Combining an evaporative system with a modern high efficiency DX chiller can dramatically cut building cooling costs. Ken Strong and Roberto Mallozzi highlight the potential of a new approach.
The power of evaporative cooling has been known for thousands of years. It is nature's original cooling system, and keeps a million panting dogs alive on a hot summer's day. Nurses have long harnessed it in the form of a moist flannel laid across a fevered brow.

Today, in the age of expensive energy and worries about carbon emissions, evaporative cooling is coming of age for use in cooling buildings and industrial processes.

It is being combined with the latest DX chiller technology, based on oil-free compressors, to produce very high efficiency systems that can dramatically cut energy costs and carbon emissions.

Air-cooled chillers, in particular, lend themselves to being adapted to harness evaporative cooling due to the large volumes of air they depend upon for operation. The principle is simple enough: to harness the natural energy transfer that takes place when a liquid evaporates from a wetted substrate or surface to the surrounding atmosphere.

The underlying science involved in the heat and mass transfer that takes place during evaporative cooling is complex. The process, involving what is known as adiabatic saturation, actually occurs without any external exchange of heat into or out of the system.

For practical purposes, however, the key is that as a result of evaporation, mass and energy are lost - resulting in the creation of a cooling effect. All things being equal, the rate of evaporation dictates the mass and energy lost and the amount of adiabatic cooling that takes place.

The challenge in harnessing adiabatic cooling, therefore, is to optimise the process of evaporation, given prevailing conditions, by controlling the supply of water to the evaporative medium and the speed of air passing over, to ensure energy use is minimised and the cooling effect maximised.

How is adiabatic cooling harnessed to best advantage? There are a number of ways of approaching this, but in the latest hi-tech systems the most effective solution is to place an evaporative medium in the vicinity of the condensing coils in order to reduce the temperature of air around the coils and enable them to operate more efficiently.

In this way, air temperature in the immediate vicinity of the coils can be reduced by up to 8K at the design condition of 35 deg C. This effectively creates a micro-climate around the coils that is cooler than the ambient condition and enables the plant to condense much more efficiently.

The evaporative substrate can be designed into a cassette-type enclosure that fits across the face of the condenser coil. This can be made from an inorganic, noncombustible material that - for obvious reasons - should not support bacterial growth.

V-bank arrangement

In the case of a typical air-cooled chiller, the cassettes containing the evaporative substrate are set in front of the face of the condenser coils in the V-bank arrangement.

For use in winter, it is desirable to have a system that allows the evaporative cassette to be removed to reduce fan pressure drop. This enables free flow of air across the unobstructed face of the condenser during the cooler season when use of evaporative cooling is much less attractive.

The substrate will also need to be periodically removed and replaced due to the build up of deposits as a result of evaporating water over a number of seasons.

Water is fed to the evaporative medium with micro jet spray nozzles using a once-through water system.

It is important to employ a means of sanitising water; this can be achieved using a high intensity UV steriliser to kill bacteria. Water from the micro spray nozzles flows onto the corrugated surface of the material, saturating it, but without passing all the way through and coming into contact with condenser coils beneath.

As warm, dry ambient air passes through the cassette it evaporates a percentage of the water, lowering the air dry bulb temperature around the coils and raising humidity. Fan motors can be controlled with a high efficiency variable speed drive to reduce energy consumption and optimise the adiabatic cooling effect.

The overall effect is to reduce the approach between the refrigerant condensing temperature and the air being delivered to the coils, reducing both condensing temperature and the air volumes required. As a result of this combined action, substantial energy savings of up to 30 per cent can be achieved.

Effective control is vital and needs to strike a balance between water usage, fan energy input and improvements in condensing efficiency achieved. As ambients and load changes, this equation will be constantly changing.

The control can be set to switch off evaporative cooling at lower ambients, when the cost of water would be greater than savings in energy.

Turbomiser chiller technology explained

The latest generation of the Turbomiser chiller uses a new kind of adiabatic technology designed to shave a further 20-30 per cent off energy running costs. When combined with its Turbocor compressors, EC fans and Liquid Pressure Amplification (LPA) system, the Turbomiser III Adiabatic Advantage chiller is said to deliver energy savings of more than 50 per cent compared with conventional screw and reciprocating chillers.

The Turbomiser III is equipped with an evaporative system on the face of condenser coils. This 'adiabatic advantage' effectively reduces ambient temperatures in the immediate vicinity of coils by up to 8 deg C, lowering condensing temperatures and dramatically improving the chiller's energy performance. The adiabatic system is fed by nebulised water, which is absorbed by a porous natural-fibre honeycomb array facing condenser coils. The system can be set to activate automatically at a predetermined external temperature.

The Turbomiser III is the fruit of a five-year development programme by Italian manufacturer Geoclima and UK companies Klima-Therm and Cool-Therm. The Turbomiser has won a string of awards, culminating recently in CIBSE's Low Carbon Technology of the Year award.

Helped by its LPA system, it is claimed to achieve EERs of 10 and above without the need for additional freecooling circuits with expensive glycol, saving both on initial cost and ongoing pump energy.

The Turbomiser III is available in capacities from 250kW to 1.5MW, and can be applied in most commercial and industrial applications. It is said to be ideal for cooling data centres, large retail developments and deep plan buildings that have a constant base load. Hospitals and hotels, with their need for all-year-round cooling, are also excellent applications.

• Ken Strong is md of Cool-Therm and Roberto Mallozzi is md of Klima-Therm