In this study, we analyze factors affecting the explosion limits of flammable refrigerants. We conclude that any method used for measuring flammable refrigerant explosion limits has its conditional restrictions. Flammable refrigerants in the atmosphere can also explode under certain conditions, when the concentration is approaching the explosion limits. An experimental study on the explosion limits of six kinds of flammable refrigerants is carried out with a mixture of refrigerant and combustible refrigerant, which has a similar effect to a flame retardant. An experimental apparatus was designed to test the explosion limits of mixtures made from three different nonflammable refrigerants and six different flammable refrigerants. Two practical models were developed to estimate the critical concentration for inhibiting explosion of refrigerant mixtures: one was made up of two flammable components with one nonflammable component, and the second was made up of one flammable component with two nonflammable components.
Some refrigerants that are currently used as substitutes for CFCs and HCFCs are flammable. If these flammable refrigerants leak into the environment and the resulting air to refrigerant ratio is within a certain limiting range, flames are able to propagate [
The lowest concentration of flammable gas in the air at which flames can propagate, is called the Minimum Concentration of Flame Propagation. Correspondingly, the highest concentration is called the Maximum Concentration of Flame Propagation. These limiting concentrations for flame propagation are also referred to as the Flammability Concentration Limit. A mixture of flammable gases with air, which is within the Flammability Concentration Limit, could ignite instantaneously and cause an explosion under certain circumstances (for example, in a sealed vessel). Therefore, at atmospheric pressure, the Flammability Concentration Limit can be called the explosion limit. It is important to determine and control the explosion limits of flammable refrigerants so that they can be safely used.
The explosion limits of flammable refrigerant gases are related to the following factors.
The molecular structure and reactivity of flammable gases are closely related to their explosion limits. As shown experimentally, hydrocarbons with a C-C single bond are very stable because the carbon bond is stable with low reactivity. Since these molecules are hard to destruct, they have a relatively narrow range between upper and lower explosion limits when compared to other hydrocarbons. In contrast, hydrocarbons with C≡C triple bonds have a relatively large range, as their carbon bonds are fragile, and their molecules are easy to break, having strong chemical reactivity [
The purity of flammable gases could affect their explosion limits, because the presence of inert gases in unpurified flammable gases will decrease the range of explosion limits. Inert gases like nitrogen, carbon dioxide and water vapor influence the explosion limits by diluting the flammable gases, isolating oxygen, and cooling the gases. When there are alkyl halides in the flammable gases, these alkyl halides can not only dilute, isolate and cool, but more importantly, they can also chemically inhibit combustion and explosion reactions. Furthermore, alkyl halides also have the ability to increase the minimum explosion limits and ignition energies of flammable refrigerants, which will significantly reduce the explosion limit range. For these reasons, most of the gas-extingui- shing systems comprise alkyl halides.
If flammable refrigerant vapors are mixed homogeneously with air and the concentration of flammable mixtures at some point reaches the explosion limits, the concentration of flammable mixtures in the whole mixing space will reach explosion limits. In this case, combustion and explosion reactions will occur in this gas mixture space at the same time and will continue without interruption, which leads to an increase in the explosion limit range. However, if they are mixed heterogeneously, the concentration of flammable mixtures in that mixture will reach or exceed the explosion limits, and the concentration of other flammable gases will not reach explosion limits. Consequently, the reactions will be interrupted, which means the mixtures will have narrower ranges of explosion limits.
Ignition sources that can cause flammable refrigerant vapors to combust and explode are mainly open fires, lighted cigarettes, lighted matches and burning candles. Electronic sparks made by electrical switches, electrical wire shorts and static electricity, and sparks generated by mechanical impacts and friction can also cause fire. Different ignition sources have different ignition temperatures or energies [
Explosion limits of flammable gases are measured in closed vessels. The geometry, size and thermal conductivity of a vessel’s wall material influence the heat dissipation capacity of a flammable mixture. If the surface area of a vessel is large, and the wall material’s thermal conductivity
Raising the temperature of the flammable mixture can increase the rate of combustion or explosion and increase the temperature of the reaction, which results in expanding the range of explosion limits. With increasing pressure of a flammable mixture [
Currently, most of the published data on explosion limits of flammable refrigerants is measured using small ignition sources (most of initiation energy is under 100 J), in small explosion vessels (0.001 - 0.005 m3), at normal temperatures. According to relevant research results, the data is close to real life situations when the initiation energy is 10,000 J using a vessel of 1 m3. Therefore, when choosing the parameters of a flammable refrigerant’s explosion limits, it is essential to determine the test conditions and consider safety factors. Based on national fire defense regulations, it is necessary to consider the mixing properties of leaked flammable gases with air, and the difference between the experimental value for the explosion limits and the actual value. Taking this into consideration, the controlling range is between 0.5 times the lower explosion limit to 1.5 times the upper explosion limit.
