By Phil Boudreau
Approximately 30 years ago, several refrigerants emerged as alternatives for chlorine-containing CFCs. Just a few examples of some of these alternatives are R401A, R401B, R402A, R402B, etc. The majority of these refrigerants were in the R400-series category and are also referred to as zeotropic refrigerants. These refrigerants were blends of pure (single) refrigerants such as R22, R124, R125, R152a, R290, etc. Each of these individual refrigerants have their own pressure-temperature characteristics. So, for a given pressure, these refrigerants change state at different temperatures. For this reason, mixing refrigerants such as these will result in a temperature glide within heat evaporators and condensers as the refrigerant within evaporators and condensers changes state.
To some extent, the maximum theoretical temperature glide will depend on the refrigerant with the lowest saturation temperature and the refrigerant with the highest saturation temperature. However, there are other factors such as the amount of each refrigerant in the blend.
For each pressure value, an R400-series refrigerant has both a bubble point temperature and a dew point temperature.
The bubble point can also be referred to as the saturated liquid temperature. Similarly, the dew point can also be referred to as the saturated vapour temperature.
In the condenser, all of the temperature glide that an R400-series refrigerant exhibits, will be present. This means that after the hot vapour from the compressor is desuperheated, it reaches its dew point temperature. As the refrigerant continues to reject heat, it changes state from a vapour to a liquid. Meanwhile the temperature gradually drops from the dew point temperature to the bubble point temperature. Once the bubble point temperature is reached, there will be no more vapour present. At this point, any further reduction in the heat content of the refrigerant will result in subcooling.
This brings up an important point about checking superheat and subcooling values in a system. Either the dew point or bubble point must be used as the reference temperature. However, if the wrong reference temperature is used, the amount of superheat or subcooling that you read will be off by the amount of temperature glide present. For example, if we want to determine the amount of subcooling present in a liquid line, we will first read the pressure. For this pressure value, there are two reference temperatures. Again, these are the bubble point temperature and the dew point temperature. When determining subcooling values we must always use the bubble point as the reference temperature in order to obtain a correct value.
Many of the printed pressure-temperature charts list only bubble point values in the higher temperature range. For example, they may list bubble point temperatures of 50°F and higher. Similarly, only the dew point temperatures are listed below 50°F. With these types of tables, it is less likely that the technician will use the wrong reference temperature. In other cases, some tables and refrigerant apps for mobile devices provide both the bubble point and dew point temperatures. Therefore, the technician must choose the correct value.
Here is an example to confirm the correct procedure for checking the amount of subcooling present in a liquid line containing R449A. The pressure reading that we measure on the liquid line is 207.8 psig. The temperature that we read is 82°F. According to the mobile app, the dew point temperature is 98.2°F and the bubble point temperature is 90°F. Since we are checking a subcooling value, we must use the bubble point as the reference temperature. Now, we simply subtract the actual temperature that we read on the thermometer, from the bubble point temperature. This results in a subcooling value of 8°R (90°F – 82°F). Note that if we were to use the dew point to determine the subcooling, we would come up with a value of 16.2°R (98.2°F – 82.0°F). This confirms that by using the wrong reference temperature, our subcooling value will appear to be much higher than what we actually have. Specifically, the subcooling will be off by the amount of temperature glide present at 207.8 psig.
Importance of technician’s diagnostics
It is important that the technician can correctly determine the evaporator, compressor suction and compressor discharge superheat. On page 54, is a similar example of a superheat calculation. For this example, we will be checking the evaporator superheat. The pressure reading that we measure at the evaporator outlet is 51.0 psig. The pipe temperature that we read is 36°F. According to the mobile app, the dew point temperature is 24.9°F and the bubble point temperature is 15.3°F. Since we are checking a superheat value, we must use the dew point as the reference temperature. Now, we simply subtract the dew point temperature from the actual temperature that we read on the thermometer at the outlet of the evaporator. This results in a superheat value of 11.1°R (36°F –24.9°F).
Note that if we were to use the bubble point to determine the superheat, we would come up with a value of 20.7°R (36°F – 15.3°F). This confirms that by using the wrong reference temperature, our superheat value will appear to be much higher than what we actually have. Specifically, the superheat will be off by the amount of theoretical temperature glide present at 51 psig.
The term theoretical temperature glide refers to the maximum amount of temperature glide present for a given pressure. In the evaporator, the refrigerant never enters at the bubble point. This is because some of the refrigerant flashed to a vapour in order to reduce the temperature of the bulk of the liquid entering the evaporator. Therefore, the actual temperature entering the evaporator (after the expansion valve), will be higher than the bubble point temperature.
When checking superheat and subcooling values, it is also very important to ensure that the pressure gauge and thermometer that we use are of the correct type for the application, are accurate and calibrated.
There are many electronic manifolds that will read the pressures and temperatures on both sides of the system while conveniently indicating the superheat and subcooling values. Although this simplifies the process, it is important that technicians know how to do this manually.