How to estimate airflow loss in a duct?

Question

Cooling systems

There is a server DLC cooler with 3 40mm fans (flow 25CFM, pressure 32mm-h2o). I want to increase its cooling capacity by adding 2 of the same additional fans. To do this, I designed an air duct:

Inlet 200x40mm, outlet 120x40mm, Total length 76mm.
As far as I understand, the duct will create resistance to the air flow.
Can you tell me how to estimate airflow loss? I want to understand how much better this solution is than 3 fans.

Answer 1

The easiest and most correct way is to measure the excess pressure at the inlet and outlet and determine the differential from the difference. also take into account that at different flow rates the difference will change.
The problem is different, the pressures are very low and it is problematic (expensive) to select a measuring instrument ...
here is something like that
ps
or you can assemble a circuit from MPXV7002DP (or similar, the main thing is that it fits by the pressure difference) + arduino

Answer 2

Losses can be estimated using the "Handbook of hydraulic resistance", author - Idelchik I.E. In your case, you will need the fifth section.
The only point is that if you have not done this before, then at first it can be difficult.
In any case, I wish you good luck!