Dearators for small back pressure turbine systems

[Impdrew]

Hi, i am new to the world of boilers and steam system design and would like to know if one still requires to place a deaerator for condensate after being used for process heat, and that has come from a back pressure (no-condensing) turbine at say 4 bar, where the steam/water remains at relatively high temperatures. The use is on very small capacity systems where having an extraction point on the turbine for bleading steam to operate the deaerator is not feasible (ie. single stage turbine - 60 KW). Little further heating of condensate is required as the temp. remains high enough to pass for minimum boiler feed water temperature. If a dearator is required, i can only consider that steam would need to be extracted from the boiler drum, prior to superheating.
Any comments or info. on how to find out about this would be gratefully received.
Thanks very much
Andrew

 

[johnp]

Please clarify what you mean by "deaerator". I am familiar with the term "aerator" which adds air to a fluid.

I would guess you are trying to get rid of any air trapped in the steam or condensate.

 

[Impdrew]

Hi johnhp
Yes that is correct. In the operation of boilers and steam turbines at high steam pressures one needs to remove air trapped in the steam/condensate by boiling and venting it. This is usually done as part of the boiler feed water heating,where the deaerator can be seen to both remove air and act as a mixing type heater. The hot steam/water is used (sprayed) into the deaerator chamber and causes the feed water within the chamber to evaporate and the air to be released and vented. In large plants, there is usually one deaearator (mixing type) heater, and a number of standard counter flow feed water heaters (i.e with a seperating surface area in between the two flows) and the hot streams are bled from extraction points along the turbine and used to heat or deaerate the condensate prior to boiler entry. The problem here is that the turbines are too small two have stages and hence extraction points.
So, if the saturated condensate at 4 bar is of a high enough temperature that it does not require heating prior to boiler entry (boiler feed water needs to be above a min. temp. to stop corrosion) does it still require deaerating? I guess the answer is probably going to be yes, and i have recently considered whether a throttle/flash chamber could be used to drop the pressure in the steam and hence flash evaporate the condensate,hence deaerating, though i have no knowledge of whether this is ever done. Otherwise, bleeding the boiler, or using an electric heater seem the only options i guess, but i would like to avoid both especially the first as it is very wasteful.
(I realise that i may be starting to answer my own question here)
Thanks
Andrew

 

[johnp]

I was hoping that would happen. You can also do a search on "deaerators" as a Keyword on this Forum, and read the comments they contain.

I don't think electric heating is the way to go. The idea is to reduce you efficiency losses as much as practical. You probably need to think a little more about the functions. On large plants you have a condenser at the low end, and a dearator at the high pressure end. If you don't use a dearator, can you get rid of the air in some other way? Can the condenser act as a dearating condenser?

Yes, a flash chamber can be used to reduce pressure. We have used it in process lines.

Keep it simple. It is a small plant.