Page 99 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
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plugging, to protect the cleanliness of optical de- . “ Pressure drop in a rotameter is constant” True/False?
vices, and to protect electrical devices from igniting & True. Pressure drop (head) will be constant across the
upon contact with combustibles. float only if flow area remains constant.
& Purge meters are quite useful in adding nitrogen gas . “A rotameter must not constrict flow.” True/False?
to the vapor spaces of tanks and other equipment. & True.
Purging with nitrogen gas reduces the possibility of
. What are the disadvantages of a rotameter?
developing a flammable mixture because it displaces
& Glass is breakable which could lead to hazardous
flammable gases.
situations when handling toxic or flammable liquids.
& The purge flow regulator is reliable, intrinsically safe
& With opaque or cloudy liquids, float will not be
and inexpensive.
visible (special rotameters are available, which are
& Figure 4.20 illustrates its working:
more expensive).
. What is the difference between correlated and direct
& No output for data transmission.
reading rotameters?
& Sensitive to differing gas types and changes in tem-
& A direct reading flow meter indicates the flow rate on
perature and pressure. In other words, as these meters
its scale in specific engineering units (e.g., mL/min or
measure volumetric flow rates, gas density variations
scf/h). Direct reading scales are designed for a spe-
will affect measurements.
cific gas or liquid at a given temperature and pressure.
& DP is high.
While it is more convenient than a correlated flow
meter, a direct reading flow meter is less accurate and & Accuracy is only 2–3% unless expensive high pre-
limited in its applications. cision rotameter is employed.
& Acorrelatedflowmeterisscaledalongeithera65 mm & Not suitable for high capacities.
or a 150 mm length, from which a reading is taken. & Accumulation of dirt or other foreign matter depos-
The reading is then compared to a correlation table ited on the float or tube walls can cause wrong
for a specific gas or liquid. This will give the actual readings.
flow in engineering units. One correlated flow meter . What type of rotameter is used for opaque or cloudy
can be used with a variety of fluids or gases. liquids?

FIGURE 4.20 Purge flow meter design.