Water Management in IDEC Systems: Balancing Performance, Water Use and Equipment Life

Indirect-direct evaporative cooling, or IDEC, uses air and water to provide cooling with much lower power consumption than conventional refrigerant-based HVAC systems. The equipment may appear simple, but the behaviour of its circulating water is not.

Only the water evaporates. Most dissolved minerals remain in the system. At the same time, dust and other contaminants can enter with the large volume of outdoor air handled by industrial evaporative cooling equipment. The water returning to the sump is therefore different from the fresh water originally supplied. This change affects cooling performance, maintenance and equipment life.

Start with the source of water

Make-up water is the fresh water added to replace evaporation and other losses. In Indian factories, its source may be single or a mixture of sources such as surface water and borewell. Its quality can also change between summer and monsoon.

Water that is suitable for drinking or general plant use is not automatically suitable for an evaporative cooling system. Water analysis should therefore form part of project design and not be postponed until operating problems appear.  Individual parameters matter, but they should not be assessed separately. Their combined effect changes as the circulating water becomes more concentrated and thus, harsher for use.

Understand the three connected risks

Scale is the mineral deposit that forms on wetted surfaces. It can interfere with water distribution, create dry areas, restrict passages and reduce heat transfer.

Corrosion is the gradual attack of metal. It is influenced by water chemistry, oxygen, temperature, deposits and the combination of materials used in the equipment.

Biofouling is the growth of bacteria, algae, fungi or slime. It can block water passages, cause odour, trap dust and promote corrosion beneath deposits.

These problems interact. A biofilm can trap minerals and dirt. Deposits can create conditions for local corrosion. Corrosion products can then worsen fouling.

Balance water conservation with concentration

IDEC systems recirculate water to reduce consumption. As evaporation continues, however, the dissolved minerals left behind become concentrated. This increase is described as the cycles of concentration. Water in the sump or tank cannot be concentrated indefinitely because industries also must comply with local effluent discharge norms including limits on the total dissolved solids (TDS).

Therefore, part of the concentrated circulating water must be removed and replaced with fresh make-up water. This controlled discharge is commonly called bleed-off or blowdown. Higher bleed-off reduces concentration of the impurities but uses more water and creates more discharge. Lower bleed-off saves water but can increase the tendency to scale and foul. The balance depends on the incoming water, evaporation rate, equipment materials and operating hours.

Manage the complete water circuit

Good water management combines analysis, filtration, controlled bleed-off, sump cleaning and even water distribution. Stagnant pockets and dry areas should be avoided. Wetted surfaces may also need periodic drying where the equipment design permits.

In water-stressed areas, poor source water may justify some form of pretreatment before use, e.g. pH control or softening to reduce hardness, or membrane treatment to reduce chloride content. Any chemicals used must be compatible with the heat exchanger, cooling media, seals and metals.

The objective is not perfect water. It is stable and economical operation of the IDEC equipment. Water analysis, cleaning access, concentration control and routine inspection should therefore be considered when selecting the IDEC equipment, not added later as maintenance instructions.