Degasser

This article covers degassers in steam and hot water systems. For other designs and applications see degassing .

Feed water tank with degasser and vapor condenser

In steam and hot water systems (e.g. in power plants), degassers are system components with which dissolved gases are removed from the make-up and circuit water or from condensates.

Gases dissolved in water, such as oxygen and carbon dioxide , cause corrosion in the systems , especially for materials made of iron . Therefore, in addition to desalination and blowdown , the degassing of the feed water and the circulating water is also important.

Degasser for two shell boilers

Steam boiler systems are almost always equipped with a deaerator (feed water deaerator) for the treatment of boiler feed water . The required quality of the water is specified in guidelines that also list the permissible residual gas content. The values ​​to be adhered to differ depending on the boiler type and pressure level. See DIN EN 12952 Part 12. For high-pressure boiler systems, the permissible guide and limit values ​​are fixed in the VGB guideline for feed water, boiler water no. R450L (VGB = VGB PowerTech ), which has now been replaced by VGB Standard 010.

Such degassers can also be used for other purposes, e.g. B. in the chemical industry, and for liquids other than water.

Procedure

Degassing is mainly carried out physically in the boiling state under positive or negative pressure.

A more recent technique is the use of membrane contactors to remove gases from liquids, but their use is limited by the temperature resistance of the membranes.

Oxygen removal can also be done chemically using chemicals. More on this under chemical degassing .

Only physical degassing in the boiling state is dealt with below.

interpretation

Physical basics

Different technical designs have been developed for the degassers for the degassing of liquids . A prerequisite for physical degassing is a disturbance of the equilibrium for the dissolved gases. This is achieved with water, for example, in that the gas phase in the degasser contains less of the gas to be removed than corresponds to the equilibrium between water and gas. Gases such as oxygen (O ₂ ) and nitrogen (N ₂ ) are easier to degas than, for example, gases such as carbon dioxide (CO ₂ ), which has a chemical-physical relationship with the water and the dissolved ingredients ( lime-carbonic acid balance ).

The physical principles of degassing are:

• Diffusion of the gases at the water / steam phase boundary
• Gas bubble formation with convective mass transfer

The following technical processes are used for degassing:

• Passing vapor bubbles through the liquid to be degassed, for example by supplying energy (colloquial: boiling)
• Droplet formation to enlarge the water surface, for example by atomization or atomization
• Reduction of the layer thickness of the water by transferring it via internals or fillers

Packing degasser

In the following, only degassers with fillers will be discussed in more detail. In practice, such degassers are equipped with nozzles or perforated plates for water distribution and a boiling device in addition to the packing. Both - water distribution and boiling - improve the degassing effect. However, this is usually not taken into account when calculating the packing degassing . In this way, an additional safety margin is achieved.

The following parameters must be taken into account when designing a low-pressure degasser that is equipped with a packing and operated with saturated steam :

• the necessary height of the packing bed = H in (m)

• the permissible surface load = in (kg / m² · h) ${\displaystyle \mathrm {\frac {G}{\phi }} }$

${\displaystyle \phi }$ = dimensionless correction factor with which the pressure and temperature-dependent density of the steam in the degasser is taken into account

• the amount of water to be degassed = G in (kg / m² · h)

• the total amount of steam required = D in (kg / m² · h)

• the ratio of the water to be degassed and the required saturated steam = in (kg / kcal kg ⁻¹ ) ${\displaystyle \mathrm {\frac {D}{G}} }$

• the necessary transmission units = HTU ( H eight of T ransfer U nits) in (m).

HTU can also be converted to HETP ( H eight E equivalent to one T heoretical P late) via 2.3 HTU . The calculation of the packing layer is then directly possible with this value. ${\displaystyle \log \beta \cdot \delta }$

• Ratio of dissolved gases before and after degasser = in (mg / mg) ${\displaystyle {\frac {C1}{C2}}}$
• required amount of swath = in (%) ${\displaystyle \beta }$

Packing :

These are specially shaped materials such as Raschig rings or Berl saddles, which greatly increase the surface that is wetted with water. The gas exchange takes place at the boundary layer of the liquid on the surface of the packing and the vapor phase.

The exchange surface of the packing is recorded with (a) in m² / m³. The values ​​are strongly dependent on the shape and dimensions of the packing. For example, Raschig rings 1/2 inch have the value a of 374 m² / m³ and for 1 inch of 190 m² / m³. The values ​​are given in tables from the manufacturers.

