Chill water Coil Airflow

[newmep]

On a large old central AHU (30000 cfm), the maintenance wants to increase the airflow. I think that the chill water coil should have been designed for a specific airflow and we can not just increase the fan cfm. Please comment.
Thanks

 

[AbbyNormal]

The airflow should have been based on what ever the sensible cooling load was.

You can change the airflow, increasing it will give you more total cooling and a reduction in latent capacity.

Increasing it too much can result in mositure carry over also.

Sometimes addressing duct leakage results in more airflow to the space.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ChrisConley]

A big issue to watch would be the velocity over the coil (as AbbyNormal alludes to). If the air over the coil velocity increases significantly over 500fpm you'll start blowing water off the coil and into the ductwork... not usually a good thing.

 

[newmep]

Chris Conley:
Is there a name for this max air velocity (max face velocity or max tube film velocity) or something technical like that.

thanks,

 

[imok2]

If the air flow face velocity is les then 500 fpm then you can increase it. With draw-through systems, limit cooling-coil average face velocity at design conditions to 500 fpm.
Because of the difficulty in predicting variations in localized coil velocities, avoid blow-through designs when possible. When a blow-through design is necessary, limit cooling-coil face velocity at design conditions to less than 500 fpm to avoid carryover from certain sections of the coil.

 

[AbbyNormal]

I prefer blow through on the high sensible loads. Means less overall air flow, less fan power, less fan heat, less cooling capacity needed.

Draw through on big sensible loads, CAN force you to overdehumidify air to compensate for fan reheat,and force you to move more air.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ChrisConley]

newmep: The 'technical' term is maximum face velocity over the coil.

Abby, I've done a couple blow through and always ended up using a profile plate (large pressure drop!) to ensure even flow over the coil.

Do you do something similar?

 

[AbbyNormal]

last blow through I did was a plenum fan

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

only ever used a profile plate when filling in forms in the design department to make "HE" direct fired make up air units

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ChrisConley]

An ex-EngA employee eh? I got my start with ICE (BMA direct fired MUA) in Winnipeg.

 

[AbbyNormal]

Was there Larry D selling the stuff?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[walkes]

Larry D only stopped selling the stuff a few years ago.

 

[ChrisConley]

You bet, I worked a little with Larry. I was more in the plant.

 

[AbbyNormal]

guy burned me a couple times

Take the "V" out of HVAC and you are left with a HAC(k) job.