HUMIDIFICATION & DEHUMIDIFICATION


How humidifiers can provide cooling recovery


Heat recovery in the winter on extract air is commonplace to reduce heating energy use, but many building operators are unaware that the same thing can be done to reduce cooling loads in the summer. Dave Marshall-George explains how humidifiers can deliver cooling to a building’s fresh air supply.


Dave Marshall- George


Sales director at Condair


www.condair.co.uk R


ecovering heat from a building’s extract air is a well-established method for reducing HVAC energy


consumption during the winter. However, recovering thermal energy for cooling in the summer also offers huge potential but is a greatly underused strategy. Now, given the relative similarity


in temperature between incoming and outgoing summer air, you might think that warm extract air from a room offers little potential for cooling incoming air, other than on the hottest of days. And by itself, you’d mostly be right. However, by adding a low energy, evaporative humidifier to the exhaust airstream prior to an energy recovery system, the temperature of the extract air can be greatly reduced. This increases the potential for energy recovery from outgoing to incoming airstreams in the summer, allowing for up to 10°C of very low energy cooling to be achieved.


Evaporative cooling When water evaporates, it uses heat energy from the air during the process. This evaporative cooling effect equates to around 0.68kW of cooling energy for every 1kg of water evaporated. A single evaporative humidifier can provide up to 1,000kg of water per hour with a resultant 680kW of evaporative cooling, whilst consuming less than 1kW of electricity. So, by adding an evaporative humidifier to the extract airstream, an energy recovery system’s potential for providing cooling in the summer becomes significant. To put some figures against this


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potential, if the air extracted from the room is 21°C and 50%RH, and is humidified to 95%RH, a run-around energy recovery system with a 90% efficiency could pre-cool incoming air of 28°C and 44%RH by up to 11°C. This level of cooling would significantly reduce a building’s HVAC energy consumption during the summer. It may not always replace the need for mechanical cooling, but it will greatly reduce the size of chiller needed and the ongoing energy consumption for mechanical cooling. This will lower a building’s carbon footprint and improve its sustainability. This type of evaporative cooling


effect is more well-known for being used directly on incoming fresh air. However, the cooling potential in a direct air strategy is limited by the humidity of the outside air and the allowable maximum humidity inside the building. But by placing the humidifier on the exhaust airstream, rather than the incoming, the starting humidity condition is never that high. This allows for a good amount of evaporation and cooling from the humidifier to be achieved. And secondly, because the humidification is occurring on the


extract air, there is no upper limit on the humidity level after the humidifier, so air can be taken right up to saturation point, with maximum cooling effect. This makes an exhaust air cooling strategy a more practical and consistent method of using humidifiers for indoor evaporative cooling than a direct air method.


Austrian example A recent successful illustration of this strategy can be seen in a new- build health clinic in Tyrol, Austria. Condair worked with energy recovery specialist, Konvekta, and supplied its Condair ME evaporative humidifiers. They cooled the extract air prior to it being run through Konvekta’s high efficiency run-around energy recovery systems. Amir Ibrahimagic, group business


development manager at Konvekta, explains: “Hospitals, research labs, and universities have a particularly high interest when it comes to energy recovery systems because they have extensive operational times, leading to high energy costs. With a good energy recovery unit, they can massively decrease their energy use. For this


Exhaust air evaporative cooling in action


project in Hull in Tyrol, we installed three energy recovery units with Condair ME evaporative humidifiers. Overall, ongoing monitoring showed a proven annual energy recovery of 92% on an annual basis across all Konvekta systems. “A single Condair ME humidifier


contributed more than 60,000kWh of cooling during this annual period from just 3,800kWh of consumed electricity. This low energy cooling significantly improved the building’s overall energy efficiency and helped our client become the most sustainable clinic of its type in all of Austria,” he concludes.


Online calculator For designers who want to know how much cooling would be available using this exhaust air evaporative cooling strategy for a specific project, Condair offers an accurate calculator on the website https://mycoolblue.com. It has worldwide weather data from over 300 locations and allows variable AHU configurations to be set. Accurate calculations are provided


for the amount of exhaust air cooling that will be delivered, alongside mechanical, in different climates and under different building operating conditions. For instance, a building in London with a room set point of 22°C, operating during office hours only, with a 3x3m evaporative humidifier and 3m/s airflow, could meet 55% of its annual cooling needs with exhaust air cooling. Whilst the same building in Sydney, Australia, would be able to achieve 36% of its cooling in the same way. As room conditions are relatively


The Condair ME provides low energy humidity control and evaporative cooling to an air handling unit or duct


consistent in air-conditioned buildings around the world, the strategy of exhaust air cooling is universal. Exhaust air evaporative cooling is a proven and effective strategy, and if more widely understood and applied, could help our industry take a further step towards its net zero goals. ■


EIBI | JULY � AUGUST 2024


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