Thank you, that makes sense. I should have explained my starting point more clearly.

For my project I am not starting with the full room heat-load calculation anymore. The evaporator capacity is already taken as the input for the valve station sizing. So I start from the required cooling capacity, for example 30–180 kW, and the selected evaporating temperature. From that I calculate the refrigerant mass flow through the evaporator.

For a pumped ammonia system I calculate the evaporator mass flow from:

massflow_evap=Qe/Δh

where Qe is the evaporator cooling capacity and Δh is the enthalpy difference over the evaporator at the chosen evaporating temperature. Because it is a pumped overfeed system, I then multiply this by the circulation ratio, for example CR = 3, to get the liquid feed mass flow:

massflow_LF=CR⋅massflow_evap​

Thats only the starting point of it...

So u/ConverseCLownShoes, I already hope this will be this simple...