EWT Water Technology – Water Treatment

EWT Water Technology


Thermal Deaeration Plant

Sieve Tray Deaerator, Boiler Feed Water Tank

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Summary:

Thermal deaeration is a physical process for removal of dissolved gases from water. For thermal deaeration, the water to be deaerated is brought to the boiling point by addition of heat.

A sieve tray deaerator is a vessel for thermal deaeration of water. Sieve tray inserts within the deaerator result in an increase of the surface of the water flowing through the deaerator, which in turn will facilitate heat and mass transfer. Usually, a boiler feed water tank (short: bfw tank) for storing and secondary deaeration of the water is arranged downstream of the deaerator.

Thermal deaeration is a continuous process. During operation, make-up water, condensate and heat (usually by injection of heating steam) flow continuously in, while deaerated boiler feed water and vent steam flow continuously out.

Thermal deaeration may occur both at higher or lower than (→ vacuum deaeration) atmospheric pressure, meaning the boiling temperature may be both higher or lower than 100 °C. The following descriptions refer to thermal deaeration at higher than atmospheric pressure only.

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Design Example:

deaerator with boiler feed water tank

Thermal deaeration plant: Sieve tray deaerator with boiler feed water tank, valves, and instrumentation.

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Process Description:

Thermal deaeration is based upon the dependency of the solubility of gases in water from the equilibrium between concentration and partial pressure, the temperature, and the boiling state. With increasing water temperature, the solubility of gases in water is reduced. In boiling water, the solubility in theory approaches zero.

The first of the aforementioned dependencies results from Henry's Law, which is often quoted in this context. According to Henry's Law, the partial pressure of a gas above a liquid is directly proportional to the concentration of the same gas in that liquid. By reducing the partial pressure of the gas above the liquid, for example by evacuation or by displacing it with another gas, the solubility of that same gas in the liquid is reduced accordingly.

The mass transfer required for deaeration occurs both by diffusion at the phase interfaces and by convective mass transfer as gas and steam bubbles leave the liquid. For removal of even trace dissolved gases as required in boiler feed water treatment, boiling of the complete liquid with maximised phase interface surface areas is required, while taking into account the modification of boiling temperature by hydrostatic pressure resulting from water depth and surface tensions at phase interface surface areas.

More detailed descriptions of this process have been published back in the middle of the previous century (H. Tietz 1950, H. E. Hömig 1959, W. Bosselmann 1960, G. Ende 1964).

A sieve tray deaerator is usually designed as a vertically aligned pressure vessel with sieve tray installations, connected to a horizontally aligned boiler feed water tank. Within the water room of the boiler feed water tank, there is some kind of provision for inflow of heat, usually a steam sparging pipe for direct injection of heating steam.

Condensate and make-up water flow through the deaerator from top to bottom, and while doing so are subject to an increase of the surface area due to being sieved by the sieve tray installations. This increased surface area will facilitate heat and mass transfer. Steam flows through the deaerator from the bottom to the top, and flows around the water to be deaerated in counter-cross-flow direction. In the upper part of the deaerator, the main part of the steam will condense while heating the inflowing water up to boiling temperature. The remaining part of the steam flows out of the deaerator at the top, at the same time venting the gases which have been removed from the water and entered the steam phase. Within the water room of the boiler feed water tank, the continuous addition of heat leads to a continuous circulation, mixing and secondary deaeration of the water.

Sieve tray deaerators as described above can operate with a high turndown ratio, and can be designed for any desired temperature difference past a certain minimum, e.g. also for cold make-up water without any prior pre-heating. Depending upon the design of the heating system, between 80 and 95% of the total volume of the downstream boiler feed water tank are usually available as useful storage volume.

Normally, thermal dearators are also being integrated into the water-steam-cycle as a boiler feed water pre-heating stage; the heat used for deaeration flows back into the steam boiler. The heating of the boiler feed water usually occurs by injection of sparging steam, however alternative means are technically feasible and also sometimes employed, for example for heat recovery.

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Applications:

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Technical Data:

boiler feed water mass flow available from approx. 1 500 up to 350 000 kg/h ≈ 0.5 ... 100 kg/s for each line
amount of lines usually 1x100 %
boiler feed water quality oxygen < 20 μg/L O2 (standard)
< 50 μg/L O2 (option)
< 10 μg/L O2 (option)
≤ 7 μg/L O2 (option)
free carbon dioxide < 1 mg/L CO2
operating pressure, absolute > 1 bar, usually either approx. 1.2 bar or approx. 3.6 bar
operating temperature > 100 °C, usually either approx. 105 °C or approx. 140 °C
minimum temperature difference * approx. 10 K
maximum temperature difference * any (only limited for a given process design)
minimum pressure difference heating steam approx. 1 bar
vent mass flow usually approx. 0.2 ... 1.0 % of the boiler feed water mass flow
heating steam mass flow usually approx. 2 ... 20 % of the boiler feed water mass flow
storage time bfw tank for smaller plants usually 30 ... 120 minutes
for larger plants usually 15 ... 30 minutes
useful volume bfw tank usually approx. 75 ... 95 % of the total volume
code for pressure vessel manufacturing • not specified, "sound engineering pratice"
• EN 13445
• AD 2000
• VGB-S-110-R-00 (replacement for VGB-R 110 L)
• GOST R (export certificate for the Russian Federation)
material options deaerator • stainless steel (e.g. 1.4301, 1.4571)
boiler feed water tank • carbon steel (e.g. S235JR, P265GH)
• low alloy steel (e.g. 16Mo3)
• stainless steel (e.g. 1.4301, 1.4571)
pipelines • carbon steel (e.g. P235GH)
• low alloy steel (e.g. 16Mo3)
• stainless steel (e.g. 1.4541, 1.4571)
valves • grey cast iron (e.g. 5.1301)
• spheroidal cast iron (e.g. 5.3103)
• cast steel (e.g. 1.0619)
• stainless steel (e.g. 1.4408)
• high temperature steel (e.g. 1.7335, 1.7357)
gaskets • ethylene propylene diene monomer rubber (EPDM)
• polytetrafluoroethylene (PTFE)
• NBR composite
• graphite composite
control options closed loop pressure control • mechanical, continuous (P control)
• electric, intermittent (on/off control, very small plants only)
• electric, continuous (PI, PID, or 3-point control)
closed loop temperature control • not recommended
closed loop level control • mechanical, continuous (P control)
• electric, intermittent (on/off control)
• electric, continuous (PI, PID, or 3-point control)
* Referring to the difference between the operating temperature and the mixed temperature of the make-up water and condensate.

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2018-05-05 • water treatment made in GermanyCompany InformationPrivacy