Deaerator type - looking for opinions?

[Imak]

Dear Colleagues Eng's:

Looking for expert opinion on what type of Deaerator to use in food processing plant environment?
Tray or spray type? Pressure or atmospheric?

I'm replacing existing tray type, but can't get any unbiased advice from manufacturers.

Appreciate everyone’s thoughts.

Rgds,
I_mak


I_mak

 

[rmw]

Depends on several things.

One is turndown ratio needed. If your application is always a constant load, a spray type sized for that particular load would work reasonably well.

If, you have a lot of variability in your boiler steam flow, tray types have a lot higher turndown ratio capabilities.

Next is what level of air (O2) removal you are seeking.

Remember that oxygen soluability in water is a function of temperature, and while I don't remember the exact numbers, I do remember learning that at 250F, (15 psig sat) the O2 soluability of water was at a practical minimum.

Some one else will have to give you the exact numbers.

Then there is the consideration of how hot your returning condensate is. If the condensate is returning so hot that it will just flash off in the DA, then it behooves you to pick an operating pressure high enough (within practical limits) to enable you to recapture this heat that would be otherwise lost.

Also to be considered, is the ratio of oxygenated make up water to returning condensate.

The tray design is there because it offers better "scrubbing" of the make up water than does a spray type. So if you have very little make up to "scrub," the spray type might be your pick. If you have a high percentage of make up, I would recommend a tray type.

Spray types are susceptible to variations in supply water pressure, as well. The spray nozzles require a specific range of water pressure to function properly, while a tray type doesn't care if a level control valve is throttling the flow, and only admitting a trickle at a given moment.

There are several threads on this site that deal with DA issues. Have you searched the site??

rmw

 

[Imak]

Thank you very Rnw.

I thought to include some particulars in my original note, I guess I should of...

Our condensate return vs. make-up ratio is 46% make-up/54% condensate. Condensate back to DA is about 230F. Make-up water is softened prior.

7 ppb dissolved oxygen is what I understand standard for DA made nowadays.

We don't have much make-up water and/or condensate rate fluctuation, except start-ups and shut-downs of course.

Our current unit is tray type atmospheric, but this is 1952 made and ppb are around 15 mark. Obvious, there is a room for improvement...

What would you say?

Thanks again for your prompt answer.

Rgds,
I_mak


I_mak

 

[Imak]

Rnw:

I can't really find threads on DA types. If you happened to run accross one please shoot me a link.

Thanks!

I_mak

 

[rmw]

Use the "advanced" site search feature and search on the word deaerator.

That produced a lot of threads for me that should give you a start while I post separately for your other questions.

rmw

 

[rmw]

What is your boiler steaming pressure, or DA heating steam supply pressure, if that is different than the boiler pressure?

rmw

 

[Imak]

Boiler pressure steam pressure 1035 kPa.

I_mak

 

[rmw]

I just wanted to make sure that I did not recommend a pressurized DA if you did not have the steam to supply it.

Now, I will have to tell you for the sake of full disclosure, that I have a personal preference for tray types, because they are much more forgiving. I have seen too many spray types sized for some specific throughput condition and later on when things got changed, they were having to operate outside their turndown ratio, with oxygen removal and water heating suffering accordingly.

It is just that the types of boiler/DA systems that I have the most experience with do not have the luxury of operating within a narrow flow range.

So, I am not saying that a spray type would not do a good job for you with what you are having to do. That is your decision to make. There are plenty of them out there doing the job they were designed to do.

I definitely recommend a pressure type, in either case, so that if your makeup flow is interrupted, you don't flash off your condensate. This assumes that your condensate return system has the capability of pushing the condensate into a pressurized system. You have to check that, too, since you have previously had an atomspheric system.

A pressure type will produce the best deaerating, reducing the need to polish it off with chemicals, and will best insure you the heat recovery from your condensate under all conditions.

I hope this information helps.

rmw

 

[Imak]

Rmw:

Indeed it does help a lot. Appreciate this very much.

Last question, what about returning condensate. In our existin unit we had it returning into the dome tray section. How's done normally? In pressurized units?

Thanks.

I_mak

I_mak

 

[rmw]

In your case, not knowing any more than I do at this time, I would bring it in over the trays. I make that statement based on visualizing your return condensate at 230F, as you stated, and a common DA operating pressure of 250F (15 Psig).

When the condensate is returned saturated at a temperature higher than the DA operating pressure/saturation temperature, it is often brought in below, or at times at an intermediate point within the zone of the trays so that the flash steam from the condensate can contribute to the heating of the colder make up which is coming in at the top.

In your case, depending upon how you pressurize the condensate to get it in, you might not be fully at or above DA saturation pressure, so that condensate will require further heating. If it was brought in below the trays, it would simply flow by gravity into the storage tank without being heated (much) and would cause the water in the storage tank to be subcooled, which is a detriment to the purpose of the DA.

I make that last warning beause if your condensate system changes to push the condensate in from traps, etc., at the steam users, then it might come back higher than 230F since it would require higher pressure to push it back than it does now.

A key question you have to answer is "how do you get the condendate pressurized into the deaerator?" If it is pumped from a condensate receiver at the temperature you state, then it is not an issue, other than making sure your pump has the head capability to push it in.

If it is pressure returned from the steam users, then the condensing temperature of the user changes, since the pressure required to push it back has to be supplied from the steam inlet side to the steam users through the traps.

rmw