Flammability Limits In Air Research Articles

Fire Safety of 1, 2 and 2l Refrigerants: Myths and Reality

For many years fire safety measures for refrigerants were based on experimental data on their burning velocity and flammability limits in air. The analysis conducted in the paper and obtained experimental results show that knowledge of the flammability limits and the burning velocities is not enough to ensure the fire safety of the refrigerant. “Low flammable” substances may form explosive mixtures with air in wide range of conditions (initial concentration, pressure, temperature, ignition source (open hydrocarbon flame, hot surface)), even being a part of non-combustible mixture. One of the most underestimated things is an influence of open hydrocarbon flame as ignition source for halogenated substances not just from Class 2L (low flammable), but also from Class 1 (non-flammable). Mixtures of Class 1, 2L and 2 (lower flammable) refrigerants with lubricant oil should be also tested more carefully for their fire and explosion safety. In some cases the danger of “low flammable” substances may be underestimated due to the mismatch between test conditions and real conditions of use. It is also important that due to peculiarities of the process of mixing “low flammable” partially halogenated substances may form explosive mixtures with air at any mass of partially halogenated substance – the influence of mass factor must be also estimated more carefully.

Estimation of upper flammability limits of C–H compounds in air at standard atmospheric pressure and evaluation of temperature dependence

This study focuses on estimating the upper flammability limits of C–H compounds. A method was developed to determine the upper flammability limits in air at standard atmospheric pressure for the following cases: (a) estimation of the UFLs of pure C–H compounds at standard ambient temperature (25°C); (b) estimation of the UFLs of binary mixtures of C–H compounds at standard ambient temperature (25°C); (c) estimation of the UFLs of C–H compounds at different initial temperatures. The method was accurate in all cases. In case (a), for a total set of 115 compounds, the absolute average relative error was 7.27% and a squared correlation coefficient of 0.9248 was obtained. In case (b), the average absolute relative error was 5.55%; in case (c) it was 2.19%.

Neural network/particle swarm method to predict flammability limits in air of organic compounds

A new neural network based on particle swarm optimization was developed to estimate lower and upper flammability limit percent in air of organic compounds using their molecular structures. The capability of this hybrid method was tested using 328 compounds in a correlated set and 90 compounds in a predicted set. The results show that the proposed method can estimate the flammability limits of organic compounds with low deviations and can be used with acceptable accuracy in thermal engineering.

Detailed simulations of the transient hydrogen mixing, leakage and flammability in air in simple geometries

During an accidental release, hydrogen disperses very quickly in air due to a relatively high density difference. A comprehensive understanding of the transient behavior of hydrogen mixing and the associated flammability limits in air is essential to support the fire safety and prevention guidelines. In this study, a buoyancy diffusion computational model is developed to simultaneously solve for the complete set of equations governing the unsteady flow of hydrogen. A simple vertical cylinder is considered to investigate the transient behavior of hydrogen mixing, especially at relatively short times, for different release scenarios: (i) the sudden release of hydrogen at the cylinder bottom into air with open, partially open, and closed tops, and (ii) small hydrogen jet leaks at the bottom into a closed geometry. Other cases involving the hydrogen releases/leaks at the cylinder top are also explored to quantify the relative roles of buoyancy and diffusion in the mixing process. The numerical simulations display the spatial and temporal distributions of hydrogen for all the configurations studied. The complex flow patterns demonstrate the fast formation of flammable zones with implications in the safe and efficient use of hydrogen in various applications.

Lower Flammability Limit of Difluoromethane and Percolation Theory

Measurements of flammability limits by the ASTM E 681 method are believed to be affected by vessel size. In order to investigate the effect of volume on measured lower flammability limits in air, experimental measurements of the lower flammability limit of difluoromethane (R-32) are made at 21°C using 3, 5, 12, and 20 L vessels. A spark ignition source is used with the voltage adjusted to the value where dielectric breakdown just begins to occur (approximately 7–12 kV). The results demonstrated a higher concentration for the lower flammability limit for the vessels smaller than 5 L suggesting that wall effects quench the flame propagation. The lower flammability limit for vessels with a 5 L or greater volume are in good agreement with those obtained in earlier investigations. For 5 L and larger vessels the lower flammability limits systematically shift to higher concentrations of difluoromethane with increasing vessel size which is consistent with a percolation model for spatial flame propagation. Extrapolation of the data to infinite vessel size using a percolation model yielded a lower flammability limit of 15.2 vol.%. The results are consistent with the work of Richard for refrigerant mixtures, R134a/R152a and R125/R152a, in large volume vessels (200 L) where effects of vessel walls are negligible.

