AHU - too much air

[md1976]

Working on a project (i'm doing electrical work) where the user had a 5 ton fan coil unit that is over capacity. it is currently running @ 128% of it's rated capacity (CFM).

The user is concerned that this is too much air and worried about the unit. I am out of my element on this one.

1. Can this hurt the unit? It does not appear to be having any issues. It is a hot water/chilled water fan coil.

2. How can it be reduced... apparently the contractor already tried to balance it back..? sheeves?

Thanks
Brian

 

[MintJulep]

Can it hurt the unit?

Fan power is proportional to the cube of the flow. So the motor may be running over its rated load.

Excess air velocity over a cooling coil can cause the moisture the condenses to become entrained in the airflow. This moisture will eventually fall out of the air stream somewhere, possibly causing corrosion, mold, water dripping etc.

How can it be reduced?

If it is a belt drive fan, chance the pulley sizes.

If it is something else, undo whatever was done to increase it.

 

[chicopee]

develop a fan performance curve and the system performance graph. At the juncture of the two curves which is the ideal operating condition, you will be in a better position to develop an opinion about the current set up.

 

[TheTick]

See if the pulleys on the fan are the adjustable type.

Are all of the outlets balanced/properly damped?

 

[zekeman]

If this is a new unit, then the HVAC guy should have supplied the proper blower motor to match the CFM requirements and usually have a 3 speed. single speed with variable pulley or variable speed motor to do the job, including balancing, etc. Which is it? And how was the CFM determined?
If we are talking single speed then:
First thing is to measure the current in the motor line using and see if it is exceeding the rated current for that motor. That will determine if it is overloaded. Using the CFM is problematical since that measurement may be inaccurate.
Also, if CFM is the issue then you can "slow" down the flow by adding dampers to the ductwork, or as others have said, to change the pulley ratio.