14 replies to this topic

[Mayank Joshi]

can someone tell me how a deaerator works.i want links of some articles on deaetor design. plz
thanking you in advance

[Zaki]

The daerator work to remove the Oxygen content from boiler feed water.The daerator working at the low pressure and high temperature. Normaly at the 0.5barg dan 110 c

[Mayank Joshi]

I do know what it does. I wanted to study in detail how it does this.what are the different kinds of deaerator?

[ankur2061]

Mayank,

There is a search feature for searching topics on the forums it. Use it and you will find a lot of information on deaerators. I am posting a link on one of the posts related to deaerators:

http://www.cheresour...__fromsearch__1

Regards,
Ankur.

[kkala]

-A deaerator expels dissolved oxygen from boiler feed water, taking advantage of decreasing O2 solubility in it as temperature becomes higher. Usual operating pressure of deaerator is in the range 0.1 - 1.0 Barg (operating temperature 102-120 oC), heating is implemented by bare steam. Practically all injected steam is used for heating, a negligible amount comes out of deaerator as a plume, along with stripped O2.
-There are two basic deaerator types
α. The tray type, as in Fig 1 of http://www.cheresour...3994-deaerator/, consisting of the surge drum and the attached deaerating tower, where stripping of O2 mainly occurs.
β. The spray type, as in Fig 2 of mentioned web reference, or http://www.stork-the...-operating.aspx, where there is no tower. Stripping of O2 occurs in the drum.
γ. In boiler process duty spec we specified spray - tray type deaerator, which seems to be close to (α), not to (β), with BFW hold up time in the surge drum about 20 min.
-Some part of the design is proprietary, e.g. Stork has patent on spray deaeration. On the other hand interest is concentrated in making the duty spec of the deaerator, not in designing it, at least here. I have heard of Romanian (old) books explaining how to design the tray tower though.
-However following links may be useful
http://www.spiraxsar...deaerators.asp , from Spirax Sarco.
http://www.cheresour...-calculations/ , from forum (vent steam)
http://www.cheresour...eam-vent-flow/ , from forum (vent steam).
http://www.altairequ...principles.pdf , packed deaerator type, etc.
-It is also noted that we specify a generous margin (3 m) between NPSHa and NPSHr of BFW pumps taking suction from deaerator in boiler duty spec. This means that that deaerator has to be adequately elevated.

Edited by kkala, 31 December 2011 - 02:41 PM.

[Mayank Joshi]

can I operate the deareator on a gauge pressure of 1.5kg /cm2 if I maintain the water inside it at a temperature of 126 'C (saturation temperature at 2.5 kg/cm2)??

Will the BFW quality suffer???

[kkala]

Some links of post no 5 (by kkala) do not work now, so this part of the text is repeated below.

-However following links may be useful
<http://www.spiraxsar...-deaerators.asp>  from Spirax Sarco. "www.spiraxsarco.com/resources/steam-engineering-tutorials/the-boiler-house/pressurised-deaerators.asp".
<http://www.cheresour...-calculations/ >, from forum (vent steam)
<http://www.cheresour...eam-vent-flow/> , from forum (vent steam).
<http://www.altairequ...principles.pdf> , packed deaerator type, etc.

Editing note.

Links in orange can be properly accessed from following post no 6, by  thorium90. Thanks thorium90 for finding these links, while I was searching for them.

Edited by kkala, 13 February 2013 - 08:06 AM.

[thorium90]

I think you meant

http://www.cheresour...e-calculations/

http://www.cheresour...team-vent-flow/

http://www.altairequ..._principles.pdf

 

(Cant figure out what is the first one)

Edited by thorium90, 13 February 2013 - 07:44 AM.

[Mayank Joshi]

my question was :

can I operate the deareator on a gauge pressure of 1.5kg /cm2 if I
maintain the water inside it at a temperature of 126 'C (saturation
temperature at 2.5 kg/cm2)??

Will the BFW quality suffer???

[thorium90]

Those pressures look normal to me. Its already at saturation and blowing steam at the top vent isnt it? Is there any reason why you feel the quality will suffer (as compared to) ?

[Mayank Joshi]

yeah it is at saturation temperature , but someone is telling me that gases will not strip out and when o2 levels have to be in ppb than even this much increase in pressure would lead to an increase in hydrazine consumption .

Also since a bit of steam is vented in dearation this rate would increase.

I just want to know that is low pressure deareation better that high pressure one or only the saturation temperature matters.???

