What Are Sub-Coolers?
A sub-cooler is a piece of gas conditioning equipment that cools a saturated gas stream further, reducing both its temperature and its water vapour content, typically using a heat exchanger to remove heat from a recirculating scrubbing liquor. It’s usually found downstream of a quench vessel, picking up where quenching leaves off — a quench brings hot gas down to its saturation point through evaporative cooling, and a sub-cooler then pushes the temperature lower still, condensing out additional water vapour in the process.
That combination of “cooler and drier” is exactly why sub-coolers matter: many downstream processes and scrubbing stages work better, or require, a gas stream that’s both cooler and less humid than a quench alone can achieve.
How Does a Sub-Cooler Work?
1. Saturated gas enters the sub-cooler. Gas that’s already been quenched (or is otherwise close to saturation) enters the vessel — commonly a packed tower, though tray towers or V-tex® scrubbers can also be configured for this duty.
2. Gas contacts cooler liquid. The gas meets colder scrubbing liquor, usually flowing in a counter-current arrangement.
3. Heat transfers from gas to liquid. The colder liquor absorbs energy from the gas, cooling the gas while the liquor itself warms up.
4. The liquor is cooled externally. The warmed liquor passes through a heat exchanger, which removes the absorbed heat so the liquor can be recirculated back into the process.
5. Water vapour condenses out. As the gas cools further, its absolute humidity drops — water vapour condenses from the gas stream and drains as effluent.
Why Cool a Gas Stream Twice?
It’s a reasonable question — if a quench vessel already cools the gas, why add another cooling stage? Two distinct reasons drive the need for sub-cooling:
– Better scrubbing performance. Many scrubbing processes work more efficiently at lower temperatures, because the vapour pressure or solubility characteristics of the target contaminant improve as temperature drops.
– A drier gas for downstream equipment. Some downstream processes — compressors, gas engines, carbon filter polishing stages — require a gas stream with less residual moisture than quenching alone leaves behind.
Design Options for Sub-Cooling
– Heat exchanger choice — plate-and-frame exchangers are the standard, cost-effective option; shell-and-tube or spiral plate exchangers are used where the scrubbing liquor carries solids or tars that would foul a simple plate design
– Cooling utility — cooling water is the typical choice where available; chilled water is used instead of, or alongside, cooling water when especially low outlet temperatures are needed
– Air blast cooling — used where no cooling water or chilled water supply exists, either directly in the sub-cooler liquor circuit or in a closed loop
– Combined sub-cooling and scrubbing — it’s common practice to combine the sub-cooling and scrubbing functions within a single vessel, rather than using two separate stages
Where Are Sub-Coolers Used?
– Flue gas abatement — improving SO₂ scrubbing efficiency by lowering process temperatures
– Biogas and syngas conditioning — condensing water ahead of downstream compressors or gas engines
– Odour control — improving scrubber performance by condensing VOCs out of the gas stream
– Carbon filter preparation — reducing gas moisture content prior to reheating for activated carbon polishing stages
– Chlorine-caustic scrubbing — removing the heat of reaction generated in the scrubbing liquor
Sub-Coolers vs. Other Gas Conditioning Equipment
A sub-cooler picks up specifically where a quench vessel’s evaporative cooling reaches its natural limit — quenching alone can only bring gas down to its adiabatic saturation temperature, and a sub-cooler’s external heat exchanger is what allows the temperature (and humidity) to be pushed lower still. The two are frequently used as a matched pair rather than alternatives to one another, with the sub-cooler positioned downstream of the quench in the overall gas cleaning sequence.
Benefits and Limitations
Benefits:
– Improves downstream scrubbing efficiency by lowering process temperature
– Reduces gas humidity for processes that require a drier gas stream
– Flexible heat exchanger and cooling utility options to match site constraints
– Condensate blowdown can sometimes be reused as make-up water for an upstream quench, reducing overall water and effluent demand
Limitations:
– Adds a heat exchanger and associated controls to the system, increasing complexity versus quenching alone
– Condensate is typically contaminated and must be managed as an effluent stream
– Fouling-prone liquors may require a shell-and-tube or spiral plate exchanger rather than the simpler, cheaper plate-and-frame design
Frequently Asked Questions
What is a sub-cooler used for?
A sub-cooler is used to reduce the temperature and water vapour content of a gas stream beyond what a quench vessel alone can achieve, either to improve downstream scrubbing efficiency or to meet drier gas requirements for later process stages.
Why does cooling a gas further also remove water from it?
As gas temperature drops, its capacity to hold water vapour drops too — once the gas is cooled below its saturation point, excess moisture condenses out as liquid, which is why sub-cooling reduces both temperature and humidity together.
Can sub-cooling and scrubbing happen in the same vessel?
Yes. It’s common practice to combine the sub-cooling and scrubbing functions within a single vessel rather than using two separate stages, particularly where a packed or tray tower configuration is already well suited to both duties.
What happens to the water condensed during sub-cooling?
The condensed water is typically contaminated and must be treated as an effluent stream. In some system designs, this condensate blowdown is reused as make-up water for an upstream quench vessel, reducing both fresh water consumption and effluent treatment demand.
Explore ERG’s Sub-Cooler Technology
For full engineering detail — heat exchanger options, cooling utility selection and water balance design — see ERG’s dedicated technical page, which remains the authoritative source on this technology:
– Sub Coolers — operating principle, design features and application examples – Quench Vessels Guide — the upstream stage sub-coolers typically follow – Packed Towers Guide — the most common vessel configuration used for sub-cooling – Carbon Filters Guide — a common downstream stage that benefits from a drier gas stream
Talk to our team about a sub-cooling solution to improve scrubbing efficiency or reduce gas humidity at your site.
