Two Formulas for Combinations of Restrictors

For parallel flow, the total Lohm rating is:

Please note that this relationship is identical to the electrical equation.

Example:

For Series Flow, the total Lohm rating is:

Please note that this relationship is not the same as in electrical problems. The difference is due to the non-linearity of

Example:

When L1 = L2 = L3 , then L T = L
N = Number of equal resistors in series

For passageway size:

D = Diameter of the actual orifices,each with a Lohm rate = L1
DT = Diameter of a single equivalent orifice, with a Lohm rate = LT

One of the reasons for using two restrictors in series is to allow fine tuning of a total resistance value. If L1 is known and is more than 90% of LT, then L2 may vary by ±5% without altering the value of LT by more than ±1%, even though the value of L2 may be as high as 40% of LT . This effect becomes even more pronounced as L1 approaches LT . To determine the intermediate pressure between two resistances in series, the following formulas may be used.

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