[2A] DISPLACEMENT VENTILATION                             

BENEFITS VS. MIXING SYSTEMS

-  Much cleaner air for occupants (IEQ)
-  Reduced cross contamination between occupants
-  Greater ventilation effectiveness
-  Reduced power consumption
-  Better acoustics (less air noise)
-  No drafts
-  Lower equipment cost

HAS IT BEEN IN USE?

Displacement systems have been used with great success in office space and classrooms since 1970, and are effective in reducing cross contamination, since there is virtually no mixing in the space. 

In a displacement ventilation system, supply air is introduced to the space at or near the floor level, at a low velocity, at a temperature only slightly below the desired room temperature. The cooler supply air "displaces" the warmer room air, creating a zone of fresh cool air at the occupied level. Heat produced in the space moves air, including contaminants, to ceiling level where it is exhausted from the space.  This is much cleaner than conventional venting systems.

Displacement ventilation systems are typically more energy efficient, with lower fan horsepower, and quieter than conventional overhead systems. They also provide better ventilation effectiveness, and thus improve indoor air quality.¹

HVAC Systems Analysis (Massachusetts Institute of Technology)

ENERGY SAVINGS AND IEQ IMPROVEMENT

In a study² comparing displacement ventilation and conventional mixing ventilation, displacement ventilation consumed 57% less energy (Table 1 below).  Indoor environmental quality (IEQ) was also improved as measured by a 57% reduction in carbon monoxide emissions.  The Local Mean Age of Air was 7-9 minutes in the occupied zone for the mixing system and 2-8 minutes for the displacement system.  The plume around the occupants provided air to the occupant breathing zone with an age of 2.5-3 minutes.  The less time the air is in the room (before breathing), the less pollutants are absorbed into the air and the higher the IEQ.

Table 1 - Predicted Benefits of Displacement Ventilation

MV - Mixing ventilation model
DV - Displacement ventilation model
vDV - Demand led displacement ventilation system.

PRODUCTIVITY AND IEQ

Table 2³ details the cost of undesirable IAQ in terms of lost worker productivity in U.S. commercial buildings.  On average the data equate to $1.97 per square foot, or $840 per worker.

Table 2 - Worker Lost Productivity Costs in USA

DESIGN CONSIDERATIONS

SYSTEM PERFORMANCE EVALUATION AND DESIGN GUIDELINES FOR DISPLACEMENT VENTILATION By Qingyan Chen and Leon Glicksman "This book presents system performance evaluation, and includes a 10-step design guideline for displacement ventilation systems for U.S. buildings.  These design guidelines present two important models: 1) to calculate the temperature difference between the head and the foot level of an occupant; and 2) one to determine the ventilation effectiveness at the breathing level. "The book notes that: A displacement ventilation system can provide a thermally comfortable indoor environment at a high cooling load through careful design.  The indoor air quality in a space with displacement ventilation is better if the contaminant sources are associated with the heat sources.  The displacement ventilation system can also save energy but requires a separate heating system if it is applied to building perimeter zones." -----  From www.ashrae.org

"Underfloor air distribution (UFAD) and displacement ventilation (DV) often are considered synonyms for describing an air distribution system that uses a raised access floor plenum. However, just because the supply air is delivered into a room or space from an access floor does not necessarily mean it is a DV system. To apply these air delivery systems properly, building design professionals must understand the difference between the two systems."⁴

BRE has completed a project to determine whether displacement ventilation can be an effective cooling method that maximises the potential for using ambient air for low-cost, energy-efficient cooling.

The limited cooling capacity of most current displacement ventilation systems restricts their use to buildings with relatively low heat gains, or where an additional cooling system is provided. This project investigated the potential to improve displacement ventilation so that it can be used as a standalone method of cooling a wider range of buildings – and to maximise the use of ambient air as a cooling source.

Findings showed that displacement ventilation on its own is able to deal with far higher heat loads than usually thought.

Tests on a number of diffusers showed that conventional wall diffusers and prototype innovative fabric diffusers have the greatest potential for providing the higher air volumetric supply rates needed to allow displacement ventilation systems to deal with higher heat loads. ⁶

    Displacement Ventilation in Classrooms

    HVAC Design - Displacement Ventilation

    Advanced Buildings - Displacement Ventilation

Additional references on displacement ventilation design:

Jackman, P. 1990. Displacement Ventilation. Berkshire, UK. BSRIA (Building Services and Research Information Association).

¹ Source: www.greenengineer.com and www.xetexinc.com
² Cullen, N., Lea, H. High Performance Displacement Ventilation Using Fabric Diffusers - A Case Study, London: CIBSE.
³ Dorgan, C., et al. 1999. Indoor Air Quality - Standards of Performance. Atlanta, GA: ASHRAE.
⁴ McDonnell, G. 2003. Underfloor & Displacement - Why They're Not the Same. Atlanta: ASHRAE Journal.
⁵ Source: Malcolm Ravenscroft Ltd., Christchurch, NZ
⁶ Source: BRE (Building Research Establishment, Ltd., Watford, UK)

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