Lecture 8: Feedwater heaters -construction

1. Construction
Feedwater heaters

2. Open Feedwater heater (types)
1. In the DA heater, the removal of the unwanted gases in the feedwater (FW) results
because of the difference in the amounts of these gases within the FW and
atmosphere around the condensate.
2. Following are types of deaerators:
a) Spray-type: FW is sprayed using nozzles into the head filled with steam.
b) Tray-type: Condensate is directed onto the top tray from where it cascades by
gravity into the storage tank. Trays are perforated and have to be fixed firmly as
they are subjected to forces created by the flowing steam.
c) Spray/tray type: FW is first sprayed into the steam-filled space and then made
to cascade down trays.
Note: Steam flow to vent condenser is about 25-30% of bled steam. To save this heat,
the incoming FW is heated by condensing the vapor. After storage tank, to prevent
cavitation during the suction of the pump that is caused by pressure falling below
the saturation pressure, the DA heater is located at a sufficient height from the
basement where the pump is installed.

3. Feedwater heater (Types)
Spray/tray
type deaerator
Tray-only
type deaerator

4. Open Feedwater heater (Type)
There is also one known as ‘Stork’ deaerator (see Fig. on next slide). There is no
deaerating head but in the space above the working water level in the top of the
storage tank (also called hotwell) there is a series of specially designed sprayers,
which are designed to provide the necessary heat transfer surface in order to heat the
incoming feedwater to as near to the saturation temperature as practicable (Note: Has
been measured to be within 1-2 °C of it). Thus, the stored water in the tank is
considered to be at saturation temperature.
The steam bubbling through the water absorbs any residual non-condensable gases
that are not liberated in the spraying process. The ‘Stork’ deaerator has no vent
condenser as the loss due to heat in the vapor/air mixture is too small to warrant its
cost.

5. Open Feedwater heater (Type)
Always positioned to be below the lowest working
feedwater level (marked by L.W. in the Fig.)
“Stork” deaerator

6. Open Feedwater heater (Construction materials)
1. Heads and tanks are cylindrical pressure vessels with dished ends
made of mild steel.
2. Spray nozzles and trays are made of stainless steel.
3. The spray water distribution system and tray support structures are
made of mild steel.
4. In the vent condenser, the tubesheet is made of mild steel in which
are fixed stainless steel tubes. Its shell and other details are made
of mild steel.

7. Closed Feedwater heater (General)
• Are classified as High-Pressure (HP) & Low-Pressure (LP) heaters.
Can be horizontal or vertical (depends on space availability).
• Can have a maximum of 3 zones wherein the condensing zone is
always present. When bled steam entering a FWH is superheated,
as is often the case in HP and some LP heaters, the heater includes
a DSH zone. As explained before, the DC zone maybe located
external to the heater.
• Vertical heaters are designed with their water head (water box) at
the bottom. It is located up only in special cases.
• Baffle plates are located in all 3 sections. The plates (a) guide the
steam/water across the tubes to provide efficient heat transfer (b)
support the tubes (c) form the ends of the DC and DSH sections.

8. Closed Feedwater heater (Types)
Horizontal
Vertical
(Notice baffle plate
arrangement)

9. Closed Feedwater heater (Figures)

10. Closed Feedwater heater (Figure)
Hemispherical
head

11. Closed Feedwater heater (Questions)
Q. In the horizontal FWH shown, why not exchange the inlet and outlet
feedwater positions?
A. There are three reasons for this:
1) Counter-flow gives better heat exchange than parallel flow.
2) DSH section is placed on the outlet end of the U-tubes in order
that the incoming superheated steam can raise the feedwater near
to or above the saturation temperature before it leaves the heater.
3) DC section is placed at the inlet end of the tubes to allow the
outgoing drain to be cooled to as near to the incoming feedwater
temperature as needed.
Q. In the vertical FWH shown, why not move the DSH section upward
so that it is located above the water level?
A. Because in that case, the feedwater must travel through tubes
immersed in water at saturation temperature. This limits the steam
TTD as, if a negative TTD were employed, the feed would tend to be
cooled back down to saturation temperature.

12. Closed Feedwater heater (Tubesheet)

13. Closed Feedwater heater (Construction materials)
HP heaters:
1. The heater shell, dished end, baffle plates etc. are all made from mild steel.
2. In the hemispherical header, the tubesheet is normally made of low carbon steel
with a forged steel or cast steel hemispherical head in which are fixed low
carbon manganese steel tubes.
LP heaters:
1. The shell, tubesheet and water header are all made from mild steel.
2. Tubes can be of stainless steel but brass can also be used depending on steam
temperature.