Our company has signed a pledge to reduce energy consumption. I am looking at using waste steam to pre-heat the feed water to the deaerator. I undertand that there is a general belief of a maximum temperature limit- about 20 oC temperature difference between the incoming feed water and the outgoing deaerated boiler feed water has to be maintained for efficient O2 stripping with steam. I wonder why this has to be so. If the feed water is preheated to a higher temperature than normal boiling point of 100 oC, it should flash (to the extent dpendent on the temperature difference) as it comes off the sprays. This steam flashing should be as effective as using raw steam to strip the O2. It should eliminate the use of raw steam for stripping. Anyone has experience in this please comment.

the removal of o2 should occur in the dearator and not in the piping system upstream of the dearator or by flashing the condensate in the dearator.  the action taken place in the dearator, that is lp steam mixing with condensate stream, is effective is removing o2 from the condensate/boiler feedwater.  i recollect (not too certain) that it is optimum to remove o2 at the operating temperatures and pressures of the dearator and not some other value.
i do not believe that what is described, as i understand, will leave boiler feedwater in the bottom of the dearator - that is preheating condensate and then flashing in dearator.  what will cool the steam to produce boiler feedwater in the lower portion of the dearator?

First of all, flashing liquid through spray valves is a good way to ruin them.  If you break the spray valves, you will have near zero performance in the unit, no matter how much steam flow you have.  FYI, 90% of deaerators are operated at 5 psig. Saturation temperature at 5 psig is 228?F (108.9?C).  If you are bringing the water in at 212?F (100?C), it's not going to flash anyway.

Secondly, pre-heating the water releases dissolved gases.  I'm guessing your pipe upstream of the deaerator is not stainless steel.  If it's not, the liberated gases are going to cause corrosion and eventual pipe failure (dangerous and expensive).  There is a reason plants remove dissolved gases IN the deaerator .... this is one of them.

Thirdly, why are you using the waste steam in a pre-heater?  A deaerator is nothing more than a direct contact heat exchanger.  If you want to use the waste steam to its full potential, use it IN the deaerator.

To answer your original question, a 20?F temperature difference between the inlet and outlet of the deaerator is desirable because it allows for enough steam flow through the unit.  Steam flow through the spray and tray section disrupts the liquid film as it falls.  This breaks the falling film into droplets (more surface area).  The smaller these droplets are, the easier the dissolved gases are to remove (less distance for diffusion).

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