Glide, Part IV: The refrigerant circuit
In this fourth and final technical article focused on glide, we analyze once again the practical implications of this phenomenon, focusing mainly on the movement of the fluid along the refrigerant circuit.
A further problem to consider, as a consequence of the use of zeotropic mixtures, occurs when the liquid fluid is allowed to accumulate along the cooling system, such as in separators and liquid collectors. The risk is represented by the possible variation in the composition of the fluid along the circuit (fractionation) and the impossibility of predicting the repercussions on the performance of the cycle, which operates, in fact, with fluids that are different with respect to the heat exchange organs. To avoid, or rather mitigate the problem, the fluid should always be kept in motion, avoiding stagnation points of the condensate, a scenario that would penalize flooded evaporators in the use of refrigerants with significant values of glyde.
In addition, it should be mentioned that a pressure drop from the refrigerant circuit, when it occurs in a component where the fluid is present in the two-phase state (evaporator, condenser, separator, liquid collector, line downstream the thermal expansion valve) causes a change in the composition of the residual charge in the circuit, depending on which phase is involved in the loss. That circumstance would require, in principle, that the subsequent topping up of the circuit be carried out with a composition fluid complementary to that remaining in the circuit, so as to restore its ideal charge composition. It is clear that this procedure is objectively difficult to implement in practice. The change in composition causes, in addition to variations in the cooling capacity and in the operating conditions of the circuit, also a loss of calibration of the thermal expansion valve, if of the thermostatic type, as it induces an alteration in the link between the dew temperature and the pressure of the fluid, on which the working principle of the thermostatic valves is based.
Francesco Viola, Thermodynamic Engineer
