Page 393 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Ejectors and Mechanical Vacuum Systems 361

2.00 2.00
1.90 1.90 2'
1.70 .
I.IO 1.80 1
0
:,
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� 1.1,0 ':
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1.40 ..
-- - I.SO 5'
i!.
- ..... 1.30 ..
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..... 1.20 =i.z
...... "1-l
,� 1.10 !�
v 1.00 =-
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.,, .90 �z
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.70 "-l
i!.-
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..I .60 8'
.so ;; 0
I .40 3
Entrainment Ratio = weight of 8 89 3
i
weight of air .30 >
a
.20 !:!
I I I I I I I is·
:,
.l I I I I I I I I I .10 ..,
ii I I I I I I I I I ;;
I
I I I I I I I I ' I
10 20 30 40 50 60 70 10 90 100 110 120 130 140 0 �
MOLECULAR WEIGHT
Figure 6-18. Molecular weight entrainment ratio curve. Reprinted by permission, Standards for Steam Jet Ejectors, 3rd Ed., Heat Exchange
Institute, 1956.
Mixture: The mixture 70°F air equivalent= 161.3 + 476 Nole: if the data for this example had been given as air
= 637.3 lbs /hr equivalent, then the water vapor portion would have been
corrected for molecular weight, using Figure 6-18.
Example No. 6-3: Actual Air Capacity For Air-Water If there is only one of the components alone in the sys-
Vapor Mixture tem its corrections are made as illustrated in the preced-
ing examples.
Some manufacturers furnish 70°F air equivalent curves
to allow the purchaser to convert performance to actual Steam and Air Mixture Temperature
plant conditions. lt is almost impossible lo operate on one
design point. For a mixture of steam and air handled by an ejector,
the temperature of the mixture in the ejector mixing
What is the expected actual capacity at 500°F of a mix-
ture rated at 70°F? chamber is calculated by [ 11]:
Air = 325 lbs/hr \V,Cp,L, + W,Cp:.ta
lm (6-1)
Water vapor = 300 lbs/hr w.c, + vV Cpa
0
Total 525 lbs/hr
tm = temp of mixture at ejector suction, °F
vV, = steam flow rate, lbs/hr
For air: From Figure 6-17, ratio= 0.897 actual air Cp, = specific heat of steam at constant pressure correspond-
capacity = 225 (0.897) = 202 lbs/hr ing Lo downstream absolute pressure (0.45 approx.)
For water vapor: From Figure 6-17, ratio = 0.859 actual water t, = temp of steam on downstream side of nozzle, °F
vapol- capacity = 300 (0.859) = 257 lbs/hr W = Air flowrate, lbs/hr
3
Total mixture actual capacity = 202 + 257 Cps = specific heat of air at constant pressure (0.24 approx.)
= 459 lbs/hr t.., = ambient air temp "F