NATURAL VENTILATION
The equation below is used in calculating ventilation (or infiltration) due to the stack effect.
Q = C x A x [h x (ti – to )] / ti
In this equation:
• Q = Air Flow Rate (CFM)
• C = constant of proportionality = 313 (This assumes a value of 65 percent of the maximum theoretical flow, due to limited effectiveness of actual openings. With less favorable conditions, due to indirect paths from openings to the stack, etc., the effectiveness drops to 50 percent, and C = 240.)
• A = area of cross-section through stack or outlets in sq ft. (Note: Inlet area must be at least equal to this amount)
• ti = (higher) temperature inside (°F), within the height h
• to = (lower) temperature outside (°F)
• h = height difference between inlets and outlets (ft)
OUTDOOR AIR
The equation for calculating outdoor quantities using carbon dioxide measurements is:
Outdoor air (in percent) = (Cr – Cs) x 100 / (Cr – Co)
Where:
Cs= ppm of carbon dioxide in the mixed air (if measured at an air handler) or in supply air (if measured in a room)
Cr= ppm of carbon dioxide in the return air
Co= ppm of carbon dioxide in the outdoor air
The auto-controller ensures that the increased ventilation is supplied only when required or needed for higher occupancies. This benefit in the energy cost s avings because of reduced cooling and heating of outdoor air during reduced occupancy rates.
DILUTION VENTILATION
Dilution ventilation is most often used to advantage to control the vapours from organic liquids such as the less toxic solvents. To determine the correct volume flow rate for dilution (Qd), it is neces sary to estimate the evaporation rate of the contaminant (qd) according to the following equation:
qd = 387 (lbs) / (MW) x (T) x (ρ)
Where
• qd = Evaporation rate in CFM
• 387 = Volume in cubic feet formed by the evaporation of one lb-mole of a substance, e.g. a solvent
• MW = Molecular weight of the emitted material
• lbs = Pounds of evaporated material
• T = Time of evaporation in minutes
• ρ = density c orrection factor
The appropriate dilution volume flow rate for toxics is:
Qd = qd x Km x 106 / Ca
Where
• Qd = Volume flow rate of air, in CFM
• qd = Evaporation rate in CFM
• Km = Mixing factor to account for poor or random mixing (note Km = 2 to 5; Km = 2 is optimum)
• Ca = Accessible airborne concentration of the material
