Smudging From Supply Diffusers

It is a mistake to think that smudging occurs strictly because of dirty filters. This is not always the case. When air is discharged from a diffuser, it sets up a secondary air flow which entrains air within the primary air discharged from the diffuser. This secondary air flow is turbulent and causes dirt particles, suspended in this secondary air flow, to be affected by the ceiling's surface effect.

The surface effect causes the suspended particles to be attracted to the ceiling and attach themselves to the surface. The effect is worse in areas with a lot of foot traffic. Heavy foot traffic causes a great number of particles to be put into suspension within the immediate air. Using low returns in areas with very heavy foot traffic and designing the returns with high face and entrainment velocities reduces the occurrence of smudging.

(see our publications #37, 29 and 84)

In facilities or areas of facilities which have normal foot traffic the smudging could be a result of particles that work their way into the building due to the building being under neutral or negative pressure as related to the building's ambient. The dirt particles after entering the building then become entrained in the diffusers air patterns and become deposited on the surface of the ceiling. Keeping the building pressurized, greatly reduces the effect of smudging.