Deaerator
Degasser are with those dissolved gases are removed from the extra -vascular water or condensate in steam and hot water systems (eg, power plants ) system components.
In water, dissolved gases such as oxygen and carbon dioxide, causing particularly for materials of iron corrosion in the equipment. Therefore, in addition to the desalination and desalination, the degassing of the feedwater and the recirculating water is important.
Steam boilers are equipped for the treatment of boiler feed water almost always with a degasser ( feedwater ). The required quality of the water is defined in policies, leading the the permissible residual gas contents. The mandatory values are different depending on boiler type and pressure rating. See DIN EN 12952 part 12 for high pressure boilers are fixed the permissible guide and limit values in the VGB Guideline for feed water, boiler water No. R450L (VGB = VGB Power Tech ).
Such degasifier can also be used as water for other purposes, for example in the chemical industry, and other liquids.
A method for degassing
Degassing is performed predominantly physically in a boiling state when over-or under pressure.
A more recent technology, the use of membrane contactors for the removal of gases from liquids, but their use is limited by the temperature resistance of the membranes.
The oxygen removal can be performed chemically with chemicals. For details, see Chemical degassing.
In the following only the physical degassing is treated in a boiling state.
Interpretation of degasser
Physical Basics
For the degassing of liquids of different technical designs have been developed for the degasser. Requirement for a physical deaeration is a disturbance of the equilibrium of the dissolved gases. This is achieved for example in water, by the gas phase from the degasser less gas has to be removed as this corresponds to the balance between the water and gas. Gases such as oxygen (O2 ) and nitrogen ( N2) are easier to degas as for example gases such as carbon dioxide (CO2), which communicates with the water and the dissolved substances in a chemical-physical relationship (lime and carbonic acid ).
Physical principles of degassing are:
- Diffusion of the gases at the phase boundaries water / steam
- Formation of gas bubbles with convective mass transfer
For degassing following technical procedures are used:
- Passage of vapor bubbles through the liquid to be degassed, for example by supplying energy ( in the vernacular: cooking )
- Drop formation to increase the water surface, for example by spraying or atomization
- Reduction of the layer thickness of the water by passage over parts or fillers
Füllkörperentgaser
The following sections discuss only degasser closer with packing. In practice, such degasser be fitted in addition to the packing with nozzles or perforated panels for water distribution and a Nachkochvorrichtung. Both - Water distribution and Nachkochung - improve the degassing. But this will not normally be taken into account when calculating the Füllkörperentgasung. This additional safety margin is achieved.
The following parameters are to be considered for the design of a low-pressure degasser, which is equipped with packing and operated with saturated steam:
- The necessary height of the packed bed = H (m)
- The permissible surface load = in (kg / m · h)
= Dimensionless correction factor by which the pressure and temperature dependent density of the steam is taken into account in the deaerator
- The amount of water to be degassed = G in (kg / m · h)
- The required total amount of steam = D in (kg / m · h)
- The ratio of water to entgasendem and necessary saturated steam = in (kg / kcal · kg -1)
- The necessary transfer units = HTU (Height of Transfer Units ) in ( m).
H.T.U can also be 2.3 · H.T.U. · To H.E.T.P. (Height Equivalent to one Theoretical Plate) are reshaped. The calculation of the packing layer is then directly possible with this value.
- Ratio of the dissolved gases before and after degasser = in (mg / mg)
- Necessary Abschwademenge = in (%)
Packing:
These are specially shaped materials such as Raschig rings or Berl saddles, which increase the water-wetted surface strong. At the boundary layer of the liquid on the surface of the packing and the vapor phase of the gas exchange.
The exchange surface of the packing is measured by ( a) in m² / m³. The values are strongly dependent on the shape and dimensions of the packing. For example, Raschig rings have 1 /2 inch to a value of 374 m² / m³ and for 1 inch of 190 m² / m³. The values are given in tables of the manufacturing companies.
The height of the packed bed depends on the type of filler ( value a), the ratio of the gases before and after degassing unit ( value C1/C2 ), the Abschwadmenge ( % value) and the temperature of the degassed water as it enters the degasser and the degassing temperature ( values in ° C). The greater the temperature difference, the greater the required quantity of steam. Typical heights of the bed are 0.8 to 3.0 m.
Wing loading:
In the deaerator of the steam flows from bottom to top, and the water to be degassed countercurrently from top to bottom through the packed bed. At too high a wing loading of the flow of steam and water is impeded. It comes to the flooding point as steam and water undisturbed in counter-current can no longer flow through the packed bed. This flooding point must be avoided. Accordingly, only a permissible surface load must be chosen when this can not occur. With increasing steam demand will reduce the permissible surface loading. In calculating a correction factor () is used which corrects the effect of the temperature of the degassing of the flood point. Common area loads are 30 - 60 t / m² · h
Gas ratio:
Content of oxygen in the water before and after degasser. 10 mg / l ( = C1) and 0.010 mg / l ( = C2) gives 10 mg / l / 0.01 mg / L = 1.103
Abschwademenge:
This is the exhaust steam discharged, which is referred to as Fegedampf and contains all the expelled gases. Customary amounts of from about 1% (=).
Calculation of Füllkörperentgasern
The calculation is performed in several steps and required for the calculation values are taken from tables. This table values have been determined experimentally. Next, the calculation steps that are carried out separately for the amount of the packing, saturated steam demand and wing loading:
- Determination of the layer height for the packing
H = 2.3 × H.T.U. · Ft (m)
H.T.U. = According to table for the selected type of filler ( HTU / m)
Ft = factor given in the table (for example, a degassing temperature of 25 ° C = 1.0 or at 100 ° C = 0.33)
- Determination of the steam demand
=
Is = enthalpy of boiling water ( in the degasser )
Ie = enthalpy of the incoming water
ID = enthalpy of the heating steam
- Determination of the surface stress
G = D * in (kg / m · h)
Entgasertypen
Smaller to medium degasser - to about 300 m³ / h Power - mainly as Degasers with internals ( trays) or packing are performed. The above illustration of the circuit schematically shows such degasser. Larger degasser up to 2000 m³ / h Power are reasons of cost, usually with spray-type Nachkochvorrichtung.
The most common method used in power plants is the degassing by heating the already desalted feed water with the connectors in the system anyway steam (thermal degassing). By heating the gases are derived along with the Fegedampf as vapors from the degasser. These are either directly outside or in larger degasser after separation and condensation of the vapor portion in the vapor condenser. The degassing is carried out at elevated pressure ( Druckentgasung ) or rarely, in negative pressure ( vacuum degassing ).
The processed and stored in the deaerator water passes through the feed water pump in the boiler.
The amount of steam required for degassing is 2-5 % of the steam generated in the boiler.