Airflow in buildings can be extremely complex, with time-dependent and multi-directional variables. The operation of mechanical cooling, heating, exhaust, in addition to other ventilation systems can have an enormous impact on pressure. Still, interstitial airflow and interstitial air pressure fields are often overlooked during planning, construction and HVAC implementation.  

Proper pressurization of buildings is crucial to indoor air quality (IAQ) management, maintenance of occupant health and comfort, not to mention maximizing energy efficiency. Lack of building pressurization management can allow infiltration of moisture, constant drafts in extreme weather and difficulty with doors at entrances and exits.

Forced vs. Induced Draft

The evaporative system is housed in what is commonly referred to as a “cooling tower.” Air is either:

• Forced into the tower by fans positioned on either side that “push” the air in (forced draft)
• Drawn into the tower by fans at the top that “pull” the air in (induced draft)

The tower is filled with a media, typically a desiccant material that holds water. The water flows down through the media before it is circulated from the bottom of the unit back up to the top again.

Counterflow vs. Crossflow

The tower’s flow designation describes the orientation of the airflow with respect to the direction of water flow inside the tower’s fill media. 

• Counterflow towers: air travels vertically through the fill, passing upward through the downward water flow
• Crossflow towers: air travels horizontally through the fill media, crossing the downward water flow. 

Counterflow towers are typically more efficient than crossflow towers.

Direct vs. Indirect vs. Hybrid

Evaporative cooling can cool air directly, indirectly or through a combination of both. 

Indirect methods, or can combine traditional style HVAC cooling coils as a hybrid form of cooling.

• Direct cooling happens through contact with water droplets, providing a humidifying effect. Direct systems mix outdoor air with return air from the building, using wetted media in the airstream to cool the blown air
• Indirect systems cool air without additional humidification by keeping moisture separate from the airstream
• Hybrid systems utilize traditional style refrigerant filled HVAC cooling coils that pull air past them, rather than through water-soaked media

Up-Draft vs. Side-Draft vs. Down-Draft

The draft designation of an evaporative cooler is based on the direction of airflow being blown out of the tower.

• An up-draft evaporative cooler is typically installed on the floor of a room and blows the cooled air upwards
• A side-draft evaporative cooler is mounted on the side of a building, and blows air into the interior horizontally, in a similar fashion to a window air conditioning unit
• A down-draft evaporative cooler is positioned on the roof of a building and blows air downward through a vent and short piece of ducting

Some evaporative units are stand-alone systems that require ductwork to be installed and utilize a strong blower fan to circulate air from a centralized location. 

Evaporative Coolers vs. Traditional HVAC Systems

The Pros and Cons of traditional HVAC systems:

Pros:

• Works in both dry and humid climates
• Can be easier to regulate temperature room by room
• Doesn’t create excess humidity

Cons:

• Have a higher cost for both purchase and installation
• Use more energy to operate
• Are more expensive to repair
• Dry air out, causing dry, itchy eyes and sinuses