediction of multiple properties. We briefly review relevant algorithms and focus on a set of Monte Carlo molecular simulation methods for accomplishing the tasks. We demonstrate their applications to the calculation of isoenthalps for the two pure fluid alternative refrigerants R134a (CH2FCF3; 1,1,1,2-tetrafluoroethane) and R143a...

Development and evaluation of animal models for sex steroid deficient dry eye

Purpose: The present study was conducted to develop animal models to mimic postmenopausal/androgen deficiency dry eye and to evaluate the expression of sex steroid receptors (NR3A1, NR3A2 and NR3C4) in the ocular tissues of the developed models. Methods: The study was conducted in healthy Wistar rats of either sex weighing 180–250g. Bilateral ovariectomy was performed in female rats and oral finasteride (dose of 1.16mg/kg/day) challenge was given to both male and female rats. Along with time tear film stability was assessed by using cotton thread method and tear breakup time (TBUT). Dew point calculation was done using August–Roche–Magnus approximation during the tear assessments to correlate environmental factors affecting the tear function tests. At the end, animals were sacrificed and ocular tissues (lacrimal gland and cornea) were subjected for the quantification of the expression of NR3A1 (ER-α), NR3A2 (ER-β) and androgen (NR3C4) receptors. Results: The impact of ovariectomy caused a significant tear film deficiency from the 20th day onwards in all female rats. The ten day finasteride administration also showed a significant tear film deficiency in both male and female rats. However, subjecting 60days post ovariectomy rats to finasteride challenge did not show any further decrease in tear flow. Gene expression analysis also revealed a significant downregulation of sex steroid receptors in ocular tissues after ovariectomy and finasteride challenge. Discussion and conclusion: From this study, it has been concluded that ovariectomized and finasteride treated antiandrogenic models produced a significant tear deficiency in the rats which can be explored for pharmacological screening of topical agents and understanding the disease process in postmenopausal and androgen deficiency dry eye disorders.

Considerations for the dew point calculation in rich natural gas

Hydrocarbon Dew Point (HCDP) in natural gas processing is a key quality parameter and a critical consideration for pipeline operations. In the present paper, a comparison between equations of state approaches is presented by estimating HCDP in rich natural gas using different approaches, i.e. PR, SRK or GERG-2004 EoS. Further, a parameter comparison is performed discussing the impact on the accuracy of estimating HCDP in natural gas using the selected approaches. The following parameters are compared: binary interaction coefficients, C7+ characterization methods, critical properties for single-carbon number groups and mixing rules. A set of considerations is presented in order to guide users to choose the most accurate right method for calculating HCDP in natural gas.

Advances in hygrothermal building component simulation: modelling moisture sources likely to occur due to rainwater leakage

Moisture control design of building envelope components by hygrothermal simulation is state-of-the-art today. For architects and engineers, there are several simulation tools available which comply with the European Standard EN 15026 and/or the North-American moisture control standard ANSI/ASHRAE 160. Numerous investigations have shown that hygrothermal simulation models are clearly superior to steady-state dew-point calculations in predicting the climate dependent moisture behaviour of construction assemblies. However, compared to real life, there remains a major deficit: most models assume a perfectly assembled and installed building component without any cracks or gaps that may cause unintended moisture entry. This draw-back has been recognized and new approaches have been developed to allow for imperfections by introducing source terms in the models, e.g. accounting for the effects of rainwater penetration. The magnitude of these moisture sources is still a matter of discussion because they should represent best practice situations and not poor workmanship. This paper gives an example of a simplified approach to consider rainwater leakage in one-dimensional hygrothermal building component simulation.

It's Not as Easy as it Looks: Revisiting Peng—Robinson Equation of State Convergence Issues for Dew Point, Bubble Point and Flash Calculations

The use of equations of state (EOS) is a ‘modern’ method of physical property prediction and here we present a detailed exposition of the use of the Peng–Robinson EOS for mixtures. This EOS can be used to determine whether a stream is in the vapor phase or liquid phase. However, for two-phase systems the approaches presented in standard textbooks for the calculation of the state frequently will not converge. We provide modifications to standard dew point, bubble point and flash algorithms to help improve the convergence of any EOS. Commercial software can show different values for stream energetic properties (entropy values) and here we utilize species formation from its elements as well as species ideal gas capacities and mixture departure functions to explain observed differences.

Investigating the dynamic changes of the vapour concentration in a Vapour Phase Soldering oven by simplified condensation modelling

The dynamic changes of the vapour concentration during Vapour Phase Soldering (VPS) process was investigated by a board level modelling approach of vapour condensation. This model is based on a process (machine) level model which calculates the vapour and temperature space in the VPS tank. The board level condensation model is embedded into the process level model and runs as a co-simulation. The condensation model applies combined transport mechanisms including heat transport by heat conduction; mass transport by phase change during the condensation of the Galden liquid; and energy transport caused by mass transport. The model uses a special dew point calculation which was developed for Galden liquid by experiments. The model can describe the temperature change of soldered assembly, and the vapour consumption around the assembly. This way the effect of the soldered assembly on the vapour space can be investigated. It was also shown that the heat conduction of the vapour space and the immersion speed of the assembly cannot be neglected.