A variety of Halon substitutes are currently used as fire-extinguishing agents. Examples include R134a in FE-24, R125 in FE-25, R227 in MT200 and CF3I [
This is the initiation process of a chain reaction, and further reaction results in the formation of free radicals:
The last step of this reaction can release a huge amount of heat and regenerate Ooo free radicals. If there are no inhibitors or buffering agents, the chain reaction will continue automatically, with the flammable gas combusting or exploding. If the combustible gas contains inhibitors, these inhibitors will be decomposed by heat. The inhibition of the chemical reaction is as follows:
The Io free radical then reacts with the combustible gas to produce HI:
HI reacts with OHo from Formula (2), removing OHo and regenerating Io. The reaction is:
Io from this regeneration and Io from decomposition of CF3I, will take part in a chain reaction, which according to Formulas (5) and (6) result in the constant elimination of active free radicals, such as OHo, Ho, Ooo. The chain reaction responsible for the suppression of fire is the opposite of the chain reaction which forms part of the process of combustion and explosion. Previous inhibition of “chain” interrupted latter a “burning chain”, to control the combustion and fire. The chemical inhibitory effect of CF3I on the flammability of hydrocarbons is also known as a negative catalytic effect.
To quantitatively analyze fire-suppressant mixtures that affect the explosion limit of flammable refrigerants, this experiment will determine the explosion limit of flammable gases by the national standard GB/T12447-90. Six flammable refrigerants, R290, R600, R600a, R32, R143a and R152a, are used in this experiment. Using R134a, R125 and R227ea to influence the explosion limits of the six flammable refrigerants, we obtained the following results.
The inhibitory effect of R134a on the six flammable refrigerants’ explosion limit is shown in
The inhibitory effect of R125 on the six flammable refrigerants’ explosion limit can be seen in
The inhibitory effect of R227ea on the six flammable refrigerants’ explosion limits is shown in
Figures 1-6 show the experimental curves for the impact of three nonflammable refrigerants on the explosion limits of six flammable refrigerants.
The inhibition efficiency of nonflammable refrigerants on the explosion limits of flammable refrigerants is influenced by: their chemical properties; temperature; ignition energy; test container volume: container geometry; the degree to which the flame has spread; and other factors. If the need for inerting concentration of HC compounds is higher than that of HFC compounds, and the ratio of F atoms in combustible HFC compounds is greater than the hydrogen atoms, the requirement for the inerting concentration will be lower. Furthermore, the inerting concentration will increase with an increase in the flammable refrigerant’s temperature. The degree to which temperature
Refrigerant code | R290 | R600 | R600a | R32 | R143a | R152a |
---|---|---|---|---|---|---|
Molecular formula | C3H8 | C4H10 | C4H10 | CH2F2 | CH2FCHF2 | CH2FCH2F |
R134a | 13.87 | 10.12 | 10.8 | 8.11 | 8.82 | 15.13 |
R125 | 10.53 | 9.50 | 9.60 | 4.32 | 5.21 | 7.15 |
R227ea | 7.86 | 7.53 | 7.59 | 3.37 | 4.23 | 5.67 |
Note: The experimental conditions are: room temperature (17˚C to 23˚C), ambient atmospheric pressure, and 200 W ignition power.
will influence the inerting concentration is based on the variety of flammable refrigerants. Nonflammable refrigerants have a large inhibitory effect on the upper explosion limit, and a small inhibitory effect on the lower explosion limit.
For a fixed test system or device, the inerting concentration will increase with an increase in ignition energy; the degree to which it will increase depends on the type of flammable refrigerant, with the impact on HFC of flammable refrigerants being significantly greater than on HC. When measuring the inerting concentration using small containers, high ignition energy will cause the inerting concentration to significantly increase. In contrast, with low ignition energy (less than 100 J), the volume of the test device has little influence on the inerting concentration. An increase in the degree of flame diffusion within the test device will result in a decrease in the inerting concentration, and this effect is huge when the ignition energy is less than 100 J.
