Parallel Downflow Deaerator 3

[BD55]

I am replacing a 25 year old deaerator that has been plagued with oxygen corrosion for quite some time.

This deaerator is a spray type counterflow deaerator with one spray nozzle. It is rated at 140,000 pounds/hour and was designed for 50 deg F water and 100% makeup feed. Some time after installation several heat exchangers were installed to preheat the makeup feed to ~200 deg F. I believe this caused the oxygen corrosion because the steam flow to the dearator was greatly reduced to the point that it could not remove the dissolved oxygen.

I plan on installing a new 140,000 pounds/hour spray/tray type deaerator. Makeup feed under normal conditions will be 60% makeup at ~200 deg F with the possibility of 100% makeup at 50 deg F.

My research indicates that a Parallel Downflow Deaerator will deaerate properly with elevated makeup feed temperature and high makeup feed volume, whereas the Counterflow type will not. I have also been told that the Parallel Downflow Type Deaerator exhibits signs of carbonic acid corrosion on its internal shell resulting from the CO2 passing over it as it leaves with the other non-condensible gasses.

Any information about the capabilities/pros and cons of a Parallel Downflow Type Deaerator will be greatly appreciated.

 

[jproj]

BD55:

Spray units are not very flexible when it comes to operating under varying conditions. The reason is that the scrubber section of the spray type deaerator has fixed orifices where the steam enters and makes contact with the water. Maintaining the steam velocity out of these orifices is crucial for proper deaeration. When you placed the pre-heaters in service (as you recognized), the deaerator steam requirement was reduced. When the steam flow decreased, the velocity out of the orifices decreased and performance suffered. Increased oxygen levels in the "deaerated" water causes your corrosion.

Regarding the new unit, I'm glad to see that you are going with a spray/tray unit. These are 1000% better than spray type units at handling load variations (yes, they are also more expensive). I would strongly recommend that you go with a counterflow unit, however.

Reason:
A deaerator is a glorified heat exchanger... nothing more, nothing less. Deaerators function because the solubility of gases (like oxygen, nitrogen, carbon dioxide) in water goes to zero as the temperature approaches the saturation temperature. Knowing this, the best way to remove gases is to raise the temperature of the inlet water to the saturation temperature at the operating pressure.

I'll take you through the operation of both parallel and counter flow units. Hopefully you'll see the reasons why counterflow is a more efficient and effective method of heat transfer.

Counterflow:
At the bottom of the tray stack, hot steam (steam with the most BTU/lb) comes into contact with hot water (which is actually a combination of inlet water and condensed steam... this water has little if any non-condensable gases in it). The steam looses some energy in heating the water (and condenses), but since the falling water is near saturation temperature, most of the steam proceeds up the tray stack. As you move up the tray stack, more steam is condensed by the falling water (thereby heating the water and releasing non-condensable gases). At the top (by the spray valves), the steam with the least energy (BTU/lb) contacts the water being sprayed in (full of non-condensable gases), further condensing and heating the inlet water. Steam that is not condensed is vented along with the non-condensable gases that were removed. All non-condensable gases are vented and never come into contact with carbon steel materials and corrosion is minimized. Also, since the steam and water are moving counter current to each other, the steam tends to disrupt the normal liquid films that develop when water cascades through the tray stack. The steam breaks the falling film into water drops and thereby increases contact area and heat transfer.

Parallel:
At the top of the tray stack, hot steam (steam with the most BTU/lb) comes into contact with cold water (loaded with non-condensable gases). Heat is transferred and steam is condensed, but since the gas and liquid are moving in the same direction, they are not being mixed and the contact area is not as large as it is in a counterflow unit. As the steam and water flow together down the tray stack, steam is condensed, water is heated and gases are released. At the bottom of the tray stack, the residual steam and non-condensable gases are released to contact the deaerator's shell and both heads since the vent is in the top head. Carbon steel and non-condensable gases DO NOT MIX (this is the reason you are removing them). Any carbon steel material in contact with these gases will corrode.

Now, I am not saying that a parallel flow unit will not deaerate water. Of course it will. In order to do so, however, the unit has to have a much larger contact area than the counterflow deaerator. This translates into more trays and more steel, which tends to cost more money.

If you need any additional help, please let me know.

Good Luck!

jproj

 

[KeItSiSt]

Ive always wondered how the steam is supposed to flow down to heat the water? Counter-flow makes much more sense and does have alot more contact area. This is why 99% of tray deaerator manufactures make the counter-flow design and have abandoned the parallel-flow. Spray/Tray deaerators are MUCH better than spray/scrubber type deaerators.