Quote Originally Posted by TRASH101 View Post
DesA
please expand on original theory
adequate mass charge implicates the full system rather than HX solely
Heat-transfer theory in the derivation of dTlm (log mean temperature difference), makes the implicit assumption that 'enough' liquid/refrigerant/vapour, whatever, is in the heat-exchanger/condenser/evaporator to achieve the required/design value.

In practice, if the incorrect amount of refrigerant is resident in the heat-exchanger during operation, it will actually have a dTlm different to the design (expected) value.

This can be seen using a low refrigeration charge, where a condenser, for instance, is most certainly not flooding, but runs at a very low dTlm value. As the system mass charge is raised, the dTlm of this condenser will begin to rise (push back) up until a certain point, after which flooding does begin to take place, & condenser surface is overtaken by liquid subcooling. At this point, the dTlm appears to begins to reduce again.

So, in the end, RHVAC systems are hugely charge-dependent. We see this a lot in air-to-water heat-pumps, for instance, where the mass charge re-distributes itself during the heat-up cycle. The effect of as little as 10g of additional charge (1-1.5%) can be clearly seen in system performance.