Figure 2-16.—Equipment-flow switch.
Venturi Flowmeter: As the coolant approaches
the contracted portion (throat) of the venturi flow-
meter, the velocity of the coolant must increase as it
flows through the contracted zone (throat). The angle
of approach is such that no turbulence is introduced
into the stream. A pressure tap is located at the side
wall in the pipe ahead of the meter, and another one is
located at the throat. The increase in velocity of the
coolant water through the throat results in a lower
pressure at the throat. The flow rate is proportional to
the difference in pressure between the two taps. The
gradual tapering of the meter walls back to pipe size
downstream of the throat allows the coolant water to
slow down with a minimum of lost energy. This
allows a recovery of nearly 99 percent of the pressure
on the approach side. To monitor the amount of flow
through the venturi flowmeter, a differential pressure
gage is used to monitor the pressure difference be-
tween the two pressure taps. A calibration chart is
usually supplied with the flowmeter to convert the
different pressure to gallons per minute (gpm), or the
Figure 2-17.—Venturi flowmeter.
face of the meter may indicate readings in gpm. Fig-
ure 2-17 shows a venturi flowmeter.
Orifice Flowmeter: The orifice flowmeter
works in the same manner as the venturi flowmeter,
but its construction is much simpler and less expen-
sive to manufacture. In place of the tapered throat, the
orifice flowmeter uses a flat plate with a hole in it,
which causes a considerable loss of pressure down-
stream. The efficiency of this type of flowrneter can
be as low as 65 percent.
Rotameter: The rotameter, shown in figure
2-18, is a variable area orifice meter that maintains a
constant differential pressure with varying flow. The
rotameter has a float positioned inside a tapered, tem-
pered glass tube by the action of the distilled water
flowing up through the tube. The flow restriction is
the space between the float and the tube wall; this area
increases as the float rises. The differential pressure is
fixed, depending on the weight of the float and the
buoyant forces resulting from the combination of float
material and the distilled water’s specific gravity. The
tapered tube of the rotameter is usually glass, with
calibration marks reading directly in gpm. The major
advantage of a rotameter over a venturi meter is the
visibility of the coolant, as it allows quick determina-
tion of excessive entrained air in the coolant.
2-18
