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 |