Modeling and Experimental Studies on Phase and Chemical Equilibria in High-Pressure Methanol Synthesis

A solution method was developed to calculate the simultaneous phase and chemical equilibria in high-pressure methanol synthesis (P = 20 MPa, 463 <T <553 K). Algorithms were developed that explicitly consider the existence of a condensed phase and include dew point calculations. A modification of the Soave-Redlich-Kwong equation of state was used to correct for nonideal effects. Binary interaction coefficients were derived from literature data on high-pressure binary vapor liquid equilibria. Predicted equilibrium conversions, with and without formation of a liquid phase, were successfully verified with new experimental results on high-pressure methanol synthesis obtained in a packed bed methanol synthesis reactor. Experimental data coincide very well with model predictions for the equilibrium conversion and gas composition. Remarkably, in some situations the model calculations appeared to predict condensation followed by a disappearing liquid phase (retrograde-like behavior) with increasing extent of the methanol synthesis reactions. Finally, at equilibrium only a gas phase remained present.

Development of a mathematical model for the quantification of fog-collection

With the impacts of climate change and impending crisis of potable water, potential sustainable sources of water are under investigations. One of such potential sources is fog water collection. In the past, collection of fog water was considered negligible and non-worthy. However, recent studies suggest that fog-water collection can be a reasonable sustainable source of water depending on favorable topographic and atmospheric conditions. This paper presents development of a mathematical model for fog-formation and eventual quantification of fog-water collection. The National Oceanic and Atmospheric Administration (NOAA) equation for dew point calculation has been modified and extended to incorporate effects of different climatic parameters on fog formation. A new parameter named Fog Potential Index (FPI), which is a function of difference between dew point and air temperature for a particular location has been introduced and defined. NOAA proposed an equation to calculate dew point based on actual water vapor pressure in the air, which can be calculated from dry bulb temperature of the air and relative humidity. Following calculation of average dew point using NOAA equation between a timestep, FPI was calculated for a particular time step. FPI for each timestep was multiplied with the humidity factor, wind velocity factor and area of the fog collector to incorporate the effects of humidity ratio, wind velocity and fog collector area in the quantification of fog water. Effect of wind velocity on fog formation has been discussed. It was found that wind velocity has got two-fold impacts on the formation of fog; (i) for a lower wind velocity upto a certain threshold value, fog formation increases with velocity as old condensed moist air is continuously replaced by new moist air, and (ii) for a higher wind velocity (beyond the threshold value), fog formation decreases with velocity due to disturbance in the formation of fog. The developed model was calibrated using fog-water collection experimental data from Asir region of Saudi Arabia.

Relationships between Climate Parameters and Forest Vegetation At and Near Digya National Park, Ghana

This paper evaluates the effect of three climate parameters on forest cover in Ghana and the normalized difference vegetation index (NDVI) at Digya National Park derived from Landsat image data. Climate data (temperature, humidity, dewpoint, rainfall) are assembled from statistics provided by Ghana's Meteorological Agency. The study introduces a weighted averaging method by computing weather information from neighbouring stations. Also, this research introduces a model of dewpoints, enabling the direct calculation of dewpoints from temperature and humidity data. The major finding is that while temperature significantly affects forest cover and Park vegetation, dew-points and rainfall do not. The paper suggests where future research may be more fruitful in analyzing the effects of climate on vegetation.

Refrigeration cycle design for refrigerant mixtures by molecular simulation

We describe a molecular simulation methodology to calculate the properties of a vapor-compression refrigeration cycle and its Coefficient of Performance, in the case when the refrigerant is a mixture. The methodology requires only a molecular force-field model for each refrigerant pure component and, for improved accuracy, an expression for the vapor pressure of each pure component as a function of temperature. Both may be constructed by means of theoretical approaches in combination with minimal amounts of experimental data, and the latter may also be estimated by empirical formulae with reasonable accuracy. The approach involves a combination of several available molecular-level computer simulation techniques for the individual processes of the cycle. This work extends our earlier study to cases when the refrigerant is a pure fluid. The mixture refrigerant simulations entail the calculation of bubble- and dew-point curves for the refrigerant mixtures, and we propose a new approach for dew-point calculations via molecular simulation. We compare results for a test case with those obtained from the Equation-of-State model used in the standard REFPROP software and with experimental data for a commercially available refrigerant mixture of R32 (CH2F2) and R143a (CH2FCF3).

Thermophysical Properties of Dry and Humid Air by Molecular Simulation Including Dew Point Calculations with the Mollier Ensemble

A pseudoensemble, the Mollier ensemble, is proposed for the dew point calculation of humid air systems. This approach combines features of the isobaric−isothermal (NpT) ensemble and the grand-canonical (μVT) ensemble. The molecular model for dry air is taken from previous work and compared to experimental data as well as to two recommended reference quality equations of state regarding vapor−liquid equilibria, thermal, and caloric properties. An excellent agreement is found for temperatures up to 1000 °C and pressures up to 200 MPa. For water, two different molecular models are considered, the popular TIP4P model from the literature and an optimized version that better describes the vapor pressure. Both water models are used in combination with the molecular dry air model to predict the compressed gas density as well as the dew point of humid air at 60 and 80 °C for elevated pressures up to 25 MPa. The results are in very good agreement with experimental data from the literature.

Calculation of Phase Equilibrium of Natural Gases with the Peng-Robinson and PC-SAFT Equations of State

Cubic equations of state (EOS), such as the Peng-Robinson EOS, are routinely used by the oil and gas industry for the design of recovery and processing operations of gases at high pressures. Nonetheless, many non-cubic equations of state, derived from statistical thermodynamics and with solid theoretical basis, have been proposed in recent years. Among them, EOS of the SAFT-family are possibly those with widest acceptance. In this work, we compare the performance of the Peng-Robinson and PC-SAFT EOS in the calculation of dew points, bubble points, and critical points of natural gases. Binary interaction parameters in both EOS were set equal to zero in all calculations to test the predictive capability of the models. Calculations were performed for 19 synthetic natural gases for which experimental data are available in literature. For most mixtures, predictions of the PC-SAFT EOS are in better agreement with the experimental data.