OA Mixing distance 1

[Clymber]

I am looking for a general rule for "thorough" mixing of outside air and return air prior to entering an air handler. The air handler in consideration has no mixing plenum, so this needs to happen upstream. To keep cost down, I would prefer to accomplish mixing without addition of an "air blender" device. What is a minimum or safe distance between the OA branch and the intake on the air handler to accomplish this?

For this example, I have ~25% outside air in a constant volume system.

TIA.

 

[ChrisConley]

I don't have a rule of thumb, but one technique you can use to promote mixing to is specify parallel blades on the mixing box, rather than opposed blades. This will help replace an air blender device.

 

[MintJulep]

This is an area where rules of thumb can get you into trouble. There are a bunch of variables involved, including physical geometry, the densities of the two air streams, and their velocities.

Non-uniform air flow, or stratified air flow entering your coil can have a significant effect on the coil performance.

An air blender is a cheap, simple static device. Will pay for itself the first time you don't have to go investigate why the unit is no performing as expected

 

[remp]

Keep the fresh air intake damper and the return air intake damper to the mixing chamber close together rather than on opposite sides of the mixing chamber, This will help get a better mix before entering the Air handling unit.

 

[lilliput1]

You don't have that much OA & it is constant volumeso it should not be too big a problem. If OA design is 10°F and indoor design is say 70°F your mixed air °F T = .25 x 10 +.75 x 70 = 55°F which is above freezing.

It would be better for the air stream to blow against each other using opposed blade damper. Another way would be to put a fan powered unit heater in the plenum to stir up the air. Put the thermostat low & set for 40°F.