HEAD FLOW METERS
Dall Flow Tube
The dall flow tube, illustrated in Figure 4,
Figure 4 Dall Flow Tube
has a higher ratio of pressure developed to
pressure lost than the venturi flow tube.
It is more compact and is commonly used
in large flow applications.
consists of a short, straight inlet section
followed by an abrupt decrease in the
inside diameter of the tube. This section,
called the inlet shoulder, is followed by
the converging inlet cone and a diverging
exit cone. The two cones are separated
by a slot or gap between the two cones.
The low pressure is measured at the
slotted throat (area between the two
cones). The high pressure is measured at
the upstream edge of the inlet shoulder.
The dall flow tube is available in medium
to very large sizes. In the large sizes, the
cost is normally less than that of a venturi flow tube. This type of flow tube has a pressure loss
of about 5%. Flow rate and pressure drop are related as shown in Equation 4-3.
= volumetric flow rate
K = constant derived from the mechanical parameters of the primary elements
DP = differential pressure