Engineering Evaluation Case: A Packed Thermal Deaerator Misses the Dissolved-Oxygen Target When Cold Feedwater Arrives

A packed deaeration tower may remove dissolved oxygen effectively under its normal design condition and then suddenly miss the outlet target when incoming water becomes colder.

The packing has not changed.

The steam supply may still appear normal.

The process driving force has changed.

Thermal deaeration depends on bringing water toward conditions where dissolved gases can be released efficiently.

Cold feedwater can increase the required heating and stripping duty substantially.

Project Situation

Consider a packed thermal deaerator treating boiler or process feedwater.

The system uses:

• hot stripping steam;
• packed contact section;
• liquid distributor;
• vent for released non-condensable gases.

During normal operation, dissolved oxygen meets the target.

During cold-weather operation or after a source-water change:

• feedwater temperature drops;
• outlet dissolved oxygen increases;
• tower differential pressure remains normal.

The customer suspects the packing.

The first investigation should focus on the thermal balance.

Cold Water Requires More Steam Condensation

The incoming water must be heated toward the required deaeration condition.

When feedwater is significantly colder, a larger fraction of the available steam can be consumed simply heating the liquid.

If steam flow or pressure does not increase correspondingly, the tower may operate below the intended thermal condition.

Packing Cannot Replace Missing Heat Duty

Packing improves:

• steam-water contact;
• interfacial area;
• distribution.

It cannot generate the thermal energy needed to raise the water temperature.

Adding more packing may therefore provide little benefit when insufficient steam is the real limitation.

Outlet Water Temperature Is an Important Diagnostic

A useful check is whether the water leaving the contact zone reaches the expected operating temperature.

If the outlet temperature is significantly below the design condition, investigate:

• steam supply;
• feedwater temperature;
• steam pressure;
• steam distribution.

If temperature is correct but oxygen remains high, other causes become more likely.

Steam Distribution Matters

Steam entering one side of a large tower may not automatically spread evenly.

Poor steam distribution can create zones where water receives:

• insufficient heating;
• insufficient stripping gas.

The tower can therefore show acceptable bulk temperature while local sections perform poorly.

The gas-entry arrangement and packing support area should be reviewed.

Non-Condensable Gas Must Have a Vent Path

Released oxygen and other gases need to leave the tower.

If the vent:

• is restricted;
• is operated too tightly;
• contains condensate blockage,

non-condensables can accumulate in the upper section.

This can reduce the effective stripping driving force.

A deaerator therefore requires both good contact and controlled venting.

Liquid Distribution Remains Important

Cold feedwater normally enters through the liquid distributor.

If some sections receive more water than others, those zones also require more local steam condensation.

Poor distribution can therefore create both:

• hydraulic maldistribution;
• thermal maldistribution.

The distributor should be checked for blockage and levelness.

Pressure Influences the Required Temperature

Thermal deaeration conditions depend on operating pressure.

A change in tower pressure changes the saturation relationship.

Therefore, temperature should not be interpreted independently from:

• absolute pressure.

A sensor may show the “usual temperature” while pressure has changed enough to alter the real condition.

Sudden Feedwater Increase Can Produce the Same Symptom

The problem is not limited to seasonal temperature.

If water flow increases while steam supply remains fixed:

• heating duty increases;
• liquid loading increases;
• contact time changes.

The tower may miss oxygen specification even though individual equipment remains intact.

Fouling Can Still Contribute

If the tower gradually loses performance rather than only during cold feed conditions, inspect for:

• distributor fouling;
• packing scale;
• blocked steam passages.

But if the performance loss tracks feed temperature closely, the thermal balance is strong evidence.

What Should Be Trending?

Useful values include:

• feedwater temperature;
• outlet water temperature;
• tower pressure;
• steam flow;
• water flow;
• vent condition;
• dissolved oxygen.

Looking at dissolved oxygen alone hides the mechanism.

Material Selection Still Matters

Deaerators operate hot and wet.

Packing and internals need suitable:

• temperature resistance;
• corrosion resistance;
• mechanical strength.

But material selection does not solve an insufficient steam-to-water balance.

Engineering Takeaway

A packed thermal deaerator is a combined heat-transfer and mass-transfer system.

Cold water can consume the available steam capacity before the packing has a chance to achieve the intended oxygen removal.