Heat Rejection Plant

Heat rejection plant takes heat out of a building's cooling system and releases it to the outside air. Water-cooled chillers reject heat through cooling towers or dry coolers; air-cooled chillers and refrigeration plant reject it through condenser coils on the machine itself. The choice of system decides the risks: cooling towers are efficient but carry a Legionella risk that is regulated in all three jurisdictions; dry and air-cooled systems avoid that risk but are larger, noisier and lose capacity on hot days.

Towers sit on roofs or at ground level and are built in galvanised steel, stainless steel, fibreglass or concrete. Inside are the fill (the surface over which water spreads), drift eliminators (which catch water droplets before they leave), the basin, distribution system, fans with motors and gearboxes or belts, and the water treatment and bleed equipment.

Every cooling tower has a water treatment system that is as important as the tower: chemical dosing (corrosion and scale inhibitors, biocides), controlled bleed-off to limit how far dissolved solids concentrate, often side-stream filtration, and monitoring. Treatment is usually provided by a specialist contractor under a service agreement, and the regulated records are theirs to keep but the owner's to hold.

What shortens life:

• Water treatment failures: scale, corrosion and biofouling all follow.
• Coastal and industrial air on coils and steel.
• Debris and nearby sources: leaves, dust from construction, exhausts drawn into towers.
• Poor siting: towers and condensers too close to walls or screens, so hot air recirculates.
• Intermittent operation with stagnant water.

Legionella growth

Legionella bacteria multiply in warm water with sediment, biofilm and stagnation, and cooling towers produce aerosols that can carry them to people nearby. Outbreaks of Legionnaires' disease have been traced to towers in all three countries, which is why towers are registered, risk-managed and tested by law. Risk rises with poor water treatment, dead legs and stagnant pipework, damaged drift eliminators, intermittent operation, and dirty fill and basins.

Scale

As water evaporates, dissolved minerals concentrate. Without enough bleed-off and treatment, they deposit as scale on fill and in condenser tubes, cutting heat transfer and raising chiller energy use.

Corrosion

Galvanised towers develop white rust when the zinc is attacked by poorly balanced water, especially in the first months; steel basins and supports then corrode. Fibreglass degrades under UV. Coils on dry coolers and condensers corrode, especially aluminium fins in salt air.

Biofouling and fill collapse

Biofilm and sediment build up on fill, adding weight until the fill sags and collapses, and providing habitat for Legionella.

Mechanical failures

Fan blades crack, gearboxes leak and wear, belts slip, motors fail, and vibration switches fail to act. A fan or gearbox failure takes a cell out of service.

Hot air recirculation

Air-cooled condensers and dry coolers placed too close together, against walls or behind solid plant screens, draw in their own hot discharge air. Capacity falls on hot days and head pressures rise.

Stagnation and dead legs

Towers that run intermittently, standby cells left full, and pipework branches that no longer carry flow hold warm stagnant water where biofilm and Legionella grow. Plant changes over the years often leave dead legs behind.

Drift and siting

Drift eliminators catch water droplets before they leave the tower. Damaged, displaced or poorly fitted eliminators release more aerosol, and towers placed near air intakes, openable windows, roof terraces or public areas expose people to it.

Water consumption

Towers consume water through evaporation, drift and bleed. Uncontrolled bleed, overflowing basins and leaks waste water and dilute treatment chemicals.

From aerial imagery and the roof

• Tower type and material; visible corrosion and staining.
• Drift and staining on surrounding roof and plant.
• Proximity of towers to air intakes, windows and occupied areas.
• Condensers boxed in by walls or screens.

On site

• Scale and biofilm on fill and basin; sludge in basins.
• Damaged or missing drift eliminators.
• Corrosion of basin, casing and supports; white rust.
• Fan and gearbox noise, oil leaks, vibration.
• Water treatment dosing equipment working, chemicals in date.
• Coil fins corroded, flattened or blocked.

