ACR Journal

June | July 2021 TRAINING 36 Bubble and dew: what’s the point Volume 7 No.4 The fundamental properties of refrigerants are overlooked by a lot of commissioning and diagnostic engineers. Superheat and subcooling values are essential to any refrigeration or air conditioning engineer as they generate a fine line between system reliability and e ective, e cient operation. Too little suction superheat and there is a risk of liquid refrigerant entering a compressor, too much and system capacity and e ciency are reduced. Reduced refrigerant subcooling lowers the amount of energy that can be absorbed in the evaporator; too much can cause liquid to pass through the evaporator and enter the compressor along with reduced condenser capacity. Azeotropic refrigerants are composed of a single refrigerant and only have only one boiling point. This means that it will condense and evaporate at the same temperature at any given pressure. Zeotropic refrigerants, however, have Mark Denford of the Grimsby Institute explains why it’s important to understand refrigerant properties and correctly apply system diagnostics. more than one boiling point; this is due to the refrigerant’s more complex chemical formula or composition. Let’s take, for example, R452A, a low global warming potential alternative for R404A and R507, and used in low-temperature refrigeration. It’s composition is 11 per cent R32, 59 per cent R125, 30 per cent R1234yf. These three refrigerants all have di erent properties for evaporating and condensing temperatures at any given pressure, so a thorough understanding of the bubble point and the dew point is important for system commissioning and diagnostics. Condensing R452A operating at a discharge pressure of 18.06 Barg (Bar Gauge) has a dew point temperature of 43.31°C, and a bubble point of 40°C. When the refrigerant is at the dew point temperature, the first part of the refrigerant composition starts to condense and the first liquid droplet forms at 43.31°C. As further energy is rejected, the refrigerant composition further condenses until all the refrigerant has turned to liquid - this is the bubble point, a term used to describe the moment when the last bubble of the vapour condenses into liquid and would occur when the refrigerant temperature reaches 40°C. When calculating system sub-cooling values, the bubble point temperature on the discharge pressure side of the condenser is used, this temperature is subtracted away from the condenser outlet liquid line temperature. Further removal of energy from this point will subcool the refrigerant liquid. Mark Denford, senior trainer consultant Refrigerant flow through a condenser, illustrating the points at which R452A refrigerant starts to condense and at which it is saturated liquid. Comparator to show refrigerant properties of a zeotropic refrigerant in a condenser