Page 174 - Fluid mechanics, heat transfer, and mass transfer

152 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

➢ It accelerates erosion on the system (e.g., changes & The supply steam conditions should not be allowed to
critical dimensions on nozzles and venturis) and vary greatly. Pressure below design can lower ca-
leads to ejector failure. pacity. Pressure above design usually does not in-
➢ Erosion is due to fast movement of water droplets crease capacity and can even lower it.

with the steam. About 3–8 C superheat is some- & Use Stellite or other hard surface material in the jet
times recommended. nozzle. For example, 316 s/s is insufficient.
➢ Erosion of steam inlet nozzle increases its & Always provide a suitable knockout drum ahead of
diameter permitting more steam to pass the ejectors. Water droplets can quickly damage an
through. This increases downstream condenser ejector. The steam should enter the drum tangentially.
pressure. Any condensate leaves through a steam trap at the
➢ Diffuser operation suffers as diameter of the bottom. It is a good idea to provide a donut baffle near
diffuser is designed to operate at a certain steam the top to knock back any water creeping up the
flow. vessel walls.
. Give a troubleshooting checklist for vacuum systems & The ejector barometric legs should go in a straight

based on ejectors. line to the seal tank. A 60–90 slope from horizontal
is best.
& Steam lines from the utility side should be well
& The suction to the ejector must be designed for half
insulated to prevent condensation. Insulation thick-
the absolute pressure of the evacuated system.
ness is a function of steam temperature as well as pipe
diameter. & Screwed fittings should never be used in any vacuum
system regardless of size.
& A steam separator is installed before the ejector to
ensure that the steam is dry. If steam quality is & For large size vapor lines and condensers (frequently
questionable, a calorimeter is used to measure the possible invacuum systems), the line, condenser, and
percentage of moisture in the steam. top of the column should always be insulated; oth-
erwise, rain or sudden weather variations will change
& The maximum allowable level of moisture in the
column control. It is possible to have more surface in
steam is 3%.
the overhead line than in the condenser.
& The steam pressure on each ejector is checked to
& Liquid traps in vacuum system piping should be
ensure that it is within limits. Checking the pressure
avoided by never going vertical after having gone
of only the main steam line will not be enough.
to horizontal.
& Water pressure should be within 15% of the design
& The vacuum system control valves should be in-
pressure and its temperature should be less than or
stalled at the highest point of a horizontal run with
equal to design temperature.
the control valve bypass in the same horizontal plane.
& Water flow rate must be adequate for proper conden-
. Summarize troubleshooting of steam jet ejectors.

sation and cooling. About 8–15 C temperature rise in
the condenser is acceptable. & Table 5.8 presents troubleshooting guidelines for
steam jet ejectors.
& The system must be checked for excessive air leak-
age. Excess air leakage is based on volume and time . Compare different types of vacuum producing equip-
taken to build up pressure from vacuum after the ment with respect to their capacities and operating
ejectors are turned off. ranges.
& The system for corroded/eroded/cracked parts, espe- & Table 5.9 presents a comparison of different types of
cially ejector nozzles, must be checked. vacuum producing equipment on pressures and ca-
pacities obtainable.
& Blocked nozzles/lines/diffusers/strainers/barometric
legs/fouled condenser should be checked. & Dry pumps work in the range from 0.05 to 760 Torr
3
with capacities of 1.4–40 m /min.
& Steam leaks between the steam chest and the nozzle,
which will increase the suction load to ejector, should & Liquid ring pumps have capacities of more than
3
be checked. 600 m /min.
& Always spares for critical items should be maintained
as inventory. 5.3 FANS, BLOWERS, AND COMPRESSORS
. Give some general rules of thumb for ejectors.
& To determine number of stages required, assume a . What are the different types of fans and blowers?
maximum compression ratio of 7:1 per stage. & Centrifugal or axial flow types.