In the records

• Tower registration and risk management plan.
• Legionella and bacterial count sampling results.
• Water treatment reports.
• Independent audit reports where required.
• Maintenance and cleaning records.

• Basin overflowing or make-up valve running constantly.
• Algae on exposed wet surfaces in sunlight.
• Visible plume or drift wetting nearby surfaces.
• Standby cells left full and stagnant; isolated pipework branches.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A mechanical services engineer and the water treatment contractor; an independent auditor where regulation requires one.

• Records review: registration, risk management plan, sampling results and trends, treatment logs, cleaning and disinfection records, audit reports.
• Tower inspection with the tower drained and isolated: fill, eliminators, distribution, basin, casing, structure, access.
• Water sampling for Legionella (laboratory culture to AS/NZS 3896 in Australia, or an equivalent accredited method) and heterotrophic plate count, with chemistry for conductivity, pH, inhibitor and biocide levels.
• System survey for dead legs, stagnant branches and standby cells.
• Siting review: distance to air intakes, windows and occupied areas; recirculation around condensers.
• Mechanical checks: fan, gearbox and motor vibration, oil condition, belt tension.
• Capacity check: the tower's ability to deliver design condenser water temperature on a hot day, against the chillers' needs; a performance test to the Cooling Technology Institute method where capacity is in doubt.
• Structural check of supports and the roof under the tower.

The report should state compliance status, the Legionella risk position, tower condition and remaining life, capacity, siting risks, and whether to refurbish, replace like for like, or move to a dry or adiabatic system.

• Compliance: register, write or update the risk management plan, restore sampling and audit schedules, keep records.
• Clean and disinfect on the regulated schedule and after positive results.
• Water treatment: automatic dosing and conductivity-controlled bleed, side-stream filtration, and monitoring.
• Remove dead legs and drain standby cells or circulate them.
• Replace fill and drift eliminators; high-efficiency eliminators where the tower is near people.
• Mechanical repairs: fans, gearboxes, motors, variable speed drives.
• Basin and casing repair: coatings, stainless basins, structural repairs.
• Relocation or screening away from intakes and occupied areas.
• Replacement with a new tower in stainless or fibreglass, or with dry or adiabatic coolers.

Water treatment, sampling and audits are recurring contracts and compliance costs. Fill and eliminator replacement are lump sums per cell. Mechanical repairs are itemised. Tower replacement is a lump-sum project including crane lift, structural works, pipework and electrical connection, controls, water treatment equipment and commissioning, with temporary heat rejection where the building cannot be without cooling.

Moving to dry or adiabatic cooling can change chiller selection and roof structure, and is usually part of a wider plant strategy rather than a like-for-like line.

Legionella compliance is code compliance and recurs; replacement is lifecycle; repairs are repairs and maintenance; a positive result is immediate.

• Australia. States regulate cooling towers under public health legislation, typically requiring registration, a risk management plan, independent audit and routine Legionella and heterotrophic plate count sampling with prescribed responses (for example the Public Health and Wellbeing Regulations 2019 in Victoria and the Public Health Regulation 2022 in New South Wales). AS/NZS 3666 sets design, maintenance and performance-based maintenance of cooling water systems.
• United Kingdom. The Notification of Cooling Towers and Evaporative Condensers Regulations 1992 require towers to be notified to the local authority; the HSE Approved Code of Practice L8 and HSG274 Part 1 set the control regime.
• United States. ASHRAE 188 sets the risk management standard for building water systems; New York City's Local Law 77 requires cooling tower registration, inspection and Legionella sampling; the Cooling Technology Institute publishes standards for tower performance and maintenance.

• Tower registration and the current risk management plan.
• Legionella and heterotrophic plate count results for at least two years, with the responses taken to any positive results.
• Water treatment service reports and chemical dosing records.
• Cleaning and disinfection records.
• Independent audit reports where required.
• Maintenance records for fans, gearboxes and motors.
• Installation date and materials of the tower.
• Drawings showing tower location relative to air intakes and occupied areas.