RACA Journal June 2016 - Page 66

Essentials Selele Mashilo Selele Mashilo has a mechanical engineering diploma from Tshwane University of Technology and a refrigeration and air-conditioning diploma from Unisa. His experience includes over a decade in government as deputy-director building services before rejoining the private sector in 1998 as HVAC&R project engineer. He is the former chairperson of the Refrigeration and Air Conditioning Empowerment Forum of SA (RAEFSA), the Air Conditioning and Refrigeration Industrial Council of SA (ACRICSA), and Black Energy Services Companies (BESCO). The science of refrigeration continued By Selele Mashilo We continue looking at what is involved in the refrigeration process and the properties of refrigerants. W e can only understand the refrigeration plant by understanding how does the refrigerant behave if acted upon by plant components. As indicated in the previous article, we are trying to control the temperature and pressure of the refrigerant in the system. By so doing, the process of heat absorption and discharge takes place. The advantage of a refrigerant is its ability to vaporise and freeze at low temperatures. Think of water which boils and freezes at very high temperatures. We know temperature is directly proportional to pressure. In other words, when temperature rises so does the pressure and vice versa. Scientific laws and processes were used during the invention of refrigeration systems which made quantifying possible. #Essentials the compressor at a saturated low vapour pressure of 500kPa, and a saturated high pressure liquid of 1 500kPa when it leaves the condenser. You need to refer to a pH diagram to read other properties of the refrigerant. Process 1-2 Looking at the diagram below, Process 1-2 is work done on the refrigerant by the compressor, thus enthalpy and temperature rise. 30kJ/kg of heat has been added and is called heat of compression. For a simple system the process takes place at constant entropy and the constant entropy line can be followed until a pressure of 1 500kPa when the refrigerant enters the condenser. The entropy will be 1.80kJ/kg.K The simple saturated refrigeration cycle Let us have a look at a p-H diagram of a specific refrigerant. We will look at a theoretical system. This is the system where the assumption is made that the refrigerant enters the compressor at saturation state and leaves the condenser at saturation state. Then other properties of the refrigerant can be read from its Mollier diagram if the evaporating and condensing pressure are known. The diagram below shows the refrigerant R407c entering 64 RACA Journal I June 2016 www.hvacronline.co.za