Deaeration of Condensate and Make-up Water
High oxygen content in the feedwater can cause corrosion of components and piping.
The deaerator should be designed to remove the maximum amount of the incondensable
gases possible from the boiler feedwater cycle and to achieve an oxygen content of < 7 ppb
(=0.005 cc/l) in the feedwater. This protects the boiler and the whole system against corrosion.
Depending on the pressure in the feed water tank/deaerator the deaeration process
takes place either under vacuum (known as vacuum deaeration) at about 60°C, 0.2 bar, or at
pressure above atmospheric pressure (over-pressure deaeration).
in Steam Turbine and Combined Cycle Power Plants
Deaeration of Condensate
If the oxigen content of condensate at the deaerator inlet is not very high, a tray type
deaerator can be used (see deaerators Fig. 007).
Tray Type Deaerators:
- Tray type deaerator as horizontal vessel over FWST
- Annular tray type deaerator as vertical dome over FWST (ALSTOM tray type Deaerator)
- Tray type deaerator as vertical dome over FWST (TPT tray type Deaerator).
The water is sprayed at the top of deaerator by means of a spring-operated sprayer or
by means of spray nozzles. The sprayer reduced the water to small droplets or to a water film.
The water flows down between trays or deflection baffles. The steam enters the deaerator in
the lower part of the deaerator and rises to the sprayed water in counterflow to the down-coming
water and takes away the gases from the water. The steam condenses practically on the sprayed
water and the incoming water is heated to the satturation temperature and deaerated.
The apparatus works as DC heater and deaerator.
The freed oxygen is discharged to atmosphere or to the condenser via the vent openings
at deaerator top, which may be fitted with throttling organ.
TPT Tray type Deaerator:
Inlet condensate: Flow rate Mc, Temperature Tc, oxygen content C1o2
Design of the TPT Deaerator:
Steam flow rate Ms: Ms = f(Mc, Tc, Ps)
Dome diameter D: D = f(Mc, Ms, Ps)
Bypass steam channel ratio: L/D = f(Ms, Ps)
Venting flow rate Mvent: Mvent = f(Mc, C1o2, C2o2)
Deflection plats number (baffles) and height H1 are function of O2 content C1o2, C2o2 and Ps.
The deaerator is laid out in such a way that the main heating steam flows through two by-pass
channels and the condensate and flush steam are connected in a counter flow.
For the design of a deaerator the following condensate and steam load capacities are computed:
a) Water load capacity
The water load capacity limit (flood limit) is put at the basis for the maximum condensate
mass flow density mc (kg/sm2) through the deaerator dome.
Since steam in the top of the dome condensed and the condensate increases the water load of
the deaerator qross-section, for the diameter definition the mass flow density mc is computed
from the sum of the condensate flow (Mc) and the steam flow (Ms).
b) Steam load capacity
The load capacity limit (flood limit) for a given deaerator type is a function of the steam
flow and the steam pressure or the density of the saturated steam.
With an excess of the Steam load capacity instability with strong steam impacts will
The deaerator masses (dome diameter D, by-pass steam chanels or their distance L, ...) are
laid out so that the flood limit is not exceeded. The calculation will be determined for the
worst operation case and from the water and steam load capacities of the deaerator
(see deaerator sketch 007a).
Tray type Deaerator on Condenser Wall:
The steam condenser have acasing wall and second casing wall mounted on the steam condenser.
The casing walls are spaced apart and from a mixing chamber and aseparation column.
The mixing chamber includes a means for spraying oxygen-rich make-up water.The steam condenser
casing wall has a steam-inlet opening located between the recciver and the separation column
though which steam flows into and through the separation column and into the mixing chamber.
Patent: US 006056806A: Deaerator on Condenser Wall
Deaeration of Make-up Water
Make-up water is usually fully saturated with oxygen (see diagram).
For make-up water with oxygen content of 9000 ppb (9 mg/lit) at deaerator inlet,
the oxygen content at the outlet of 5 to 7 ppb (0.005 to 0.007 mg/lit) should be achieved.
For a lower mass flow rate of make-up the water can be sprayed to the cycle in the condenser neck.
for Make-up Water and Condensate
The deaerator is arranged as cylindrical dome over FWST or as separate deaerator.
The mixture of Make-up water and condensate is sprayed at the top of the deaerator (Fig. 007).
By means of a distributor the sprayed water is distributed across the dome cross-section.
The water trickles down through the mass transfer packing elements and forming a large
interfacial area between the water and the surrounding steam phase. A venting system
extracts all gases from the vapor phase. The freed non-condensable gases is discharged to
atmosphere or to the condenser via the vent openings at deaerator top.
Since the packing elements are expensive, the packing deaerator is used for deaeration of
water under extreme parameters (high oxygen content and low pressure).
The disadvantage of this deaerator type (especially in case of vacuum deaeration) that a
large packing diameter is necessary because of the large volume flow of the heating steam.
The large packing volume leads to higher deaerator costs.
TPT Packing Deaerator
for make-up water and condensate
The Make-up water is sprayed in a chamber at the top of the deaerator dome. During direct
introduction of steam to the upper chamber the water is heated and partly deaerated.
The heated water flows downward to the distributor and distributed across the dome cross-section
The condensate can be introduced into the dome by spray nozzles above the Packing or by a
spraying valve above the distributor. The make-up water and the condensate trickle down through
the packing elements in counter flow to the second steam flow part.
The steam flow through the packing is reduced and thus arises a small packing diameter.
The packing diameter D is laid out so that the flood limit is not exceeded. The calculation will be
determined for the worst operation case and from the water and steam load capacities of the
By means of a venting system with a venting condenser the gases is extracted from the deaerator
through several venting stages. A small bypass make-up flow is selected, which is sprayed by a
small nozzle into the vent condenser. By means of openings at the top of the upper chamber and
the top of the packing chamber the steam/air flows (Vent 1 and Vent 2) are throttled to the vent
condenser. The throttle organs of the vent lines 1 and 2 are adjusted in such a way that the
pressure Pv of the vent condenser and the desired steam/air flows (Vent 1 and Vent 2) results in.
The non-condensable gases (Vent Mv) is discharged from the vent condenser via a throttle organ
to the condenser.
Packing Deaerators for CCPP:
Patent: EP 077500 A2: Packing Deaerator with counter-flow
Spray type deaerator in FWST
The deaeration occurs by blowing steam through the feedwater in FWST.
This deaeration can be used as final or after deaeration step. The spray type deaerator has
the disadvantage of steam pressure loss which reduces the saturation temperature.
TPT can help you in deaerator questions like:
- New deaeration concepts for normal and special cases
- Deaerator design
- Failure analysis
- Improvement of deaerator design
- Solution of deaerator problems