The boiling of a refrigerant is a complicated process. It changes with induced pressure drop, the type or blend of refrigerants, the temperature difference between the hot and cold sides and other factors. The following subchapters will discuss the most important factors influencing the evaporation process.
Temperature profile inside the evaporator
The boiling temperature of a refrigerant at a certain pressure is called the saturation temperature. At the saturation temperature, any additional energy absorbed by the refrigerant will transform liquid to gas. If the pressure is constant, the temperature will remain at the saturation temperature until no liquid remains. Only at the point where no liquid refrigerant is present can the temperature of the vapor increase, i.e. become superheated. A popular example to illustrate this is water, which boils at 100°C at sea level. The temperature will not increase even if the water boils vigorously. Hence, 100°C is the saturation temperature of water at a pressure of 1 atm. To change the evaporation temperature, the pressure must be changed. At the top of a mountain, where the atmospheric pressure is lower, the saturation temperature of water is lower, and it starts to boil at temperatures lower than 100°C.
Figure 6.8 illustrates the temperature profiles inside an evaporator for an evaporating and superheating refrigerant compared with the secondary fluid. Refrigerant a evaporates at a higher evaporating pressure and thus saturation temperature than refrigerant b.
Influence of Superheating on the Evaporation Process The latent energy absorbed in the evaporation represents the majority, approximately 95%, of the total heat of absorption (THA). Superheating of the vapor represents the remaining heat transfer to the refrigerant. The evaporation process accounts for most of the heat transfer area of the evaporator. Although the superheating only accounts for approximately 5% of the total heat of absorption, the gas heating process normally takes up 10-25% of the total heat transfer surface. This imbalance in the required heat transfer surface versus energy transfer can be explained by the differences in heat transfer coefficients between boiling liquid refrigerant and one-phase gas heating. During the evaporation process, the BPHE wall is in contact with boiling liquid refrigerant made turbulent by the vapor formed. When the liquid phase has boiled off, the energy is instead transferred through the vapor, i.e. a one-phase process, with a greatly reduced heat transfer coefficient as a result. Because the temperature of the vapor increases, the driving force for heat transfer is gradually reduced.
A BPHE designed with a very high level of superheating (see Figure 6.9) will therefore require a proportionately very large heat transfer area for vapor heating. Similarly, a BPHE designed with a normal level of superheating that is forced to operate with too much superheating will have less heat transfer area available for evaporation. The result is a decreased evaporation temperature, which leads to reduced system capacity and efficiency.
Effect of pressure drop
When the refrigerant flows through the winding channels inside the BPHE, turbulence is created and pressure drop is induced. Hence, the pressure will decrease as the refrigerant passes through the BPHE. As stated above, the evaporation temperature is constant at constant pressure for a pure refrigerant. Decreasing pressure at a later stage in the evaporator means a decreasing saturation temperature, because the force required to hold the refrigerant molecules together as a liquid will be smaller. The evaporation temperature therefore decreases as evaporation progresses through the unit.
The pressure drop through the BPHE channels depends on the characteristics of the refrigerant used and many other factors. The pressure drop increases with higher inlet vapor quality, mass flow and level of superheating. A lower evaporation temperature, which corresponds to a lower evaporation pressure, also results in a higher pressure-drop due to the larger specific volume of the vapor formed. The pressure drop is proportional to the velocity squared, so a higher vapor content in the refrigerant will result in a larger pressure drop. Because the specific volume of the vapor increases with decreasing pressure, a lower evaporation temperature also gives a higher pressure-drop for the same mass flow. As an example of how sensitive different refrigerants are to pressure drop it is possible to compare what temperature change a certain pressure drop create. 10kPa pressure drop in a standard chiller case lead to a loss in temperature of 0.8K for R134a and 0.4K for R410A.
For an azeotropic refrigerant, i.e. a refrigerant without glide, the pressure drop will result in the inlet saturation temperature being higher than the outlet evaporation temperature (see Figure 6.10). The outlet saturation temperature, which is often referred to as the evaporation temperature, is in fact the minimum evaporation temperature inside the BPHE evaporator. Normally, the saturation temperature differs by only one or a few degrees over the evaporator.