Page 193 - Chemical Process Equipment - Selection and Design

7.7. EJECTOR AND VACUUM SYSTEMS 165
a. Estimated Air Leakages Through Connections.
Valves, Stuffing Boxes Etc. of Process
Equipmenta EXAMPLE 7.12
Equivalent Air Rate
Estimated Suction gases are at the rate of 120lb/hr at 300°F and have a
Average
Air Leakage molecular weight of 90. The temperature factor is not known as a
Type Fitting Whd function of molecular weight so the value for air will be used. Using
Eqs. (7.61) and (7.62),
Screwed connections in sizes up bo 2 in. 0.1
Screwed connections in sizes above 2 in. 0.2
Flanged connections in sizes up to 6 in. 0.5 rn = 120(0.375) ln(90/2)[1- 0.00024(300-70)]
Flanged connections in sizes 6 in. to 24in. including 0.8 = 161.8 lb/hr equivalent air.
manholes
Flanged connections in sizes 24 in. to 6 ft 1.1
Flanged connectiomns in sizes above 6 ft 2.0
Packed valves up to in. stem diameter 0.5
Packed valves abomve a in. stem diameter 1 .o
Lubricated plug valves 0.1 STEAM CON SUM PTlON
Petcocks 0.2
Sight glasses 1 .o The most commonly used steam is 100 psig with 10-15" superheat,
Gage glasses including gage cocks 2.0 the latter characteristic in order to avoid the erosive effect of liquids
Liquid sealed stuRing box for shaft of agitators, 0.3 on the throats of the ejectors. In Figure 7.31 the steam
pumps, etc. (per in. shaft diameter) consumptions are given as Ib of motive steam per Ib of equivalent
Ordinary stuffing box (per in. of diameter) 1.5
Safety valves and vacuum breakers (per in. of 1 .o air to the first stage. Corrections are shown for steam pressures
nominal size other than 100 psig. When some portion of the initial suction gas is
condensable, downward corrections to these rates are to be made
a For conservative practice, these leakages may be taken as SUP- for those ejector assemblies that have intercondensers. Such
piementan/ to thcise from Eq. (7.59). Other practices allow 5 Ib/hr for corrections and also the distribution of motive to the
each agitator stuffing box of standard design; special high vacuum
mechanical seals with good maintenance can reduce this rate to individual Stages are problems best Passed on bo ejector
1-2 Ib/hr. manufacturers who have experience and a body of test data.
[From C.D. Jackson, Chern. Eng. Prog. 44,347 (1948)l.

1,000

coefficient is a function of the process pressure as follows:
100

Pressure (Torr) >90 20-90 3-.20 1-3 <I
k 0.194 0.146 0.0825 0.0508 0.0254
10
For each agitator with a standard stuffing box, 5 Ib/hr of air leakage
is added. Use of special vacuum mechanical seals can reduce this
allowance to 1-2 lb/hr. w
For a conservative design, the rate from Eq. (7.59) may be c I
3
-8 0
supplemented with values based on Table 7.11. Common practice is (D a
to provide oversize ejectors, capable of handling perhaps twice the
standard rates of the Heat Exchange Institute.
Other Gases. The gas leakage rate correlations cited are based 0.100
on air at 70°F. For other conditions, corrections are applied to
evaluate an effeictive air rate. The factor for molecular weight M is

fM = 0.375 ln(M/2) (7.60) 0 010
POUNDS MOTIVE STEAM PER POUND IIR LOAD
and those for temperature T in "F of predominantly air or
predominantly steam are

fA = I - 0.010024(T - 70), for air, (7.61) Figure 7.31. Steam requirements of ejectors at various pressure
fs = 1 - O.O1DO33( 7 - 70), for steam. (7.62) levels with appropriate numbers of stages and contact interconden-
sers. Steam pressure 100 psig, water temperature 85°F. Factor for
65 psig steam is 1.2 and for 200 psig steam it is 0.80 (Worthington
An effective or (equivalent air rate is found1 in Example 7.12. Corp).