Application of Thermochemical Energy Hazard Criteria to the Prediction of Lower Flammability Limits of Hydrocarbons in Air

The energy hazard criteria of ASTM program CHETAH have been tested as criteria of flammability of gaseous mixtures containing an hydrocarbon vapour and air as oxidiser, at lean concentrations. A method for calculating the lower flammability limits in air, at atmospheric pressure and ambient temperature, is proposed as a result of a statistical study, which involved 67 combustible compounds. CHETAH's fourth energy hazard criterion was found to be the most reliable flammability criterion. The method is predictive since all the input data may be obtained with very good accuracy using a group contribution method. It is reliable, predicted limits being more frequently underevaluated, by 0. 11 mol% on average, than overevaluated. The method is also precise, with a standard deviation of the estimates of 0. 14 mol%. It is easily applicable to fuel mixtures and gives the same results as Le Chatelier's law.

Factors Affecting the Solubility of Gaseous HCFC and HFC Blowing Agents

The majority of the current production of rigid polyurethane foams utilizes HCFC-141b (1,1-dichloro-1-fluoroethane) as either the blowing or co-blowing agent to expand the foam. With the phase-out of hydrochlorofluorocarbons (HCFCs) scheduled for the year 2003, the usage of HCFC-141b will need to be reduced or terminated at that time. The existing options for rigid polyurethane foam blowing agents after the phase-out will be hydrofluorocarbon (HFC), carbon dioxide, or hydrocarbon based compounds. One possible candidate for HFC blowing agents is HFC-134a (1,1,1,2-tetrafluoroethane). Reasons for its use include its commercial availability, zero ozone depletion potential, low toxicity, low foam k factors at low temperatures, better dimensional stability, and the lack of flammability limits in air [1,2]. The major challenge in the use of HFC-134a is the difficulty in foam processing due to its low boiling point and moderate solubility in rigid foam polyols. During foam production, the reacting mixture rapidly expands and froths. This expansion can lead to isolated large cells in a matrix of smaller cells (worm hole formation) and can adversely affect the k factor and quality of the foam. A slower expansion is desirable to achieve better processibility and the lowest k factor possible. One way to influence the expansion is to increase the intermolecular interactions between the blowing agent and the foaming mixture. It may be possible to increase these intermolecular forces through the judicious selection of polyols and additives. Moreover, if a good understanding of the intermolecular interactions is gained, an opportunity exists to design rigid foam polyols to meet processing difficulties.

Effect of low temperature on the rich flammability limits in air of hydrogen and some fuel mixtures containing hydrogen

The rich flammability limits in air of hydrogen at atmospheric pressure and for upward flame propagation were established consistently under isothermal initial conditions at low temperatures extending down to −60°C. Moreover, the lean limits were determined down to −100°C. The flammability limits of a range of binary combinations of hydrogen with methane, ethylene and propane were also established. It is shown that the rich flammability limits of hydrogen-ethylene mixtures deviate very significantly from the commonly used predictive technique of Le Chatelier's rule in a wide range of fuel composition. Considerable deviation was also observed for hydrogen-propane and hydrogen-methane mixtures containing relatively small concentrations of propane or methane. The extent of the relative deviation was found to be virtually independent of the initial temperature of the mixtures for the conditions considered.

Effect of low temperature on the rich flammability limits of some gaseous fuels and their mixtures

The rich flammability limits in air of methane, ethane, propane, butane, ethylene and propylene at atmospheric pressure and for upward flame propagation were established consistently under isothermal initial conditions at low temperatures extending down to −60°C. In addition, the flammability limits of a range of binary combinations of their mixtures were also established. On the basis of the experimental data obtained, some guidelines for predicting the rich flammability limits are presented and discussed.

Correlating the Flammability Limits of Fuel-Diluent Mixtures

A simple method based on analysis of experimental flammability limits data is described for the calculation of the lean and rich flammability limits in air of common gaseous fuels (e.g., hydrogen, carbon monoxide, ethylene and propane) with the addition of diluents (e.g., nitrogen, carbon dioxide and helium) and various combinations of their mixtures.

The lean flammability limits in air of methane, hydrogen and carbon monoxide at low temperatures

The lean flammability limits in air for methane, hydrogen and carbon monoxide at atmospheric pressure were established under isothermal initial conditions at temperatures extending down to −130°C. Simple guidelines are then suggested for predicting these limits on the basis of the calculated flame temperature of the limiting mixtures. Estimates were also made of the extent of changes in these limits due to heat transfer over a wide mixture temperature range.

Flammability Limits in Air

Flammability Limits in Air

Seminar on chemical induction of genetic changes in Aspergilli

The rich flammability limits in air of hydrogen at atmospheric pressure and for upward flame propagation were established consistently under isothermal initial conditions at low temperatures extending down to −60°C. Moreover, the lean limits were determined down to −100°C. The flammability limits of a range of binary combinations of hydrogen with methane, ethylene and propane were also established. It is shown that the rich flammability limits of hydrogen-ethylene mixtures deviate very significantly from the commonly used predictive technique of Le Chatelier's rule in a wide range of fuel composition. Considerable deviation was also observed for hydrogen-propane and hydrogen-methane mixtures containing relatively small concentrations of propane or methane. The extent of the relative deviation was found to be virtually independent of the initial temperature of the mixtures for the conditions considered.