Page 169 - Fluid mechanics, heat transfer, and mass transfer

EJECTORS 147

& For large systems, 8–10 kg/h may be a reasonable & The individual compression ratio of an ejector is a
figure for air leakage. function of cooling water temperature, steam use and
& Solubility of dissolved gases plus any gases produced condensation profile of vapors handled.
in chemical reactions in the feed streams and the & The first-stage ejector, tied directly to column dis-
evacuated equipment can be a rough guide in esti- charge, will have a compression ratio set primarily by
mation of noncondensables. intercondenser cooling water temperature. Intercon-
& Condensables may be estimated from a knowledge of densers are positioned between ejectors.
saturation conditions of such vapors in the . What is the recommended pressure for ejection of
noncondensables. noncondensables into the atmosphere from the last
. What are the operating conditions that must be supplied stage of the ejector system?
to the vendor while ordering an ejector system? & About 800 mmHg abs.
& Flows of all components to be purged from the . Classify ejectors stating their relative characteristics
system (often air plus water vapor). and usefulness.
& Temperature and pressure entering the ejectors and & Ejectors can be classified as single stage or multi-
pressure leaving if not atmospheric. stage. Multistage ejectors may be further divided into
& Temperature and pressure of steam available to drive condensing or noncondensing types.
the ejectors. & The single-stage ejector, the simplest and the most
& Temperature and quantity of cooling water available common type, is generally recommended for pres-
for the intercondensers and cooling water allowable sures ranging from atmospheric pressure (101.3 kPa
pressure drop for the intercondensers. abs) to 13.3 kPa (100 Torr) abs. Discharge is typically
at or near atmospheric pressure.
& Thevendor will convert the component flow data into
an air equivalent. Because ejectors are rated on air & A three-stage ejector requires 100 kg steam/kg air to
handling ability, the vendor can then build up a maintain a pressure of 133.3 Pa (1 Torr). A five-stage
system from his standard hardware. ejector produces a pressure of 6.7 Pa (0.05 Torr).
& The vendor should provide air equivalent capability & Multistage noncondensing ejectors are usually of
data with the equipment he supplies. two-stage type although six-stage units have been
used successfully.
& Air equivalent can be determined with the following
equation: & Multistage noncondensing ejectors are used to pro-
duce suction pressures lower than 10–15 kPa (3 or
p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4 in.Hg) abs.
e R ¼ F ð0:0345 M W Þ ð5:23Þ
& Steam consumption in a multistage noncondensing
where e R is the entrainment ratio (or air equivalent). It ejector is relatively high. Each successive stage is
is the ratio of the weight of gas handled to the weight required to handle the load plus the motive steam
of air that would be handled by the same ejector from the previous stage.
operating under the same conditions; M W is the gas
& Multistage noncondensing ejectors are frequently
molecular weight.
used when low first cost is more important than
F is 1:00; for M W 1 30; long-range economy. They are also used for intermit-
ð5:24Þ tentserviceorwhencondensingwaterisnotavailable.
F ¼ 1:076 0:0026; for M W 31 140:
& Multistage condensing ejectors are available in two
. Give the Heat Exchanger Institute Standard equation for through six stages. Intercondensers (surface or direct
steam jet ejectors. contact) between stages condense steam from the
preceding stage, reducing the load to be compressed
2 0:5
W ¼ 892:4 C d D ðP s =V g Þ ð5:25Þ in the succeeding stage.
n
where W is the motive steam (lb/h); C d is the nozzle & Four, five, and six-stage ejectors are used to achieve
discharge coefficient; D n is the nozzle throat diameter suction pressures as low as 5 mmHg abs. Under such
(in.); P s is the motive steam pressure at ejector, psia; V g vacuum conditions, pressure between the prelimi-
3
is the motive steam specific volume (ft /lb). nary stages is too low to permit condensation of
. What is ejector compression ratio? ejector steam, and only the final two stages are fitted
with condensers.
& The ratio of discharge pressure to suction pressure is
& Multistage condensing ejectors remove condensable
the ejector compression ratio. The values normally
vapor ahead of a given ejector stage. They also permit
vary from 3 to 15.