Outside Air Dampers Icing Up

[SRBKDW]

An owner has asked me to look at an existing condition in a very cold weather climate facility. The main mechanical room is located on the outside wall of the building to facilitate economizer operation. The outside air louvers and outside air dampers are located on one of the mechanical room's exterior wall. The entire mechanical room acts as a mixing plenum for return air and outside air. Note the return air (conditioned air) is humidified in winter to ~40% RH. You can picture what happens. When the humidified air touches the outside air dampers the water condenses on the dampers. It then drips down towards the bottom of the dampers and then ices up on both the lower portion of the dampers and on the floor of the mechanical room. The ice is up to two inches thick in spots and obviously hinders the dampers operation. Are there "heated" dampers or similar for cold weather applications. Any other ideas for dealing with this situation are greatly appreciated. Thanks.

 

[lilliput1]

Can you put the humidifier downstream of the heating coil & cooling coil? What is the design minimum temperture of the mixed air in the mechanical room? Will you get saturation w/ humidity at 40% RH of the design indoor temperature? What is the design indoor temperature & minimum temperature downstream of the heating & cooling coil?

 

[SRBKDW]

The humidifier is downstream of the cooling coil. It's the previously humidified return air that is causing the problem I believe when it condenses on the cold outside air dampers. The supply air temperature setpoint is 50 degrees.

 

[imok2]

Can you partition the return air from directly mixing at the OSA dampers and get a better mix with the Drier OSA and absorb more moisture from the return air? A better mix with OSA would probably help the situation. Or maybe extend the return air duct closer to the supply fan!

 

[wilg]

Either go with imok's idea of separating
the two air streams (possibly with ducting)
reduce the outside air volume,lower the
space %RH, or lower the return %RH prior to air
entering the Mechanical room.
Another idea is to position a unit heater
to discharge directly toward the OA damper
and place a condensate pan under the damper.
Using the mechanical room as a mix box is

 

[ChasBean1]

I don't know if I'd reduce outside air. Is the plenum (MER)sufficiently negative? If there's neutral or fluctuating pressure between the MER and OA, that could definitely contribute to the phenomena. I wonder what the return fan volume versus supply fan volume and room pressure is(?)

I also like the idea of building a duct doghouse (extend ductwork from the OA damper bank inward into the plenum). An approx ~4 or more foot structure could minimize venturi effect of drawing moist air against the dampers via OA intake velocity. It could also offer a location for placement of a flow station if desired.

 

[imok2]

ChasBean1: Now that you mentioned flow station what do you think of the following:
International Standard ISO 3966, Measurement of fluid flow in closed conduits - Velocity area method using Pitot static tubes (1977)3, defines the currently accepted method for traversing ducts. The Standard recommends a minimum of 25 points be measured in rectangular ducts. The ISO Standard has also been adopted as the basis of ANSI/ASHRAE Standard 1111.
But get this!!!Tests conducted at the U.S. Army Corps of Engineers’ Construction Engineering Research Laboratory (UACERL) evaluating sensor density of traverses and various airflow measuring devices, concluded that low sensor density (air) probes (3 sensors in a 22" x 40" duct) resulted in the same accuracy as a 35 point traverse measurement .
Any comments?

 

[ChasBean1]

I think it would be easy to obtain similar results between two or more devices if they are in a straight run of duct with uniform, turbulent flow. At a large OA intake such as mentioned above though? I'd prefer more points. Such an application could have areas at the opening with flow reversals, etc. that would result in measurement error... There's also potential for more error due to range - OA intakes are typically ~500 fpm intake velocity where ducts might have 1,000-3,000 fpm, which would define with less fluctuation the difference between the static & total pressures. 3 sensors at an OA intake location wouldn't cut it.