Blow-Through type air handlers, with the cooling coils placed downstream of the supply fan, can be problematic in humid climates when final filters (or other significant pressure drop) are downstream of the coils. The air leaving the cooling coils is typically saturated (dry bulb temp = wet bulb temp), so that the pressure drop of the filters causes further water vapor to come out of the airstream – and into the filters. This results in wet filters with shortened life or worse, a fertile place for biological growth.
AHU-19 at Shands Healthcare is a large (156,000 cfm) built-in-place air handler, constructed in a blow-through configuration and having final filters. The final filters were perpetually wet, leading to excessive pressure drop in the filters, and requiring frequent (and expensive) replacement. The solution to the problem: Desaturation coils. Desaturation coils are essentially standard chilled water cooling coils, but with additional rows on the leaving air side of the coil that are piped (internal to the coil) to route return water through these rows. No additional piping or controls are required. The warm (relatively speaking) return water serves to lift the leaving air condition a few degrees off the saturation point (dry bulb temp > wet bulb), elimi- nating wet final filters.
Fortunately there was sufficient space in the unit such that
new desaturation coils could be installed (in a new coil rack)
while the unit was in operation. Final tie-in/change-over of
the main supply and return chilled water was made during
a period that permitted economizer cooling for the short
outage involved.
A Perfect Solution
Desaturation Coils work as combination cooling and reheat coils to restore your application’s LAT and humidity to the desired levels. This is accomplished through an energy and cost- efficient process on the air side and the in the water loop of the coil.
