Displacement ventilation systems differ from conventional HVAC systems in several important ways. In a conventional HVAC system, air is supplied at the ceiling, at a relatively high velocity, at a temperature about 20°F below the desired room temperature. The supply air mixes with the room air to provide a nearly uniform temperature throughout the space. Because of the mixing effect, "used" room air re-circulates, resulting in relatively low ventilation efficiency. 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 one of fresh cool air at the occupied level. Heat and contaminants produced by activities in the space rise to the ceiling level where they are exhausted from the space. Here are some placement options:
- Fabric diffusers can be placed along the ceiling in round, half-round, and quarter-round shapes, providing versatility in design and can be made to fit any environment with regard to wall shape or obstructions.
- Fabric diffusers can run vertically from the ceiling supply duct. In this case, the last 7 feet or so of the vertical duct can be your diffuser.
- Fabric diffusers can run inside a vertical column. The last 7 feet of the column will be cut out to allow for the insertion of a grid cover for aesthetics, fit flush with the shape of the column.
- Fabric diffusers can run outside a vertical column and fit flush with its shape. Diffuser color can be designed to match exactly that of the column.
- For cooling applications, fabric ducts can run the length or width of the ceiling, supplying a pool of low-velocity, cool fresh air. As this supply of air is heavier than the air below, it will sink. Thus, the placement of the return or exhaust is crucial to the directional movement of air.
- Air quality is calculated to maintain OSHA minimum levels of particulate in the indoor air.
- Heat loads are determined and air-change requirements are determined.
- Air displacement vents, fresh air make-up units, exhausters, heat sources and control Systems are sized and selected.
- Building height at walls and center, floor dimensions, and layout
- Floor velocity (Total airflow / Total floor area) calculated to be ~ 100 FPM
- Supply and vent locations determine the direction of the low-velocity airflow
- Equipment, ductwork, piping, and other utilities are laid out in the plans.
- Hotter exhaust temperatures provide for the recovery of more energy, making air to air energy exchangers extremely effective, putting investment return on a fast track
- Economic analysis of fuel and electric consumption are compared with installation and material costs to compute payback times.