[Dr Krska]

There are several different types of deaerator depending on the application. They include low temperature vacuum, gas stripping or steam stripping packed column deaerators, low pressure spray deaerators and "High Level" deaerators. From the information provided I assume we are talking about a High Level deaerator which are so called as they are normally set at high elevation to provide static head so they can free flow into a depressurised boiler to keep it cool. High Level deaerators usually comprise a spray section on top of a tray or packed section. Steam enters below the tray/packed section which heats the water and strips the dissolved gasses. A vent condensor is normally located above spray section, which condenses most of the stripping steam to reduce the vent losses. The vented steam with liberated dissolved gasses flow out through a fixed orifice, which maintains the pressure in the deaerator. The vent orifice is usually sized to vent about 0.1% of design mass flow of feed water. Increasing the vent rate will reduce the reduce the O2 concentration, reducing it will increase it. 0.1% is a typicalcompromise between performance and lost heat. The feed water is split between the main sprays and vent condensor. An optimum split wiil exist, this typically lies between 8 and 12% feeding the vent condensor. High Level deaerator normally incorporate a large storage volume for boiler run down directly underneath deaerator or incoprated into a common vessel. A steam sparge is normally applied to the stored water to maintain its temperature and keep the dissolved gas concentration low. This type of deaerator, by its simple design, will always operate at the water saturated vapour pressure. Up to a point the hotter the water is the better the deaeration will be, the reduced solubility of the dissolved gasses more than offsets increased partial pressure of te dissolved gasses. A typical operating temperature of 125 DegC should provide an O2 concentration of about 0.005 to 0.007 mg/l. The mechnical integrity of the deaerator should be considered before increasing it operating temperature/pressure. If the deaerator isn't providing adequate performance, then it is likley that some internal component is worn out or damaged.

[Mayank Joshi]

Thanks a lot .
Do you have any pdf which I can refer to about deareators??

[Bobby Strain]

http://www.kansascit...y-deaerator.htm

 

Try this company. Once upon a time they had an online sizing program.

 

Bobby

[kkala]

1. Outlet water O2 concentration of 0.005 - 0.007 mg/l reported by Dr Krska (post no 12) is same as locally specified for (spray-tray) deaerators treating demineralized water at 103 oC. Not verified by measurement during boiler performance tests. Hydrazine dosing (post no 11) is expected not to be sensible to changes in O2 concentration as above. Hydrazine is added in excess, to cover its thermal decomposition and reactions beyond O2 removal. So inefficient (to some extent) deaeration may not be "caught" by hydrazine dosage.
2. Impression is that deaerator efficiency concerning O2 will not practically change by increasing operating pressure from 1.5 to 2.5 kgf/cm2 a. Once a boiler supplier proposed to increase deaerator temperature to (say) 130 oC to eliminate boiler feed water preheater (downstream BFW pumps). Deaerator performance would not be sacrificed, but we could not check that (see above). This is an uncertain indication; a general justification would be needed, considering a pressure deaerator with deaerating tower (High Level, per post no 12).

α. Equilibrium concentration (Ce) of O2 in water for a specific temperature depends on O2 partial pressure in the vapor phase (Henry's law). Vapor phase contains H2O(g), O2, etc, where O2=stripped O2. Increasing temperature to 126 oC will increase H2O(g) pressure to ~2.5 kgf/cm2 a. Absolute O2 pressure in vapor phase will not change, or it will increase quite little due to temperature increase. Thus Ce will remain as before or decrease quite a little.

β. Above concerns equilibrium concentrations. Actually water in deaerating tower contains more air in dilution than what equilibrium would dictate under operating conditions (*).  This "surplus"  air (and O2 content) is vigorously "shaken" out by stripping steam. 

For constant stripping steam mass flow, increasing its pressure to 2.5 kgf/cm2 a decreases its volumetric flow rate by about 40%. This may or may not decrease rate of stripped O2. If it does, increasing vent rate by 40% would probably restore previous O2 removal efficiency. Specific advice from suppliers would be useful, or even from members having expertize.

γ. My impression is that deaerators of above type have great flexibility, on the other hand required vent rate is referred as % of incoming water.

Comments on this rough (incomplete) explanation are welcomed, as well as additions.

-It is noted that above results in higher design temperature and pressure to be looked into (post no 12).

 

(*) Betz, Handbook of Industrial Water Conditioning, Boiler Water Systems, Boiler Feedwater Deaeration (Betz, Trevose, 1976).
     

Edited by kkala, 16 February 2013 - 12:14 PM.