Flow Factor "fM" (Multi-Orifice)

When multiple orifices appear in series or when a restrictor has several stages, as in a visco Jet, there is a non-uniform distribution of the overall pressure drop through the restrictor.

The effect of the above flow behavior is that the gas flow rate of a multi-orifice device is higher than would be expected from a single-orifice device of the same lohm rate, and at the same pressure conditions.

This characteristic is reflected in the flow factor, " fM", which reaches a maximum value of 1.3 at a pressure ratio of 3/1. See the graph below for values of " fM", at any pressure ratio for multi-orifice restrictors.

Sonic flow: for P1 / P2 > 3, use fM = 1.3

EXAMPLE: What multi-orifice restriction will permit a flow of 0.5 std L/min. of hydrogen at 70°F, with supply pressure at 40 psig, discharging to atmosphere.

K = 1030 (click here for details)
T1 = 70°F, fT = 1.0 (click here for details)
P1 = 40.0 + 14.7 = 54.7 psia
P2 = 14.7 psia
P1/ P2 = 54.7 / 14.7 = 3.72
fM = 1.30
Q = 0.50 std L/min

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