Sub Coolers

Sub‑coolers for gas temperature and humidity control

ERG sub‑coolers are used to cool saturated gas streams, reducing gas temperature and condensing water vapour to condition the gas for downstream treatment stages or to reduce visible stack plume. They are frequently installed in combination with quench vessels as part of integrated gas cleaning and abatement packages for flue gas, process gas and syngas treatment.

Operating principle

Sub-cooling can be carried out using a packed tower (the usual configuration) or with a tray tower or V-tex®.

The hot gas is cooled by direct contact with colder liquor, usually in a counter-current configuration. The colder liquor absorbs energy from the gas, thereby cooling it – and in turn the liquor temperature increases. The energy is then removed from the liquor by passing it through a heat exchanger. As the gas temperature is reduced, this also condenses water vapour from the gas stream, and so the absolute humidity (ie the kg of water vapour per kg of dry gas) of the gas is reduced.

The heat exchanger configuration depends upon the type of scrubbing liquor being used, the available cooling utility and the amount of sub-cooling required.

It is also common to combine sub-cooling and scrubbing within the same vessel.

Applications for sub‑cooling

Sub‑cooling is used wherever the gas temperature needs to be reduced. This is usually either:

To cool the gas and the scrubbing liquid because the scrubbing process operates more efficiently at lower temperatures, for example due to vapour pressure or solubility characteristics of the contaminant being removed; or
To remove water vapour from the gas because downstream processes require a drier gas.

ERG has extensive experience with sub‑cooling technology, including:

Increasing SO₂ scrubbing efficiency in flue gas abatement systems.
Condensing water from biogas or syngas to assist downstream compressors or gas engines.
Improving the performance of odour scrubbers by condensing VOCs.
Reducing gas water content prior to reheating for activated carbon filter polishing.
Removing the heat of reaction from scrubbing liquor for chlorine‑caustic scrubbing.

Design features and cooling options

Choice of heat exchanger

Most applications use a plate‑and‑frame heat exchanger with cooling water as the cooling utility, which is usually the most cost‑effective solution. However, alternative options are available where required, such as:

Shell‑and‑tube or spiral plate heat exchangers for applications where the scrubbing liquor contains solids and/or tars and a simple plate exchanger would foul.
Air blast coolers for applications where there is no cooling water or chilled water available; these may be installed directly within the sub‑cooler liquor circuit or indirectly in a closed cooling loop to provide cooling water to the sub‑cooler heat exchanger.

Choice of cooling utility

Cooling water is typically the preferred utility where it is available. Where especially low outlet process gas temperatures are required, chilled water can be used instead of, or in combination with, cooling water.

Blowdown control and water balance

Sub‑cooling the process gas condenses water which must be properly controlled. This condensate is typically contaminated and needs to be treated as an effluent. In some applications, the effluent blowdown from the sub‑cooler can be arranged as the water make‑up to an upstream quench, minimising both process water consumption and effluent treatment demands from the abatement package.

ERG sub‑coolers are provided as part of complete scrubbing packages, with the necessary sump tank, pump, heat exchanger, pipework, fan, ductwork and controls. In the majority of applications, the sub‑cooler is combined with other ERG heat and mass transfer equipment (quench, venturi scrubber, tray tower, packed tower, V‑tex® scrubber) as part of an integrated gas cleaning system.