The Benefits of Displacement Ventilation

One of the most common forms of air distribution for HVAC system is overhead mixing, which is why you will often see air distribution devices--diffusers--blowing tempered air on commercial facilities such as offices, schools and hospitals. 

Diffusers use high pressure to throw the tempered air into a room, mix with the room air and subsequently heat or cool the space.

• Less energy because the space is condition only when occupied

• Less energy from reduced fan power by blowing that air at a slower pace

• Increased comfort of occupants because less air movement 

• Decreased contaminant risks, because there is no mixing of air.  

This technology has its flaws, as the mixed air can be subjected to airborne contamination and consequently lead to poor indoor air quality. Another concern with mixing ventilation is the location of the diffuers. It is common for diffusers to be placed 8-12 ft above the floor. This means that energy is wasted heating and cooling unoccupied space.  

For these reasons, an increasing number of energy and HVAC engineers are turning to alternative designs  in order to temper the space in more effective, efficient and clean ways. 

A popular alternative to this traditional mixing ventilation is a method referred to as displacement ventilation.  Displacement ventilation  is an air distribution technology that introduces cool air to a space at a low velocity through larger diffusers usually located near floor level.  

By utilizing buoyancy forces in a room, generated by heat sources such as people, lighting, computers, electrical equipment, exc., this system is able to remove contaminants and heat from the occupied space.  The conditioned air is then able to migrate naturally to the heat sources of occupants throughout the room. This is due to the fact that heat drives air movement, and since our bodies naturally give off a thermal plume of consistent heat, the conditioned air finds its way to us and cools us off. 

As air conditioned air enters the space, it displaces the room air through natural buoyancy and exhausts through a high point in the room.  Because the the air is not mixed contaminant particles can be removed. 

Displacement Air Ventilation is Recommended

Displacement ventilation is a recommended ventilation system for a variety of facilities because of the above mentioned benefits.  The ASHARE has passed Addendum G to standard 170-2008 "Ventilation of Healthcare Facilities" and recognizes the use of displacement ventilation in healthcare facilities, likewise The Collaborative for High Performance Schools recommends displacement ventilation as the preferred distribution method in education facilities.

Sources: 

Erway, Joel. Engineering.com " Rethinking Air Distribution with HVAC Systems" 15 October 2013. 

Engineering Guide-Displacement Ventilation Guide"