Currently the most common cycle used in automotive applications is the vapour compression cycle, e.g. R12 and R134a closed systems. To understand the vapour compression cycle and
other cycles it is important to understand the types of changes that a refrigerant goes through when used in an A/C system.This is explained using a pressure/enthalpy diagram (Figure 1.53).
A refrigerant in the ‘subcooled’ liquid region point a is at a temperature which is below its boiling point. If heat is continually added while maintaining constant pressure, the refrigerant’s temperature and enthalpy will increase. Its state will eventually approach ‘saturated liquid’ point b.This is where the liquid will start to vaporise.As the heat is continually added the liquid vaporises and continues to increase in enthalpy but not increase in temperature.The ‘saturated liquid’ vaporises until it becomes a ‘saturated vapour’ point b–c.The ‘saturated vapour’ at point c has no liquid because it has completely vaporised.The heat that is absorbed through the transition from ‘saturated liquid’ to ‘saturated vapour’ is called the latent heat of evaporation (heat is absorbed without any increase in temperature).With additional heat still available to the ‘saturated vapour’ the temperature increases causing the refrigerant to become ‘superheated vapour’ point d.
At this point the heat addition causes an increase in temperature. The saturated liquid and saturated vapour curves meet at a point called the ‘critical point’.This point has a corresponding critical pressure and critical temperature.Above the critical pressure the refrigerant is in a state called the ‘supercritical region’.The supercritical region is where heat addition or removal does not cause a distinctive liquid vapour phase transition.
The ideal vapour compression cycle
The following applies the vapour compression cycle as an ideal cycle within an automotive A/C system.The Figure 1.54 pressure/enthalpy diagram shows the beginning of the cycle as the refrigerant enters the compressor as a ‘saturated vapour’ at point 1. The refrigerant is compressed adiabatically (the compressor is 100% efficient and no heat is removed by the process) and becomes a ‘superheated vapour’ due to the increase in pressure, temperature and enthalpy
as shown by point 2. The refrigerant by this point is above the temperature of the outside air. The efrigerant leaves the compressor and enters the condenser (heat exchanger). The condenser allows the heat to transfer to the outside air effectively removing enough heat to change into a saturated vapour from a superheated vapour point 2a.This has lowered the temperature of the refrigerant. Now the refrigerant is a ‘saturated vapour’ the pressure and temperature are kept constant but heat is still being removed and only the enthalpy continues to decrease.The vapour begins to condense to liquid (latent heat of condensation). Condensation continues until all the vapour is a ‘saturated liquid’ point 3. The refrigerant leaves the condenser as a saturated liquid and travels to an expansion valve or fixed orifice tube.The refrigerant now undergoes an ‘isenthalpic’ expansion process (constant enthalpy). The process significantly reduces the temperature and pressure of the refrigerant while the enthalpy remains the same.A small amount of liquid refrigerant (flash gas) vaporises during expansion but most of the refrigerant is liquid at a temperature lower than that of the outside air (air inside the vehicle or entering the vehicle) point 4. The refrigerant flows through the evaporator which acts as a heat exchanger that transfers heat from the air flowing through its fins to the refrigerant flowing through its coils.The refrigerant absorbs the heat increasing in enthalpy while the temperature and pressure remain the same. The liquid refrigerant vaporises until it becomes a ‘saturated vapour’. The saturated vapour then travels to the compressor to start the cycle again.
The real world operation
The real world operation of the A/C system deviates from the ideal cycle. It is difficult for condensation and evaporation to end exactly on the liquid/vapour saturation lines.This is particularly difficult due to the fact that the system has to operate under so many varying conditions. To ensure an acceptable performance under all loads the condensers are designed to ‘subcool’ the refrigerant to a certain amount to ensure that only liquid refrigerant flows to the expansion device for optimum performance (also one of the jobs of the receiver drier). If vapour flows to the expansion device it reduces the flow of refrigerant significantly. Evaporators are generally designed to slightly ‘superheat’ (TXV systems) the refrigerant to ensure that only vapour flows to the compressor and no liquid (except for oil circulation 3%). There are also pressure drops across components like the condenser and evaporator which cause deviation from the ideal constant pressure process. Fixed Orifice Valve (FOV) systems use a fixed orifice diameter which is designed for optimum flow at high compressor and vehicle speeds. Poor performance at idle conditions can occur where there is a possibility of the evaporator being excessively flooded. Some flooding is advantageous with the FOV system.This is to increase cooling capacity and reduce ‘hot spots’, which are areas of reduced heat transfer caused by poor refrigerant distribution. Too much flooding reduces compressor performance.
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