The height of the packing depends on the type of packing (value a), the ratio of the gases upstream and downstream of the degasser (value C1 / C2), the amount of waste (% value) and the temperature of the water to be degassed when entering the degasser and the degassing temperature (values ​​in ° C). The greater this temperature difference, the greater the amount of steam required. Usual heights of the fill are 0.8 - 3.0 m.

Wing loading :

In the degasser, the steam flows from bottom to top and the water to be degassed flows in countercurrent from top to bottom through the packing layer. If the surface loading is too high , the flow of steam and water is hindered. The point of flooding occurs because steam and water can no longer flow undisturbed in counter-current through the packing layer. This flooding point must be avoided at all costs. Accordingly, only a permissible surface loading may be selected at which this cannot occur. As the demand for steam increases, the permissible surface load decreases. A correction factor ( ) is used in the calculation , which corrects the influence of the degassing temperature for the flooding point. Usual surface loads are 30 - 60 t / m² · h. ${\displaystyle \phi }$

Gas ratio :

Content of oxygen in the water before and after the degasser. At 10 mg / l (= C1) and 0.010 mg / l (= C2) the result is 10 mg / l / 0.01 mg / l = 1 · 10 ³

Waste volume :

This is the amount of exhaust steam escaping, which is also known as sweeping steam and which contains all of the expelled gases. Quantities of around 1% (= ) are common. ${\displaystyle \beta }$

calculation

The calculation is carried out in several steps and the values ​​required for the calculation are taken from tables. These table values ​​were determined experimentally. The following are the calculation steps that are carried out separately for the height of the packing, saturated steam requirement and surface loading:

• Determination of the layer height for the packing

${\displaystyle H=2{,}3\cdot H.T.U.\cdot F_{t}}$ in (m)

HTU = according to the table for the selected type of packing (HTU / m)

F _t = factor, according to table (e.g. for a degassing temperature of 25 ° C = 1.0 or at 100 ° C = 0.33)

• Determination of the steam requirement

${\displaystyle {\frac {D}{G}}={\frac {i_{s}-i_{e}}{i_{D}-i_{s}}}+\beta }$

i _s = enthalpy of the boiling water (in the degasser)

i _e = enthalpy of the entering water

i _D = enthalpy of the heating steam

• Determination of the wing loading

${\displaystyle G=D\cdot \phi }$ in (kg / m² · h)

Types

Small to medium-sized deaerators - up to around 300 m³ / h capacity - are predominantly designed as trickle deaerators with internals (floors) or fillers. The circuit diagram above shows such a degasser. Larger degassers with a capacity of up to 2000 m³ / h are usually spray degassers with a boiling device for reasons of cost.

The most common process in power plants is degassing by heating the already desalinated feed water with the steam that is already present in the system (thermal degassing). When heated, the gases are removed from the degasser together with the sweeping steam as vapors . These are either discharged directly into the open or, in the case of larger degassers, only after the vapor fraction has been separated and condensed in the vapor condenser . The degassing is carried out at overpressure ( pressure degassing ) or less often at negative pressure ( vacuum degassing ).

The water prepared and stored in the degasser is fed into the steam boiler via the feed water pump .

The amount of steam required for degassing is 2–5% of the steam generated in the boiler.

See also

• Continuous degasser
• Vacuum mixer

Individual evidence

1. ^ Fritz Mayr , Kesselbetriebstechnik, Verlag Dr. Ingo Resch, 10th edition, 2003, page 392.
2. ^ HE Hömig , Physicochemical Basics of Speisewasserchemie, VGB , Vulkan-Verlag Essen, 2nd edition, 1963, p. 313.
3. ^ HE Hömig , Physicochemical Basics of Speisewasserchemie, VGB, Vulkan-Verlag Essen, 2nd edition, 1963, p. 317.
4. ^ HE Hömig , Physicochemical Basics of Speisewasserchemie, VGB, Vulkan-Verlag Essen, 2nd edition, 1963, p. 317.
5. ^ HE Hömig , Physicochemical Basics of Speisewasserchemie, VGB, Vulkan-Verlag Essen, 2nd edition, 1963, p. 314.
6. ^ Fritz Mayr , Kesselbetriebstechnik, Verlag Dr. Ingo Resch, 10th edition, 2003, page 394.

literature

Fritz Mayr , Kesselbetriebstechnik, Verlag Dr. Ingo Resch, 10th edition, 2003, pages 392-394 HE Hömig , Physicochemical Basics of Speisewasserchemie, VGB , Vulkan-Verlag Essen, 2nd edition, 1963, pp 299-325

Thomas Melin, Robert Rautenbach , Membrane Process, Springer-Verlag (VDI book), 3rd edition, 2007