STEEPED in selling the benefits of evaporative cooling, Stephen Fairgrieve of Jaeggi Guentner is quick to point out how dry coolers require large surface areas and have relatively high energy consumption, even with higher water temperatures, in comparison to the better energy efficiency and significantly smaller surface areas of evaporative coolers.
'Evaporative cooling can achieve lower cooling water return temperatures only

4-5°C above the design wet bulb temperature (typically 19 to 22°C for the UK), enabling lower condensing temperatures,' he says. 'And remember, each 1°C reduction in condensing temperature equals 3% energy saving for the chiller.'

Many types of traditional evaporative coolers are available, for example open and closed wet cooling towers or evaporative condensers.

However these traditional evaporative systems also have some big disadvantages, according to Fairgrieve. Depending upon operating condition and design, a visible vapour plume is emitted, which is often considered visually undesirable. In addition, relatively large fresh water quantities are needed to reconcile the evaporation and draining losses.

In 1992, with the support of the Swiss Energy Commission, Jaeggi developed and patented a hybrid cooling concept. The Jaeggi hybrid dry cooler is said to combine the advantages of conventional dry and wet cooling systems, without any of their disadvantages.

Customers get highly efficient cooling technology with lowest energy and water consumption. In addition hybrid coolers are extremely quiet and guaranteed to operate without vapour plumes. Due to the integrated 'plug & play' HybriMatic control, compact cooler dimensions and low weight, they can be installed easily into modern building systems - whether new build projects, refurbishments or retrofits.
Hybrid dry coolers are always optimised for water-saving operation.

During colder annual operating hours or partial load operation, the coolers operate as pure dry coolers, ie with convective heat transfer to the ambient air. The wetting cycle intelligently activates only if the chilled water demand cannot be achieved in dry mode. Then the coolers utilise the energy efficient natural evaporation principle, saving around 70-90% of the annual water consumption and reducing associated costs compared with wet cooling towers.

'The energy efficiency of hybrid dry coolers can be impressive,' maintains Fairgrieve.

The pictured hybrid cooler HTK 2.4/9.0 is used for building cooling and requires only 3 x 4.8kW for the fans and 2 x 0.7kW for the wetting pumps - a total 15.8kW for a cooling performance of 1,900kW (water flow 230m3/h at warm water temperature 35°C, return cold water temperature 28°C, ambient wet bulb temperature 21.5°C).

In partial load operation or with lower ambient temperatures the fan speed is reduced by the HybriMatic control using frequency inverters.

'Since the absorbed fan power falls by the cube of the fan speed, the COP value then rises exponentially. In the above example, the cooler thereby reaches a COP value of 590 at half fan speed.'

As soon as the switchpoint for dry operation is reached, the cooler intelligently switches the wetting off and then operates in dry mode without water consumption. The switchpoint for dry operation lies, usually, between 10 and 20°C ambient temperature.

Typical operation

A typical operational characteristic is represented in the table above for a hybrid dry cooler in the UK. The point of switching for dry mode, in this example, is 18°C, with an all year round constant cooling load of 1000kW.

In the above example, the hybrid dry cooler results in a yearly water consumption (evaporation and draining) of 888m3 and a waste water discharge of 297m3. This replaces a wet cooling tower using 14,534m3 fresh water, with a resulting waste water discharge of 4,845m3.

'Often due to short term business strategies, purchasing decisions are based only on capital outlays and so allegedly 'cheaper' cooling systems are installed,' says Stephen Fairgrieve. 'This causes high operating costs and are detrimental to the environment by higher energy consumption.

'However, with well planned installations and modern hybrid coolers, businesses can save energy and reduce costs - as well as preserve the environment.'