POWER February 2012 - 79

PLANT DESIGN
lent to 67.4/18 = 3.744 kg-moles/hr, where
18 is the molecular weight of water vapor.
■ Total number of kg-moles/hr = 1.055 +
3.744 = 4.799 kg-moles/hr.
■ Units conversion: 4.799 kg-moles/hr x
22.4 = 107.49 standard cubic meters/hr.
■ Units conversion: Convert standard cubic
meters to pump test suction conditions
of 33.9 mbar pressure and 20C: 107.49 x
(1,013/33.9) x (293/273) x (1/60) = 57.455
m3
/min. Note: 33.9 mbar = 1 inch of mercury
and 1 atm = 1,013 mbar.
Pump test results provide the following data:
■ The capacity of the vacuum pump was 29
m3
/min at 33.9 mbar and 20C.
■ The seal water temperature was 25C, the
same as expected during plant operation.
It appears that the HEI requirement for air
removal is not being met, as the HEI standard
requires a pump capacity of 57.455 m3
/min.
The test results found the pump capacity was
only 29 m3
/min. The pump capacity, which
is unknown by most designers, must now be
adjusted by a pump-specific " condensation
bonus factor, " which provides added pump
capacity due to moisture condensation inside
the pump casing.
For example, if a condensation bonus factor
of 0.5 is used, then 57.455 x 0.5 = 28.7 m3
/min,
and because this value is equal to or exceeded
by the pump test capacity of 29 m3
/min, the HEI
requirement is considered to be met.
In other words, the pump capacity is governed
by the value of the condensation bonus
correction factor that is determined by the
pump vendor. This factor is generally proprietary
to each vendor and difficult for the
purchaser to verify, especially if these factors
are not published. Note that the condensation
bonus factor for LRVPs is applied to the
volumetric flow and not the mass flow.
Hybrid Arrangements
In the hybrid arrangement, an air jet ejector is
used as the first stage of the unit (Figure 3).
The ejector uses part of the pump discharge
air as the motive air, and the ejector discharge
is directed to the inlet of the vacuum pump.
Consequently, the first-stage ejector boosts
the pump suction pressure, thus requiring a
smaller pump with lower operating/capital
costs.
The first-stage ejector also helps to minimize
pump cavitation, as the pump is no longer
operating at low suction pressures, which
could cause vaporization of the seal fluid
within the pump casing. Cavitation occurs
at low suction pressures and high seal water
temperatures.
In the hybrid arrangement, the air removal
capacity is governed by the mass flow capability
through the ejector, and the " condensation
bonus " discussed above is not directly
applicable to this equipment arrangement.
However, a different type of condensation
bonus is used, taking advantage of a few degrees
of adiabatic cooling within the ejector,
which reduces the water vapor load. This, in
turn, reduces the loading on the downstream
vacuum pump. Note that the amount of adiabatic
cooling within the ejector is empirical
and can vary from vendor to vendor. The
water vapor loading is directly related to the
temperature and can be determined as (18/29)
x (Pw
/Pt - Pw), where Pt
is the total pressure
3. Best of both worlds. This schematic shows a hybrid arrangement using an air ejector
and a vacuum pump. Source: Bechtel Power Corp.
Suction from
condenser
Jet ejector
Discharge to
atmosphere
Motive air
For More Information
Additional information on vacuum system
design requirements is available from:
■ Heat Exchange Institute, Standards for
Steam Surface Condensers, 10th ed.
■ Heat Exchange Institute, Performance
Standard for Liquid Ring Vacuum
Pumps, 3rd ed.
■ Dekker Vacuum Technologies, " Effect
of Saturated Air Service on Capacity of
Liquid Ring Vacuum Pumps. "
■ Gardner Denver Nash, Vacuum Systems
Handbook, 2006.
associated with dry air plus water vapor and
Pw
is the saturation pressure of water vapor at
a reduced temperature inside the ejector.
The HEI standard requires removal of water
vapor plus dry air while the ejector-pump
system test is carried out with dry air at 33.9
mbar and room temperature. However, the
effective water vapor loading inside the ejector
is reduced due to cooling while the dry air
loading remains the same. Therefore, for consistent
comparison of test results against the
standards, the reduced vapor loading in the
ejector is first converted to equivalent mass of
dry air (by multiplying by a ratio of air-water
molecular weights: 29/18), and the original
amount of dry air specified in the standard
is added to it. This total mass amount (kg/hr)
is then converted to standard volumetric flow
(m3
/hr) and adjusted to pump test pressure/
temperature conditions for direct comparison
to the dry-air test results.
Air-washer
separator
Pump
Seal water cooler
Seal water return to pump
February 2012 | POWER
www.powermag.com
Closing Thoughts
The HEI standards stipulate the capacity requirement
for air-venting equipment for the
condenser. However, equipment provided by
the vendor is generally sized at a somewhat
lower capacity, taking into consideration the
condensation bonus. It is difficult for the end
user to verify the condensation bonus, as it is
generally proprietary and specific to the vendor's
equipment. However, vendors should
be asked to justify, with data, their selection
of the condensation bonus for your project. If
the condensation bonus selected by the equipment
vendor is too optimistic, proper venting
of the condenser will not occur, and design
heat transfer rates may not be achieved. ■
-S. Zaheer Akhtar, PE (szakhtar@
bechtel.com) is technical advisor to the
manager of engineering on assignment
from Bechtel Power Corp. to Power
Generation Engineering and Services Co.
(PGESCo). Magdy Mahmoud is manager of
engineering for PGESCo, Cairo, Egypt.
79
http://www.powermag.com
POWER February 2012

Table of Contents for the Digital Edition of POWER February 2012
