The airside is usually considered clean service, although some customers will specify an airside-fouling factor as a result of airborne contaminants.  Usually a fouling factor is only included for the process side of the cooler. The fouling factor is actually a resistance to heat transfer and is actually added to the other resistances due to the heat transfer coefficients

 There is no limit on the total size.  However, there is a limit on the width of one bay and height during transportation as restricted by roads and bridge heights.  Most manufacturers limit the width of one bay to 16 ft., which often coincides with the maximum 50HP belt drive system