Page 465 - Chemical process engineering design and economics

444 Chapter 8

By dividing Equation 8.25 by Equation 8.24, the flow rate of the standard
fluid, at standard conditions, in terms of the flow rate of the actual fluid, at actual
conditions, is given by
TPPS-PS P r 2
Qs = Ql ———— — I (8.26)
I p F - p p s )
provided the rotameter coefficient is independent of the fluid being metered, i.e.,
CR ~ CRS- Equation 8.26 may be used for either liquid or gases, but for gases it
may be simplified because p s « PF and p « p F. Thus, Equation 8.26 reduces to

PFS P
Qs = Q — — (8.27)
PF Ps
If the ideal gas law is obeyed, then p = M P / R T. Substituting this equation
into Equation 8.27, the flow rate at standard conditions,

PFS T P M T s "1 1/2
Qs = Q — I — —— -I (8-28)
PF I P s Ms T )
where M is the molecular weight of the gas.
Thus, sizing rotameters requires using either Equation 8.26 or Equation 8.28
to calculate the flow rate of the standard fluid. Then use Table 8.3, supplied by a
manufacturer, to select a rotameter. The procedure for sizing a rotameter is illus-
trated in Example 8.3.
Example 83 Rotameter Sizing___________________________

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3
Find the rotameter size required to meter 1.5 gal/min (5.68xlCT m /min) of carbon
tetrachloride at 20 °C (68 °F).
To select a rotameter from Table 8.3 first calculate an equivalent flow rate of
water from Equation 8.26. In Table 8.3 stainless steel floats are used. The density
3
of stainless steel is 8.02 g/cc (501 lb/ft ) and the density of carbon tetrachloride is
1.60 g/cc (99.9 lb/ft3). After substituting numerical values into Equation 8.26, the
volumetric flow rate of water,
[(8.02-1.00) 1.60 1 1/2
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3
Qs = 1.50 I ——————— —— I = 1.984 gal/min (7.51xlO~ m /min)
L (8.02-1.60) 1.00 J
Therefore, from Table 8.3 select a 1A inch rotameter having a maximum flow
rate of 2.44 gal/min (9.24 1/min). This rotameter size is somewhat larger than
needed, allowing for a safety factor.

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