Engineering Evaluation Case: A Direct-Contact Cooling Tower Uses Dirty Process Water and Gradually Loses Cooling Performance

Direct-contact cooling towers and quench-contact towers rely on the same fundamental advantage as packed absorbers:

large gas-liquid contact area.

When the cooling liquid contains:

• suspended solids;
• hardness;
• oil;
• dissolved salts,

that same surface area can become a fouling surface.

A tower can therefore lose thermal performance even though the packing is not chemically damaged.

Project Situation

Consider a packed direct-contact cooler handling hot process gas.

Originally, relatively clean water is used.

To reduce water consumption, the plant later introduces recycled process water.

The water carries:

• fine solids;
• higher dissolved salts;
• trace oil.

After several months, operators observe:

• warmer outlet gas;
• increasing bed pressure drop;
• greater pumping demand;
• visible deposits on removed packing.

The problem is now both thermal and hydraulic.

Cooling Depends on Effective Wetted Area

Packing creates area for:

• sensible heat transfer;
• evaporation;
• gas-liquid contact.

If deposits cover the packing, the geometric area may remain unchanged while the effective wetted surface decreases.

Water can channel through preferred paths.

Other areas become poorly wetted.

The result is less effective cooling.

Evaporation Concentrates Dissolved Solids

Direct-contact cooling often evaporates part of the circulating water.

The vapor leaves with the gas.

Dissolved salts remain.

This creates the same concentration mechanism seen in some scrubbers, but here evaporation may be a major part of the intended cooling duty.

A water source that begins with moderate dissolved solids can become highly concentrated in the loop.

Scaling Changes Both Heat Transfer and Hydraulics

Deposits can:

• insulate the liquid from the packing surface;
• reduce void space;
• increase pressure drop.

Therefore, cooling performance and gas-flow capacity can deteriorate simultaneously.

The plant may see:

• hotter gas outlet;
• higher fan load

at the same time.

Solids Can Collect at the Distributor

Fine distributor openings may plug before the main bed becomes heavily fouled.

Once distribution becomes uneven:

• some packing dries;
• local hot zones develop;
• scaling accelerates in concentrated regions.

The distributor should therefore be part of the fouling investigation.

Oil Creates a Different Surface Problem

Even a thin oily layer can change:

• water spreading;
• surface wetting.

The packing may remain hydraulically open but lose thermal contact efficiency.

This is different from hard mineral scale.

Deposit analysis helps distinguish the two mechanisms.

More Open Packing Can Increase Run Length

In dirty-water service, packing selection may favor:

• larger openings;
• lower solids retention;
• easier washability.

This can reduce clean-state surface area per cubic meter.

The design trade-off is:

maximum clean efficiency versus stable long-term operation.

Blowdown Becomes a Thermal-Process Variable

If water evaporates continuously, dissolved solids need a controlled exit.

Blowdown can limit concentration.

The required rate depends on:

• makeup quality;
• evaporation;
• contaminant input.

Insufficient blowdown can make scaling inevitable regardless of packing type.

Water Treatment May Be More Valuable Than New Packing

Depending on the fouling source, useful measures can include:

• filtration;
• solids settling;
• oil separation;
• hardness control.

The packing supplier should identify the sensitivity, while the plant determines the appropriate water-treatment system.

Cleaning Should Restore Both Flow and Wetting

After cleaning, evaluate:

• pressure drop;
• water distribution;
• cooling approach.

A pressure-drop recovery without thermal recovery may indicate that the remaining surface condition is still poor.

Establish a Clean Thermal Baseline

At commissioning, record:

• inlet gas temperature;
• outlet gas temperature;
• gas flow;
• water inlet temperature;
• water flow;
• bed pressure drop.

These values provide a reference for future fouling diagnosis.

Engineering Takeaway

In direct-contact cooling, fouling does not only block gas passages.

It reduces the active surface responsible for heat transfer.