Ventilation System Comparison

The charts below show the advantages of using displacement ventilation.

Overhead (Mixing) Underfloor Air Distribution (Mixing Displacement) Lower Wall (Displacement)
Description Diffusers located in the ceiling deliver 55°F air at velocity of 400-700 ft. per minute (fpm). Objective is a well mixed airspace. Diffusers mounted in the floor deliver 65°F air at about 100-200 fpm velocity. Air pattern causes some mixing in the occupied space, but a higher temperature near the ceiling. Diffusers mounted near the floor level deliver 65°F air and less than 75 fpm velocity. Air flow causes a thermally stratified space and vertical air movement towards the return.
Supply conditions Normally 55°F in cooling. Typically 60°F-64°F in cooling. Some temperature rise will occur in the underfloor plenum. Typically 63°F-68°F in cooling.
Architectural requirements Space above ceiling for ductwork and ceiling diffusers. Minimum ceiling height of 8-9 ft. recommended. A raised access floor is used as an air plenum and for wiring and communications. Possibility to reduce floor-to-floor height slightly. Minimum ceiling height of 9 ft. is recommended. Higher ceilings are preferred. Diffusers may take up some wall space. Floor-to-floor height is not necessarily impacted.
Thermal comfort Even temperatures throughout the space in cooling with proper design. Good thermal comfort with proper airflow. Potential for individual temperature control. Very good thermal comfort in cooling with proper design. Some potential for drafts near the diffusers.
Ventilation effectiveness FAIR-Supply air mixes with room air to dilute contaminants. GOOD-Better than overhead distribution, but some mixing occurs in the occupied zone. VERY GOOD-Supply air is delivered to occupants, and contaminants are displaced to the upper unoccupied zone.
Acoustic performance Diffusers can be a noise source if the air velocity is too high. Quieter due to low air velocity. Also quieter due to lower air velocity at the diffusers.
Applications Any Offices or any space with open floor plans. Schools, restaurants, theaters, atria, and other spaces with high ceilings.

Economic Comparison

Welding Facility 240’ Length X 210’ Width X 34’ Height
Design for 40 MIG Welders

Information Conventional Air Displacement Difference
Ventilation CFM Required 135,000 82,629 -52,371
Annual Fuel MBH Consumed 24, 376 12,720 -11,656
Annual Fuel Cost $127,457 $66,511 -$60,946
Installation Cost $160,000 $175,000 $15,000
Payback Time… 1/4 years
Air Conditioning Load (TR) 511 Tons 311 Tons -200 Tons
Rotary Exchanger
Deduct for Energy Recovery -146 Tons
Remaining Chiller Load (TR) 165 Tons