Page 413 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Ejectors and Mechanical Vacuum Systems 381

( • Determine operating steam pressure, psig.
u"
In�- J • Determine/ establish required final suction pressure
p"
0 in vessel/system, inches Hg abs.
Solving: t = (6- 26)
In[ ,v J • Establish discharge pressure required (usually to
Rp, v' � � atmosphere), psig.
• Note that the performance is specific Lo the ejector
ln(Q.01) used.
760
(15 ) ln ( 500 ) Example 6-14: Evacuation of Vessel Using Steam Jet for
500 + 0.5 Pumping Gases
The performance and procedure use the data of Pen-
ln (0.0000131579) -11. 23841862
t= berthy for this illustration (by permission):
(15) In (0. 999000999) 15 (-0.0009995003)

t = 749.6 sec= 12.49 min Evacuating-Selection Procedure
Evacuation With Steam Jets Refer to U Evacuation Time chart.

Rough Estimate of System Pumpdown Using Steam jets [24 j Step 1. Determine evacuation time in minutes per
hundred cubic feel.
The remarks presented earlier regarding the use of
steam jets for pumping down a system apply. The method Step 2. Go to the left-hand column in table, final Suction
of power [35] presented by Reference [24] is: Pressure (hs). Read across to find evacuation time equal
to or less than that determined in Step 1. Read to the top
t = [2.3 - 0.003 (I'',)] V /wj (6-27) of table and note unit number. See Table 6-15.

where t = lime required to evacuate a system from atmos- Step 3. Read Steam Consumption of unit selected off
pheric pressure Lo the steady state operating pres- Capacity Factor Chart. See Table 6-16.
sure, min
P', = design suction pressure of ejector, torr Evacuating-�LE:
V = free volume of the process system, cu ft
"'.i = ejector capacity, 70°F dry air basis, lb/hr To evacuate 3000 cubic foot vessel full of air at atmos-
pheric pressure:
This assumes dry air with no condensables and negligi- Operating Steam Pressure, PSJG (hm) 100
ble pressure drop through the system to the ejector. Also, Final Suction Pressure, inches Hg abs (hs) 5
the jet air handling capacity is assumed approximately Time to evacuate, hrs 2.5
twice the design capacity, and air inleakage during evacu- Discharge Pressure (hd) atmosphere
ation is negligible.
When considering time to evacuate a system using a Step 1. Determine evacuation time in minutes per
steam jet, first recognize that securing reasonable accura- hundred cubic feet.
cy is even more difficult than for a positive displacement
pump. The efficiency of the ejector varies over its operat-
2.5 hr X 60
ing range; therefore as the differential pressure across the ------- = 5 min I 100 cu ft
unit varies, so will the volume handled. Consequently, 30 (hundred) cu fl
evacuation time is difficult to establish except in broad Step 2. Go to the final pressure on left of Evacuation Time
ranges. The above relations can be adopted to establish chart (5 in. Hg hs). Read across and find evacuation time
the order of magnitude only. equal lo or less than 5 minutes. See Table 6-15.
A recommended evacuation calculation is given in Ref:
erence [ 19]. This is specific to Penberthy equipment, but is The U-2 will evacuate the tank in 5.33 minutes per hun-
considered somewhat typical of other manufacturers. dred cubic feet and the U-3 will complete the evacuation
in 3.42 minutes per hundred cubic feet.
• Establish suction load of air to be evacuated in cubic
feet volume of vessel/ system. Step 3. Read steam consumption of selected unit off
• Establish the required time to evacuate, in minutes. Capacity Factor Chart. See Table 6-16. The unit to select