Many homes in steamy hot tropical regions are multi-story open structures. They are cooled with natural air displacement. Built into a high river bank with large louvered openings at ground level, facing a moist riverbed with the outlet high on the opposite top side above the bank for the prevalent breeze to induce the pressurized cool air up through the house. Also known as Natural Convection for re-circulated air, this type of air movement is caused by differences in the relative weight or pressure of a given volume of air.

Natural Convection Vertical Air Displacement Function
The principle behind natural air displacement can be readily observed on a psychometric chart. One pound of dry air at 800 F occupies 13.36 ft3 whereas 600 F air occupies 13.20 ft3. Inverting these quantities, one cubic foot of 800F dry air weighs 0.0735 lb/ft3 and 600F air weighs 0.0763 lb/ft3, The pressure exerted by the 600F dry air is greater than the 800F air. The magnitude of this air pressure difference can be illustrated by imagining a 10 ft X 10 ft air opening with still 600F air pushing with 7.63 lbs. but the same 800F air pushing with only 7.37 lbs. The cooler air flows in a stream to the warmer air in a natural attempt to equalize pressure..
These conditions often exist in high bay industrial plants. Provided fans or open doors do not blow or mix the plant air, this still air seeks its own thermal level with 800F air at the ceiling and 600F air on the floor. Then when a 10 ft X 10 ft opening placed low near the floor lets tempered 600F outdoor air slowly flow in, a standing pool of fresh air above the floor level will form. As heat from processes, people or heaters increase the air temperature, this heated air at say 800F flows to the peak of the ceiling where a stack or roof opening vents this warm air outdoors. This natural draft system is powered solely by the heat added in the plant to increase the air temperature resulting in a lower air pressure at the ceiling than at the floor. These natural convection and air displacement systems work well when 1) Cool air enters low at floor level where it stays, 2) Fresh air enters slowly to prevent mixing with plant air and 3) Heat from any source is added within the ventilated area.

Conventional and Air Displacement Comparison
Conventional "Mixed/Diluted-Air" systems typically filter or exhaust polluted air. Filter units re-circulate the cleaned air but when exhausters are used, some type of fuel-fired make-up air units blow in heated air to keep the plant warm. Neither method provides high quality air in a welding environment because each only mix or dilute the smoky air generated by the welders with some clean supply air resulting in less smoky air throughout the plant. Filters do not purge the chemical gases but allow them to build-up concentration with the re-circulation. Another adaptation of the "Mixed/Diluted-Air" principle is a horizontal, forced air displacement system where air is "Displaced" by higher velocity air from the unit and "Throws" the primary heated air horizontally and high across the plant, inevitably inducing secondary plant air into itself to mix and dilute the plant air to an averaged temperature and pollutant level. Operating costs for all adaptations of "Mixed/Diluted-Air" systems are high. Plant air is rarely exceptionally clean because the design criteria only calls for enough clean air to mix with the polluted air to result in an average air dilution level to meet the minimum acceptable pollution requirements . (Insert" Conventional "Mixed/Diluted" Air" diagram}

Natural Air Displacement Ventilation systems by contrast use "Separated/Displacement Air" to provide exceptionally clean breathing air at much lower operating costs. Nature's way of moving heated air to the ceiling by natural convection without blowing air around is used. This natural or vertical air displacement system distinguishes itself from forced, horizontal air displacement systems because 1) air movement is by gravity and 2) clean air below is separated from polluted air above. Thermal layers of air from floor to roof keep the clean air near the floor and the dirty, contaminated warm air moving to the roof peak.

Fresh, outdoor ventilation air is tempered to within 5 F of desired plant temperature by a fresh air make-up air unit, distributed through extremely low velocity (10 fpm) floor mounted air displacement vents and allowed to accumulate a literal "pool" of high quality fresh air at floor level to breathe in the welding area. The high temperature of the welding arc pulls the heavier fresh air off the floor into it, rapidly heats and expands this air which entrains the generated smoke from the arc and both speed directly to the ceiling in a plume of smoke. A roof exhauster continually purges this concentrated hot air at the ceiling. Cooler, heavier fresh ventilation air remains on the floor separated from the hot, lighter polluted air at the ceiling. Radiant heaters warm the employees while they breath only uncontaminated air from the lake of clean fresh-air immersing them. Sizing is based on enough ventilation air to maintain a 6-7 foot high layer or pool of clean air. Excessive fresh air is not required since all the concentrated, polluted air is exhausted from the ceiling, never to be re-circulated.