Which damper arrangement promotes mixing by keeping the outside air velocity high as it enters the air handling unit?

Which damper arrangement promotes mixing by keeping the outside air velocity high as it enters the air handling unit?

• A. Two-section outside air damper with one stage opening before the other
• B. A single outside air damper
• C. Return air damper fully open
• D. Outside air damper closed during operation

Answer: (A)

Mixing inside the air handling unit hinges on outside air entering with enough momentum to create a turbulent jet that entrains and blends with the return air in the mixing box. A two-section outside air damper that opens one stage before the other creates that high-velocity entry first. When the first stage opens, outside air rushes in at a higher velocity, forming a jet that effectively stirs the incoming air and quickly mixes it with the return air. Once mixing has begun, the second stage can open to achieve the desired total outside-air amount, but the initial high velocity has already promoted thorough blending. A single outside air damper doesn’t generate that same high-velocity jet, and keeping the return damper fully open or outside air closed reduces the opportunity for mixing.

Mixing inside the air handling unit hinges on outside air entering with enough momentum to create a turbulent jet that entrains and blends with the return air in the mixing box. A two-section outside air damper that opens one stage before the other creates that high-velocity entry first. When the first stage opens, outside air rushes in at a higher velocity, forming a jet that effectively stirs the incoming air and quickly mixes it with the return air. Once mixing has begun, the second stage can open to achieve the desired total outside-air amount, but the initial high velocity has already promoted thorough blending. A single outside air damper doesn’t generate that same high-velocity jet, and keeping the return damper fully open or outside air closed reduces the opportunity for mixing.