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The loss factors are given for mass flow rates from port_a to port_b as:

A_a < A_b (Idelchik 1994, diagram 4-1, p. 208):
   zeta = dp/(rho_a*v_a^2/2)
        = (1 - A_a/A_b)^2 for Re_a ≥ 3.3e3 (turbulent flow)
   zeta = 30/Re           for Re_a < 10    (laminar flow)

A_a > A_b (Idelchik 1994, diagram 4-9, p. 216 and diagram 4-10, p. 217)
   zeta = dp/(rho_b*v_b^2/2)
        = 0.5*(1 - A_b/A_a)^0.75 for Re_b ≥ 1e4 (turbulent flow)
   zeta = 30/Re                  for Re_a < 10  (laminar flow)