A Simplified System of Defining Fluid Resistance
Over the years, The Lee Company has developed the Lohm system for
defining and measuring resistance to fluid flow. Just as the "ohm"
defines electrical resistance, the "Lohm" or "liquid ohm" can be used
as a measure of fluid resistance.
The Lohm is defined such that 1 Lohm will flow 100 gallons per minute
of water with a pressure drop of 25 psi at a temperature of 80°F. Since
resistance is inversely proportional to flow, by definition:
By using Lohms, one can specify performance without concern for
coefficients for discharge, passageway geometrics, physical dimensions
or tolerances. The resistance of any flow can be expressed in Lohms and
confirmed by actual flow tests.
Lohm Laws generalize the Lohm definition and allow the system designer
to specify Lohm requirements for particular application based on the
desired pressures and flow rates. Click here for a graph relating Lohms
to hole diameter and flow coefficient, Cv.
Liquid Flow Lohm Laws (Gas Flow Lohm Laws can be found on another page.)
The Lohm Laws predict the actual performance of fluidic devices beyond
the definition conditions of water at 25 psid and 80°F. In Liquid Flow
several variables must be related, including:
I = Flow rate
H = Differential pressure
V = Viscosity correction factor. V factors compensate for the
interaction of viscosity and device geometry and are unique to each
class of device. See "V" factors graph for typical Lee orifices. Use
1.0 for water @ 80°F
S = Specific gravity. Use 1.0 for water @ 80°F
K = A constant to take care of units of measure. Use 20 for psi and gpm.
The Lohm Law for Liquid Flow is:
also:
d = orifice diameter (inches)
Cd = coefficient of discharge
Cv = flow coefficient
For special flow requirements, The Lee Company can determine the required Lohm rating.
Units Constant K
The Units Constant K eliminates the need to convert pressure and flow parameters to specific units such as PSI and GPM.
Example:
An orifice must flow 43in.3/min. of water at a head of 300 kPa. What Lohm rate is required?
Solution:
First, the appropriate K is selected from the table: K = 1760.
Second, the Lohm Formula is solved using the K value:
K for various flow and pressure units :
| gpm |
20 |
76.2 |
7.62 |
.24 |
75.4 |
13.2 |
2.78 |
| L/min. |
75.7 |
288 |
28.8 |
.91 |
285 |
50 |
10.5 |
| mL/min. |
75 700 |
288 000 |
28 800 |
911 |
285 000 |
50 000 |
10 500 |
| in3/min. |
4620 |
17 600 |
1760 |
55.6 |
17 400 |
3040 |
642 |
| ft3/min. |
2.67 |
10.2 |
1.02 |
.032 |
10 |
1.76 |
.372 |
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