RACA Journal January 2020 | Page 69

Support GRANT LAIDLAW Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale. He holds a Bachelor of Business Administration and an associate degree in educational administration. He has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for 17 years and has been a Johannesburg committee member for the past eight (chairman in 2011 and 2012) . Currently he is the treasurer (Johannesburg council) as well as president (national council). THE NECESSITY OF SUBCOOLING By Grant Laidlaw Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten. Karel asks: Grant,can you explain subcooling please? I feel that this is an important aspect of the system which is not well understood. Why is subcooling necessary? T he term subcooling is the condition where the liquid refrigerant is colder than the minimum temperature (saturation temperature) required to keep it from boiling or refers to a liquid existing below its normal boiling point. The amount of subcooling, at a given condition, is the difference between its saturation temperature and the actual liquid refrigerant temperature. Condenser subcooling ensures that there is liquid exiting from the condenser so that only liquid refrigerant and not vapour exits the condenser. Let us look at why subcooling is desirable. Subcooling increases the efficiency of the system since the amount of heat being removed per kilogram of refrigerant circulated is greater. In other words, less refrigerant is pumped through the system to maintain the refrigerated temperature required. Subcooling is beneficial because it prevents the liquid refrigerant from changing to a gas before it gets to the evaporator. This is very important as vapour entering the evaporator can damage the expansion device casing wire drawing on the needle and seat, resulting in metal fatigue and possible failure. Subcooling reduces the amount of time that the compressor must run to maintain the temperature. The amount of capacity boost resulting from each degree of subcooling varies with the refrigerant being used. Pressure drops in the liquid piping and vertical risers can reduce the refrigerant pressure to the point where it will boil or ‘flash’ in the liquid line. This change of phase causes the www.hvacronline.co.za refrigerant to absorb heat before it reaches the evaporator. Inadequate subcooling, therefore, prevents the expansion valve from properly metering liquid refrigerant into the evaporator, resulting in poor system performance. The amount of liquid subcooling not only affects system capacity but also the effectiveness and capacity of expansion- type metering devices. In the system, subcooling is sensible heat taken away from the 100% saturated liquid point in the condenser. Liquid subcooling then occurs from the 100% saturated liquid point in the condenser to the metering device. Once all the saturated vapour in the condenser phase has changed to saturated liquid, subcooling will start to occur if any sensible heat is taken away. Remember, sensible heat is heat that causes a temperature change. Any drop in temperature of saturated liquid in the condenser will be considered liquid subcooling. We find that we can identify subcooling as condenser subcooling and total subcooling. CONDENSER SUBCOOLING Condenser subcooling is liquid subcooling present in the condenser. It can be measured by subtracting the condenser-out temperature from the condensing temperature. When subcooled, the refrigerant is not generating or losing any vapour pressure. Consequently, there is no pressure-temperature relationship and a pressure-temperature chart cannot be used. The condenser-out temperature must be measured with either a thermistor or a thermo-couple fastened to the condenser outlet. The condensing temperature, on the other hand, can be acquired from the condensing pressure read from a gauge on the high side of the refrigeration system because a pressure-temperature relationship does exist in a saturated condition. RACA Journal I January 2020 67