Final answer:
The subcooling method is used to check the charge of a system equipped with a TXV, as it provides an accurate measure of the refrigerant amount by assessing liquid temperature below saturation point. This is because TXV systems regulate flow based on superheat, so subcooling becomes the appropriate charging check method.
Step-by-step explanation:
The subcooling method should be used to check the charge of a system equipped with a Thermostatic Expansion Valve (TXV). Subcooling is the process of lowering the temperature of the liquid refrigerant below its saturation temperature at a given pressure. This is necessary for systems with a TXV because the valve controls the refrigerant flow based on the superheat, making the superheat method unsuitable for checking the charge in these systems.
To properly assess the charge, technicians will look for a consistent subcooling value across the system, indicating that the correct amount of refrigerant is present. This method aligns with ensuring that the properties of the charge align with the expected values from technical documents or performance data, such as in Equation 20.4.31, which must be balanced, just like the charges and numbers of atoms in chemical equations. Similarly, when calculating the work done by the system in thermodynamics, such as represented by the area of a right triangle on a P-V diagram, one must ensure that the measurements, perhaps from a pressure gauge, reflect the actual behavior of the system during expansion.