Hydrocarbon refrigerants are environmentally friendly and many countries have started using them in refrigeration and air-conditioner systems. Propane, a typical hydrocarbon, does not damage the ozone layer and its effect on global warming is very small. Use of propane as a refrigerant is gaining favour, especially in systems designed for R-22. The main drawback of using propane as a refrigerant is its flammability. To reduce the risk posed by propane, the total mass of propane in the system should be minimized without degrading the system performance. This work deals with how the mass of propane within a capillary tube can be reduced without compromising the system performance. In the first step, the mass flow rate and inlet and outlet conditions of the refrigerant within the capillary tube of a commercial propane air-conditioner under normal working conditions were measured. In the next step, a parametric MATLAB code was developed to determine the required length and volume of the capillary tube, and the velocity and mass of the propane within the capillary tube. The input parameters of the code are the inside diameter, roughness, shape, mass flow rate, and inlet and outlet conditions. The script was run for sixteen common types of capillary tubes, including the one in our laboratory. According to the results, it is possible to considerably reduce the propane mass within the capillary tube using a small inside diameter helical capillary tube with a decreased coil diameter. However, the results should not be extended to an air-conditioner in which the distance between the outlet of the condenser and the inlet of evaporator is fixed and longer than the calculated length.
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