![]() The height of the hot air column from ground floor to outlet air duct above roof is approximately 8 m. The friction coefficient used is 0.019 which is appropriate for normal galvanized steel ducts.Ĭalculate the air flow caused by natural draft in a normal family house with two floors. The calculator below can be used to calculate the air flow volume and velocity in a duct similar to the drawing above. Q = air volume (m 3/s) Natural Draft Air Flow and Velocity Calculator V = 1/2 (4)Įquation (4) can also be modified to express the air flow volume through the duct Σ ξ = minor loss coefficient (summarized) Air Flow and Air VelocityĮquation (1) and (3) can be combined to express the air velocity through the duct The major and minor loss in the system can be expressed asĭp = λ (l / d h) ( ρ r v 2 / 2) + Σξ 1/2 ρ r v 2 (3)ĭp = pressure loss (Pa, N/m 2, lb f/ft 2) The natural draft force will be balanced to the major and minor loss in ducts, inlets and outlets. Make a Shortcut to this Calculator on Your Home Screen?.The calculator below can be used to calculate the natural draft pressure generated by the inside and outside temperature difference. Ρ = (1.293 kg/m 3 ) (273 K) / (273 K + t) (2)Įquation (1) above can easily be modified by replacing the densities with equation (2). With air density 1.293 kg/m 3 at 0 oC - the air density at any temperature can be expressed as G = acceleration of gravity - 9.81 (m/s 2) Density and Temperature static pressure and pressure head in fluidsĮquation (1) can be modified to SI pressure units:.H = height between outlet and inlet air (m) Ρ h2o = density water (in general 1000 kg/m 3) The natural draft head can be calculated asĭh mmH2O = 1000 h (ρ o - ρ r) / ρ h2o (1)ĭh mmH2O = head in millimeter water column (mm H 2O) Warmer outside air flows in to the upper parts of the building. If the outside air temperature is higher than the inside air temperature - the inside air is more dense than the outside air - and the air flows down inside in the building. The colder outside air will flow in to the lower parts of the building. If the inside air temperature is higher than the outside air temperature, the inside air density is less than the outside air density, and the inside air will flow up and out of the upper parts of the building. The direction of the air flow depends on the temperature of the outside and inside air. The temperature difference between the outside air and the inside air creates a "natural draft" forcing air to flow through the building.
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