Well, can it? Chaz Rehal of Carel UK shows how a novel dual zone system can be used for humidity control or just for adiabatic cooling - and does the maths.
Many consultants are familiar with adiabatic cooling using plate heat exchangers or direct atomisation into a duct or room.

However a novel dual zone solution has been developed for air handling units which maximise both direct and indirect adiabatic cooling thus providing potentially huge savings on the mechanical cooling.

In some applications it is possible to save more than 80 per cent of the mechanical cooling demand. The dual zone system known as humifog Multizone, can be used for humidity control or purely for adiabatic cooling.

Let us consider that for every litre of water atomised and evaporated you contribute approx 690 W of cooling power to the system. This cooling power can come from as little as 2.5 W specific humidifier power per litre of water.

Incredible payback

The system is able to achieve this incredible payback of energy by atomising the water into tiny droplets approx 10-15μm in size which have a superior evaporation efficiency. As the droplets evaporate, they take energy from the surrounding air to change from the liquid state to a vapour state, thus as a consequence the air is cooled. Since water has a high latent heat of vaporisation, the cooling effect is also very high.

The pumping unit uses an inverter driven motor to supply water at 75bar to an atomising rack with several stainless steel nozzles where the water is atomised into a fine mist/fog inside the ahu. Here it evaporates, cooling and humidifying the air.
In comfort applications it is necessary to limit the humidity level to around 60 per cent on the supply air. However this still allows considerable cooling on hot days up to a maximum of 9°C on London's average driest day of the year.

When the humidity outside does not allow direct adiabatic cooling it is possible to use the indirect method cooling the exhaust air which, in turn, passes through a heat exchanger and cools the fresh air intake. Since the exhaust air will be expelled the adiabatic cooling can be maximised up to around 95%RH or more. In this case it is possible to see adiabatic cooling up to 10-12°C depending on the application, but we must consider the efficiency of the heat exchanger in this situation.

The most energy efficient strategy is to provide priority (see schematic) to the direct adiabatic cooling since it has the highest COP (690 W cooling from 6 W/L including water treatment) Next priority should be the exhaust cooling since it will still have a very high COP compared with DX cooling and finally when adiabatic can not be used or meet the target, mechanical cooling is used.

This design has incredible benefits when looking at applications such as data centres and other cooling hungry applications.

A study using UK climatic data for the last 10 years into adiabatic cooling for data centres has shown that in London more than 80 per cent of the mechanical cooling can be reduced. In fact in London 69 per cent can be managed completely by adiabatic cooling with a further 22 per cent using both adiabatic and mechanical, leaving 9 per cent only requiring mechanical cooling alone. What's more, the study showed that in some UK locations 100 per cent of the cooling demand can be managed only by adiabatic cooling, in theory providing a chillerless data centre.

The study uses the new 2008 ASHRAE design standards for Class 1&2 data centres. However the target supply condition was set at 23°C max instead of the permissible 27°C since most UK clients and consultants are not happy to supply at the top limit.

Considering the average data centre can use 40 per cent or more of its total power input for cooling and air conditioning there is a huge energy and cost saving potential.

Furthermore, as a consequence of the adiabatic cooling process the air is also humidified. Since minimum humidity levels are required by ASHRAE to reduce static discharges, the designer is able to aid both cooling and humidification using the same process.

Short of moving the data centre site location to a country with more favourable external conditions there is currently no other system we are aware of that can supply such energy savings on UK Data Centre cooling.

Carel UK 0208 391 3540 email humidification@careluk.co.uk