Evaporator (Cooling) Coils & Moisture Carryover

Moisture carryover is present on DX (Evaporator) Coils or Chilled Water coils where dehumidification happens.  Many people do not think it is a problem…until you have moisture running down ductwork or spewing all over the inside of an air handler.

Most coil manufacturers do not typically build Evaporator Coils over 50” tall. Applications that call for a 96” high coil, will be built with 2 - 48” high coils and stack them with an intermediate drain pain. This is done for a couple reasons: first, the shipping & handling is far easier and there is less chance for damage before the coil even gets to the jobsite. Second, the drain pan in the bottom of the unit for a 96” high coil would be enormous, and it would be practically “raining” off the top of a coil 96” high.

Air velocity for Evaporator Coils should never be higher than 550 feet/min!  Anything higher, and you are asking for complications.

Entering air temperatures of 80/67 of return air in the Northeast carry far less moisture than an outside 95/78 entering air temperature in Florida. Outside air always has more moisture.

Your location plays a part as well. The drain pans will need to be sized differently. Florida’s will be much larger in size.

Fin design is irrelevant when it comes to moisture carryover. Whether you have copper corrugated fins, or aluminum flat fins, plate fins or even the old-fashioned spiral fins, none of it has any effect on moisture carryover.

Lastly, be careful when installing a new Evaporator Coil in a system. Many end users like to increase the airflow on older coils because those old coils can act like filters, the fins are covered in dirt/dust, and you are not getting the same airflow through the coil. This dirt on the coil also semi-prevents moisture carryover. When that brand new chilled water coil is installed, the airflow might be higher than that 550 ft/minute and that, of course, will cause moisture carryover problems. 

y simply react to the air across the outside of the coil and whatever is running through the inside of the coil. Coils are 100% a function of your entire system, as well as the installation in general.