Refrigeration is, and has been for some considerable time, and essential aspect of domestic and industrial life. Refrigeration units rely almost exclusively on the cooling effects of a liquid as it expands into the gas phase in a closed loop system. Chloro-fluoro-cabon (CFC) type gases such as R12 (dichlorofluoromethane) have traditionally been used as the refrigerant gas.

The harmful environmental impact of CFC gases has been well documented since Sherwood Rowland and Mario Molina initially linked them with ozone layer depletion over twenty years ago. The ozone layer around the earth acts as a shield protecting us from harmful ultraviolet radiation from the sun. Life would cease to exist if this layer was absent. Exposure to increased levels of ultraviolet radiation can lead to increased incidences of such ailments as skin cancer and cataracts, and may also lead to suppression of the immune system in humans.

Although CFCs are heavier than air, they can persist long enough in the earth’s atmosphere to diffuse into the stratosphere (the second lowest layer of the earth’s atmosphere). This process may take from 2 to 5 years. When they reach the stratosphere, ultraviolet radiation form the sun causes the CFCs to break apart with the release of Chlorine atoms. These atoms can subsequently react with ozone to the extent that one chlorine can break apart more than 100,000 ozone molecules. Chlorine which is produced by natural sources such as volcanoes is readily dissolved in atmospheric moisture and is washed out of the atmosphere in rain. CFCs are not broken down in the lower atmosphere, and do not dissolve in water.

Legislation restricting, or in some cases banning the use of CFCs completely is currently being implemented. The Montreal Protocol directed, for instance, that CFCs should not be produced in the U.S. from 1995. This has resulted in the replacement of CFCs by the more environmentally friendly, but also more expensive, hydro-fluoro-carbon (HFC) derivatives. There is, therefore, both a legislative and an economic incentive for industrial refrigeration plant managers to reduce or eliminate refrigerant gas leaks. Formerly over 70% of the refrigerant gas produced was used for topping up existing refrigeration systems. Additionally a leaking system has to work harder to produce the same cooling effect, thereby adding extra costs in terms of power consumption.

The new HFC derivatives are significantly more environment friendly than the traditional CFCs. The scope, which a particular compound has to deplete the ozone layer, has been quantified in terms of the Ozone Depletion Potentials (ODP). CFCs such as R12 have ODPs of 1.0 compared to values of 0.0 for HFCs. The value for R12 reflects its chlorine content and atmospheric lifetime of approximately 120 years. An equivalent term to describe the effect of a substance on global warming is the Global Warming Potential (GWP). The GWP index for R12 is 3.0 compared to a value of 0.3 for a HFC such as R134a. The lower values for the HFCs are due to their different chemical structures and shorter atmospheric lifetimes (approximately 16 years).

The Biomedical and Environmental Sensor Technology (BEST) Centre at the University of Limerick, in collaboration with Murco Ltd.,Dublin, is currently involved in the characterisation and development of refrigerant gas sensors, especially those sensors used to detect the new HFC type gases such as R134a (tetrafluoroethane) and R125 (pentafluoroethane). A schematic representation of the dedicated gas test rig used in this work is shown in figure 1.

Figure 1. Schematic of Refrigerant Gas Sensor Test Rig

Two types of commercially available tin oxide sensor have been studied. Tin oxide is used in such sensors because it gives a reversible resistance change which may be correlated with the level of gas present. However, tin oxide reacts with a wide range of gases. Consequently, much of the sensor development work in this area of sensor technology is related to improving the specificity of the devices. As well as establishing the sensitivity and selectivity of these sensors to HFC type refrigerants, long-term stability of the devices and their immunity to interferent gas effects, e.g. humidity may be readily evaluated using the semi-automated test rig. An advantage of the arrangement shown is that the performance of the sensors is evaluated under gas-flow conditions which are in general more likely to occur in the field than the static conditions used to generate manufacturer’s data sheets. 

One would expect that increasing the flowrate would change the overall resistance of the sensor due to a cooling effect. It is interesting to note, however, that once the sensor resistance is normalised, there is very little difference in the response of the sensor under flow conditions (typically 200ml/min) to that obtained under the static test regime, figure 2. The selectivity problem associated with this sensor type is confirmed when the sensor is exposed to a combustible gas such as methane (figure 3), as a significant response is noted. The overall objective of the joint Murco-BEST project is to develop sensitive and selective refrigerant gas sensors, which are commercially viable.