The Baltimore Aircoil TrilliumSeries range of adiabatic coolers and condensers is a technological advance for small and mid-size air-conditioning and refrigeration systems says its developer, BAC.

Adiabatic cooling involves the pre-cooling of air before it is directed to a finned heat exchange coil, where the fluid in the coil is cooled or refrigerant condensed by sensible heat transfer. The medium for pre-cooling the air is water and by depressing the air temperature the capacity of the finned coil is increased.

This is not a new concept, but it has been characterised by poor efficiency, unreliable thermal performance, poor control and short operating life. BAC’s development programme for the TrilliumSeries Cooler and Condenser has addressed all these weaknesses.

Configuration

The TrilliumSeries comprises two finned coils in a V-shape configuration with an induced draught, multiple fan air moving system above the V-shape plenum. The pre-cooling pads are located vertically in front of the finned coils.

The unit can be operated either adiabatically or totally dry. In adiabatic mode, a small flow of water is used to cool the incoming air. A portion of this evaporates, with the remainder going to drain or to a water reclamation process.

The pre-cooling evaporative pads are cellulose paper impregnated with phenol resin. The pad has a unique flute angle design that consistently achieves up to 80% humidification efficiency or 6-8º depression of the ambient air temperature. For example, if the design ambient dry bulb is 32°C and the design wet bulb temperature is 22°C, the air will be pre-cooled to 24°C, ie 80% of the differential between the dry bulb and the lower wet bulb temperature.

The water distribution system above each bank of evaporative pads is enclosed so there is no possibility of splash-out or windage losses. Flow is such that about a third of the water evaporates at design load while the rest ensures the pads are fully wetted to wash away any potential fouling or light scaling. The overfeed falls into an enclosed gutter at the base of the unit where it immediately drains at the lowest point.

A controls package with the TrilliumSeries matches cooling capacity to load by varying fan speed in dry mode and only introducing adiabatic pre-cooling when required. The unit runs dry for as long as possible, using the variable speed fan drive to match capacity to required condenser water temperature or condensing pressure. Adiabatic pre-cooling is activated and stopped by a preprogrammed combination of fan speed signal and ambient temperature signal.

Other features of the controls package intelligence are the means to allow only one period of pre-cooling per day to eliminate stop/start of the water flow which could lead to scaling of the pads. The controller also switches between sides to ensure each bank of pads receives the same running hours of pre-cooling. The controls package ensures all the water distribution piping is drained after each adiabatic cycle.

Benefits

  • Highly efficient air pre-cooling giving up to 40% additional capacity over a fully dry air cooled alternative.
  • Up to 80% humidification of air compared with industry norm of 50-70%.
  • Cooling performance verified by Eurovent Standard.
  • Once-through, minimum flow water system with no scaling, corrosion or microbiological growth potential and so no requirement for water treatment.
  • Intelligent controls activate adiabatic pre-cooling only when required.
  • Year round lower condensing temperatures using pre-cooling mode only when needed, saving chiller power and reducing emissions.
  • Low noise levels verified by Eurovent standard.
  • No use of toxic chemicals.
  • Water savings of at least 70% compared with cooling towers.
  • No potential for legionella proliferation and no generation of aerosols or water droplets thus avoiding any risk of legionellosis.
  • Capital cost payback of less than two years compared with a conventional dry air cooler or condenser.

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Building Sustainable Design