From the above analysis we can conclude that non-combustible HFC compounds in a small proportion cannot effectively inhibit the explosion limit of an HC refrigerant. Whereas using CF3I and other efficient fire-extinguishing agents in a relatively small proportion can successfully control the explosion limit of HC compounds. According to 1 to 3 experimental data,
sion limits of R290 and R600. The curves obtained are the same with the changing of Figures 1-6, which indicates the versatility of dealing with experimental data in this test.
A key problem when faced with safely using combustible refrigerant mixtures is the question of how to determine the inhibiting explosion limit concentration of mixtures with combustible components. In this paper, a new method is put forward, which makes the number of components of the working group equal to the number of non- combustible workers. When combining one combustible component with one noncombustible component to form a new combustible mixture, we can use the correlation equations to compute the volume of each pair such that the ratio of each pair’s concentrations reaches the critical concentration for combustion.
Let A and B be flammable refrigerants, and C be a nonflammable refrigerant. The volume of the components is VA, VB, VC. When the flammable refrigerant’s VA/VB has been determined, we can use the equations below to calculate the volume of the combustible component VA, VB and nonflammable component’s critical volume VCR.
Using Equations (7) through (11), the following equation group can be derived:
VCAR is the volume of component C (%), when the binary mixture of C and A reaches the ratio (RAC defined below) of the critical flammable volume. Similarly, VCBR is the volume of component C (%), when the binary mixture of C and B reaches the ratio of critical flammable volume. RAC is the critical flammable volume ratio of A and C mixed refrigerants; RBC is the critical flammable volume ratio of B and C mixed refrigerants. Formula (12) is an iteration form with different values of X, and
Let A, B and C represent the flammable refrigerant and the two nonflammable refrigerants respectively, and VA, VB and VC represent their separate volumes. When the nonflammable refrigerant VB/VC is known, VA, VB and the critical volumetric fraction VAR of A can be calculated using the following equations:
Using Equations (13) through (17), we derive the following equation group:
VABR is the volume ratio of the explosion limit (%) when the mixture with A and B reaches the critical suppression explosion concentration. VACR is the critical volume ratio of the explosion limit (%) of the mixture with A and C. Equation (18) is the iterative form with a different value for Y.
A model for estimating the two mixtures’ critical suppression explosion concentrations is established. One of the mixtures consisted of two flammable components with one nonflammable component, and the other consisted of one flammable and two non-
flammable components. The critical concentration ratio can be obtained from the explosion limit curve described in this study.
Since the model calculation is based on experimental data, the calculation results are consistent with experimental results. The relative error between the calculated results and the experimental results is generally less than 10%. The model is used in conjunction with the explosion limit curves of 7 refrigerants and 14 groups of mixed refrigerants described in this paper, and can be used to evaluate critical suppression explosion concentrations of ternary mixtures with 7 refrigerants and any nonflammable refrigerants. Using this calculation method to guide the critical inhibition explosion concentration experiments for ternary mixtures with nonflammable refrigerants, can greatly reduce the number of experiments and hence the cost.
From the above analysis we can draw the following conclusions:
1) The explosion limit of flammable refrigerants in all published papers, is generally obtained using a small explosion vessel and a small ignition source [
2) As there are many methods to determine flammable gas explosion limits [
3) A model to estimate two mixtures’ critical suppression explosion concentrations is established. The model of two mixtures is two flammable components with one nonflammable component and one flammable component with two nonflammable components. The critical inhibition explosion concentration ratio determined by using this model can be used for inhibition of flammable refrigerant mixtures.
4) Flammable refrigerants such as R290, R600 and R600a are ideal substitutes for CFCs and HCFCs, but they can only be used in refrigerators with small volume since their flammability limits their use [
Tian, G.S., Li, X.Q., Gao, Y.F. and Zhang, F.X. (2016) Theoretical and Experimental Study of Explosion Limits and the Inhibition of Flammable Refrigerants. Journal of Software Engineering and Applications, 9, 501-515. http://dx.doi.org/10.4236/jsea.2016.910033
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