An Experimental Study of the Lower Flammability Limit of LPG/Halocarbon Mixtures Using the Tubular Burner Apparatus

The motivation for this project is to develop a hydrocarbon-based (ER12) refrigerant that has a lower flammability limit in excess of 5% refrigerant in air. Optimisation of the hydrocarbon content in the mixture, favourable refrigeration properties, minimal toxicity and benign environmental impact form additional constraints. A variety of compounds were considered as potential flammability reducing additives. The flammability of the various mixtures was evaluated using the tubular flame burner technique. The compounds tested in the initial screening study included CO2, CF3I, CHF2Cl (R22), C3HF7 (R227ea), C3F8 (R218), C4F10 (R3110), C4F8O, SF6, CBr2F2, 1-bromopropane, 2-bromopropane, CH2Br2 and CH2BrCl. Strong flammability reduction performance was observed for CBr2F2, CH2Br2, CF3I, C4F10, SF6 and C3F8. Less effective compounds included CO2, C3HF7 (R227ea), CHF2Cl (R22), C4F8O, and CH2BrCl. Estimation of potential non-flammable ER12/additive compositions based on the flammability tests revealed the best additives to be CBr2F2, CH2Br2 and CH2Br2 each with an ER12 content above 20 mass percent. It is concluded that a practical single additive/ER12 mixture with higher than 20% ER12 content is unlikely. Compounds containing bromine and iodine possess superior chemical suppression effectiveness with the strength of that effect increasing in proportion to the level of bromination/iodination. Highly fluorinated compounds have excellent suppression efficiency due to the relatively low atomic weight of fluorine. The presence of hydrogen in an additive compound dramatically reduces the suppression efficiency. Inclusion of oxygen and sulphur in compounds can potentially give very good flammability reduction effects. The major environmental and parameters of atmospheric lifetime, ozone depletion potential (ODP) and global warming potential (GWP) were compiled for the screened and targeted additive compounds, with some of the more favourable species shown to possess relatively high atmospheric lifetime and global warming potential despite having zero ozone depleting effect. The toxicological and safety aspects of the compounds are also discussed as an additional means of evaluation of potential additives.