ACR Journal

CMYK / .ai CMYK / .ai CMYK / .ai acrjournal.uk 37 TRAINING Evaporating R452A, operating with suction pressure of 1.5barg, has a bubble point temperature of -26.27°C and a dew point temperature of -22.37°C. This means the first part of the refrigerant composition starts to evaporate at -26.27°C. The first vapour bubble forms at -26.27°C, and as it continues to absorb energy, the refrigerant evaporates until the last composition of the liquid has turned to vapour. This moment is called the dew point - a term used to describe when the last droplet of the liquid refrigerant evaporates into vapour, and this is when the refrigerant temperature reaches -22.37°C. Any further absorption of energy will cause the vapour to become superheated. When calculating superheat values, we subtract the dew point temperature at the evaporator outlet pressure from the evaporator outlet suction line temperature. Glide Zeotropic refrigerants have a mixture of refrigerants that make up their composition, so it is important to know that the di… erence between a refrigerant bubble and dew point temperature is referred to as glide. In the evaporator, the glide is the di… erence between the bubble point (the temperature at which liquid starts to evaporate) and the dew point (the point in which all liquid has turned to vapour). In the case above, the glide would be 3.9 Kelvin (K) (-26.27°C - -22.37°C) and would result in an average evaporating temperature of -24.32°C; halfway between bubble and dew point temperatures. Regarding the condenser, the glide is still the di… erence between bubble and dewpoint. Comparator Let’s look at this on a refrigerant comparator, traditionally a handheld slider, but now much easier to use via apps such as the Danfoss Refrigerant Slider. If a system operates with suction pressure of 1.5 Barg, as it enters the evaporator, it starts to evaporate at a bubble point temperature of -26.27°C. Changing over to the dew point at the same system pressure, the last part of the refrigerant mixture liquid is evaporating at -22.37°C; this is the dew point. It is important to remember that when we are calculating our superheat value at this point, it is our dew point temperature that we need to subtract from our evaporator outlet suction line temperature. The di… erence between the bubble point and dew point at this moment is our temperature glide. With regards to the condenser side of our system, operating with a discharge pressure of 18.06 barg the bubble point is 40°C, and our dew point is 43.31°C. Therefore, as the refrigerant enters the condenser, the first part of our refrigerant starts to condense (dew point). As further energy is rejected, the refrigerant continues to condense, and the bubble point is reached – the point at which the last vapour bubble condenses. It is important to remember that when we calculate our subcool value, our bubble point temperature is subtracted from our condenser outlet liquid line temperature. Only by having a firm understanding of the properties of the refrigerants we use can engineers truly commission and diagnose the systems on which they are working. Automated system diagnostics are a great feature to bridge the skills gap that we currently have, but nothing can replace dedicated fundamental training. Refrigerant flow through an evaporator, illustrating the points at which R452A refrigerant starts to evaporate and at which it is completely vapour Comparator to show refrigerant properties of a zeotropic refrigerant in an evaporator

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