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Direct evaporative cooling

Evaporative cooling strategies for the Gulf

Mahmoud Widyan, General Manager at Condair Middle East*, looks at how humidifiers can be used in different evaporative cooling strategies.

01 April 2020

Some 680 W of energy is required to evaporate one kilogramme of water into an atmosphere. For steam humidifiers, this energy typically comes from the electricity or gas being used to boil the water. However, for cold water humidifiers, this energy is taken from the air in the form of heat, which results in a drop in air temperature. So for every kilogramme of water evaporated, 680 W of evaporative cooling is achieved.

As well as increasing the humidity, a large commercial cold water humidifier delivering 1,000 kg of moisture per hour to the atmosphere is providing 680 kW of evaporative cooling. As some humidifiers can do this while operating on less than 0.3 kW of electricity, their potential for delivering low-cost cooling is substantial.

Mahmoud Widyan

So how can this potential be realised in a modern building services heating, ventilation and air-conditioning (HVAC) system?

Let’s look as some evaporative cooling strategies:

• Air-handling unit (AHU) direct evaporative cooling: This is the most common form of evaporative cooling strategy used in AHUs. The humidifier adds water to the incoming air stream, raising its humidity and reducing its temperature. This conditioned air is then simply supplied to the room with a high percentage of the room air that is exhausted rather than re-circulated to maintain the ideal humidity in the room.

The advantage of this strategy is that 100 per cent of the cooling effect is felt by the room. A disadvantage is that the amount of cooling that can be achieved is dependent on the supply condition of the incoming air. Air that has a high level of humidity already will not be able to absorb much moisture, so will not be subject to the cooling effect of the humidifier.

• AHU exhaust air evaporative cooling: This strategy uses the exhaust air in combination with a heat recovery unit to pre-treat the supply air prior to it being introduced to the room. When the air is extracted from the room, it is typically warmer than the incoming air and unsuitable for use in cooling it. However, by humidifying this air, and economically reducing its temperature below that of the incoming air, it can be run through a heat recovery unit and its cool thermal energy transferred to the incoming air.

Exhaust air evaporative cooling.

During the summer months, this can reduce the temperature of the incoming air by several degrees and lower the need for additional DX cooling, thus reducing a HVAC system’s energy costs. During colder periods, the warm exhaust air can be used without the humidifier to pre-heat the incoming air, thus reducing heating costs.

As there is no mixing of the humidified exhaust air and the incoming fresh air, cooling occurs irrespective of the incoming air’s humidity level.

• AHU indirect evaporative cooling: This strategy uses the outside air to cool an internal environment, without allowing external and internal airstreams to directly mix. The outside air is drawn through a heat recovery unit then immediately exhausted, while internal air is drawn from the room and circulated through the heat recovery unit before being re-introduced to the room. The outside air’s thermal energy is thus transferred to the internal air via the heat recovery unit without the two streams directly mixing.

By humidifying the outside air prior to it entering the heat recovery unit, its temperature is reduced and cooling is applied to the internal atmosphere. This enhances the capacity of the system and makes it effective even during periods when the outside temperature is warmer than the desired internal room condition.

Indirect evaporative cooling.

• Direct air evaporative cooling: For buildings and areas that do not have a central air-conditioning system, direct air evaporative cooling can be a very economical and achievable way to reduce the temperature. Spray humidifiers deliver moisture directly to the atmosphere using a series of spray nozzles. Strategically located throughout the space, spray nozzles deliver cooling at a fraction of the cost of traditional DX air-conditioning.

As with in-duct direct evaporative cooling, the amount of cooling achieved is dependent on the humidity levels of the area, as high humidity will prevent further moisture being absorbed and its resultant cooling effect. This strategy is best employed within an area that has very active ventilation and a high level of air exchange to keep the ambient humidity low enough to continually absorb moisture.

This strategy can also be employed outdoors, as was the case when Condair created the world’s largest evaporative cooling system around the Prophet’s Mosque in Madinah, Saudi Arabia. Across the 15-hectare site, nozzles release up to 50,000 litres of moisture per hour, providing 34,000 kW of evaporative cooling to the desert air and reducing its temperature by up to 10 deg C.

*Condair Middle East is the UAE-based office of Condair, which manufactures and supplies a comprehensive range of in-duct and direct air humidifiers, and offers expert technical advice on how they can be employed to meet a building’s HVAC objectives for either humidity control or cooling.