increasing flow will decrease AHU capacity? 1

[naifmbo]

is that true?
increasing water flow for high value will decreases the AHU capacity?Why?

thanks.

 

[mdonner]

Water coil Heat transfer coefficient increases with water flow.
Heat transfer rate (water to air) is directly proportional to the product of water flow rate and water temp rise through water coil.
This means the AHU water coil has a larger capacity with increased water flow rate.

 

[Macbeth]

I agree with more water equals more AHU capacity, but with the faster flow a lower DT (Delta Temp) is produced a crossed the coil. If the chiller is designed with X GPM at Y DT and BTU = GPM * DT then lower DT equals lower capacity.

 

[mdonner]

To Macbeth
I have to disagree with your theory.
Lower DT means constant "capacity" with increased GPM across the coil. Actually you refer to constant prevailing "load" and not to increased potential "capacity".
With DESIGN heat load and increased GPM (beyond design value), DT will decrease compared to designed DT, but chiller LOAD will not change (capacity will increase a bit due to better heat transfer coefficient).
But coil increased capacity means that it can absorb larger heat load, keeping DT at design value.
Copressor load will be the problem: it may not be able to handle the increased capacity, impose lower DT and force the system to balance at a new working point (pressures, temperatures and capacity).