Icepak FAQ: How to determine System pressure drop from Simulation results
Icepak reports static pressure values. The sytem pressure drop is actually the difference in total pressures. If you have a fan using fan curve, it is very simple. YOu just need to lookup the operating point. If you are imposing a fixed flow rate.... Let's say that the cross sectional area at the inlet and outlet approximately the same. Then, you just need to report for average pressures at then inlet ant the outlet and determine the difference. If the areas are different, then you need to be careful. Static pressure can even increase from the inlet to the outlet if the area at inlet is smaller than that at the outlet (good ol' Bernoulli's law...). In this case, I have 2 advices: you may have to put your sytem inside an enclosure that has a duct infront of the inlet and behind the outlet. Put free openings at both these ends. The length of the ducting should be such that the distribution of velocity is fairly uniform. Now, if you simulate this ducted model, the static pressure difference between the duct end openings is the system pressure drop. If you already have a big model, extending the domain may not help to speed up the solution time. In that case I have another Idea. You don't have to duct the flow into the sytem. The system pressure drop is the difference in pressure between the total pressures: Ptot = Pstatic (reported by icepak) + 0.5 * Density of fluid * V^2. This will work if you have fairly uniform pressure distribution at the inlet and the outlet. 

